1-5 Dynamic Genomes and the Creation of Genetic Diversity

Question 1

How is the process of bacterial homologous replication carried out?

Answer 1

• rec enzymes catalyze genetic exchange
• 2 homologous, double-stranded sequences align
• A nick is made in one strand, allowing it to invade and anneal to the complementary strand of the homologue (strand exchange)
• The displaced strand is nicked and anneals to the other homologue
• Ends are ligated, forming the Holliday junction
• Complex strand crossover
• Crossing strands cut and ligated to each other

Question 2

How is eukaryotic homologuous recombination carried out?

Answer 2

• Initiates with a double strand break
• Rad50 complex cuts out the 5’ ends, leaving 3’ overhangs
• Rad51 facilitates strand invasion/exchange
• Ligase and resolvase connect the ends

Question 3

What is the life cycle of bacteriophage lambda?

Answer 3

After the lambda virus attaches to a host cell and injects lambda DNA, the lambda DNA circularizes, and there are two pathways:

1. Prophage pathway: lambda DNA integrates into and replicates with the host chromosome
2. Lytic pathway: viral proteins synthesized to form new viruses, rapid DNA replication, packaging into viruses, release by lysis

Question 4

When are recombination enzymes induced in human reproduction?

Answer 4

During meiosis.

Question 5

What is a retrotransposon?

Answer 5

A small, mobile DNA element that moves via an RNA intermediate. They transpose ONLY in the germline.

Question 6

What are the three retrotransposons in humans?

Answer 6

1. The LINE-1 elements: 6-8kb in length, common, encode for their own reverse transcriptase, are still capable of transposition
2. The SINEs: 100-300bp in length, less common than LINE-1, use LINE reverse transcriptase to move
3. The Alu sequences: 300bp, comparatively rare, most can’t transpose

Question 7

How do retrotransposons contribute to human genetic diversity?

Answer 7

• Can disrupt gene function by inserting within an exon
• Can affect gene expression (e.g., decreasing rates of transcription elongation)
• Can create sites for unequal crossovers

Question 8

What are unequal crossovers, and what are their potential consequences?

Answer 8

Misalignments during recombination that result in two homologues of unequal lengths. They can be caused by transposon insertions that flank genes.

They can result in gene families (e.g., globin gene family), exon amplification and deletion (e.g., dystrophin gene), or exon shuffling.

Question 9

How many possible outcomes are there from independent assortment during meiosis?

Answer 9

2ⁿ, where n = the number of homologous pairs.

In humans, this means that there are 2²³ = 8.4 million possible gametes for each parent alone.

Question 10

How do retroviruses contribute to genetic instability in humans?

Answer 10

Viral reverse transcriptase converts single-stranded RNA from a retrovirus to double-stranded DNA, which can be inserted into a host chromosome by viral integrase. Potential consequences include:

• Disruption of gene function due to insertion in an exon
• Inappropriate activation of neighboring genes, due to active viral promoters
• Uptake into the viral genome of neighboring genes, which can be passed into the next host cell

Question 11

How is rotation at the Holliday junction achieved?

Answer 11