3.4.3 meiosis Flashcards
1
Q
what is the use of meiosis
A
- In sexual reproduction two gametes fuse to give rise to new offspring
- To maintain the chromosome number in the adults of a species the number must be halved at some stage in the life cycle
- Meiotic division halves the chromosome number
2
Q
Where does meiosis occur
A
ovaries and testes
3
Q
prophase I
A
- chromosomes condense, nuclear envelope disappears
- Homologous pairs - bivalents formed, chromosome from both parent cells join by overlapping
- where the chromosomes overlap is called crossing over where you get an exchange of allelles between mother and fathers genes
- there can be between 0-3 crossovers on each chromosome
- This creates chiasma - different genetic material in one structure
- start of genetic variation
4
Q
metaphase I
A
- Pairs of homologous chromosomes move to the equator of the cell
- spindle fibres produced by the centriole attach to the centromere
5
Q
anaphase I
A
- homologous chromosomes move to the opposite pole of the cell
- the centromere does not split
- Independent segregation of chromosomes - random assortment of chromosomes, second chance for genetic variation, completely random
6
Q
telophase I
A
- Chromosomes unwind
- Nuclear envelope reappears
- cytoplasm and organelles split
- folding of the cell membrane
7
Q
cytokinesis I
A
-cell divides into two, genetically different but still diploid cells
- they stay close together
8
Q
prophase II
A
- chromatin shortens and thickens
- nuclear envelope disappears
9
Q
metaphase II
A
- chromosomes line up at the equator
- centrioles produce spindle fibres which attach to the centromere
10
Q
anaphase II
A
- spindles contract, centromere splits and one of each chromatid goes to either pole
- independent segregation of chromosomes
11
Q
telophase II
A
- chromatids get longer and thinner, chromatin appears
- nuclear envelopes appears
- organelles and cytoplasm split
- membrane folds in
12
Q
Cytokinesis II
A
- 4 daughter cells called tetrad
- genetically different from each other and the parent cell
- haploid cells, one of each chromosome
13
Q
what stages does the genetic variation occur in meiosis?
A
prophase I
anaphase I
anaphase II
14
Q
When does interphase occur in meiosis
A
Once at the beginning before prophase I
15
Q
Meiosis in pollen
A
- Undergoes meiosis to form genetically different tetrads
- Theses genetically different cells undergo mitosis
- They are able to mass produce cells while also having genetic variation
16
Q
non-disjunction
A
- Failure of chromosomes to separate and segregate into daughter cells
- Can occur during meiosis I or II
- monosomic - successeful, 1 chromatid of each pair in each cell
- disomic, 2 chromatids of one pair in one cell
- nullsomic, no chromatids of a specific pair in the cell
17
Q
What is the cause of mutation during meiosis
A
weak spindle fibres
