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Biology · Heredity, Variation & Evolution

Cell division, reproduction and continuity of life

A cell must distribute DNA when it divides. A sexually reproducing lineage must also avoid doubling its chromosome sets with every fertilisation. Those two accounting tasks are connected, but mitosis and meiosis distribute chromosomes differently.

The earlier lessons on biological scales and cells provide useful background: recognise a cell, its DNA and the difference between an individual and a population. The DNA and nucleotide explanation in Chemistry of Life is optional support. You need fractions such as one half and one quarter; the worked cases explain the calculations before using them. Detailed molecular copying, Mendelian inheritance and evolutionary inference return in later chapters.

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By the end, you can

Follow copying, chromosome segregation and fertilisation, and distinguish a probability model of transmission from guaranteed offspring counts.

01

Copy DNA before distributing it

In the ordinary eukaryotic cell cycle used here, DNA replication occurs before mitosis. Each replicated chromosome has two sister chromatids. Mitosis separates those copies into daughter nuclei; division of the cell's contents, cytokinesis, commonly follows or overlaps with it.

The model begins with two chromosome pairs: four chromosomes and four nuclear DNA molecules. After copying, there are still four replicated chromosomes but eight DNA molecules. After mitosis and completed cell division, each daughter cell has four chromosomes and four DNA molecules.

Chromosome counting and DNA-molecule counting therefore need stated stages. An X-like drawing of a replicated chromosome shows two associated chromatids; it is not evidence that every chromosome is always X-shaped. Copying and segregation are normally accurate, but errors can occur.

Count chromosome sets and DNA copies separately

Count chromosome sets and DNA copies separatelyA starting diploid cell with two homologous pairs leads through DNA copying to two lanes. The mitotic lane branches into two cells retaining two sets; the meiotic lane branches first into two cells and then four retaining one set. Each cell carries chromosome and DNA-molecule counts alongside visible single or paired chromatid symbols.Before copyingNumber of cells: 1Chromosomes per cell: 4DNA molecules per cell: 4After copyingNumber of cells: 1Chromosomes per cell: 4DNA molecules per cell: 8After mitosis and celldivisionNumber of cells: 2Chromosomes per cell: 4DNA molecules per cell: 4After first meioticdivisionNumber of cells: 2Chromosomes per cell: 2DNA molecules per cell: 4After second meioticdivisionNumber of cells: 4Chromosomes per cell: 2DNA molecules per cell: 2Single chromatidTwo associated sisterchromatids
Standard two-pair diploid count model at completed stages. Omit crossing over from this inventory diagram. Four meiotic cell products do not imply four equivalent mature gametes in every organism. The mitotic and meiotic lanes are alternative routes; do not add their outputs together.
Read the diagram in words

Before copying: one cell, four chromosomes and four DNA molecules. After copying: four chromosomes and eight DNA molecules. Mitosis gives two cells with four of each. First meiosis gives two cells with two chromosomes and four DNA molecules each; second meiosis gives four cells with two of each.

Count chromosome sets and DNA copies separately
Completed stageNumber of cellsChromosomes per cellDNA molecules per cellChromosome sets per cellChromatids per chromosomeChromosomes per set
Before copying144212
After copying148222
After mitosis and cell division244212
After first meiotic division224122
After second meiotic division422112
Count chromosome sets and DNA copies separately
FromTo
Before copyingAfter copying
After copyingAfter mitosis and cell division
After copyingAfter first meiotic division
After first meiotic divisionAfter second meiotic division
Single chromatid
1
Two associated sister chromatids
2
02

Reduce a set, then combine sets

Meiosis includes one preceding round of DNA copying and two successive divisions. In the standard model, homologous chromosomes separate in the first division; sister chromatids separate in the second. The products have one chromosome set rather than the original two.

Our four-chromosome model gives two cells after the first meiotic division, each with two replicated chromosomes and four DNA molecules. After the second division it gives four haploid cells, each with two chromosomes and two DNA molecules. This inventory does not imply that all four products become equivalent mature gametes in every life cycle.

In many animals, meiosis contributes to forming eggs or sperm, and fertilisation combines two haploid sets into a diploid set. Plants and other organisms organise these steps differently; meiosis can produce spores rather than directly produce gametes. Mitosis can also occur in haploid cells. The deeper life-cycle chapter develops these variations.

Cell division can reproduce a single-celled organism or add cells to a multicellular one. Growth by cell division does not make each new body cell a new organism. Asexual reproduction maintains lineage continuity without a union of gametes, and bacteria divide without eukaryotic mitosis.

03

A grid lists possible allele unions

For a one-locus diploid cross, give each parent genotype Aa. Assume each produces A and a gametes with equal probability and that the two gamete contributions combine independently. Each offspring then has four equally likely allele-union paths: A with A, A with a, a with A, or a with a.

The two middle paths both give genotype Aa. Thus the genotype probabilities are one quarter AA, one half Aa and one quarter aa. These are probabilities for the stated model, not an instruction that every four offspring must contain one of each homozygote and two heterozygotes.

No phenotype is assigned in this grid. Converting it to a trait ratio would need a dominance relationship and other genotype-to-phenotype conditions. Altered segregation, mutation or genotype-dependent survival could also change the counts at a later stage.

Two paths produce the same heterozygous genotype

Two paths produce the same heterozygous genotypeA two-by-two allele-union grid labels Parent 1 gametes A and a across columns and Parent 2 gametes A and a down rows. Cells show AA, Aa, Aa and aa with one-quarter probability per union path. The two Aa cells share an outline pattern, independent of colour.Parent 1 gameteParent 2 gameteAaAaAA0.25Aa0.25Aa0.25aa0.25Probability under the stated modelAA0.25Aa0.5aa0.25
Aa × Aa, equal segregation and independent gamete union, with no locus mutation or genotype-dependent loss before counting. No phenotype, dominance or guaranteed four-offspring pattern is assigned.
Read the diagram in words

One A–A union gives AA, two opposite A–a unions give Aa, and one a–a union gives aa. The genotype probabilities are one quarter, one half and one quarter.

Probability under the stated model
Parent 2 gameteParent 1 gamete: AParent 1 gamete: a
Parent 2 gamete: AAA: 0.25Aa: 0.25
Parent 2 gamete: aAa: 0.25aa: 0.25
Probability under the stated model
GenotypeProbability
AA0.25
Aa0.5
aa0.25

Worked example

Count the two paths to a heterozygote

Parent 1 contributes A with probability 1/2 and Parent 2 a with probability 1/2, giving probability 1/4 for that path. The opposite path, a from Parent 1 and A from Parent 2, also has probability 1/4. Add them: 1/4 + 1/4 = 1/2 for Aa. Eight offspring would have an expected four Aa under repeated independent applications of the model, but a particular group of eight can differ.

Check your understanding

  1. In the stated model, does DNA copying change four chromosomes into eight chromosomes before division?

    Show reasoning

    No. Before the sister chromatids separate, four replicated chromosomes contain eight DNA molecules. Chromosome and DNA-molecule counts refer to different units.

  2. Why does fertilisation not double the chromosome-set number generation after generation in this diploid sexual model?

    Show reasoning

    Meiosis reduces two sets to one in the relevant reproductive pathway, and fertilisation combines two one-set contributions to restore two sets.

  3. Does the Aa × Aa grid guarantee exactly one aa among four offspring?

    Show reasoning

    No. It assigns probability 1/4 to aa for each offspring under the model. A small realised group need not match the expected ratio.

What to carry forward

State what is copied, what is separated and which stage is being counted.

References

Resources supporting this lesson’s explanations and examples.

  1. Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStax4 sources
    1. Biology 2e — The Cell Cycle (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxTextbook chapteropenstax.orgRead source
    2. Biology 2e — The Process of Meiosis (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxTextbook chapteropenstax.orgRead source
    3. Biology 2e — Characteristics and Traits (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxTextbook chapteropenstax.orgRead source
    4. Biology 2e — Prokaryotic Cell Division (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxTextbook chapteropenstax.orgRead source
  2. Talking Glossary — Genes, alleles and inheritance terminologyNational Human Genome Research InstituteScientific glossaryTalking Glossary — Genes, alleles and inheritance terminologyAlleleChromosomeGenotypePhenotypePolygenic traitMutationGenomeLocusMitochondrial DNAgenome.gov