Cell Division

Cells are continually shed and replaced, and every cell comes from a pre-existing cell. Division serves three purposes:

1. Growth - producing more cells.
2. Repair and replacement - cells have a finite lifespan.
3. New offspring - asexual organisms reproduce by dividing.

Before any division, the cell doubles its DNA so that each daughter cell receives a complete, identical copy of the genetic information. In eukaryotes this occurs during the S phase of the cell cycle; prokaryotes replicate their chromosome as the first step of fission. Prokaryotic cells divide by binary fission; eukaryotic somatic cells divide by mitosis; meiosis is a special division used only in sexual reproduction.

BINARY FISSION (prokaryotes)

Binary fission is the simplest mechanism of division: one bacterial cell splits into two genetically identical daughter cells. It is fast — there is no cell cycle controlling it, and under favourable conditions cell number doubles about every 20 minutes. The trigger is size: as the cell grows it reaches a critical surface-area-to-volume ratio beyond which exchange with the environment becomes too inefficient, so the cell must divide. It occurs in two main stages — DNA replication, then cell splitting:

1. The circular chromosome (DNA molecule) attaches to the inner layer of the cell membrane, and replication begins there.
2. DNA replication completes: two circular chromosomes, both attached to the membrane.
3. The cell elongates, and growth drags the chromosomes to opposite ends.
4. A protein Z-ring forms at the midline, guiding new membrane and cell wall that split the cell into two identical daughters.

BINARY FISSION (prokaryotes) — the DNA attaches and replicates at the membrane, the cell elongates, and a Z-ring completes the split into two identical daughter cells.

CHROMOSOMES: KEY TERMS

• Chromatin - the loose form of DNA in a non-dividing eukaryotic nucleus.
• Chromosome - the condensed, visible, thread-like form that chromatin takes as division approaches.
• Sister chromatids - the two identical strands of a replicated chromosome, produced in S phase.
• Centromere - the region where sister chromatids are joined.
• Homologous pair - one maternal and one paternal copy of each chromosome type, similar in length and centromere position (the sex chromosomes are the exception). A homologous pair carries genes for the same characteristics, but possibly different versions of each gene — alleles.
• Diploid (2n) - a cell holding both members of every homologous pair; haploid (n) - a cell holding one of each. Humans: 2n = 46 (23 pairs); n = 23, the number in a sperm or egg cell.

MITOSIS (eukaryotes)

Mitosis is how eukaryotic somatic (body) cells divide for growth and repair — asexual division that never involves the germ line. One division produces two genetically identical, diploid daughter cells with the same quantity and quality of chromosomes as the parent cell. It occurs in four main stages (PMAT), all energy-dependent:

Cytokinesis then cleaves the cytoplasm to form the two daughter cells: animal cells pinch inward along a cleavage furrow, while plant cells (whose walls cannot pinch) build a cell plate between the daughters. The period between one cytokinesis and the next prophase is the interphase, during which DNA replicates and the cell prepares to divide again.

MEIOSIS

Germ-line cells are the diploid cells from which gametes are made. Meiosis takes them through two divisions. Meiosis I is the reduction division: homologous pairs separate, halving the chromosome number. Meiosis II separates the sister chromatids, like a mitosis performed on haploid cells, with cytokinesis in between. The result is four genetically different, haploid gametes.

The genetic variation is generated in Meiosis I:

1. Crossing over (prophase I) - replicated homologous chromosomes pair up (synapsis), and equivalent sections of non-sister chromatids are exchanged: the chromatids break at a defined point — the chiasma (plural chiasmata) — and rejoin at the same point, swapping genetic material.
2. Independent assortment (metaphase I) - each homologous pair aligns at the equator randomly and independently of every other pair. There is an equal chance the maternal or paternal chromosome faces either pole, so each gamete receives a fresh shuffle of whole chromosomes in addition to the crossing-over swaps.

CROSSING OVER (prophase I) — non-sister chromatids of a homologous pair break and rejoin at the chiasma, exchanging equivalent segments.

Fertilisation completes the cycle: a haploid sperm and a haploid egg join, and the nucleus of each fuses to form a diploid zygote — restoring 2n = 46 with a new combination of alleles.

Mitosis produces 2 identical diploid cells; meiosis produces 4 genetically different haploid gametes.

In summary: asexual reproduction (binary fission, mitosis) → clones, with variation only by mutation; sexual reproduction → crossing over + independent assortment + random fertilisation → the genetic variation on which evolution depends.