GAG WK4 Flashcards
What is meiosis
cell divides twice to produce 4 cell containing half of the original amount of genetic information
How many gamete cells form from meiosis
4 daughter cells
What are homologues
any pair of homologous chromosomes
Explain what is meant by meiosis is reductional
meiosis reduces the chromosome number by half
Explain what is meant by mitosis is proliferative
mitosis maintains the chromosome number
What is the difference between diploid and haploid cells
diploid = cells w/ 2 sets of chromosomes
haploid = cells w/ 1 set of chromosomes
What are the features of mitosis
- produces 2 diploid daughter cells
- no homologous pairing
- maintains the same number of chromosomes
What are the features of meiosis
- produces 4 haploid daughter cells
- 2 rounds of cell division
- reduces the amount of chromosomes
What is the main difference between meiosis I and meiosis II
meiosis I = homolgous pair up and separate
meisois II = sister chromatids separate
What are telomeres
protective ends of chromosomes
What are centromeres
region where sister chromatids are attached
What are microtubules
protein structures that pull chromosomes apart
What are kinetochores
protein complex on the centromere where microtubules attach
What is the main reason why proper chromosome segregation is important
it prevents abnormal chromosome numbers
List the processes necessary for correct chromosome segregation
- chromosome pairing (synapsis)
- crossing-over
- spindle fibre attachment
- checkpoint mechanisms
- cohesion degradation
Outline the importance of chromosome pairing (synapsis) in chromosome segregation
Proper pairing ensures each gamete gets one chromosome from each pair, preventing aneuploidy (ex: Down syndrome)
Outline the importance of crossing-over in chromosome segregation
it ensures chromosomes stay together until properly aligned and separated
Outline the importance of spindle fibre attachment in chromosome segregation
- microtubules must attach to kinetochores on chromosomes to pull them to opposite poles
- errors in attachment can lead to mis-segregation and non-disjunction
Outline the importance of checkpoint mechanisms in chromosome segregation
- Prevents Premature Separation (delays anaphase until all chromosomes are aligned properly)
- detects errors (misaligned chromosomes are detected)
- reduces risk of aneuploidy
Outline the importance of cohesin degradation in chromosome segregation
cohesin must degrade to allow chromatids to separate
What is the law of segregation
- each parent carries 2 alleles for a gene
- alleles segregate during meiosis
- ensures each gamete only gets 1 allele
Outline the law of independent assortment
- genes for different traits on different chromosomes are inherent independently
Outline linkage
2 genes on the same chromosomes can be inherited together (unless separated by crossing over)
What increases the odds of linkage
if the genes are sitting close to each other
What are the assumptions of Hardy-Weinberg equilibrium
- random mating (no mating preference)
- no selection (no trait gives an advantage)
- no mutation + genetic drift + meiotic drive (random segregation of chromosomes)
What is the Hardy-Weinberg equilibrium
A principle stating that allele and genotype frequencies in a population remain constant from generation to generation, under certain conditions
What is the purpose of the Hardy-Weinberg equilibrium
- Acts as a baseline to detect evolutionary changes.
- Helps estimate allele frequencies in populations.
- Useful in predicting carrier frequencies of genetic disorders
What are unliked genes
genes that are inherited independently from each other
What is a genotype
specific combination of alleles an individual inherits
What is a phenotype
observable trait resulting from that genotype
What is Rec8
specific cohesin protein that holds sister chromatids together during meiosis
What is the purpose of cohesin
- ensures sister chromatids remain attached until the right time in cell division
- preventing premature separation
What is Mps3
- a protein that marks telomeres at nuclear periphery
- this marking helps facilitate the pairing of homologous chromosomes
Explain the importance of dynamic movement of chromosomes during meiosis
-it is necessary for formation of side to synaptonemal complexes, which help hold homologous chromosomes together and promote recombination
What happens when chromosome movement during meiosis is disrupted?
It causes errors in chromosomes segregation
Why do chromosomes undergo structural changes?
It allows them to come closer together, making it easier for homologous chromosomes to align
What is the bouquet formation in chromosome pairing?
When Telomeres cluster at one side of the nucleus
What is the purpose of a bouquet shape during chromosome pairing?
It helps bring homologous chromosomes near each other
What is the Rabl configuration?
The central mirrors are positioned on one side of the nucleus with chromosome arms extending to the opposite side
What is the purpose of the Rabl configuration in chromosome pairing?
It maintains approximately of the homologous chromosomes
How does homologous recombination contribute to chromosome pairing?
Homologous recombination uses DNA sequence, similarity between homologs to promote pairing and enable genetic material exchange
Why are pairing centers important in chromosome pairing?
- initiate the pairing of homologous chromosomes
- stabilises the association between homologues
What is the synaptonemal complex
Protein structure that forms homologous chromosomes
What is the function of synaptonemal complex
Stabilises the pairing and facilitates genetic recombination
Outline prophase I stages
- thin ribbon
- paired ribbon
- thick ribbon
- double ribbon
What happens during the thin ribbon stage
Initial pairing begins
What happens during the paired ribbon stage
Paired Ribbon: The synaptonemal complex forms.
What happens during the thick ribbon stage
Thick Ribbon: Chromosomes are fully paired and organised.
What happens during the double ribbon stage
final arrangement before segregation
What is the function of enzyme spo11
- creates double-strand breaks in DNA
- initiates homologous recombination
How does Spo11 help chromosome pairing **
- double-strand breaks are repaired using sequences from homologous chromosomes
- ensures correct pairing
What is SMC3
protein part of a complex called cohesin
What is the function of SMC3
- holds sister chromatids together
- helps maintain structure of paired homologous chromosomes
Why is chiasma formation important in the segregation of homologous chromosomes
- chiasma helps stabilise the chromosomes during segregation
- ensures homologous pairs stay connected until pulled apart
How do microtubules help segregation of homologous chromosomes
- microtubules pull homologous chromosomes toward opposite side of the cell
- chiasma and cohesin help resist these pulling forces until the right moment
What is the chiasmata
physical links between homologous chromosomes due to crossovers during prophase I
What is the function of chiasmata
counteract the pulling forces exerted by microtubules during chromosome segregation
What is the link between chiasmata and crossovers
- The number of chiasmata indicates how often crossovers occur
- If there are fewer chiasmata (therefore fewer crossovers) = a higher chance chromosomes won’t be distributed correctly
- leads to missegregation
What is mono-orientation
- each kinetochore attached to microtubules from only 1 pole
- use of monopolin protein complex at kinetochore
why is that sister kinetochore must exhibit mono-orientation
for correct separation of homologous chromosomes
What happens to cohesion during anaphase I
- cohesion on chromosome arms is removed
- allows sister chromatids to separate whilst keep centromeric cohesion intact
What happens to cohesion during anaphase II
- centromeric cohesion is removed
- allows complete separation of sister chromatids
What is the role of rec8
- ensures homologous chromosomes stay connected while the arm cohesion is lost
What is the role of separase
- a protease that cleaves the Rec8 subunit of cohesion
- allows chromatids to separate when cell is diving
How is separase kept inactive
- securin is a protein that binds to separate and keeps it inactive
- so the chromatids don’t separate too early
How is separase activated
- securin is degraded when all chromosomes are aligned during metaphase
- ensures separate can now cleave Rec8
- only when the cell is ready to move to anaphase
Why is it that at anaphase I, centromeric cohesion is kept phosphorylated by Sgo1-PP2A
preventing premature separation of sister chromatids during meiosis I
What is meiotic recombination
homologous chromosomes exchange genetic material
What are the functions of meiotic recombination
- promotes homologous pairing
- chiasma formation
- genetic diversity
What are meiotic recombination hotspots
thousands of hotspots where recombination is more likely to occur
How does Spo11 promote DNA breaks
it alters the shape of DNA substrates
Where does Spo11 activity happen the most
meiotic hotspots
What are the features of meiotic recombination hotspots
- over 200 DNA breaks per cell
- distributed across genome but in a non-random manner
- thousands of hotspots exist where recombination is more likely
What are hotspots influenced by
- methylation marks
- intergenic regions = non-coding areas between genes can affect hotspot designation
What is PRDM9
gene that determines hotspot specificity and it can turn Spo11 into a site-specific nuclease
What is PRDM9 function
it directs Spo11 to break DNA at certain spots determining where recombination happens
Why are recombination patterns in terms of reproduction
- 2 populations w/ different PRMD9 alleles cannot interbreed
- they have incompatible recombination patterns
What is being activated during DNA damage (DNA breaks)
ATM and ATR are activated (kinases)
What happens if ATM and ATR become mutated
they can lead to defects in DNA repair and meiotic progression
Outline how ATM and ATR are activated
- phosphorylation of chromatin
- transient meiotic arrest for DNA repair
- Promotion of DNA repair mechanisms to resolve breaks and maintain genomic integrity
What happens when Spo11 is knocked out in mice?
- mouse becomes infertile
- Chromosomes are unable to pair correctly because there are no DNA breaks for recombination
- Without recombination, no chiasmata are formed
- cell undergoes apoptosis or produces defective gametes, causing infertility
What is Rad51
Rad51 is a protein that helps chromosomes pair up by promoting recombination.
What is the role of Rad51 in meiosis?
It repairs many DNA breaks, but only a few crossovers occur
How many crossovers do most chromosomes experience?
Most chromosomes experience 1 crossover
How are crossovers regulated during meiosis?
- Crossovers happen in specific locations
- crossover in one area of the chromosome can prevent crossovers nearby
- helps ensure that crossovers are spaced out properly to prevent too many in one region
