Topic 3.3: Meiosis Flashcards
Meiosis
Reduction division of a diploid cell to produce four haploid cells (gametes) that are genetically distinct
Cellular divisions in meiosis
a) Meiosis I separates homologous chromosomes
b) Meiosis II separates sister chromatids
Mitosis vs Meiosis (Discopug)
a) Divisions
b) Independent assortment
c) Synapsis
d) Crossing over
e) Outcome
f) Ploidy
g) Use
h) Genetics
a) Divisions
b) Independent assortment
c) Synapsis
d) Crossing over
e) Outcome
f) Ploidy
g) Use
h) Genetics
a) 1 | 2
b) No | Yes (Metaphase I)
c) No | Yes
d) No | Yes (Prophase I)
e) 2 | 4
f) Diploid | Haploid
g) Body | Sex
h) Identical | Variation
DNA Duplication in Meiosis
DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids
Stages of Meiosis
a) P-I
b) M-I
c) A-I
d) T-I
e) P-II
f) M-II
g) A-II
h) T-II
Prophase I
a) Chromosomes condense
b) Nuclear membrane dissolves
c) Homologous chromosomes form bivalents
d) Crossing over occurs
Metaphase I
a) Spindle fibres from opposing centrosomes connect to bivalents
b) Bivalents align along the middle of the cell
Anaphase I
a) Spindle fibres contract and split the bivalent
b) Homologous chromosomes move to opposite poles of the cell
Telophase I
a) Chromosomes
decondense
b) Nuclear membrane may reform
c) Cell divides (cytokinesis)
Prophase II
a) Chromosomes condense
b) Nuclear membrane dissolves
c) Centrosomes move to opposite poles (perpendicular to before)
Metaphase II
a) Spindle fibres from opposing centrosomes attach to chromosomes (at centromere)
b) Chromosomes align along the cell equator
Anaphase II
a) Spindle fibres contract and separate the sister chromatids
b) Chromatids (now called chromosomes) move to opposite poles
Telophase II
a) Chromosomes decondense
b) Nuclear membrane reforms
c) Cells divide (cytokinesis) to form four haploid daughter cells
Crossing over
a) Crossing over occurs via synapsis in Prophase I
b) Homologous chromosomes form bivalents (or tetrads)
c) As a result of this exchange of genetic material, new gene combinations are formed on chromatids (recombination)
Chiasmata
The points where genetic information has been exchanged between the homologous pair
Recombinants
The non-sister chromatids that have exchanged DNA
Random Assortment
a) The homologous pairs orient randomly in Metaphase I
b) This means there is an equal chance of a resulting gamete containing either the maternal or paternal chromosome
Random assortment and potential gamete combinations in humans
As humans have a haploid number of 23, consequently there are 2^23 potential gamete combinations (>8 million)
What does the halving of the chromosome number allow?
a) It allows for a sexual life cycle with the fusion of gametes.
b) This acts as a further source of genetic variation
Three main sources of genetic variation from sexual reproduction
a) Crossing over (in prophase I)
b) Random assortment of chromosomes (in metaphase I)
c) Random fusion of gametes from different parents
Non-disjunction
Chromosomes failing to separate, resulting in gametes with extra or missing chromosome
When may the failure of chromosomes to separate occur?
a) Failure of homologues in Anaphase I (4 affected daughter cells)
b) Failure of sister chromatids in Anaphase II (2 daughter cells being affected)
Down Syndrome
a) If a gamete with an extra chromosome fuses with a normal gamete, the resulting zygote will have three copies
b) Trisomy 21
How does parental age influence the chances of non-disjunction?
Older parents are at a higher risk of non-disjunction events
