Genetic diversity can arise as a result of mutation or during — AQA A-Level Biology

A gene mutation is a change to the base sequence of the DNA in a chromosome, producing a new allele of that gene. Because the base sequence determines the sequence of codons on mRNA, a change can alter which amino acid is inserted at that point in the polypeptide. A different amino acid sequence changes the primary structure. Since the R groups of the amino acids determine where ionic bonds, hydrogen bonds and disulfide bridges form, the tertiary structure may also change. For an enzyme, this altered 3D shape means the active site may no longer be complementary to its substrate, preventing enzyme-substrate complexes from forming. Some mutations are silent: because the genetic code is degenerate, a changed triplet may still code for the same amino acid, so the primary structure is unchanged.

Mutations can arise spontaneously as copying errors during DNA replication. A base substitution replaces one base with another, affecting one triplet only. It may be silent because the genetic code is degenerate, meaning the new codon can still specify the same amino acid. Alternatively, a different amino acid may be inserted, which can alter the tertiary structure, or the triplet may become a stop codon. A base deletion removes a base, causing every base after it to shift back into a new triplet. This frameshift means that most codons downstream are altered, so most amino acids after the deletion are changed and the protein is often non-functional. Because of this frameshift effect, deletions are usually more damaging to the final polypeptide than substitutions.

The code is degenerate because most amino acids are specified by more than one triplet (codon), often differing at the third base. A substitution that produces a synonymous codon therefore leaves the amino acid sequence (primary structure) unchanged, so tertiary structure and function are usually unaffected: a silent mutation. For example, if a codon table shows that a third-base change still specifies the same amino acid, the substitution is silent. A substitution at the first or second base is more likely to change the amino acid, altering folding and possibly function. Degeneracy buffers organisms against some copying errors, but not all substitutions are harmless: one creating a stop codon truncates the polypeptide. Degenerate (more than one codon per amino acid) must be distinguished from universal (the same codons specify the same amino acids in almost all organisms).

Mutations occur spontaneously at a low rate, and mutagenic agents raise that rate. Ionising radiation such as X-rays and gamma rays, and ultraviolet light, damage DNA directly; ultraviolet light makes adjacent thymine bases bond together, distorting the helix so that replication mis-copies it. Chemical mutagens work in other ways. Base analogues such as 5-bromouracil are incorporated in place of normal bases and then pair wrongly, alkylating agents add chemical groups that change base pairing, and intercalating agents slot between the bases and push the strands apart so extra bases are inserted or lost, causing frameshifts. Some mutagens, such as benzopyrene in tobacco smoke, are also carcinogens because the mutations they cause may fall in proto-oncogenes or tumour suppressor genes.

Chromosome mutations change the number of chromosomes rather than the base sequence. They arise when homologous chromosomes fail to separate at anaphase I, or when sister chromatids fail to separate at anaphase II. This is non-disjunction: both structures are pulled to the same pole, so one daughter cell gains an extra chromosome and another is left one short. A gamete with n plus one chromosomes fertilised by a normal gamete gives a zygote with forty-seven chromosomes in humans, a trisomy, as in Down's syndrome where chromosome 21 is present three times. Non-disjunction of a whole set produces polyploidy, which is common and often viable in plants but almost always lethal in animals.

Meiosis produces genetically distinct daughter cells via two key mechanisms. First, independent segregation occurs at metaphase I. Homologous chromosome pairs align randomly on the equator; which chromosome goes to which pole is independent of other pairs. For example, humans have 23 pairs, yielding $2^{23}$ (over 8.3 million) possible chromosome combinations. Second, crossing over happens in prophase I. Homologous chromosomes form bivalents and chromatids twist at chiasmata, exchanging equivalent sections of DNA. Because homologues carry different alleles, this creates entirely new combinations of alleles on a single chromosome, increasing genetic diversity.

Meiosis begins with a diploid cell whose DNA has already replicated, so each chromosome exists as two identical sister chromatids joined at a centromere. In meiosis I, homologous chromosomes pair up as bivalents, crossing over occurs at chiasmata, and the pairs are then pulled apart, one whole chromosome from each pair going to each pole. This is the reduction division: the two cells produced are haploid, carrying one chromosome from each homologous pair, though each chromosome is still two chromatids. Meiosis II resembles mitosis: the chromatids of each chromosome are separated, giving four haploid daughter cells in total from one diploid parent. Which homologue of each pair goes to which pole in meiosis I is independent of every other pair, so the combinations of maternal and paternal chromosomes differ between cells, and crossing over has already exchanged alleles between them.

Meiosis produces four genetically different haploid cells, whereas mitosis produces two genetically identical diploid cells. In diagram questions, after meiosis I, draw half the original chromosome number, each still as two chromatids (one from each homologous pair). After meiosis II, draw the same number but as single chromosomes. In unfamiliar life cycles, meiosis occurs exactly where the chromosome number halves, regardless of whether the resulting cells are called gametes or spores. Finally, random fertilisation further increases genetic variation because any genetically unique male gamete can fuse with any genetically unique female gamete, creating novel allele combinations in the zygote.