Lecture 16 Flashcards
When is homologous recombination important?
Meiosis: generating diversty and chromosome segregation
Mitosis: repair DNA damage and stalled replication forks
What does crossing over take place between?
Non sister chromatids
Recombination
reassortment of genetic material already present
Meiotic recombination
aspect of sexual reproduction that can enhance genetic variability
Sister chromatids
each of 2 identical copies of a replicated chromosome. Must be linked at the centromere.
bivalent
4 chromatids
Diad
2 chromatids
Monad
one chromatid
First stage of prophase one
Leptotene
Leptotene
means thread
chromosomes become visible
2nd stage of prophase one
Zygotene
Zygotene
zygo=pair
Chromosomes pair
Synapsis occurs
synapsis
pairing of homologous chromosomes
3rd stage of prophase I
Pachytene
Pachytene
bivalents appear
Synaptonemal complex
Synaptonemal complex
forms between synapsed chromosomes and allows homologous pairing along the entire length
believed to be necessary for recombination to occur
4th stage of prophase I
Diplotene
Diplotene
Chiasmata form
Chiasmata
sites of contact
5th stage of prophase I
Diakinesis
Terminalization
Chiasmata move out to the termini of chromosomes
Axial element
proteinaceous structure around chromsomes that condenses at the start of synapsis
Lateral element
structure in the synaptonemal complex that forms when a pair of sister chromatids condense
Holiday Model steps
- Endonuclease nicks strand & denaturation occurs
- Joint molecule forms (complex strands reassociated and are sealed by ligase)
- Strands opposite crossover strands are nicked by endonuclease
- Rotation of the axis (branch migration can occur)
- New DNA synthesis & gaps filled
- Seals by ligase
- Have heteroduplex DNA
Basic holliday model steps
- breakage
- reunion
- reciprocal exchange
Branch migration
recombinant joint can move along the chromosome
Topoisomerases
relieve tension from the twisting of DNA by breaking then remaking bond
Type 1 topoisomerase
breaks single strand of DNA
Type 2 topisomerase
breaks double strand of DNA
Robertsonian fusion
2 acrocentric chromosomes recombine and fuse to metacentric
Chromosome fusion
2 NON homologous chromosomes fuse into 1
Chromosome fission
A chromosome splits into 2 chromosomes
Where are robertsonain changes common?
plant and animal kingdom