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M3

Module 3 · Passing it on

Passing it on

DNA replication, cell cycle & mitosis, meiosis, Mendelian inheritance, dominant/recessive alleles, and pedigree analysis. Module 3.

Semi-conservative replication

Before a cell divides, DNA replication ensures that every daughter cell inherits a complete genome. It happens in S phase, the middle stage of interphase in the cell cycle (between G1 growth and G2 preparation; see the Mitosis tab for the full cycle).

The double helix unwinds and each strand serves as a template for building a complementary new strand, following the base-pairing rules: A pairs with T, and G pairs with C. This process is called semi-conservative replication: each resulting DNA molecule retains one parental strand paired with one newly synthesised strand.

But how do we know?

In 1958, Meselson and Stahl grew E. coli in medium containing ¹⁵N (heavy nitrogen) until all DNA was labelled, then switched to normal ¹⁴N medium and sampled after each generation. Using density-gradient centrifugation to separate DNA by weight, they found that after one generation all DNA had hybrid (intermediate) density, and after two generations the sample was 50% hybrid + 50% light: matching semi-conservative replication exactly and ruling out both rival models.

Replication fork with enzymes and Meselson–Stahl density bands supporting semi-conservative replication

Enzymes of replication

A team of enzymes works at the replication fork: the Y-shaped opening where the two strands peel apart. Helicase unwinds the double helix. Primase then lays down a short RNA primer, because DNA polymerase cannot begin a new strand from nothing; it can only extend an existing one.

DNA polymerase builds in one direction only (5′→3′): the leading strand is built continuously towards the fork, while the lagging strand is built in short sections called Okazaki fragments going away from the fork. DNA ligase joins these fragments together. As it works, DNA polymerase also proofreads each newly added base, keeping replication errors extremely rare.

Semi-conservative replication

Before a cell divides, DNA replication ensures that every daughter cell inherits a complete genome. It happens in S phase, the middle stage of interphase in the cell cycle (between G1 growth and G2 preparation; see the Mitosis tab for the full cycle).

The double helix unwinds and each strand serves as a template for building a complementary new strand, following the base-pairing rules: A pairs with T, and G pairs with C. This process is called semi-conservative replication: each resulting DNA molecule retains one parental strand paired with one newly synthesised strand.

But how do we know?

In 1958, Meselson and Stahl grew E. coli in medium containing ¹⁵N (heavy nitrogen) until all DNA was labelled, then switched to normal ¹⁴N medium and sampled after each generation. Using density-gradient centrifugation to separate DNA by weight, they found that after one generation all DNA had hybrid (intermediate) density, and after two generations the sample was 50% hybrid + 50% light: matching semi-conservative replication exactly and ruling out both rival models.

Replication fork with enzymes and Meselson–Stahl density bands supporting semi-conservative replication

Enzymes of replication

A team of enzymes works at the replication fork: the Y-shaped opening where the two strands peel apart. Helicase unwinds the double helix. Primase then lays down a short RNA primer, because DNA polymerase cannot begin a new strand from nothing; it can only extend an existing one.

DNA polymerase builds in one direction only (5′→3′): the leading strand is built continuously towards the fork, while the lagging strand is built in short sections called Okazaki fragments going away from the fork. DNA ligase joins these fragments together. As it works, DNA polymerase also proofreads each newly added base, keeping replication errors extremely rare.

Every tissue in your body, skin, muscle, bone marrow, depends on a reliable way to produce new cells. Mitosis is that process: one cell divides to give two genetically identical daughter cells, each with the same full set of diploid chromosomes as the parent. Mitosis drives growth during development, tissue repair after injury, and the steady replacement of worn-out cells such as red blood cells and skin cells. This section also introduces how mitosis compares to the very different process of meiosis.

The purpose of mitosis

Mitosis serves three main purposes. First, growth: as a developing organism grows from a fertilised egg into a full body, mitosis multiplies cell numbers rapidly. Second, tissue repair, when you cut yourself, mitosis produces fresh cells to seal the wound. Third, cell replacement: millions of cells, especially blood cells and skin cells, are shed and renewed every day through mitosis.

Some organisms also use mitosis for asexual reproduction, producing offspring that are genetic copies of the parent.

Diploid daughter cells

Mitosis produces two diploid daughter cells, each with the same number of chromosomes as the parent. In humans that means each daughter receives 46 chromosomes (23 pairs), the full complement.

Both daughters are genetically identical to each other and to the parent cell. There is no shuffling of genes and no crossing over; the genetic blueprint is copied exactly.

Meiosis is the specialised cell division that produces haploid gametes (eggs and sperm) for sexual reproduction. Where mitosis yields two genetically identical diploid daughters, meiosis involves two successive divisions that halve the chromosome number and reshuffle alleles through crossing over and independent assortment. This process explains Mendelian inheritance: why a heterozygous parent (Aa) produces a 1:1 ratio of A and a gametes, and why random fertilization gives offspring genotypes of AA, Aa, or aa. In spermatogenesis all four meiotic products become sperm; in oogenesis only one becomes the egg, with the others discarded as polar bodies. Use the stepper below to trace a single gene through replication, pairing, segregation, and fertilization.

Crossing over in Prophase I: non-sister chromatids from paired homologs exchange DNA segments.

Maternal homologPaternal homolog

Step 1 of 10

Diploid parent cell

A somatic cell, an ordinary body cell, not a gamete, is diploid (2n), meaning it carries two copies of each chromosome, one inherited from each parent. Matching chromosomes from different parents form homologous pairs. At our gene locus (the specific spot where this gene sits), one homolog carries allele A and the other carries allele a. Together, this parent is heterozygous (Aa) at that locus. It has two different versions of the gene.

But how do we know?

In 1902, Sutton and Boveri proposed the chromosome theory: the idea that genes are carried on chromosomes. They showed that Mendelian segregation lines up with the way homologous chromosomes pair and then separate during cell division. Morgan (1910) took this further by mapping the white (w) eye mutation to the X chromosome in Drosophila fruit flies. That was direct evidence for sex-linked inheritance, and it also launched classical gene mapping using crossing over.

Key distinction: Meiosis I vs II

Anaphase I separates homologous chromosomes: the reduction division that halves the chromosome number (2n → n).
Anaphase II separates sister chromatids using the same mechanism as mitosis. Chromosome number stays haploid (n).
Crossing over in Prophase I shuffles alleles between homologs, creating recombinant chromatids and recombinant gametes.
Independent assortment and random fertilization further increase genetic diversity beyond what segregation alone would produce.

A pedigree chart is a family diagram that tracks genotypes and phenotypes (what you can observe) across generations. Use the calculator to predict offspring probabilities from parental genotypes. You can also compare how autosomal dominant and autosomal recessive traits show up differently in a family tree.

Pedigree diagram

Click a symbol to cycle through genotypes: unknown → AA → Aa → aa

Unaffected (no copies of the risk allele) Carrier (has one copy, often no symptoms) Affected (shows the trait)

Punnett square builder

A Punnett square predicts offspring genotypes from two parent genotypes by listing every combination of the alleles each parent can pass on. The interactive uses a monohybrid cross for eye colour: B (brown) is dominant over b (blue). Choose each parent's genotype and read the genotype ratio, phenotype ratio, and plain-language summary below the grid.

How to use the interactive

Set Parent 1 and Parent 2 genotypes (homozygous dominant, heterozygous, or homozygous recessive).
Inspect the 2×2 grid for offspring genotypes (BB, Bb, bb) and phenotype badges.
Compare the genotype ratio and phenotype ratio bars, hover cells for percentage tooltips.
Use Explain this result to connect gamete contributions to the observed 3:1 or other ratios.

Loading Punnett square builder…

Check your understanding

Eight questions on DNA replication, mitosis, meiosis, and Mendelian inheritance. Explanations follow any misses.

What separates during anaphase I vs anaphase II?

Anaphase I: homologous chromosomes; anaphase II: sister chromatidsAnaphase I: sister chromatids; anaphase II: homologous chromosomesBoth separate sister chromatidsBoth separate homologous chromosomes

Select all differences between mitosis and meiosis described in this module. (Select all that apply.)

Mitosis: one division; meiosis: two divisionsMitosis: two diploid daughters; meiosis: four haploid gametesCrossing over occurs in meiosis but not in mitosisMitosis shuffles alleles; meiosis copies DNA exactly

Select all that apply.

Put the main stages of mitosis in order.

1.Metaphase: chromosomes line up at the equator
2.Prophase: chromosomes condense
3.Anaphase: sister chromatids separate
4.Telophase: new nuclei form; cytokinesis splits the cell

At metaphase I, the random orientation of each homolog pair is called independent assortment.

TrueFalse

In autosomal dominant inheritance, how many copies of the risk allele are usually enough to show the trait?

One copy (heterozygous Aa can be affected)Two copies only (aa required)Three copiesZero copies

DNA is replicated during S phase of interphase, before mitosis or meiosis begins.

TrueFalse

What does semi-conservative replication mean?

Both parent strands stay together; a new helix is built separatelyEach new DNA molecule keeps one original strand and one new strandOld DNA is cut up and mixed randomly into both daughtersOnly half the genome is copied each time

After one generation in light nitrogen, Meselson and Stahl found all DNA at intermediate (hybrid) density, supporting semi-conservative replication.

TrueFalse

Answer every question to submit.