Genetics Chapter 13.6 and 19.5 Flashcards
Functions of homologous recombination
- Creating tension between opposite poles during chromosome segregation
- Shuffling allelic combinations
- DNA damage repair
Homologous Recombination
recombination occurs during pachytene (prophase) of meiosis I
Sister chromatid exchange
- 2 sister chromatids break & rejoin with one another
- Physically exchanging regions of the parental strands in the duplicated chromosomes
Genetic recombination
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
Holliday model of recombination
- Homologous chromosomes align
- Single strand nicks at identical locations
- Strand invasion and ligation
- Branch migration and heteroduplex formation
What is Double-strand break model
Double-strand break model: More Recent Models for Homologous Recombination
Refinement of the Holliday model
Double-strand break model steps
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
Heteroduplex
method detecting the sequence differences between normal DNA and the DNA tested
Displacement loop
non-coding region and acts as a promoter for both the heavy and light strands of mitochondrial DNA contains essential transcription & replication elements
RecBCD
complex scans DNA and finds DSBs.
degrades the DNA to make single stranded regions.
RecA
is loaded onto the single stranded DNA and promotes strand invasion to make a D loop.
RuvABC
binds to the Holiday junctions
RuvAB
promotes branch migration.
RuvC
is an endonuclease that resolves the Holiday junctions.
Gene conversion
When one allele is converted to the other allele on the homologous chromosome
How does Gene conversion occur
DNA mismatch repair
DNA gap repair synthesis
how non-Mendelian inheritance in Neurospora crassa led to the discovery of gene conversion
- 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
Why is Neurospora important in genetics?
- 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
Mismatch Repair
Removes incorrect bases that DNA polymerase missed during proofreading and is present in all species
Explain how DNA repair mechanisms recognize the parental versus daughter strand
DNA Methylation- how the cell can distinguish parent versus daughter strand
The most common type of DNA methylation is 5-methylcytosine (5-mC)
Explain two consequences of unrepaired DNA damage
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
DNA Repair
Living cells contain several DNA repair systems that can fix different types of DNA alterations
DNA repair process
- An irregularity in DNA structure is detected
- The abnormal DNA is removed
- Normal DNA is synthesized
Direct Repair
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
