Chromosome evolution Flashcards
Define a telomere.
A repetitive nucleotide sequence at the end of a chromosome that prevents it from deterioration.
What happens to the telomere at each cell division?
They shorten
What would lengthening the telomeres do?
Reverse ageing.
Define a centromere.
A region, of no defined sequence, that joins sister chromatids.
There is a high sequence turnover rate at the centromeres. Why?
The spindles fibres attach here at meiosis and pull them apart.
What is a telocentric chromosome?
The centromere is at the end.
What is an acrocentric chromosome?
The centromere is near the end.
What is a metacentric chromosome?
The centromere is in the middle.
What is a submetacentric chromosome?
The telomere is near the middle.
Generally gene location and function are independent. Give 2 exceptions to this rule.
- When genes convey positional information, i.e. hox gene clusters display co-linearity
- Tight linkage: in linkage disequilibrium genes are not broken up in recombination and inherited as blocks.
What does positive selection at a linkage site cause?
A selective sweep, which results in a loss of genetic diversity.
Selection only acts on individual genes. True or false?
False, it can act on whole chromosomes.
Do larger chromosomes carry more genes?
Generally but not always.
Define a karyotype.
The number of chromosomes in a cell, represented as ‘n’ for haploid and ‘2n’ for diploid.
Define polyploidy.
The multiplication of the entire set of chromosomes.
What is a) allopolyploidy and b) autopolyploidy?
a) A hybrid resulting from the combination of 2 distinct genomes
b) A duplication of the self-genome, resulting from a problem at meiosis
Which form of polyploidy is more common?
Allopolyploidy.
Why are polyploid plants artificially selected for by humans?
They have a higher chromosome number so produce more protein per cell.
Why are only polyploids with an even number of chromosomes viable?
Uneven numbers results in unequal pairing at meiosis.
Polyploids often return to diploidy because it is stable. Quadravalents will return to bivalence. Define bivalence.
When 2 copies of chromosomes line up together at the spindle.
Does translocation occur between homologous chromosomes?
No: it results in incorrect pairing at recombination due to similarity between chromosomes, non-homologous sections are exchanged.
Fusions of chromosomes can occur. Between which types of chromosome is this most common and why?
Between telo and acrocentric chromosomes: the centromeres are at the end so they combine to form a super-centromere.
What does the fusion of 2 telocentric chromosomes form?
A metacentric chromosome with a single, functioning centromere.
What does the fusion of 2 metacentric chromosomes create? What will happen?
A dicentric chromosome prone to breakage.
This leads to sterility as the 2 centromeres will line up at different spindle fibres.
What happens in an inversion?
Chunks of chromosome flip over, reversing the gene order. This forms inversion loops that cannot recombine and thus blocks of genes are conserved.
Define co-adapted alleles.
Those that have been conserved together in an inversion loop.
Sometimes there can be crossing over within the inversion loop. What happens?
It can lead to deletions, resulting in a 50% reduction of viable gametes.
How many remaining regions are there on the Y chromosome of placental mammals that can recombine?
2.
What happens at recombination with segmental duplications?
Incorrect pairing: this results in one gamete lacking the region (lethal) and the other has 2 copies (deleterious if dose sensitive).
Give an example of a segmental duplication that has been favoured by humans?
The Belgian Blue is a mutant Aberdeen Angus that has a double-dose of myosin, so is favoured for meat production.
