Reproduction, meiosis and DNA

Sexual reproduction joins two gametes (sex cells): sperm and egg in animals, pollen and egg cells in flowering plants.

Each offspring gets a mix of genetic information from two parents, so the offspring vary.

Asexual reproduction needs one parent and no gametes. The new cells are made by mitosis, so the offspring are genetically identical clones. Strawberry runners, bacteria splitting in two and potato tubers are all asexual.

Biology only: sexual reproduction is slower but gives variation, so some offspring may survive if the environment changes. Asexual reproduction is faster and needs no mate.

Gametes are made by meiosis in the testes and ovaries (anthers and ovaries in plants).

The genetic information is copied, then the cell divides twice, giving four gametes. Each has one chromosome from every pair, so half the normal number, and each is genetically different.

At fertilisation two gametes join, so the full number is restored. The new cell then divides by mitosis and its cells differentiate as the embryo develops.

1. Horse body cell: 64 chromosomes (32 pairs)
2. Gamete from meiosis: 64 ÷ 2 = 32 chromosomes
3. Fertilisation: 32 + 32 = 64 again

Which chromosome of each pair a gamete gets is random, and each pair is independent of the others, so every pair doubles the number of possible gametes.

number of different gametes = 2ⁿ (n = number of chromosome pairs)

1. Fruit fly: 8 chromosomes, 4 pairs
2. 2⁴ = 2 × 2 × 2 × 2 = 16 different gametes

Use the number of pairs, not the number of chromosomes: 2⁸ would be wrong for the fruit fly.

DNA is a polymer made of two strands twisted into a double helix.

A chromosome is one very long DNA molecule, found in the nucleus. A gene is a short section of DNA that codes for a particular sequence of amino acids, to make one protein. The genome is all the genetic material of an organism.

Studying the human genome helps to find genes linked to diseases, to understand and treat inherited disorders, and to trace how humans migrated long ago.

Biology only: DNA is made of repeating nucleotides, each a sugar, a phosphate and one of four bases (A, C, G, T). A always pairs with T and C with G, so in any DNA sample %A = %T and %C = %G. Three bases code for one amino acid; a copy of the gene (mRNA) is read at the ribosomes, where carrier molecules bring the amino acids in order.

Edexcel and OCR name the two stages of protein synthesis and the molecules in each.

Transcription happens in the nucleus. The enzyme RNA polymerase binds to the non-coding DNA just in front of a gene. The two DNA strands separate, and RNA polymerase moves along the gene making a strand of messenger RNA (mRNA) that is complementary to the template strand. RNA has the base uracil (U) where DNA has thymine, so a T on the DNA gives an A on the mRNA, and an A on the DNA gives a U.

The mRNA is small enough to leave the nucleus. It attaches to a ribosome in the cytoplasm.

Translation happens at the ribosome. The mRNA is read in codons, sequences of three bases, each coding for one amino acid. Transfer RNA (tRNA) molecules each carry one particular amino acid and have an anticodon, three bases that pair with a codon. The amino acids are joined in the order of the codons, and the chain folds into the protein.

1. Template DNA: TAC GGA TTC
2. mRNA (A to U, T to A, C to G, G to C): AUG CCU AAG
3. First codon AUG pairs with the tRNA anticodon UAC
4. 9 bases ÷ 3 = 3 codons, so 3 amino acids

A codon is on the mRNA; an anticodon is on the tRNA. Neither has any T: RNA uses U.