Origins of variation in genomes and recombination Flashcards
Mitosis vs meiosis, homologous recombination DNA repair/meiotic, recombination and genetic analysis.
What kind of cells does mitosis happen in?
Somatic cells.
Represent the number of chromosomes in mother and daughter cells for diploid and haploid organisms in mitosis using n.
2n = 2n + 2n (diploid organisms)
1n = 1n + 1n (haploid organisms)
What is the final product of mitosis?
Genetical identical daughter cells.
What kind of cells does meiosis happen in?
Haploid gamete production.
Represent the number of chromosomes in mother and daughter cells in meiosis using n.
2n = n x 4
What is the final product of meiosis?
Genetically different daughter cells.
How many divisions are there in mitosis?
One - homologous chromosomes separate.
How many divisions are in meiosis?
Two - recombination and homolog separation.
Meiosis generates new combinations of…
alleles.
When does homologous recombination occur in meiosis?
Prophase of meiosis 1 where DNA has already replicated.
Before meiosis 1, homologous chromosomes pair. What happens if non-homologous chromosomes are present?
Leads to sterility.
Why can’t crossing over between organisms with different numbers of chromosomes produce viable offspring?
Chromosomes can’t line up during meiosis so no haploid gametes are produced.
Crossing over is visible as…
chiasmata.
When does crossing over occur in eukaryotes?
Meiosis.
How come crossing over also occurs in prokaryotes?
It’s a universal process.
Homologous recombination involves active breaking and joining of…
DNA.
What results in a reciprocal exchange of genetic material and a genetic rearrangement between chromosomes?
Homologous recombination in meiosis 1.
Why is homologous recombination important in meiosis?
It is important in evolution to bring together new combinations of alleles and DNA sequences.
Describe, step by step meiotic homologous recombination.
(If confused, look at summary sheet).
- Starts with a pair of homologous
chromosomes. - Spo11 and Mre11 nuclease complex cut a
chromosome to form a double-strand
break. - Spo11 leaves and Mre11 nuclease
complex caps 5’ end of strands leaving 3’
ends exposed. - Rec-A strand exchange protein loaded
and Mre11 nuclease complex leaves. - Strand invasion and formation of a d
loop. - DNA synthesis and DNA ligation forms a
double holiday junction (2 D-loops). - Chromosomes with no crossover due to
breaking and joining of DNA strands in the
same place.
OR - Chromosomes with crossing over due to
breaking and joining of DNA strands in
different places.
The Rec-A filament searches for a homologous sequence in an intact DNA duplex.
Once homology is found, Rec-A facilitates strand invasion, where the ssDNA invades the homologous DNA duplex, leading to strand exchange.
Which two ways can cells repair DNA double-strand break?
Non-homologous end joining.
Homologous recombination.
Explain why non-homologous end joining evolved to homologous recombination for repairing a double-strand break.
There is a loss of nucleotides due to degradation from ends.
End joining means deletion of DNA sequence = consequences for the cell.
Homologous recombination is a universal process in which the double-strand break is accurately repaired using information from…
A sister chromatid.
Give the mechanism of double-strand break repair by homologous recombination.
- Exonuclease degrades 5’ ends of double
strand break. - Complementary strand invades damaged
DNA forming a heteroduplex structure (D-
loop). - DNA synthesis and migration of branch
point by enzymes. - Continued branch migration, followed by
pairing of newly synthesised DNA with top
strand and top-strand DNA synthesis. - DNA ligation to accurately repair double
strand break.
The basic mechanism of homologous recombination to repair a double-strand break is different to crossing over in meiosis.
How?
DNA repair is non-reciprocal and localised (only a small exchange of genetic material).
DNA repair also means a potential loss in heterozygosity.
