Gene duplication and repetitive DNA

Question 1

Discuss the C value paradox

Answer 1

C-Value = the amount of DNA in a haploid nucleus for a given species
Paradox = The complexity of the DNA code is not correlated to the size of the genome

Question 2

Describe the two ways that satellite DNA can differ between individuals

Answer 2

1. Replication Slippage - dissociation of DNApolymerase during replication and it doesn’t correctly stick back. Nascent DNA could rehybridize with an identical sequence earlier or later in the sequence. = Daughter strand is shorter or longer than template
2. Unequal crossing over - crossovers occur between misaligned repeats on sister chromatids, one gamete will have more and one gamete will have less copies of the repeat.

Question 3

How can satellite DNA affect disease? (use specific example)

Answer 3

Huntingtons disease - There is a trinucleotide repeat in huntingtin gene (CAG). Codes for polyglutamine tract
Proteins with expanded CAG repeats are degraded into toxic fragments.

Question 4

Describe the key steps of SSLP DNA fingerprinting (simple sequence length polymorphism)

Answer 4

1. extract DNA
2. Digest with a convenient restriction enzyme (endonuclease)
3. Separate the fragments on a gel
4. Southern Blot using a minisatellite sequence probe
5. Observe characteristic bands for individuals

Question 5

what is the difference between orthologues and paralogues??

Answer 5

Orthologues= evolved by speciation, eg a-tubulin in species 1 and a-tubulin in species 2 (ie same gene in diff species)
Paralogues = evolved by gene duplication. eg a-tubulin and b-tubulin in species two. (gene  is duplicated in same species and differences accumulate in two genes)

Question 6

What are the possible fates of a duplicated gene?

Answer 6

1. Psuedogene - 2 copies of gene reduces selective pressure and 2nd copy mutated and loses function but remains in genome
2. Neofunctionalization - one cope of gene remains normal and one copy gains entirely new function
3. Subfunctionalization - each copy specialises

Question 7

What is a conventional pseudogene?

Answer 7

Accumulated mutations prevent one of the duplicated genes from functioning.
no selective pressure due to two copies of the gene, the gene can lose function all together and degrade

Question 8

What is a processed pseudogene?

Answer 8

Generated by reverse transcription of functional mRNA and insertion of the cDNA into genome by line proteins.
Inserted sequence do not have correct processing signals (eg introns) so generally do not function

Question 9

what is the development of the human globin gene an example of?

Answer 9

subfunctionalization

Question 10

How does adult and foetal haemoglobin differ?

Answer 10

Foetal haemoglobin has a much higher affinity for Oxygen than adult haemoglobin. This allows oxygen to diffuse from maternal blood into foetal blood

Question 11

Describe the evolution process of globin gene family

Answer 11

1. unequal crossing over between two transposons
2. chromosome with 2 b-globin genes are passed onto germline
3. the 2 copies evolve independently, genetic paralogues.

Question 12

What are the various fates of meiotic disjunction?

Answer 12

Gametes can be formed which are 2n
This can combine to form 3n with a normal 1n gamete = triploid
Or can combine with 2n of another species = tetraploid.

Question 13

Discuss the stability of duplicated genomes

Answer 13

even number of duplicated genomes are likely to be more stable than odd number (eg3n)
however triploid does seem to be fairly stable in plants

Question 14

What effects can duplicated genomes have?

Answer 14

Entire duplicated genome means there are two copies of each gene so selective pressure is reduced.
This can allow for the divergence and speciation of two copies.
Lots of duplicated material is lost by mutation or deletion over evolutionary time (diploidization)