meiosis

Question 1

what does meiosis consist of?

Answer 1

• 2 successive nuclear divisions, namely meiosis I & meiosis II

Question 2

how are a pair of homologous chromosomes similar?

Answer 2

• they contain the same number & type of gene, but may be of different alleles

Question 3

are a pair of homologous chromosomes genetically identical?

Answer 3

• NO, but they are similar

Question 4

what does the term ‘homologous chromosomes’ refer to?

Answer 4

• it refers to a pair of chromosomes with the same arm length & shape,
• same centromere position
• same sequence of genes along the chromosome
  -same staining pattern

Question 5

how many chromosomes are there in the cell at the start of interphase?

Answer 5

• 46

Question 6

what is the ploidy of the cell at the start of interphase?

Answer 6

• 2n, diploid

Question 7

what is the amount of DNA in the cell at the start of interphase?

Answer 7

• 2X

Question 8

how many chromosomes are there in the cell at the end of interphase?

Answer 8

• 46

Question 9

what is the ploidy of the cell at the end of interphase?

Answer 9

• 2n, diploid

Question 10

what is the amount of DNA in the cell at the end of interphase?

Answer 10

• 4X

Question 11

what is the number of chromosomes per cell at the end of meiosis I?

Answer 11

• 23

Question 12

what is the ploidy of the cell at the end of meiosis I?

Answer 12

• n, haploid

Question 13

what is the amount of DNA in the cell at the end of meiosis I?

Answer 13

2X

Question 14

what is the number of chromosomes per cell at the end of meiosis II?

Answer 14

23

Question 15

what is the ploidy of the cell at the end of meiosis II?

Answer 15

n, haploid

Question 16

what is the amount of DNA in the cell at the end of meiosis II?

Answer 16

• X

Question 17

what does the term ‘bivalents’ refer to?

Answer 17

• it describes homologous chromosomes that pair up during prophase I of meiosis, in a process known as synapsis

Question 18

what does ‘synapsis’ mean?

Answer 18

• it is a process during prophase I of meiosis, whereby homologous chromosomes pair up & are physically connected to each other

Question 19

what does the term ‘crossing over’ refer to?

Answer 19

• the exchange of corresponding sections between chromatids of a pair of homologous chromosomes when they are in synapsis

Question 20

what is a chiasma?

Answer 20

• it is an x-shaped structure formed between chromatids of a a pair of homologous chromosomes, & the site where corresponding sections of homologous chromosomes break & rejoin
• which enables the exchange of genetic material to occur between homologous chromosomes in cross over

Question 21

what does the term ‘non-sister chromatids’ refer to?

Answer 21

• they are chromatids of a pair of homologous chromosomes
• they have the same number & sequence of genes, but may carry different alleles

Question 22

when is the only time non-sister chromatids can be used?

Answer 22

• when describing crossing over during prophase I of meiosis!!

Question 23

what happens during prophase I?

Answer 23

• chromosomes become visible due to condensation of chromatin
• homologous chromosomes pair up in synapsis, with each pair of homologous chromosomes forming a bivalent
• crossing over occurs, & chiasmata may form between chromatids of a pair of homologous chromosomes
• centrosomes migrate to opposite poles of the cell, & the spindle fibres extend from each pole towards the equator of the cell
• nucleolus disappears, & the nuclear envelope breaks down, with the nuclear lamina and the nuclear pore complexes disintegrating, and the nuclear envelope fragments forming vesicles

Question 24

what happens during metaphase I?

Answer 24

• the spindle fibres attach themselves to the kinetochore at the centromere of the chromosome
• the homologous chromosomes arrange themselves 90° to the spindle axis, forming 2 rows at the equator of the cell
• the arrangement of each pair of homologous chromosomes is completely independent of the arrangement of other pairs

Question 25

what happens during anaphase I?

Answer 25

- homologous chromosomes separate & move to opposite poles of the cell, centromere first, due to the shortening of the spindle fibres - the cell elongates as non-kinetochore fibres lengthen

Question 26

what happens during telophase I?

Answer 26

- the chromosomes reach opposite poles of the cell, & the nucleolus in each nucleus reappears - the nuclear envelope reforms around each group of chromosomes at each pole, where the nuclear membrane vesicles fuse together, & the nuclear lamina and nuclear pore complexes reassemble - the spindle fibres break down

Question 27

how many daughter cells are formed at the end of meiosis I & after cytokinesis?

Answer 27

- 2!!!

Question 28

what happens in prophase II?

Answer 28

- in cells where telophase I & interphase occur, the nucleolus disappears, and the nuclear envelop breaks down, where the nuclear pore complexes & nuclear lamina dissociate, and the nuclear envelop fragments turn into vesicles - if centrosomes are present, they migrate to opposite poles of the cell - the spindle fibres develop perpendicular to the spindle axis of meiosis I

Question 29

what happens in metaphase II?

Answer 29

- spindle fibres attach to the kinetochore at the centromere of the chromosome, - while the chromosomes arrange themselves 90° to the new spindle axis in a single row, at the equator of the cell

Question 30

what happens in anaphase II?

Answer 30

- the centromere of each chromosome divides, causing the chromatids of each chromosome to separate - the chromatids move to opposite poles of the cell, centromere first, due to the shortening of the spindle fibres

Question 31

what happens in telophase II?

Answer 31

- the chromatids reach the opposite poles of the cell & become the chromosomes of the daughter cells - the chromosomes uncoil & become chromatin, while the nucleolus of each nucleus reappears & the spindle fibres break down - the nuclear envelop reforms around the chromosomes at each pole, with the nuclear pore complexes & nuclear lamina reassembling and the nuclear membrane vesicles fusing together

Question 32

how many daughter cells has the parent cell divided to form at the end of meiosis II & cytokinesis?

Answer 32

- 4

Question 33

what are the significances of meiosis?

Answer 33

- it gives rise to genetic variation between gametes through crossing over of homologous chromosomes & independent assortment of bivalents - prevents the doubling of chromosome number upon fusion of gametes

Question 34

what are the 3 process in meiosis that can bring about genetic variation?

Answer 34

1. crossing over between homologous chromosomes during prophase I 2. independent assortment of homologous chromosomes at the equator of the cell during metaphase I & separation of homologous chromosomes during anaphase I 3. independent assortment of chromosomes at the equator of the cell during metaphase II, & separation of chromatids of the chromosomes during anaphase II

Question 35

how does crossing over between homologous chromosomes during prophase I bring about genetic variation?

Answer 35

- because there is an exchange of genetic material between homologous chromosomes

Question 36

how does meiosis bring about genetic variation?

Answer 36

- the 3 processes lead to new combinations of alleles

Question 37

how does meiosis prevent the doubling of chromosome number upon fusion of gametes?

Answer 37

- meiosis produces 4 haploid gametes, which are genetically non-identical - as each gamete has half the number of chromosomes of the parent cell, during fertilisation, the nuclei of 1 male & 1 female haploid gamete fuse to restore the diploid number of chromosomes in the zygote - if meiosis did not occur, then fusion of gametes would result in doubling of the number of chromosomes for each successive sexually reproduced generation