Chromosome evolution

Question 1

Define a telomere.

Answer 1

A repetitive nucleotide sequence at the end of a chromosome that prevents it from deterioration.

Question 2

What happens to the telomere at each cell division?

Answer 2

They shorten

Question 3

What would lengthening the telomeres do?

Answer 3

Reverse ageing.

Question 4

Define a centromere.

Answer 4

A region, of no defined sequence, that joins sister chromatids.

Question 5

There is a high sequence turnover rate at the centromeres. Why?

Answer 5

The spindles fibres attach here at meiosis and pull them apart.

Question 6

What is a telocentric chromosome?

Answer 6

The centromere is at the end.

Question 7

What is an acrocentric chromosome?

Answer 7

The centromere is near the end.

Question 8

What is a metacentric chromosome?

Answer 8

The centromere is in the middle.

Question 9

What is a submetacentric chromosome?

Answer 9

The telomere is near the middle.

Question 10

Generally gene location and function are independent. Give 2 exceptions to this rule.

Answer 10

1. When genes convey positional information, i.e. hox gene clusters display co-linearity
2. Tight linkage: in linkage disequilibrium genes are not broken up in recombination and inherited as blocks.

Question 11

What does positive selection at a linkage site cause?

Answer 11

A selective sweep, which results in a loss of genetic diversity.

Question 12

Selection only acts on individual genes. True or false?

Answer 12

False, it can act on whole chromosomes.

Question 13

Do larger chromosomes carry more genes?

Answer 13

Generally but not always.

Question 14

Define a karyotype.

Answer 14

The number of chromosomes in a cell, represented as ‘n’ for haploid and ‘2n’ for diploid.

Question 15

Define polyploidy.

Answer 15

The multiplication of the entire set of chromosomes.

Question 16

What is a) allopolyploidy and b) autopolyploidy?

Answer 16

a) A hybrid resulting from the combination of 2 distinct genomes
b) A duplication of the self-genome, resulting from a problem at meiosis

Question 17

Which form of polyploidy is more common?

Answer 17

Allopolyploidy.

Question 18

Why are polyploid plants artificially selected for by humans?

Answer 18

They have a higher chromosome number so produce more protein per cell.

Question 19

Why are only polyploids with an even number of chromosomes viable?

Answer 19

Uneven numbers results in unequal pairing at meiosis.

Question 20

Polyploids often return to diploidy because it is stable. Quadravalents will return to bivalence. Define bivalence.

Answer 20

When 2 copies of chromosomes line up together at the spindle.

Question 21

Does translocation occur between homologous chromosomes?

Answer 21

No: it results in incorrect pairing at recombination due to similarity between chromosomes, non-homologous sections are exchanged.

Question 22

Fusions of chromosomes can occur. Between which types of chromosome is this most common and why?

Answer 22

Between telo and acrocentric chromosomes: the centromeres are at the end so they combine to form a super-centromere.

Question 23

What does the fusion of 2 telocentric chromosomes form?

Answer 23

A metacentric chromosome with a single, functioning centromere.

Question 24

What does the fusion of 2 metacentric chromosomes create? What will happen?

Answer 24

A dicentric chromosome prone to breakage.

This leads to sterility as the 2 centromeres will line up at different spindle fibres.

Question 25

What happens in an inversion?

Answer 25

Chunks of chromosome flip over, reversing the gene order. This forms inversion loops that cannot recombine and thus blocks of genes are conserved.

Question 26

Define co-adapted alleles.

Answer 26

Those that have been conserved together in an inversion loop.

Question 27

Sometimes there can be crossing over within the inversion loop. What happens?

Answer 27

It can lead to deletions, resulting in a 50% reduction of viable gametes.

Question 28

How many remaining regions are there on the Y chromosome of placental mammals that can recombine?

Answer 28

2.

Question 29

What happens at recombination with segmental duplications?

Answer 29

Incorrect pairing: this results in one gamete lacking the region (lethal) and the other has 2 copies (deleterious if dose sensitive).

Question 30

Give an example of a segmental duplication that has been favoured by humans?

Answer 30

The Belgian Blue is a mutant Aberdeen Angus that has a double-dose of myosin, so is favoured for meat production.