Somatic vs Gametic Cells

The dot point names two things to compare — structures and roles — so the answer must cover both. A comparison of chromosome number alone addresses a fraction of the question.

6 marks. Similarities in one sentence, then chromosome number, then how each is produced, then specialisation, then role and fate.

Both are eukaryotic cells of the same organism, both carry that organism’s DNA and the same set of genes, both are bounded by a cell membrane and contain cytoplasm and organelles, and both arise ultimately from the division of pre-existing cells.

Somatic cells are diploid, written 2n, carrying two complete sets of chromosomes, one inherited from each parent, which is forty-six chromosomes in twenty-three pairs in humans. Gametes are haploid, written n, carrying a single set of twenty-three chromosomes, one member of each pair.

Somatic cells are produced by mitosis, which yields two daughter cells genetically identical to the parent cell and to each other. Gametes are produced by meiosis, which halves the chromosome number and, through crossing over and independent assortment, yields four daughter cells that are genetically different from the parent and from one another.

Somatic cells are structurally diverse and specialised for the function of their tissue: a nerve cell is elongated for conduction, a red blood cell loses its nucleus to carry more haemoglobin, a muscle cell is packed with contractile filaments. Gametes are specialised instead for fertilisation: a sperm cell is small and motile with a flagellum, many mitochondria in its midpiece to power swimming, and an acrosome containing enzymes to penetrate the egg, while an egg cell is large and non-motile, stocked with cytoplasm and nutrients to support the early development of the embryo.

The role of somatic cells is to build, operate, grow and repair the body of the individual, and they die with that individual. The role of gametes is reproduction: a haploid sperm fuses with a haploid egg at fertilisation to form a diploid zygote, restoring the full chromosome number and beginning a new organism, so gametes are the only cells whose genetic contribution passes to the next generation.

A short question that rewards a precise chain. Explain here means show what would happen if gametes were diploid, because the necessity of haploidy is only visible against the alternative.

4 marks. State what fertilisation does to chromosome number, run the counterfactual, state the consequence, then add why halving must be meiosis rather than any halving.

Fertilisation is the fusion of two gametes, so the chromosomes of both are combined in the single cell that results. Whatever number each gamete carries, the zygote receives the sum of the two.

If human gametes were diploid, carrying forty-six chromosomes each, the zygote would have ninety-two, its offspring in turn one hundred and eighty-four, and the number would double in every generation. Since gametes are haploid with twenty-three, the zygote receives forty-six, which is exactly the diploid number characteristic of the species, and that number is therefore maintained unchanged across generations.

A constant chromosome number matters because the quantity of DNA determines the balance of gene products a cell produces, and cells cannot function with arbitrary extra copies of every gene; additional chromosome sets disrupt development, and in animals such conditions are almost always fatal. A species is also defined in part by its karyotype, and individuals whose chromosome numbers do not match generally cannot produce viable offspring together.

The halving must also be systematic rather than arbitrary. Meiosis separates homologous pairs so that each gamete receives exactly one member of every pair, and therefore one complete set of the organism’s genes. A cell given any random twenty-three chromosomes would carry two copies of some genes and none of others, and would not be viable, so it is the orderly separation of homologues at meiosis I, not merely the reduction in number, that makes a functional haploid gamete.