Reproduction, meiosis and DNA

Biology · Inheritance, variation and evolution. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Sexual reproduction joins two gametes, so the offspring get a mixture of alleles and show variation. Asexual reproduction needs one parent and uses mitosis only, so the offspring are genetically identical clones.

Gametes are made by meiosis. The cell copies its chromosomes, then divides twice to give four cells. Each has half the normal chromosome number and each is genetically different. Fertilisation restores the full number, and the new cell then divides by mitosis.

DNA is a polymer of two strands in a double helix. A gene is a small section of DNA that codes for one protein. The order of bases decides the order of amino acids.

Rules to remember

Human body cells have 46 chromosomes (23 pairs); human gametes have 23.
Meiosis: one cell → four genetically different gametes, each with half the chromosome number.
Sexual reproduction gives variation, which helps a species survive change. Asexual reproduction is fast, needs one parent and no mate, but gives no variation.
Genome = all the genetic material of an organism.
Each nucleotide has a sugar, a phosphate and one of four bases: A, C, G, T. A pairs with T and C pairs with G.
Three bases code for one amino acid.
A mutation is a change in the base sequence. Most have little or no effect on the protein; a few change its shape so it no longer works.

Common mistake

Students mix up mitosis and meiosis. Meiosis happens only in the reproductive organs to make gametes; growth of the embryo after fertilisation is by mitosis.

Worked example

A daffodil can reproduce asexually, when its bulb divides, and sexually, by making seeds. A new disease spreads through a field of daffodils. Which offspring are more likely to include some survivors?

Plants from divided bulbs are clones: they have the same alleles as the parent.
If the parent has no resistance, none of the clones has it either.
Seeds are made by fusing gametes, so the seedlings have different mixtures of alleles.
With variation, a few seedlings may have alleles that give resistance.

Answer: The plants grown from seeds, because sexual reproduction gives variation.

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