End of CSB- Genetic recombination

Question 1

What is homologous recombination?

When does it occur?

Important for __ and __ but can also cause __

Answer 1

Genetic exchange between homologous chromosomes (where DNA sequences are similar)

Can occur during gametogenesis, and in dividing somatic cells.

Important for genetic diversity, DNA damage repair, but can also cause LOH

Question 2

Harmful consequences of homologous/general recombination

Answer 2

Deletions, inversions, translocations, etc

Tumors

Question 3

Site-specific recombination

Answer 3

Integration of phage DNA into bacterial genomes via specific nucleotide sequences and enzymes.

Question 4

The recombining sites in site-specific recombination usually

Answer 4

share a small amount of sequence similarity

Question 5

Medical relevance of site-specific recombination

Answer 5

Can result in antibiotic resistance or toxin expression

Question 6

Transposons

Answer 6

DNA inserts that comprise ~50% of our genome; they don’t replicate or survive on their own

Question 7

Rec A and RecBCD are involved in

Answer 7

homologous recombination during gametogenesis during the first separation and division of prophase I

Question 8

Loss of heterozygosity

Answer 8

Deletion within a cell of the second of two alleles of a tumor suppressor gene

Question 9

Explain the mechanism of homologous recombination in DNA replication/repair

Answer 9

1. DNA lesion (like a thymidine dimer) creates a bulge in the DNA that stalls replication machinery
2. The fork regresses so that the undamaged newly synthesized strand can be used as a template
3. REverse branch migration moves the replication fork back to its original position, with the new strand already synthesized beyond the lesion –> replication continues

Question 10

Two causes of LOH

Answer 10

• Nondisjunction/mis-segregation: failure of spindle-assembly checkpoint during mitosis resulting in 3:1 segregation of sister chromatids
  
  • Random loss of one of the 3 chromosomes can result in either a heterozygous (+/o) or a homozygous mutant (o/o).
• Homologous recombination: recombining homologous chromosomes can cause a homozygous wild type (+/+) or a homozygous mutant (o/o)

Question 11

Mechanism of transposition

Answer 11

1. Transposase makes staggered cuts in some target site
2. A transposon comes in and binds its flanking terminal repeats to the staggered ends o teh target site, thus inserting itself.
3. DNA poly fills in staggered ends, creating a duplication of the insertion site flanking the transposon

Question 12

Replicative vs non-replicative/conservative transposons

Answer 12

Replicative transposition makes a copy of the DNA via a transcription and a reverse transcriptase mechanism - the copy gets inserted somewhere in the genome while the original stays where it was

Nonreplicative transposition is ‘cut and paste’ - the original is removed from wher eit was and inserted somewhere else

Question 13

Retrotransposons use what type of transposition?

Answer 13

Replicative - a copy is made and is integrated elsewhere in the genome

Question 14

LINEs, SINEs, and LTRs are examples of

Answer 14

retrotransposons (replicative transposition)

SINEs are the most abundant sequence in the human genome and has been useful for evolutionary studies

Question 15

RAG1 and RAG2

Answer 15

Recombinase proteins descended from DNA transposons; responsible for antibody diversity; performs conservative/nonreplicative transposition

Question 16

Alu elements

Answer 16

primate-specific SINEs

Question 17

Translocation

Answer 17

Abnormal reciprocal exchange between nonhomologous chromosomes

Question 18

Recombination signal sequences (RSS) in V-J recombination in antibody rearrangement

Answer 18

RSS (relics of DNA transposons) exists between each and every variable and joining domain. RAG1 & 2 mediate V-J recombination through these intervening RSS sequences

Question 19

How does transposition contribute to evolution?

Answer 19

Homologous recombinations can occur between Alu elements interspersed among exons –> exon shuffling

Question 20

Chronic Myelogenous Leukemia (CML/AML) - what is the cause?

Answer 20

9:22 chromosomal translocation in cells of myeloid cell lineage

The resulting chromosome is the philadelphia chromosome, which expresses a BCR:Abl fusion protein deregulated and constitutively active in Abl kinase activity –> abnormal growth

Question 21

Imatinib

Answer 21

Binds the active site of the Abl kinase domain of the BCR:Abl fusion protein to inhibit further phosphorylation

Question 22

B cell lymphoma

Answer 22

18:14 translocation of BCL-2 into a region expressing high levels of Ig proteins –> overexpresses BCL-2 and provides resistance to apoptosis in the B cell lymphoma

Question 23

Burkitts lymphoma

Answer 23

Translocation of the Myc transcription factor on chromosome 8 into chromosome 2, 14, or 22, where there’s a high level of expression of Ig proteins.

Overexpression of Myc result sin active cell cycle progression in a B cell lineage.