Genetic Diversity- Mutations and Meiosis — AQA A Level Biology 7402 Revision Notes & Topical Questions

A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide. Mutations occur continuously and at random. Because the DNA base sequence determines the sequence of amino acids in a protein, mutations can sometimes lead to a change in the polypeptide.

A substitution is a mutation where one base is randomly swapped for a different base.

Natural mechanisms ensure the accuracy of DNA replication: proofreading and repair of damaged DNA. When the mutation rate rises above the normal (usually low) rate, these mechanisms become ineffective. Mutagenic agents are environmental factors that increase the mutation rate, such as high-energy radiation (UV, X-rays) and certain chemicals.

Mutations can occur at different levels, within chromosomes or at the genetic level. Chromosome mutations involve a change in the number of chromosomes. Non-disjunction occurs when chromosomes fail to separate during meiosis, so gametes have an extra or a missing chromosome (for example Down syndrome, trisomy 21).

Meiosis produces daughter cells that are genetically different from each other and from the parent cell, due to independent segregation and crossing over. The AQA specification uses independent segregation of homologous chromosomes; independent assortment is often accepted, but segregation is more precise.

Meiosis is a nuclear division that produces haploid cells from diploid cells. It produces gametes in animals and plants for sexual reproduction (in plants, spores in some life cycles). It resembles mitosis but has two divisions, meiosis I and meiosis II.

In meiosis I, homologous chromosomes pair, cross over and are separated, so the cell divides by cytokinesis into two haploid daughter cells. They are haploid because each contains half the number of centromeres (each chromosome is still duplicated, with two chromatids). There is no interphase between meiosis I and meiosis II, so DNA is not replicated again.

The second division is almost identical to mitosis.

Chromosomes are counted by centromeres, so after DNA replication in interphase the number of chromosomes stays at 46, because there are still 46 centromeres. However, each chromosome now consists of two sister chromatids, giving 92 chromatids. You need to be able to complete diagrams showing chromosome content of cells after the first and second meiotic divisions, given the parent cell.

Genetically different offspring can be advantageous for natural selection. Meiosis increases the diversity of gametes by crossing over and independent segregation of homologous chromosomes; random fertilisation then adds further variation.

With n as the number of homologous pairs, for humans the haploid number is 23 (diploid 46), so 2²³ = 8 388 608 possible chromosome combinations in gametes (ignoring crossing over).

For a plant with diploid number 16, haploid n = 8 and 2⁸ = 256 different combinations in each gamete type. After random fertilisation of an ovule and pollen nucleus the number of combinations is 256 × 256 = 2¹⁶ = 65 536, so the formula is (2ⁿ)² = 2²ⁿ.

Cells in meiosis can be observed and photographed with specialised microscopes.

Mitosis and meiosis are both forms of nuclear division. Mitosis ends with two daughter cells that are genetically identical to each other and to the parent cell. This is important so that growth and cell replacement can occur continually in the body, and for asexual reproduction. Meiosis produces four haploid cells that are genetically different, for sexual reproduction.

A common question asks you to identify where meiosis occurs in an unfamiliar life cycle. Meiosis always involves a reduction division, a nuclear division that reduces the chromosome number of a cell. When the ploidy of the cell is halved (diploid to haploid), meiosis has just occurred. When ploidy returns to diploid, fertilisation has occurred.