Lecture 1 - genetic variation vs. genome stability

Question 1

When does crossing over occur?

Answer 1

At Metaphase I

Question 2

What does crossing over acheive in meiosis?

Answer 2

• increase genetic variation

- required for correct chromosome segregation in many organisms

Question 3

What is Neurospora crassa?

Answer 3

A model organism for recombination study, first described by Payen in 1843 and established by Nernard O Dodge

Question 4

What are features of the Neurospora life cycle?

Answer 4

• heterothallic life cycle
• male and female spores, microconidium, macroconidium
• leads to branched multonucleate mycelium
• diploid stage only in young ascus

Question 5

What did Beadle and Tatum use Neurospora crassa to demonstrate?

Answer 5

Used neurospora crassa to demonstrate one-gene one-enzyme concept 1958

Question 6

How did BEADLE and TATUM use Neuropora craasa to demonstrate one-enzyme one-gene concept?

Answer 6

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

Question 7

Why is Neurospora crassa a model organism for recombination study?

Answer 7

because linear octad can track products of meiosis

1) ascus initial
2) ascus (germination)
3) Meiosis I
4) Meiosis II
- 8 actospores

Question 8

What would be shown if crossing over occured at the 2 chromosome stage?

Answer 8

matermal and paternal chromosomescross over before duplicating meansing that all subsequent divisions would contain mixture of maternal and paternal DNA

Question 9

What would be shown if crossing over occured at the 4 chromosome stage?

Answer 9

maternal and paternal homologs replicate, homologous chromosomes lines up followed by crossing over, separation LEAD TO one full maternal, one full paternal two mixtures

Question 10

When does crossing over occur?

Answer 10

Crossing over occurs at the 4 chromatid phase shown by Beadle in 1946 by crossing WT with cys-3

Question 11

If three chromatids are involved in CO, how many have recombination?

Answer 11

3

Question 12

What is gene conversion (abnormal segregation)?

Answer 12

nonreciprocal transfer of genetic information from one molecule to its homolog, may occur in the absence of crossing over

Question 13

When gene conversion occurs without crossing over what is shown?

Answer 13

• one chromosome completely retained
• opne difference at paired loci
  2: 6, one pair not same

Question 14

What pattern is shown in a fungal octad?

Answer 14

a second division segregation pattern

Question 15

What ratios are show in normal segregation?

Answer 15

4:4

Question 16

When did Robin Holliday propose the original method for homologous recombination of the holliday junction?

Answer 16

1964

Question 17

When does holliday junction homologous recombination occur?

Answer 17

during meiosis

Question 18

What is homolgous recombination characterised by?

Answer 18

exchange of genes being a maternal chromatid and a parental chromatid of a homologous chromosome pair

Question 19

How does the holliday juction explain gene convers

Answer 19

explains gene conversion using formation of heteroduplexes and their repair

Question 20

What is the process of the formation of a holliday junction?

Answer 20

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

Question 21

What is a heteroduplex?

Answer 21

where polymorphic loci are different on paired chromosomes

Question 22

what outcomes are possible to holliday junction resolution?

Answer 22

1) Non crossing over - cut in centre of recombined strands, ligate
2) Crossing over - cut in centre of non recombined strands, ligate

Question 23

How can holliday junctions be repaired and what is the evidence?

Answer 23

-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)