1-Meiosis Flashcards
where does it occur
- occurs in the gonads
- germline cells (not somatic cells)
- eggs and sperm
- germline cells (not somatic cells)
goals of meiosis
- to produce 4 progeny cells (gametes—eggs and sperm)
- with exactly half the number of chromosomes as the parent cells (2n to n)
- with genetic differences (gametes have unique genotypes)
interphase
prior to the start of meiosis, the cell goes through an interphase period, in which the DNA replicates, and the cell is checked to make sure that it is ready to divide
Prophase I
synapsis happens
- homologous chromosomes come together
- become tightly associated along their lengths
Synapsis
The pairing of two chromosomes that occurs during meiosis
Late prophase I
crossing over and recombination happens!!
difference between crossing-over and recombination
crossing-over is a process
Recombination is the outcome
crossing-over
- specialized protein deliberately break the DNA molecules at the same location in 2 non-sister chromatids of homologous chromosomes
- during DNA repair, rather than joining the broken segment back to the chromatid to which it was originally attached, the broken segment is segment is joined to the chromatid of the other homolog
- results in the exchange of genetic material between non-sister chromatids
Recombinant chromatids:
chromatid with a combination of maternal AND paternal alleles
non-recombinant chromatids:
chromatid with alleles from ONLY the maternal or paternal side
CHIASMA (pl. chiasmata)
- contact point between non-sister chromatids
- the sites of DNA breakage
- EVIDENCE!!! that crossing-over has occurred
- in Prophase I, protein complex breaks down and homologs chromosomes start to separate from each other
- BUT!! are held together at the chiasma until Anaphase I
metaphase I
- linked homologous pairs line up on either side of the metaphase plate
- their spatial arrangement (which side of metaphse plate) is random and independent of other homologous pairs
independent assortment of homologous chromosomes
random arrangement of chromosomes at the metaphase plate, important for genetic variation
the possible number of alignments at the metaphase plate formula
2^n, where n=number of chromosomal pairs
Anaphase I
- spindle fibres (microtubules contract
- homologous chromosomes separate and move towards poles
Telophase I and Cytokinesis I
- nuclear envelope may reform, forming 2 nuclei!!
- Cytokinesis—creates two cells
- the cells are now haploid!!!
- one of each chromosome type
- combination of maternal and paternal chromosomes
- sister chromatids still attached at centromere
meiosis 1 is compelte
MEIOSIS II
- very similar to mitosis!!
- sister chromatids are separating
- but now… the cell is haploid! (if the parent cell was diploid)
end product of Meiosis II
- 4 haploid daughter cells
- half the genetic content of the parent cell
- genetically different from each other!!
number of possible gamete genotypes formula
2^n
n=number of heterozygous pairs (genes not linked, or linked but crossing over can occur)
what affects the probability of crossing-over
- physical distance between genes affects the probability of crossing over
- genes that are far apart = lots of opportunities for crossing-over
- genes that are close together are less likely to cross over
- scientists cannot predict where and when crossing-over will occur
frequency of parental genotypes > increased likelihood of crossing-over
sources of GENETIC VARIATION
Crossing-over and recombination
independent assortment of homologous chromosomes
random fusion of eggs and sperm
