Lecture 1 - genetic variation vs. genome stability Flashcards
When does crossing over occur?
At Metaphase I
What does crossing over acheive in meiosis?
- increase genetic variation
- required for correct chromosome segregation in many organisms
What is Neurospora crassa?
A model organism for recombination study, first described by Payen in 1843 and established by Nernard O Dodge
What are features of the Neurospora life cycle?
- heterothallic life cycle
- male and female spores, microconidium, macroconidium
- leads to branched multonucleate mycelium
- diploid stage only in young ascus
What did Beadle and Tatum use Neurospora crassa to demonstrate?
Used neurospora crassa to demonstrate one-gene one-enzyme concept 1958
How did BEADLE and TATUM use Neuropora craasa to demonstrate one-enzyme one-gene concept?
Used mutational analysis to study biochemical pathways
1)exposed WT conidia to X-rays/UV
2) Crossed with WT of opposite sex
3)produced fruiting body
4) separated ascospores and grew in different conditions
-minimal medium
-amino acids
-vitamins
-complete
ALSO grew by crossing WT with pantothenicles and grown first with pantothoenic acid, then without, half grew linearly
Why is Neurospora crassa a model organism for recombination study?
because linear octad can track products of meiosis
1) ascus initial
2) ascus (germination)
3) Meiosis I
4) Meiosis II
- 8 actospores
What would be shown if crossing over occured at the 2 chromosome stage?
matermal and paternal chromosomescross over before duplicating meansing that all subsequent divisions would contain mixture of maternal and paternal DNA
What would be shown if crossing over occured at the 4 chromosome stage?
maternal and paternal homologs replicate, homologous chromosomes lines up followed by crossing over, separation LEAD TO one full maternal, one full paternal two mixtures
When does crossing over occur?
Crossing over occurs at the 4 chromatid phase shown by Beadle in 1946 by crossing WT with cys-3
If three chromatids are involved in CO, how many have recombination?
3
What is gene conversion (abnormal segregation)?
nonreciprocal transfer of genetic information from one molecule to its homolog, may occur in the absence of crossing over
When gene conversion occurs without crossing over what is shown?
- one chromosome completely retained
- opne difference at paired loci
2: 6, one pair not same
What pattern is shown in a fungal octad?
a second division segregation pattern
What ratios are show in normal segregation?
4:4
When did Robin Holliday propose the original method for homologous recombination of the holliday junction?
1964
When does holliday junction homologous recombination occur?
during meiosis
What is homolgous recombination characterised by?
exchange of genes being a maternal chromatid and a parental chromatid of a homologous chromosome pair
How does the holliday juction explain gene convers
explains gene conversion using formation of heteroduplexes and their repair
What is the process of the formation of a holliday junction?
1) ssnicks are induced in chromatid homologs leading to strand exchange and ligation to opposite strand
2) branch migrates along the chromosome, forming a heteroduplex
3) the structure flips and twists leading to antiparallel strands and formation of X structure
What is a heteroduplex?
where polymorphic loci are different on paired chromosomes
what outcomes are possible to holliday junction resolution?
1) Non crossing over - cut in centre of recombined strands, ligate
2) Crossing over - cut in centre of non recombined strands, ligate
How can holliday junctions be repaired and what is the evidence?
-repair to parental
-no repair
-complete repair to one chromosome
-repair to one chromatid only
Evidence from octad (Neurospora) and tetrad (yeast) analysis
6:2 = gene conversion
5:3 and 4:4 =post meitotic segregation (sectored colonies in yeast tetrads)
