Recombination

Question 1

Essential cellular processes involving genetic recombination (4)

Answer 1

genetic variation
DNA repair
genetic exchange
gene expression regulation

Question 2

What is genetic recombination?

Answer 2

It is a large-scale rearrangement of DNA molecules. It also involves physical exchanges of DNA sequences between chromosomes.

Question 3

Explain the Law of Independent Assortment.

Answer 3

Exchanges of non-sister chromatids between homologous chromosomes during meiosis can generate gametes with independently assorting alleles.

Question 4

Explain bacterial conjugation.

Answer 4

It is the transfer of a copy of DNA (plasmid) in bacteria from a donor cell to a recipient cell using a thread-like structure called pilus, resulting in two donor cells.

Question 5

Types of Recombination

Answer 5

1. Homologous Recombination
2. Site-Specific Recombination
3. Illegitimate Recombination

Question 6

Recombination requiring an extensive homology.

Answer 6

Homologous Recombination

Question 7

Recombination that does not require an extensive homology (very short homologous region).

Answer 7

Site-specific Recombination

Question 8

Recombination that does not require homology.

Answer 8

Illegitimate Recombination

Question 9

Explain the breakage and reunion of DNA molecules experiment by Meselson and Weigle (1961).

Answer 9

One strain was radioactively labeled with heavy isotopes (C-13 and N-15) as indicated by the marker alleles c- mi-, and another strain was labeled with light isotopes (C-12 and N-14) as indicated by the marker alleles c+ mi+. Upon simultaneous infection of E. coli with these phages, they found out that there are hybrid products containing the heavy c- and light mi+ alleles, and some with the heavy c+ and light mi- alleles. This demonstrates homologous recombination.

Question 10

cross-over points

Answer 10

chiasmata (sing. chiasma)

Question 11

T or F: Chiasmata are observable under the microscope.

Answer 11

True

Question 12

Explain how DNA cross-over occurs during the diplotene stage of Prophase I.

Answer 12

Two homologous pairs of sister chromatids align side by side. These 2 homologs are connected at a certain point called chiasma, where they can exchange DNA segment from the chiasma to the end of the chromosomes.

Question 13

This is where strands from two different DNA DNA helices have base-paired.

Answer 13

heteroduplex joint

Question 14

Key steps of homologous recombination

Answer 14

1. alignment of two homologous DNA molecules
2. introduction of breaks in the DNA
3. strand invasion: a Holliday structure is introduced
4. branch migration: movement of the Holliday junction
5. resolution: cleavage of the Holliday junction; vertical or horizontal cutting generates two separate duplex DNA molecules

Question 15

PRO : RecA as EU : ________

Answer 15

Rad51
Dom1

Question 16

PRO : RecBCD as EU : _________

Answer 16

MRX protein
Rad52 & Rad59

Question 17

PRO : RuvAB complex as EU : _________

Answer 17

unknown

Question 18

PRO : RuvC as EU : ____________

Answer 18

Mus81

Question 19

PRO : _______ as EU : Rad51

Answer 19

RecA

Question 20

PRO : ________ as EU : Rad59

Answer 20

RecBCD

Question 21

PRO : ________ as EU : Rad52

Answer 21

RecBCD

Question 22

Function: RecBCD

Answer 22

processes DNA breaks to generate single strand cuts for invasion;
assembly of strand exchange protein

Question 23

recombination hotspots in the bacterial genom

Answer 23

Chi site

Question 24

Function: RecA

Answer 24

catalyzes strand invasion (and for pairing of homologous DNA)

Question 25

pairing of the homologous DNA

Answer 25

synapsis

Question 26

Function: RuvA & RuvB

Answer 26

catalyzes branch migration;
recognition of Holliday junction;
rotates the helices

Question 27

Function: RuvC

Answer 27

for resolution;
endonuclease

Question 28

enzyme for integration of lambda DNA into the E. coli chromosome

Answer 28

integrase (int) and integration host factor (IHF)

Question 29

enzyme for removal of lambda DNA from the E. coli chromosome

Answer 29

integrase (int), excisionase (xis), and integration host factor (IHF)

Question 30

(1) Tn
(2) Ty
(3) SINES

Answer 30

1. bacterial transposons
2. yeast transposons
3. short interspersed nuclear elements

Question 31

Classification of transposable genetic elements

Answer 31

1. DNA transposons (“cut and paste”)
2. retrotransposons (“copy and paste”)

Question 32

Describe the mechanism of DNA transposons.

Answer 32

DNA transposons are cut from the donor DNA. The resulting intermediate DNA is then integrated into the target DNA, forming the new DNA with the cut transposable mobile element. This can be described as a “cut and paste” mechanism.

Question 33

Describe the mechanism of retrotransposons.

Answer 33

RNA polymerase generates an RNA intermediate from the retrotransposon region of the donor DNA. The RNA intermediate is reversed transcribed (by reverse transcriptase) forming the DNA intermediate, which is then integrated into the target DNA forming the new DNA with the copied transposable mobile element. This can be described as a “copy and paste” mechanism.