Genetics Chapter 13.6 and 19.5

Question 1

Functions of homologous recombination

Answer 1

1. Creating tension between opposite poles during chromosome segregation
2. Shuffling allelic combinations
3. DNA damage repair

Question 2

Homologous Recombination

Answer 2

recombination occurs during pachytene (prophase) of meiosis I

Question 3

Sister chromatid exchange

Answer 3

• 2 sister chromatids break & rejoin with one another
• Physically exchanging regions of the parental strands in the duplicated chromosomes

Question 4

Genetic recombination

Answer 4

leads to new combination of genes

This is why it’s called “recombinant” = non-parental
Important when we consider multiple genes

NOTE: a new combination of 2 or more alleles can arise when homologous recombination takes place

Question 5

Holliday model of recombination

Answer 5

1. Homologous chromosomes align
2. Single strand nicks at identical locations
3. Strand invasion and ligation
4. Branch migration and heteroduplex formation

Question 6

What is Double-strand break model

Answer 6

Double-strand break model: More Recent Models for Homologous Recombination
Refinement of the Holliday model

Question 7

Double-strand break model steps

Answer 7

Unlikely to get nicks at the same site in each chromatid
Instead, it is more likely for one DNA helix (not both) to incur a single nick or a double-strand break
1. Strand degradation
2. Strand invasion (displacement loop)
3. Gap repair fills in the gaps and ligates the backbone
4. Branch migration and heteroduplex formation

Question 8

Heteroduplex

Answer 8

method detecting the sequence differences between normal DNA and the DNA tested

Question 9

Displacement loop

Answer 9

non-coding region and acts as a promoter for both the heavy and light strands of mitochondrial DNA contains essential transcription & replication elements

Question 10

RecBCD

Answer 10

complex scans DNA and finds DSBs.
degrades the DNA to make single stranded regions.

Question 11

RecA

Answer 11

is loaded onto the single stranded DNA and promotes strand invasion to make a D loop.

Question 12

RuvABC

Answer 12

binds to the Holiday junctions

Question 13

RuvAB

Answer 13

promotes branch migration.

Question 14

RuvC

Answer 14

is an endonuclease that resolves the Holiday junctions.

Question 15

Gene conversion

Answer 15

When one allele is converted to the other allele on the homologous chromosome

Question 16

How does Gene conversion occur

Answer 16

DNA mismatch repair
DNA gap repair synthesis

Question 17

how non-Mendelian inheritance in Neurospora crassa led to the discovery of gene conversion

Answer 17

• 2 haploid mycelia of opposite mating types (A & a)
• These 2 haploids fuse to form a diploid Aa nucleus
• Nucleus undergoes 2 mitotic divisions to form 4 haploid gametes
• A and a products of the second division remain in linear order in the ascus
• A further mitotic division produces ascospores
• The release of ascospores & germination completes the life cycle

Question 18

Why is Neurospora important in genetics?

Answer 18

• Neurospora are haploid, so metabolic pathways are controlled by a single allele
• The separation of the products of second meiosis at either end of the ascus→4 identical spores at either end crosses

Question 19

Mismatch Repair

Answer 19

Removes incorrect bases that DNA polymerase missed during proofreading and is present in all species

Question 20

Explain how DNA repair mechanisms recognize the parental versus daughter strand

Answer 20

DNA Methylation- how the cell can distinguish parent versus daughter strand

The most common type of DNA methylation is 5-methylcytosine (5-mC)

Question 21

Explain two consequences of unrepaired DNA damage

Answer 21

Diseases Involving NER Genes: Xeroderma pigmentosum (XP) and Cockayne syndrome (CS)

A common characteristic in both syndromes is an increased sensitivity to sunlight→increased probability of developing skin cancer

Question 22

DNA Repair

Answer 22

Living cells contain several DNA repair systems that can fix different types of DNA alterations

Question 23

DNA repair process

Answer 23

1. An irregularity in DNA structure is detected
2. The abnormal DNA is removed
3. Normal DNA is synthesized

Question 24

Direct Repair

Answer 24

In a few cases, the covalent modifications of nucleotides can be reversed by specific enzymes

Photolyase can repair thymine dimers

Uses light to split the dimers - photoreactivation

Question 25

Nucleotide Excision Repair

Answer 25

Can repair many types of DNA damage
NER is found in all eukaryotes and prokaryotes

1. Uvr A/B complex scans DNA for damage
2. Once damage is found, UvrA releases and UvrC binds.
3. UvrC cuts the sugar/phosphate backbone on both sides of the DNA damage
4. UvrD helicase removes damaged DNA piece
5. DNA polymerase and DNA ligase repair the region

Question 26

Nonhomologous End Joining (NHEJ)

Answer 26

• When DNA has a double stranded break
• Ends are bound by proteins which recruit complexes to make a “bridge”
• DNA polymerase repairs the overhangs and DNA ligase seals the ends

Question 27

In eukaryotes, the enzyme ______ removes the RNA primers.

Answer 27

flap endonuclease