Recombination

Question 1

When does crossing over ocurr

Answer 1

between two chromosomes at prophase 1 of meiosis

Question 2

Chiasma

Answer 2

the point where homologous chromosomes (maternal and paternal) cross over. Chiasmata is plural, multiple cross over events

Question 3

Recombination

Answer 3

The rearrangement of genetic material, esp. by crossing over in chromosomes. Evolved as a mechanism to repair damaged DNA

Question 4

Single stranded break

Answer 4

originally thought that one strand from each chromosome was broken. These strands are of the same polarity and so invade the other chromosome. Nicks made in sister strands. Rejoin in a process called invasion.

Question 5

Holliday structure in single strand breaks and resolution

Answer 5

After strands invade and nicks are ligated a four branched holliday structure is formed. This can be resolved by cutting in two ways. if the two invading strands are cut the resultant is not a true recombinant. If the non invading strands are cut the result is a true recombinant

Question 6

Heteroduplex DNA

Answer 6

one half from one parental duplex and one half from the other, with a hybrid (heteroduplex) section in between in true recombinant

Question 7

Branch migration

Answer 7

holliday structure is dynamic and can move from the cross over point

Question 8

2 stranded break

Answer 8

Reality is we now think this is the case. Chromosome A breaks in both strands. as these occur frequently

Question 9

Steps in double stranded break repair. Homologous recombination

Answer 9

Double stranded break, limited degradation from 5’ end by exonuclease, RecA (bacteria) or Rad51 (humans) strand invasion, 3’ end of invading strand extended by polymerase, the displaced loop can be used as a template to extend the strand that was left by the invading strand, the invading strand joins its original partner and the ends are ligated, two holliday structures are resolved by cleaving phosphodiester bonds

Question 10

RecA

Answer 10

Coats invading DNA to help it find its complementary sequence. All animals have homologues of RecA. Has two binding sites so can bind single strand and double strand. The search process induces stretching of the DNA duplex, which enhances sequence complimentarity recognition. 38KD protein. invasion from right to left

Question 11

Gene conversion

Answer 11

If crossover occurs at the point of a gene, a mismatch of the DNA of the two inner chromatids happens. Mismatch excision repair occurs at heteroduplex joint and results in ratio of alleles in 3:1

Question 12

Recombination in repeat sequences

Answer 12

In certain sequences on the same chromosome with similarity they can recombine if proofreading fails or be repaired by mismatch repair

Question 13

Causes of mutations

Answer 13

UV, toxins, radiation, chemicals

Question 14

Deamination

Answer 14

Cytosine to Uracil. Can be repaired (back to CG) or may be paired with an A leading to mutation

Question 15

Depurination

Answer 15

A may be depurinated. It could be removed to give a frameshift mutation and deletion of AT pair or it could be repaired to be unchanged

Question 16

Pyrimidine dimer

Answer 16

disruption by UV light can form pyrimidine dimers (C,T) that form a lesion in the DNA.

Question 17

Excision repair with pyrimidine dimer humans

Answer 17

In humans recognised by XP-C and 23b complex. TFIIH recruited which partially unwinds helix, XP-G and RPA bind and continue helicase. XP-F and XP-G cut the strand (exonuclease) and gap is filled by polymerase.

Question 18

Excision repair with dimer

Answer 18

UvAB complex binds and recruits UVrC (a leaves the complex), cuts the complex and UVrD and polymerase join, ends are ligated, repair complete

Question 19

Base excision repair

Answer 19

Dna glycosylase removes base but leaves deoxyribose, AP endonuclease and phosphodiesterase remove the sugar, DNA polymerase adds new base in

Question 20

Double strand break repair

Answer 20

Double stranded break. Loss of nucleotides from degradation. Ends ligate (non homologous end joining) or they are restored in a process involving homologous recombination (homologous end joining)