grade 12 meiosis

Question 1

what is meiosis

Answer 1

a type of cell division whereby a somatic diploid cell undergoes two cell divisions, meiosis I and meiosis II and divides to form four different haploid sex cells.

Question 2

what is the difference between diploid and haploid cells?

Answer 2

diploid cells have two sets of chromosomes, where each chromosome has a homologous partner. haploid cells only have one set of chromosomes, chromosomes in haploid cells have no homologous partners

Question 3

what takes place before meiosis (DURING INTERPHASE)

Answer 3

DNA replication takes place. the result is two sets of chromosomes consisting of two identical chromatids joined together with a centromere.

Question 4

what happens in the first meiotic division

Answer 4

the number of cells is doubled, but the number of chromosomes is not, this results in half as many chromosomes in each cell

Question 5

what happens in the second meiotic division

Answer 5

the number of chromosomes does not get reduced

Question 6

what happens in prophase 1

Answer 6

• chromatin network becomes shorter and thicker
  -chromosomes become visible
• chromosomes lie in pairs next to each other, which are called homologous chromosomes
• homologous chromosomes exchange segments of their chromosomes during crossing over
• chromatids touch at chiasmata on homologous chromosomes where crossing over takes place
• chromatids break at the chiasmata
  -chromosomes of homologous pairs recombine
• homologous chromosomes exchange segments of the chromatids.

Question 7

what is the important of crossing over

Answer 7

• it promotes genetic variation

Question 8

what happens during metaphase 1

Answer 8

• spindle fibres are spread over the whole cell
• homologous chromosomes arrange themselves randomly (random arrangement) and spindle fibres are attached to the centromeres of the chromosomes

Question 9

what is the importance of random arrangement

Answer 9

it contributes to genetic variation

Question 10

anaphase 1

Answer 10

-spindle fibres contract and pull homologous pairs apart
- one full chromosome (2 chromatids joined by a centromere) of each pair moves the opposite poles of the cell

Question 10

telophase 1

Answer 10

• chromosomes reach the poles of the cell
• poles only have half of the original chromosome number
• cell membrane pinches in the middle and the cytoplasm divides (a.k.a cytokinesis)

Question 11

what happens at anaphase II

Answer 11

• spindle fibres start to contract, pulling the centromeres apart, the centromeres divide
• chromatids are pulled to the opposite poles of each cell

Question 12

what is the result of meiosis 1

Answer 12

two cells with half the chromosome (not 2n, but n)

Question 13

what happens at telophase II

Answer 13

-single stranded (unreplicated) chromosomes reach the poles
- a new nucleus is formed
- the cell membrane of each cell pinces at the middle
- cytoplasm divides by cytokinesis
- four haploid(n) cells are formed
- each gamete only has half the chromosome number of the original cell (in humans its 23 chromosomes)
- 4 gametes are all genetically different.

Question 14

genetic variation is introduced through…

Answer 14

• crossing over (prophase I)
  the random arrangement of chromosomes at the equator (metaphase I & II)

Question 15

similarities between mitosis and meiosis

Answer 15

• cell division occurs
• creates new cells
• starts with a parent cell

Question 16

differences between mitosis and meiosis

Answer 16

mitosis
- only occurs in somatic cells
- creates body cells
- only one cell divison occurs
-creates 2 identical diploid cells

meiosis
- only occurs in reproductive organs
- creates gamates
- 2 cell divisions occur
- creates 4 halpoid cells which are all genetically different

Question 17

chromosome definition

Answer 17

a threadlike structure made up of DNA and protein found in the nucleus of most living cells, carrying genetic information in the form of genes

Question 18

chromatid definition

Answer 18

one of the two identical strands of a replicated chromosome

Question 18

chromatid definition

Answer 18

one of the two identical strands of a replicated chromosome

Question 19

centromere definition

Answer 19

region where the two chromatids of a chromosome are held together

Question 20

homologous chromosomes definition

Answer 20

a pair of chromosomes of the same shape, size and having similar genes for each characteristic occupying the same position

Question 20

bivalent

Answer 20

a pair of homologous chromosomes which lie next to each other and are physically in contact with one another at a point where crossing over might occur.

Question 20

unreplicated chromosomes definition

Answer 20

an unreplicated ‘chromosome’ has a single double -stranded DNA molecule

Question 21

replicated chromosomes

Answer 21

a replicated ‘chromosome’ has two identical double - stranded DNA molecules

Question 22

interphase definition

Answer 22

the phase in the cell cycle when DNA replication occurs

Question 23

diploid (2n) definition

Answer 23

two complete sets of chromosomes in a cell

Question 24

haploid (n)

Answer 24

one complete set of chromosomes in a cell

Question 25

gene

Answer 25

a segment of DNA in a chromosome that contains the code for a particular characteristic

Question 26

centrosome

Answer 26

organelle found in only animal cells (contains 2 centrioles)

Question 27

centriole definition

Answer 27

structures formed when the centrosome divides into two, they move to opposite ends of the cell during cell division

Question 28

karyotype

Answer 28

a representation of the number, shape and arrangement of a full set of chromosomes in the nucleus of a somatic cell

Question 28

crossing over

Answer 28

overlapping of homologous chromosomes resulting in the exchange of genetic material during prophase I

Question 29

chiasma

Answer 29

point where two chromatids overlap during crossing over

Question 30

autosome definition

Answer 30

the first 22 pairs of chromosomes which control the appearance, structure and functioning of the body.

Question 31

gonosomes definition

Answer 31

the pair of chromosomes (XX or XY) responsible for sex determination

Question 32

somatic cells (body cells)

Answer 32

any cells in an organism excluding male and female gametes - they are diploid (have 2 sets of chromosomes) and are produced through mitosis

Question 33

sex cells( gametes)

Answer 33

specialised cells called gametes (sperm cell and egg cell) they have a haploid number of chromosomes and are produced through meiosis

Question 34

non- disjunction

Answer 34

when homologous chromosome pairs fail to separate in meiosis

Question 35

karyokinesis

Answer 35

karyo means nucleus
kinesis means division
karyokinesis is the process of division of the nucleus of a cell

Question 36

cytokinesis

Answer 36

the process of division during which the cytoplasm of a single cell divides into 2 daughter cells.

Question 37

what is a chromosome

Answer 37

a highly condensed form of DNA which becomes visible as chromosomes just before the cell divides

Question 38

difference between somatic cells and sex cells

Answer 38

somatic cells are body cells and sex cells are used for sexual reproduction

Question 39

the difference between gonosomes and autosomes

Answer 39

gonosomes determine your sex and all other chromosomes in your cells are called autosomes in humans we have 23 pairs of chromosomes in each cell, 22 pairs of autosoms and 1 pair of gonosomes

Question 40

where does meiosis take place in males

Answer 40

the testes

Question 41

where does meiosis take place in females

Answer 41

the ovaries