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Genetics > Origins of variation in genomes and recombination > Flashcards

Origins of variation in genomes and recombination Flashcards

Mitosis vs meiosis, homologous recombination DNA repair/meiotic, recombination and genetic analysis.

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Biology 101 flashcards
1
Q

What kind of cells does mitosis happen in?

A

Somatic cells.

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2
Q

Represent the number of chromosomes in mother and daughter cells for diploid and haploid organisms in mitosis using n.

A

2n = 2n + 2n (diploid organisms)
1n = 1n + 1n (haploid organisms)

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3
Q

What is the final product of mitosis?

A

Genetical identical daughter cells.

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4
Q

What kind of cells does meiosis happen in?

A

Haploid gamete production.

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5
Q

Represent the number of chromosomes in mother and daughter cells in meiosis using n.

A

2n = n x 4

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6
Q

What is the final product of meiosis?

A

Genetically different daughter cells.

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7
Q

How many divisions are there in mitosis?

A

One - homologous chromosomes separate.

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8
Q

How many divisions are in meiosis?

A

Two - recombination and homolog separation.

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9
Q

Meiosis generates new combinations of…

A

alleles.

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10
Q

When does homologous recombination occur in meiosis?

A

Prophase of meiosis 1 where DNA has already replicated.

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11
Q

Before meiosis 1, homologous chromosomes pair. What happens if non-homologous chromosomes are present?

A

Leads to sterility.

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12
Q

Why can’t crossing over between organisms with different numbers of chromosomes produce viable offspring?

A

Chromosomes can’t line up during meiosis so no haploid gametes are produced.

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13
Q

Crossing over is visible as…

A

chiasmata.

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14
Q

When does crossing over occur in eukaryotes?

A

Meiosis.

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15
Q

How come crossing over also occurs in prokaryotes?

A

It’s a universal process.

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16
Q

Homologous recombination involves active breaking and joining of…

A

DNA.

17
Q

What results in a reciprocal exchange of genetic material and a genetic rearrangement between chromosomes?

A

Homologous recombination in meiosis 1.

18
Q

Why is homologous recombination important in meiosis?

A

It is important in evolution to bring together new combinations of alleles and DNA sequences.

19
Q

Describe, step by step meiotic homologous recombination.

(If confused, look at summary sheet).

A
  1. Starts with a pair of homologous
    chromosomes.
  2. Spo11 and Mre11 nuclease complex cut a
    chromosome to form a double-strand
    break.
  3. Spo11 leaves and Mre11 nuclease
    complex caps 5’ end of strands leaving 3’
    ends exposed.
  4. Rec-A strand exchange protein loaded
    and Mre11 nuclease complex leaves.
  5. Strand invasion and formation of a d
    loop.
  6. DNA synthesis and DNA ligation forms a
    double holiday junction (2 D-loops).
  7. Chromosomes with no crossover due to
    breaking and joining of DNA strands in the
    same place.
    OR
  8. 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.

20
Q

Which two ways can cells repair DNA double-strand break?

A

Non-homologous end joining.

Homologous recombination.

21
Q

Explain why non-homologous end joining evolved to homologous recombination for repairing a double-strand break.

A

There is a loss of nucleotides due to degradation from ends.
End joining means deletion of DNA sequence = consequences for the cell.

22
Q

Homologous recombination is a universal process in which the double-strand break is accurately repaired using information from…

A

A sister chromatid.

23
Q

Give the mechanism of double-strand break repair by homologous recombination.

A
  1. Exonuclease degrades 5’ ends of double
    strand break.
  2. Complementary strand invades damaged
    DNA forming a heteroduplex structure (D-
    loop).
  3. DNA synthesis and migration of branch
    point by enzymes.
  4. Continued branch migration, followed by
    pairing of newly synthesised DNA with top
    strand and top-strand DNA synthesis.
  5. DNA ligation to accurately repair double
    strand break.
24
Q

The basic mechanism of homologous recombination to repair a double-strand break is different to crossing over in meiosis.

How?

A

DNA repair is non-reciprocal and localised (only a small exchange of genetic material).
DNA repair also means a potential loss in heterozygosity.

25
Q

In which cells does homologous recombination occur? Why?

A

Polypoid plant cells (contain more than two complete sets of chromosomes).
Somatic mammalian cells.

This process requires a sister chromosome/homologous chromosome so can’t occur in haploid cells.

26
Q

When heteroduplex structure forms in homologous recombination, what is synapsis catalysed by?

Synapsis = formation of heteroduplex with invading strand.

A

RAD51 (RecA in bacteria) and other proteins.
Bind to 3’ end of single stranded damaged DNA and forms a dynamic 3-stranded structure with double stranded DNA before searching for homology.

Homology = homologous region that will complementary pair with damaged DNA strand.

27
Q

What did Robin Holiday hypothesise in 1964?

A

Formation of holiday junctions during generation of crossing over.
Also hypothesised they’d occur in DNA repair as well.

Dynamic picture on PowerPoint = 4 way zip.
Double holiday junction = 2 D-loops.

28
Q

Genes on a chromosome are linked genetically and…

A

physically.

29
Q

Why can you use recombination frequency of two linked genes to map genes in transmission genetics?

A

For 2 linked genes:
Recombination frequency is proportional to distance apart.

30
Q

Homologous recombination is exploited for…

A

gene targeting.

31
Q

How can you target a specific gene for mutation in vivo using homologous recombination?

A

Introduce DNA containing homologous sequences and a selectable marker.
Recombination integrates DNA into chromosome.
Select for cells with marker.

32
Q

How can gene targeting help society?

A

Helps understand genetic disorders.

33
Q

How many crossovers happen when circular DNA is inserted into a gene in gene targeting?

A

One (single crossover).

34
Q

How many crossovers happen when linear DNA is inserted into a gene in gene targeting?

A

Two (double crossover).

35
Q

Linear DNA gene targeting is used for yeast gene deletion and knockout mice. What does this mean for their genome?

A

One or more genes are turned off.

36
Q

Homologous recombination also occurs in bacteria.
How do bacterial genomes evolve?

A

Chromosome = circular DNA.
Other DNA = plasmid.
At region of homology (similarity), homologous recombination occurs and bacteria gain new genes from plasmids.

37
Q

How can genetic engineering occur in bacteria?

A

Insert homologous DNA, genetic recombination occurs, complete sections of replacement with other DNA in chromosome.

38
Q

How are recombinant bacteriophages formed?
Why does it take place in a bacterium?

A

When viruses infect bacteria, phage DNA inserted into cell.
Crossing over occurs (single crossover).

They don’t have the apparatus for homologous recombination.

Genetics (10 decks)

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