Meiosis Flashcards
What is meiosis and what is the purpose
To produce gametes
Daughter cells with half as many chromosomes-haploid
So when sperm and egg join at fertilization, the cell is diploid
At what stage in meiosis are the cells considered haploid? Why?
After anaphase 1
Reduction in chromosomes number, each cell has 23 chromosomes
The chromosome is maternal or paternal, one chromosome from a homologous pair in that cell (so haploid) EVEN THOUGH there are 2 sister chromatids
What is each meiotic division for
Meiosis 1: Reduction in chromosome number to make haploid cells
Meiosis II: this division is needed so sister chromatids separate (balances amount of DNA) EQUATORIAL DIVISION
What are the 3 main contribution to genetic variability
- Crossing over: mat+pat homologous pairs carry different alleles on their chromatids- exchange segments during prophase 1
- Independent assortment: mat+pat chromosomes mix and separated randomly at anaphase 1
- Random fertilisation: random egg and sperm fuse
What is it called when non homologous pairs cross over
Type of mutation=translocation
What complex allows crossing over to occur
Structure
Synaptonemal complex
Has one central element, recombination nodule along central element, 2 protein axes and transverse filaments from the chromatids
What are the stages of prophase 1
Leptotene: Chromatin condense to chromosomes, forms lateral elements. The 2 ends of each homologous chromosome are attached to nuclear envelope
Zygotene: Growing of central element. Initiation of synapsis (pairing)- forms synaptonemal complex
Pachytene: Crossing over occurs. Chromosomes are tetrads (bivalents) Number of crossing over=chiasmata
Diplotene: Synaptonemal complex disorganizes. Chiasmata become VISIBLE
Diakinesis: telomeres detach from nuclear envelope and nuclear envelope disintegrates. Tetrads are visible now. non sister chromatids still held by chiasmata
How are chromosomes separated in meiosis 1
- homologous pairs of chromosomes held together by chiasmata
- Microtubules attach to fused kinetochores of sister chromatids
- Anaphase 1, the chiasmata separates by separase to pull apart the mat+pat bivalents (homo chrom)
How are chromosomes separated in meiosis ii
- sister chromatids are held together by the centromere by cohesin
- Microtubules attach to INDIVIDUAL kinetochores
- SISTER CHROMATIDS are pulled apart to opposite poles
Why in anaphase 1 do sister chromatids hold together
SHUGOSHIN prevents separase action on kinetochore of centromere
Prevents phosphorylation of centromere cohesisn to prevent opeing of the ring therefore sister chromaids remain intact
(althought cohesin arms get separated by separase)
What are some meiotic problems
If one of the chromosomes form a loop trying to cross over, this causes deletions and duplication
translocations
inversions- segments breaks of chromosome and reattaches in reverse position
What happens if there is no chiasmata
aneuploidy: ONLY A PROBLEM WITH OOCYTE
if theres aneuploidy, it is definetely due to mother egg
which process causes gene mutations
spermatogenesis because the process is fast- 74 days
sperm has way more meiotic divisions in their lifetime than oocyte
No destruction of chiasmata so never aneuploidy
What part of the paternal component is the centrosome (MTOC)
centrioles from sperm tail
(all of sperm acc penetrates the egg)
What is the development of the synaptonemal complex
The lateral elements of synaptonemal complex start to form in leptotene and central elements form in zygotene so in Pachy tene complex is fully formed and crossing over can occur
