Quick questions on Mutation, gamete variation and the source of new alleles: HSC Biology Module 6

Meiosis produces haploid gametes from a diploid parent, reducing chromosome number by half. Three processes generate variation.

Semi-conservative replication. Each daughter DNA molecule retains one original strand and one newly synthesised strand. This conserves the parental sequence.

Even with proofreading and repair, mistakes accumulate. Mutagens (UV, chemicals, viruses) further increase the rate. In humans, each newborn carries roughly 60 to 100 new mutations not present in either parent. Most are in non-coding regions and have no effect; some are mildly deleterious; a small fraction are advantageous.

Natural selection requires three things: heritable variation, differential reproduction and inheritance of the advantageous variant.

A genome that mutates too much loses its information; a genome that mutates too little cannot adapt. Real organisms balance the two with high-fidelity replication (conservation) plus a small residual rate of mutation (variation). The mutation rate is itself an evolved property.

During metaphase I, the homologous chromosome pairs line up at the spindle equator independently of one another. Either the maternal or the paternal chromosome of each pair can face either pole. With $n$ chromosome pairs, this produces $2^n$ possible combinations.

During prophase I, homologous chromosomes pair up (synapsis) and exchange segments at chiasmata. This recombines maternal and paternal alleles within each chromosome, generating combinations that were not present in either parent. Crossing over essentially makes the $2^n$ figure an underestimate; the actual number of unique gametes is astronomically larger.

Any one of the millions of possible sperm can fertilise any one of the possible eggs, multiplying the variation across the population.

Each daughter DNA molecule retains one original strand and one newly synthesised strand. This conserves the parental sequence.

DNA polymerase has a 3' to 5' exonuclease activity that removes incorrectly added bases during synthesis, reducing the error rate from about 1 in $10^5$ (unaided) to 1 in $10^7$.

After replication, mismatch repair proteins recognise base mismatches and excise the wrongly inserted base, reducing the error rate further to about 1 in $10^{10}$.

Meiosis can only shuffle what already exists.

Meiosis can shuffle the chromosomes, but every gamete still carries only the green allele. The population cannot adapt to the darker environment.

A rare mutation produces a "brown" allele. On the new dark bark, brown beetles are camouflaged and survive predation better. The brown allele rises in frequency over generations.

It does not. Meiosis recombines existing alleles into new combinations; only mutation creates new alleles.