Week 10 Flashcards
Meiosis
two phases
meiosis I is a modified form of mitosis
alignment of two replicated homologous chromosome, synapsis
synaptomenal complex forms during prophase I of meiosis
Meiosis I
monopolar attachment of microtubules to the centromeres of homologous chromosomes to move chromosomes apart
Meiosis II
Splitting of the sister chromatids of the homologous chromosomes
Recombination
Occurs during the meiosis I at the tetrad
physical exchnage of genetic information
What processes give rise to recombinants
Recombination
Law of independent assorment (Mendel’s second law)
Law of independent assortment
Mendel’s 2nd law
random alignment of chromosomes at the metaphase plate
50% chance assortment
Recombination and gene mapping
the number of recombinations is proportional to the distance between two genes on the same chromosome
map the relative position of genes on the chromosome
Two point test cross
Cross a diploid heterozygous for alleles at two genes
cross back to a test strain homozygous for the recessive alleles in the two genes
follow what happened in the meiosis of the F1 heterozygotes in creating the gametes for the test cross
Three-point test cross
cross parents homozygous for 3 dominant alleles and recessive alleles to create a heterozygote
cross the heterozygote with a homozygote for 3 recessive alleles
Chance of a double crossover
product of two single crossovers
rare
Interference
A single crossover event supresses the occurence of a near by crossover event
Following meiosis in human and fruitflies
not possible due to the loss of genetic information, sperm swim away and oogenesis only produces 1 viable oocyte (genetic info is lost in 3 polar bodies)
Tetrad Analysis (purpose)
Allows you to conclude that recombination occurs after DNA replication and map the position of centromeres
Yeast life cycle
yeast have a viable haploid stage and they can fuse to give rise to a diploid, the diploid is an ascospore that undergoes meiosis and contains four haploid spores
Dissolve ascus cell wall to release spores
use a glass needle to pick and plate the four haploid spore
2:2 segregation
Mendel’s first law of segregation
Unlinked genes on different chromosomes
will assort independently of one another, mendel’s second law.
Unlinked genes
PD = NPD (50/50) due to random alignment at the metaphase plate
Tetratype
All four spores have different genotypes of which two have the same as parents and two do not
recombination during meiosis I leads to tetretypes
Recombination before DNA replication
results in all non parental ditypes instead of tetratypes
NPD>T
This is not observed
No crossing over events
Parental ditypes
Linked genes
PD»NPD
Single crossover event
tetratype
Double crossover event (2-strands)
Parental ditype
Double crossover (3-strand)
tetratype
Double crossover (4-strand)
non parental ditype
Calculating recombination frequency
NPD all four spores recombinant
T two of the four spores recombinant
RF = NPD + 1/2 T/Total tetrads x 100% mu
Mapping centromeres
can only map when we have ordered tetrads
During Meiosis
Centromeres are pulled to opposite poles
Recombination between a gene and a centromere
RF= 1/2(# of tetrads with 2nd division events)/Total tetrads x 100% or mu
Model for recombination
Double-strand break model
double stranded breaks a recombinant genic
homologous chromosomes differe at a few points
double stranded breaks are very dangerous, one mechanism of repair is to employ homologous recombination
Recombination steps
Double stranded break
Resection of 5’ ends (Degredation)
One of the single stranded ends finds homologous sequence on another homologous chromosome and base pairs displacing the other strand
Holliday Junction
Two strands binding to the other two strands of the homologous chromosome
mismatches can occur
Heteroduplex
The section of the chromosome involved in binding with the other homologous chromosome
When do you observe 3:1 segregation patterns in tetrad analysis
Gene conversion
Breaks mendel’s law of segregation
replacement of 1 piece of information with another
3:1 segregation in a tetrad analysis
first evidence of heteroduplex formation
Mismatch in a heteroduplex
DNA sequence variation between homologous chromosomes resulting in mismatches in the heteroduplex, repairs will occur at random
mismatches are changed to the recessive allele 3 out of 4 of the time
repairing one allele to another 3 to 1 segregation
Role of recombination
exchange of information between homologous chromosomes
do crossovers (chiasma) hold the tetrad together?
are crossovers important for segregation?
what does the synaptomenal complex do?
Synaptomenal complex
structure holds the homologous chromosomes together during prophase of meiosis I
contains Zipper proteins
Random Crossovers
If there was no interference small chromosomes will have less crossovers than large chromosomes regulating crossovers through interference will spread crossovers across the genome
zip1 mutant phenotype
recombination. ZIP1 and SC not required for recombination
No interference. ZIP1 and SC required for negative interference
No interference and loss of small chromosomes (non-disjunction). Spreading X overs is required for segregation during meiosis
