Explain it to me like I'm…High-school biology — proper terms explained
Glossary
Clear biology definitions for terms used throughout the explorers, searchable and grouped by topic.
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Hover highlighted terms anywhere in the app for a quick definition, or search and filter the full list below. Terms are grouped by topic so you can browse related ideas together.
DNA, RNA & proteins
Amino acid
Monomer with an amino group, carboxyl group, and unique side chain.
Also known as: amino acids
Anticodon
A three-nucleotideNucleotideThe building block of DNA and RNA: a sugar, a phosphate group, and a nitrogenous base (A, T/U, G, or C). sequence on tRNAtRNATransfer RNA: an adapter molecule that carries a specific amino acid and presents its anticodon to match an mRNA codon. that base-pairs with a complementary mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. codonCodonThree nucleotides in mRNA that code for one amino acid or a stop signal. during translationTranslationAssembly of a protein at the ribosome using mRNA as instructions..
Base pair
Hydrogen bonding between complementary bases (A with T, G with C) that holds the two DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. strands together.
Also known as: base pairing, complementary base pairing
Central dogma
The flow of genetic information from DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. → RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA. → proteinProteinA folded chain of amino acids that performs a specific function in the cell.. It describes how stored information becomes functional molecules in the cell.
Chromosome
A single, long DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. molecule packaged with histoneHistoneProteins that package DNA; their tags control gene access. proteinsProteinA folded chain of amino acids that performs a specific function in the cell. into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. (23 pairs), each carrying hundreds to thousands of genes.
Also known as: chromosomes
Codon
Three nucleotidesNucleotideThe building block of DNA and RNA: a sugar, a phosphate group, and a nitrogenous base (A, T/U, G, or C). in mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. that code for one amino acidAmino acidMonomer with an amino group, carboxyl group, and unique side chain. or a stop signal.
Also known as: codons
DNA
Double-stranded molecule storing genetic information as A, T, G, C bases.
Also known as: deoxyribonucleic acid
Double helix
The twisted ladder structure of DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. formed by two antiparallel nucleotideNucleotideThe building block of DNA and RNA: a sugar, a phosphate group, and a nitrogenous base (A, T/U, G, or C). strands wound around each other.
Genetic code
The set of rules mapping each mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. codonCodonThree nucleotides in mRNA that code for one amino acid or a stop signal. to a specific amino acidAmino acidMonomer with an amino group, carboxyl group, and unique side chain. or stop signal. It is nearly universal across life.
Hemoglobin
Oxygen-carrying proteinProteinA folded chain of amino acids that performs a specific function in the cell. in erythrocytes (α and β subunits).
Also known as: haemoglobin
Missense mutation
Point mutationMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual. changing a single amino acidAmino acidMonomer with an amino group, carboxyl group, and unique side chain. in the proteinProteinA folded chain of amino acids that performs a specific function in the cell..
Mitochondrial DNA
A small circular genome inside mitochondria, inherited almost exclusively from the mother. Because it does not recombine, it accumulates mutationsMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual. in a clock-like way and is widely used to trace maternal lineages and estimate evolutionary timings.
Also known as: mtDNA, mitochondrial genome
mRNA
Messenger RNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome.: carries a geneGeneA stretch of DNA that codes for a functional product, usually a protein.'s code from nucleus to ribosomeRibosomeCell structure that reads mRNA and links amino acids into proteins..
Also known as: messenger RNA, messenger RNA (mRNA)
Nucleotide
The building block of DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. and RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA.: a sugar, a phosphate group, and a nitrogenous base (A, T/U, G, or C).
Also known as: nucleotides
Peptide bond
A covalent bond formed between the carboxyl group of one amino acidAmino acidMonomer with an amino group, carboxyl group, and unique side chain. and the amino group of the next, linking amino acidsAmino acidMonomer with an amino group, carboxyl group, and unique side chain. into a chain. Peptide bondPeptide bondA covalent bond formed between the carboxyl group of one amino acid and the amino group of the next, linking amino acids into a chain. Peptide bond formation in the ribosome is catalysed by ribosomal RNA (rRNA), making the ribosome a ribozyme. formation in the ribosomeRibosomeCell structure that reads mRNA and links amino acids into proteins. is catalysed by ribosomal RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA. (rRNA), making the ribosomeRibosomeCell structure that reads mRNA and links amino acids into proteins. a ribozymeRibozymeCatalytic RNA molecule: evidence for the RNA world..
Also known as: peptide bonds, peptide
Polypeptide
A linear chain of amino acidsAmino acidMonomer with an amino group, carboxyl group, and unique side chain. linked by peptide bondsPeptide bondA covalent bond formed between the carboxyl group of one amino acid and the amino group of the next, linking amino acids into a chain. Peptide bond formation in the ribosome is catalysed by ribosomal RNA (rRNA), making the ribosome a ribozyme.. One or more folded polypeptides form a functional proteinProteinA folded chain of amino acids that performs a specific function in the cell..
Also known as: polypeptide chain
Protein
A folded chain of amino acidsAmino acidMonomer with an amino group, carboxyl group, and unique side chain. that performs a specific function in the cell.
Also known as: proteins
Purine
A class of nitrogenous bases with a two-ring structure: adenine (A) and guanine (G) in DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases..
Also known as: purines
Pyrimidine
A class of nitrogenous bases with a single-ring structure: cytosine (C), thymine (T) in DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases., and uracil (U) in RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA..
Also known as: pyrimidines
Ribosome
Cell structure that reads mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. and links amino acidsAmino acidMonomer with an amino group, carboxyl group, and unique side chain. into proteinsProteinA folded chain of amino acids that performs a specific function in the cell..
Also known as: ribosomes
RNA
Ribonucleic acidRNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA.: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. carries the proteinProteinA folded chain of amino acids that performs a specific function in the cell.-coding message from DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases..
Also known as: ribonucleic acid
Transcription
Making an RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA. copy of a geneGeneA stretch of DNA that codes for a functional product, usually a protein. from a DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. template.
Translation
Assembly of a proteinProteinA folded chain of amino acids that performs a specific function in the cell. at the ribosomeRibosomeCell structure that reads mRNA and links amino acids into proteins. using mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. as instructions.
tRNA
Transfer RNAtRNATransfer RNA: an adapter molecule that carries a specific amino acid and presents its anticodon to match an mRNA codon.: an adapter molecule that carries a specific amino acidAmino acidMonomer with an amino group, carboxyl group, and unique side chain. and presents its anticodonAnticodonA three-nucleotide sequence on tRNA that base-pairs with a complementary mRNA codon during translation. to match an mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. codonCodonThree nucleotides in mRNA that code for one amino acid or a stop signal..
Also known as: transfer RNA
Origin of life
Abiogenesis
Origin of lifeAbiogenesisOrigin of life from non-living matter: still being researched. from non-living matter: still being researched.
Also known as: origin of life
LUCA
Last universal common ancestorLUCALast universal common ancestor of all current life. of all current life.
Also known as: last universal common ancestor
Ribozyme
Catalytic RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA. molecule: evidence for the RNA worldRNA worldHypothesis that RNA was the first self-replicating molecule of life..
Also known as: ribozymes
RNA world
Hypothesis that RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA. was the first self-replicating molecule of life.
Epigenetics
Chromatin
DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases.–proteinProteinA folded chain of amino acids that performs a specific function in the cell. complex determining geneGeneA stretch of DNA that codes for a functional product, usually a protein. accessibility.
DNA methylation
Methyl groups added to DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases., often repressing transcriptionTranscriptionMaking an RNA copy of a gene from a DNA template..
Also known as: methylation
Epigenetics
GeneGeneA stretch of DNA that codes for a functional product, usually a protein. regulation without changing DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. sequence.
Genomic imprinting
Parent-specific silencing of one alleleAlleleAlternative forms of a gene at the same spot on a chromosome..
Also known as: imprinting
Histone
ProteinsProteinA folded chain of amino acids that performs a specific function in the cell. that package DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases.; their tags control geneGeneA stretch of DNA that codes for a functional product, usually a protein. access.
Also known as: histones
X-inactivation
Silencing one X chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. in females for equal dosage.
Also known as: X chromosome inactivation, Lyonization
Meiosis & cell division
Anaphase
Stage when sister chromatidsSister chromatidsThe two identical chromatids produced when a chromosome is replicated. They separate from each other in anaphase II. (or homologsHomologChromosome pair with the same genes but possibly different alleles. in meiosisMeiosisCell division producing haploid gametes through two rounds of division. I) separate.
Aneuploidy
An abnormal chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. number: having extra or missing copies of one or more chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes.. Usually caused by non-disjunctionNon-disjunctionFailure of chromosomes or sister chromatids to separate correctly during meiosis I or II (or mitosis), producing cells with too many or too few chromosomes. during meiosisMeiosisCell division producing haploid gametes through two rounds of division.. Trisomy 21 (Down syndrome) is a well-known example.
Also known as: aneuploid
Cell cycle
Ordered stages G1G1First interphase stage: cell growth before S phase replication., S, G2G2Second interphase stage after replication; prepares for division., and M phaseM phaseMitosis plus cytokinesis: the division phase of the cell cycle. by which a cell grows and divides.
Also known as: cell-cycle
Centromere
The constricted region where sister chromatidsSister chromatidsThe two identical chromatids produced when a chromosome is replicated. They separate from each other in anaphase II. are attached and where spindle fibers attach during cell division.
Chromatid
One of the two identical DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. strands making up a duplicated chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes., joined at the centromereCentromereThe constricted region where sister chromatids are attached and where spindle fibers attach during cell division..
Also known as: chromatids
Crossing over
DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. exchange between homologous chromosomesHomologChromosome pair with the same genes but possibly different alleles. in prophaseProphaseMitosis stage when chromosomes condense and the spindle forms. I.
Also known as: crossover
Cytokinesis
Division of the cytoplasm after nuclear division.
Diploid
Two complete sets of chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. (2nDiploidTwo complete sets of chromosomes (2n), one from each parent.), one from each parent.
Also known as: 2n
G1
First interphaseInterphaseG1, S, and G2 stages between divisions; DNA is copied in S phase. stage: cell growth before S phaseS phaseSynthesis phase of interphase when the genome is replicated. replication.
Also known as: G1 phase, Gap 1
G2
Second interphaseInterphaseG1, S, and G2 stages between divisions; DNA is copied in S phase. stage after replication; prepares for division.
Also known as: G2 phase, Gap 2
Haploid
One set of chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. (nHaploidOne set of chromosomes (n); gametes are haploid.); gametesGameteA haploid reproductive cell (egg or sperm) that fuses at fertilization. are haploidHaploidOne set of chromosomes (n); gametes are haploid..
Also known as: n
Homolog
ChromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. pair with the same genes but possibly different allelesAlleleAlternative forms of a gene at the same spot on a chromosome..
Also known as: homologs, homologous chromosomes, homologous
Independent assortment
The random orientation of homologHomologChromosome pair with the same genes but possibly different alleles. pairs at metaphaseMetaphaseStage when chromosomes align at the cell equator. I, which shuffles maternal and paternal chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. into gametesGameteA haploid reproductive cell (egg or sperm) that fuses at fertilization. independently of other chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. pairs.
Interphase
G1G1First interphase stage: cell growth before S phase replication., S, and G2G2Second interphase stage after replication; prepares for division. stages between divisions; DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. is copied in S phaseS phaseSynthesis phase of interphase when the genome is replicated..
M phase
MitosisMitosisOne division producing two genetically identical diploid daughter cells. plus cytokinesisCytokinesisDivision of the cytoplasm after nuclear division.: the division phase of the cell cycleCell cycleOrdered stages G1, S, G2, and M phase by which a cell grows and divides..
Also known as: M-phase, mitotic phase
Meiosis
Cell division producing haploidHaploidOne set of chromosomes (n); gametes are haploid. gametesGameteA haploid reproductive cell (egg or sperm) that fuses at fertilization. through two rounds of division.
Metaphase
Stage when chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. align at the cell equator.
Mitosis
One division producing two genetically identical diploidDiploidTwo complete sets of chromosomes (2n), one from each parent. daughter cells.
Non-disjunction
Failure of chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. or sister chromatidsSister chromatidsThe two identical chromatids produced when a chromosome is replicated. They separate from each other in anaphase II. to separate correctly during meiosisMeiosisCell division producing haploid gametes through two rounds of division. I or II (or mitosisMitosisOne division producing two genetically identical diploid daughter cells.), producing cells with too many or too few chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes..
Prophase
MitosisMitosisOne division producing two genetically identical diploid daughter cells. stage when chromosomesChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. condense and the spindle forms.
Recombinant
A chromatidChromatidOne of the two identical DNA strands making up a duplicated chromosome, joined at the centromere. or gameteGameteA haploid reproductive cell (egg or sperm) that fuses at fertilization. carrying a new combination of allelesAlleleAlternative forms of a gene at the same spot on a chromosome. that was not present on either original homologHomologChromosome pair with the same genes but possibly different alleles..
Also known as: recombinant chromatids, recombinant gametes
S phase
Synthesis phaseS phaseSynthesis phase of interphase when the genome is replicated. of interphaseInterphaseG1, S, and G2 stages between divisions; DNA is copied in S phase. when the genome is replicated.
Also known as: S-phase, synthesis phase
Sister chromatids
The two identical chromatidsChromatidOne of the two identical DNA strands making up a duplicated chromosome, joined at the centromere. produced when a chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. is replicated. They separate from each other in anaphaseAnaphaseStage when sister chromatids (or homologs in meiosis I) separate. II.
Synapsis
The pairing of homologous chromosomesHomologChromosome pair with the same genes but possibly different alleles. during prophaseProphaseMitosis stage when chromosomes condense and the spindle forms. I, allowing crossing overCrossing overDNA exchange between homologous chromosomes in prophase I. to occur.
Telophase
Final mitosisMitosisOne division producing two genetically identical diploid daughter cells. stage before cytokinesisCytokinesisDivision of the cytoplasm after nuclear division.; nuclei re-form.
Tetrad
A group of four chromatidsChromatidOne of the two identical DNA strands making up a duplicated chromosome, joined at the centromere. formed when homologous chromosomesHomologChromosome pair with the same genes but possibly different alleles. pair during prophaseProphaseMitosis stage when chromosomes condense and the spindle forms. I (two chromatidsChromatidOne of the two identical DNA strands making up a duplicated chromosome, joined at the centromere. per homologHomologChromosome pair with the same genes but possibly different alleles.).
Inheritance & genetics
Allele
Alternative forms of a geneGeneA stretch of DNA that codes for a functional product, usually a protein. at the same spot on a chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes..
Also known as: alleles
Autosomal
Referring to a geneGeneA stretch of DNA that codes for a functional product, usually a protein. on a non-sex chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. (autosome). AutosomalAutosomalReferring to a gene on a non-sex chromosome (autosome). Autosomal genes are inherited equally by males and females. genes are inherited equally by males and females.
BRCA1 / BRCA2
BRCA1BRCA1 / BRCA2BRCA1/BRCA2 DNA repair genes; inherited mutations raise breast/ovarian risk (BRCA1 higher ovarian risk than BRCA2). Figures vary by study./BRCA2BRCA1 / BRCA2BRCA1/BRCA2 DNA repair genes; inherited mutations raise breast/ovarian risk (BRCA1 higher ovarian risk than BRCA2). Figures vary by study. DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. repair genes; inherited mutationsMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual. raise breast/ovarian risk (BRCA1BRCA1 / BRCA2BRCA1/BRCA2 DNA repair genes; inherited mutations raise breast/ovarian risk (BRCA1 higher ovarian risk than BRCA2). Figures vary by study. higher ovarian risk than BRCA2BRCA1 / BRCA2BRCA1/BRCA2 DNA repair genes; inherited mutations raise breast/ovarian risk (BRCA1 higher ovarian risk than BRCA2). Figures vary by study.). Figures vary by study.
Also known as: BRCA, BRCA1, BRCA2
Dominant
AlleleAlleleAlternative forms of a gene at the same spot on a chromosome. expressed when at least one copy is present.
Fertilization
Fusion of two gametesGameteA haploid reproductive cell (egg or sperm) that fuses at fertilization. to form a diploidDiploidTwo complete sets of chromosomes (2n), one from each parent. zygoteZygoteDiploid cell formed when two gametes fuse..
Gamete
A haploidHaploidOne set of chromosomes (n); gametes are haploid. reproductive cell (egg or sperm) that fuses at fertilizationFertilizationFusion of two gametes to form a diploid zygote..
Also known as: gametes
Gene
A stretch of DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. that codes for a functional product, usually a proteinProteinA folded chain of amino acids that performs a specific function in the cell..
Genotype
The allelesAlleleAlternative forms of a gene at the same spot on a chromosome. an organism carries at a locusLocusThe specific physical location of a gene on a chromosome. Two homologous chromosomes carry the same locus, but alleles may differ. (e.g. AA, Aa, aa).
HBB
Beta-globinHBBBeta-globin gene; part of adult hemoglobin. geneGeneA stretch of DNA that codes for a functional product, usually a protein.; part of adult hemoglobinHemoglobinOxygen-carrying protein in erythrocytes (α and β subunits)..
Also known as: beta-globin, β-globin
Heterozygous
Carrying two different allelesAlleleAlternative forms of a gene at the same spot on a chromosome. at a locusLocusThe specific physical location of a gene on a chromosome. Two homologous chromosomes carry the same locus, but alleles may differ. (e.g. Aa).
Also known as: heterozygote
Homozygous
Carrying two identical allelesAlleleAlternative forms of a gene at the same spot on a chromosome. (AA or aa).
Also known as: homozygous dominant, homozygous recessive
Locus
The specific physical location of a geneGeneA stretch of DNA that codes for a functional product, usually a protein. on a chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes.. Two homologous chromosomesHomologChromosome pair with the same genes but possibly different alleles. carry the same locusLocusThe specific physical location of a gene on a chromosome. Two homologous chromosomes carry the same locus, but alleles may differ., but allelesAlleleAlternative forms of a gene at the same spot on a chromosome. may differ.
Also known as: gene locus
Penetrance
How often a genotypeGenotypeThe alleles an organism carries at a locus (e.g. AA, Aa, aa). actually leads to the expected condition.
Recessive
AlleleAlleleAlternative forms of a gene at the same spot on a chromosome. expressed only when two copies are present.
Sickle cell disease
RecessiveRecessiveAllele expressed only when two copies are present. disorder from HbSSickle cell diseaseRecessive disorder from HbS β-globin; sickled red cells block vessels. β-globin; sickled red cells block vessels.
Also known as: sickle cell anaemia, sickle cell anemia, HbS
Tumor suppressor
GeneGeneA stretch of DNA that codes for a functional product, usually a protein. that prevents uncontrolled cell division or fixes DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. damage.
Also known as: tumor suppressor gene
Zygote
DiploidDiploidTwo complete sets of chromosomes (2n), one from each parent. cell formed when two gametesGameteA haploid reproductive cell (egg or sperm) that fuses at fertilization. fuse.
Population genetics
Allele frequency
Proportion of allelesAlleleAlternative forms of a gene at the same spot on a chromosome. in a population that are a specific variant (p and q).
Balancing selection
Selection maintaining multiple allelesAlleleAlternative forms of a gene at the same spot on a chromosome., e.g. HbSSickle cell diseaseRecessive disorder from HbS β-globin; sickled red cells block vessels. where malaria is endemic.
Fitness
An individual's relative contribution to the next generation. Here we model viabilityFitnessAn individual's relative contribution to the next generation. Here we model viability: the probability of surviving to reproduce.: the probability of surviving to reproduce.
Also known as: viability, relative fitness
Genetic drift
Random changes in allele frequencyAllele frequencyProportion of alleles in a population that are a specific variant (p and q). due to chance in small populations.
Hardy–Weinberg equilibrium
Model where alleleAlleleAlternative forms of a gene at the same spot on a chromosome. frequencies stay constant: AA = p², Aa = 2pq, aa = q².
Also known as: Hardy-Weinberg, H-W, H-W equilibrium
Natural selection
Survival and reproduction differences that change alleleAlleleAlternative forms of a gene at the same spot on a chromosome. frequencies over time.
Random mating
Partners are chosen without regard to genotypeGenotypeThe alleles an organism carries at a locus (e.g. AA, Aa, aa).. Required for Hardy–Weinberg equilibriumHardy–Weinberg equilibriumModel where allele frequencies stay constant: AA = p², Aa = 2pq, aa = q². at a single locusLocusThe specific physical location of a gene on a chromosome. Two homologous chromosomes carry the same locus, but alleles may differ..
Gene editing & CRISPR
Antisense oligonucleotide
A short synthetic DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases./RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA. strand that base-pairs with a target mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. and triggers its destruction (via the enzyme RNase H) or blocks it, lowering how much proteinProteinA folded chain of amino acids that performs a specific function in the cell. is made: geneGeneA stretch of DNA that codes for a functional product, usually a protein. silencing without editing DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases..
Also known as: antisense oligonucleotides, ASO, ASOs
Cas9
RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA.-guided nuclease that creates a double-strand breakDouble-strand breakThe most dangerous class of DNA damage: both strands of the double helix are severed. Repaired by homologous recombination (using a sister chromatid template; often involves BRCA1/2) or by the faster but error-prone NHEJ pathway. at the target site.
Also known as: CRISPR-associated protein 9, SpCas9
CRISPR
RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA.-guided bacterial system adapted for programmable genome editingGene editingTargeted modification of DNA sequence in living cells..
Also known as: CRISPR-Cas, CRISPR-Cas9
Gene editing
Targeted modification of DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. sequence in living cells.
Also known as: genome editing, genome engineering
Gene therapy
Treatment of disease by altering the genetic material in a patient's cells: adding a functional geneGeneA stretch of DNA that codes for a functional product, usually a protein., silencing a harmful one, or editing the sequence directly. Somatic gene therapyGene therapyTreatment of disease by altering the genetic material in a patient's cells: adding a functional gene, silencing a harmful one, or editing the sequence directly. Somatic gene therapy (non-heritable) is approved for clinical use; germline editing remains largely restricted. (non-heritable) is approved for clinical use; germline editingGermline editingGenome editing of gametes, embryos, or cells that give rise to germ cells, such that changes could be inherited by subsequent generations: widely restricted in human medicine. remains largely restricted.
Germline editing
Genome editingGene editingTargeted modification of DNA sequence in living cells. of gametesGameteA haploid reproductive cell (egg or sperm) that fuses at fertilization., embryos, or cells that give rise to germ cells, such that changes could be inherited by subsequent generations: widely restricted in human medicine.
Also known as: heritable genome editing, embryo editing
GMO
A genetically modified organismGMOA genetically modified organism: a plant, animal, or microbe whose DNA has been deliberately altered using biotechnology. Examples include Bt crops (pest-resistant) and Golden Rice (vitamin A precursor). Major scientific bodies consider approved GM foods as safe as conventional equivalents.: a plant, animal, or microbe whose DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. has been deliberately altered using biotechnology. Examples include Bt crops (pest-resistant) and Golden Rice (vitamin A precursor). Major scientific bodies consider approved GM foods as safe as conventional equivalents.
Also known as: genetically modified organism, genetically modified organisms, transgenic, transgenic organism
Guide RNA
Engineered RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA. complementary to the target DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. sequence.
Also known as: gRNA, sgRNA, single guide RNA
HDR
Homology-directed repairHDRHomology-directed repair using a donor DNA template. using a donor DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. template.
Also known as: homology-directed repair, homologous recombination repair
NHEJ
Break repair that often adds or removes a few letters (indels).
Also known as: non-homologous end joining
Off-target editing
Unintended edits at partially matching off-target sites.
Also known as: off-target effects, off-target cleavage
PAM
Protospacer adjacent motifPAMProtospacer adjacent motif required for Cas9 binding (e.g. NGG). required for Cas9Cas9RNA-guided nuclease that creates a double-strand break at the target site. binding (e.g. NGG).
Also known as: protospacer adjacent motif
RNA interference
A natural geneGeneA stretch of DNA that codes for a functional product, usually a protein.-silencing pathway: a small double-stranded RNARNARibonucleic acid: usually single-stranded nucleic acid that uses uracil (U) instead of thymine (T). mRNA carries the protein-coding message from DNA. (siRNARNA interferenceA natural gene-silencing pathway: a small double-stranded RNA (siRNA) loads into the RISC complex, which uses one strand as a guide to find and cut the matching mRNA, reducing protein output.) loads into the RISC complex, which uses one strand as a guide to find and cut the matching mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome., reducing proteinProteinA folded chain of amino acids that performs a specific function in the cell. output.
Also known as: RNAi, siRNA, small interfering RNA, shRNA
Synthetic biology
An engineering-based approach to biology that designs new genetic systems from standardised, modular DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. parts (such as BioBrick components). Applications range from producing medicines in engineered microbes to constructing minimal synthetic cells.
Transgene
A geneGeneA stretch of DNA that codes for a functional product, usually a protein. introduced into an organism from an external source, often a different species, conferring a new function. The core element of most genetically modified organismsGMOA genetically modified organism: a plant, animal, or microbe whose DNA has been deliberately altered using biotechnology. Examples include Bt crops (pest-resistant) and Golden Rice (vitamin A precursor). Major scientific bodies consider approved GM foods as safe as conventional equivalents..
Also known as: transgenes
Viral vector
A modified virus stripped of disease-causing genes and loaded with therapeutic DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. to deliver it into target cells. Adeno-associated virusViral vectorA modified virus stripped of disease-causing genes and loaded with therapeutic DNA to deliver it into target cells. Adeno-associated virus (AAV) is widely used for in vivo delivery; lentiviral vectors typically integrate stably into the host genome and underpin ex vivo cell therapies such as CAR-T. (AAVViral vectorA modified virus stripped of disease-causing genes and loaded with therapeutic DNA to deliver it into target cells. Adeno-associated virus (AAV) is widely used for in vivo delivery; lentiviral vectors typically integrate stably into the host genome and underpin ex vivo cell therapies such as CAR-T.) is widely used for in vivo delivery; lentiviral vectors typically integrate stably into the host genome and underpin ex vivo cell therapies such as CAR-T.
Also known as: viral vectors, AAV, adeno-associated virus, lentiviral vector
Mutations, repair & cancer
Double-strand break
The most dangerous class of DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. damage: both strands of the double helixDouble helixThe twisted ladder structure of DNA formed by two antiparallel nucleotide strands wound around each other. are severed. Repaired by homologousHomologChromosome pair with the same genes but possibly different alleles. recombination (using a sister chromatidChromatidOne of the two identical DNA strands making up a duplicated chromosome, joined at the centromere. template; often involves BRCA1BRCA1 / BRCA2BRCA1/BRCA2 DNA repair genes; inherited mutations raise breast/ovarian risk (BRCA1 higher ovarian risk than BRCA2). Figures vary by study./2) or by the faster but error-prone NHEJNHEJBreak repair that often adds or removes a few letters (indels). pathway.
Also known as: DSB, double-strand breaks
Frameshift mutation
An insertion or deletion of a number of bases not divisible by three, shifting the reading frame of all downstream codonsCodonThree nucleotides in mRNA that code for one amino acid or a stop signal.. Typically produces a non-functional proteinProteinA folded chain of amino acids that performs a specific function in the cell. and often triggers nonsense-mediated mRNAmRNAMessenger RNA: carries a gene's code from nucleus to ribosome. decay.
Also known as: frameshift
Germline mutation
A mutationMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual. arising in germ cells (eggs, sperm, or their precursors) and therefore present in every cell of any offspring who inherits it. The basis of heritable genetic disease.
Also known as: germline mutations, germline variant
Mismatch repair
A post-replication DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. repair system that corrects base mismatches and small insertions/deletions missed by proofreading. Inherited defects in MMRMismatch repairA post-replication DNA repair system that corrects base mismatches and small insertions/deletions missed by proofreading. Inherited defects in MMR genes (MLH1, MSH2, etc.) cause Lynch syndrome, the most common hereditary colorectal cancer predisposition. genes (MLH1, MSH2, etc.) cause Lynch syndromeMismatch repairA post-replication DNA repair system that corrects base mismatches and small insertions/deletions missed by proofreading. Inherited defects in MMR genes (MLH1, MSH2, etc.) cause Lynch syndrome, the most common hereditary colorectal cancer predisposition., the most common hereditary colorectal cancer predisposition.
Also known as: MMR, Lynch syndrome, HNPCC
Mutagen
A physical or chemical agent that raises the rate of mutationsMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual.: examples include UV radiation, ionising radiation, and chemical carcinogens such as tobacco smoke components.
Also known as: mutagens
Mutation
A change in the DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutationsMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual. are neutral; some disrupt geneGeneA stretch of DNA that codes for a functional product, usually a protein. function and cause disease; rarely a mutationMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual. is beneficial. Germline mutationsGermline mutationA mutation arising in germ cells (eggs, sperm, or their precursors) and therefore present in every cell of any offspring who inherits it. The basis of heritable genetic disease. can be inherited; somatic mutationsSomatic mutationA mutation arising in a non-germline body cell after fertilisation. Not transmitted to offspring, but accumulates in tissues with age and is the primary driver of cancer. stay within the individual.
Also known as: mutations
Oncogene
A geneGeneA stretch of DNA that codes for a functional product, usually a protein. that drives cell proliferation when mutated or overexpressed (arising from a normal growth-promoting proto-oncogeneOncogeneA gene that drives cell proliferation when mutated or overexpressed (arising from a normal growth-promoting proto-oncogene). Oncogene mutations are dominant: one altered copy is sufficient. Classic examples include mutant RAS (one of the most commonly mutated genes in human cancer) and amplified MYC.). OncogeneOncogeneA gene that drives cell proliferation when mutated or overexpressed (arising from a normal growth-promoting proto-oncogene). Oncogene mutations are dominant: one altered copy is sufficient. Classic examples include mutant RAS (one of the most commonly mutated genes in human cancer) and amplified MYC. mutationsMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual. are dominantDominantAllele expressed when at least one copy is present.: one altered copy is sufficient. Classic examples include mutant RAS (one of the most commonly mutated genes in human cancer) and amplified MYC.
Also known as: oncogenes, proto-oncogene, proto-oncogenes
Somatic mutation
A mutationMutationA change in the DNA sequence of a cell: from a single base substitution to large chromosomal rearrangements. Most mutations are neutral; some disrupt gene function and cause disease; rarely a mutation is beneficial. Germline mutations can be inherited; somatic mutations stay within the individual. arising in a non-germline body cell after fertilisation. Not transmitted to offspring, but accumulates in tissues with age and is the primary driver of cancer.
Also known as: somatic mutations
Two-hit hypothesis
Knudson's 1971 model: both copies of a tumour suppressor geneGeneA stretch of DNA that codes for a functional product, usually a protein. must be inactivated for its brake on cell division to be fully lost. Explains why inherited cancer syndromes (e.g. retinoblastoma) show earlier onset and bilateral tumours compared with sporadic cases.
Also known as: Knudson model, two-hit model
Human evolution & ancestry
Admixture
Interbreeding between two genetically distinct populations, mixing their geneGeneA stretch of DNA that codes for a functional product, usually a protein. pools. In human evolutionary genetics, refers especially to the geneGeneA stretch of DNA that codes for a functional product, usually a protein. flow that occurred when modern humans encountered NeanderthalsNeanderthalAn archaic human species that lived in Europe and western Asia until ~40,000 years ago. Ancient DNA shows that modern humans interbred with Neanderthals after leaving Africa; most people of non-African ancestry carry ~1–2% Neanderthal DNA. and DenisovansDenisovanA group of archaic humans identified from ancient DNA in Denisova Cave, Siberia. Melanesians, Papuans, and Aboriginal Australians carry the highest levels of Denisovan ancestry (~5%); other Southeast Asian populations carry smaller amounts, inherited from interbreeding tens of thousands of years ago. outside Africa.
Ancient DNA
DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases. recovered from archaeological or palaeontological specimens (bones, teeth, or sediment). Pioneered by Svante Pääbo, who sequenced the NeanderthalNeanderthalAn archaic human species that lived in Europe and western Asia until ~40,000 years ago. Ancient DNA shows that modern humans interbred with Neanderthals after leaving Africa; most people of non-African ancestry carry ~1–2% Neanderthal DNA. genome in 2010 and was awarded the Nobel Prize in Physiology or Medicine in 2022.
Also known as: aDNA, palaeogenomics
Denisovan
A group of archaic humans identified from ancient DNAAncient DNADNA recovered from archaeological or palaeontological specimens (bones, teeth, or sediment). Pioneered by Svante Pääbo, who sequenced the Neanderthal genome in 2010 and was awarded the Nobel Prize in Physiology or Medicine in 2022. in Denisova Cave, Siberia. Melanesians, Papuans, and Aboriginal Australians carry the highest levels of DenisovanDenisovanA group of archaic humans identified from ancient DNA in Denisova Cave, Siberia. Melanesians, Papuans, and Aboriginal Australians carry the highest levels of Denisovan ancestry (~5%); other Southeast Asian populations carry smaller amounts, inherited from interbreeding tens of thousands of years ago. ancestry (~5%); other Southeast Asian populations carry smaller amounts, inherited from interbreeding tens of thousands of years ago.
Also known as: Denisovans
Haplogroup
A cluster of related genetic variants inherited together on a non-recombining region: typically mitochondrial DNAMitochondrial DNAA small circular genome inside mitochondria, inherited almost exclusively from the mother. Because it does not recombine, it accumulates mutations in a clock-like way and is widely used to trace maternal lineages and estimate evolutionary timings. (for maternal lineages) or the Y chromosomeChromosomeA single, long DNA molecule packaged with histone proteins into a compact structure that can be seen under a microscope. Human body cells contain 46 chromosomes (23 pairs), each carrying hundreds to thousands of genes. (for paternal lineages). HaplogroupHaplogroupA cluster of related genetic variants inherited together on a non-recombining region: typically mitochondrial DNA (for maternal lineages) or the Y chromosome (for paternal lineages). Haplogroup distributions map ancient human migration routes. distributions map ancient human migration routes.
Also known as: haplogroups
Neanderthal
An archaic human species that lived in Europe and western Asia until ~40,000 years ago. Ancient DNAAncient DNADNA recovered from archaeological or palaeontological specimens (bones, teeth, or sediment). Pioneered by Svante Pääbo, who sequenced the Neanderthal genome in 2010 and was awarded the Nobel Prize in Physiology or Medicine in 2022. shows that modern humans interbred with NeanderthalsNeanderthalAn archaic human species that lived in Europe and western Asia until ~40,000 years ago. Ancient DNA shows that modern humans interbred with Neanderthals after leaving Africa; most people of non-African ancestry carry ~1–2% Neanderthal DNA. after leaving Africa; most people of non-African ancestry carry ~1–2% NeanderthalNeanderthalAn archaic human species that lived in Europe and western Asia until ~40,000 years ago. Ancient DNA shows that modern humans interbred with Neanderthals after leaving Africa; most people of non-African ancestry carry ~1–2% Neanderthal DNA. DNADNADouble-stranded molecule storing genetic information as A, T, G, C bases..
Also known as: Neanderthals, Homo neanderthalensis
Out-of-Africa
The model describing how anatomically modern humans dispersed from Africa beginning ~50,000–70,000 years ago. Supported by declining genetic diversity with increasing geographic distance from Africa (the serial founder effect).
Also known as: Out of Africa model, Out-of-Africa dispersal
Bioethics & society
Bioethics
The study of ethical questions arising from biology, medicine, and biotechnology. Guided by four core principles: autonomy (the right to make informed choices), beneficence (doing good), non-maleficence (avoiding harm), and justice (fairness).
Eugenics
A discredited movement, coined by Francis Galton in 1883, that aimed to "improve" human populations by controlling reproduction. Applied as coercive state policy in the early 20th century, including forced sterilisation and Nazi racial programmes, causing enormous harm. A warning against misusing genetic knowledge.
GINA
The Genetic Information Nondiscrimination ActGINAThe Genetic Information Nondiscrimination Act (USA, 2008): prohibits discrimination based on genetic information in group health insurance and employment. Does not cover life insurance, disability insurance, or long-term care insurance. (USA, 2008): prohibits discrimination based on genetic information in group health insurance and employment. Does not cover life insurance, disability insurance, or long-term care insurance.
Also known as: Genetic Information Nondiscrimination Act
Informed consent
Voluntary agreement to a procedure or research participation after being fully informed of its purpose, risks, and alternatives. A cornerstone of medical ethics, established in part through the Nuremberg Code (1947) following Nazi medical abuses.