2.6.3 meiosis

Question 1

haploid

Answer 1

having only 1 set of chromosomes - represented by symbol ‘n’

Question 2

homologous chromosomes

Answer 2

matching chromosomes - contain same gene at same places (loci)
–> may contain diff. alleles for some of genes

Question 3

meiosis

Answer 3

type of nuclear division resulting in formation of cells containing half number of chromosomes in parent cell

Question 4

how does sexual reproduction increase genetic variation

Answer 4

• involves combining of genetic material from 2 (usually) unrelated individuals of same species
• via process of fertilisation

Question 5

how does genetic variation within a population increase its chances of survival during environmental changes

Answer 5

some individuals will have characteristics which enable them to be better adapted to the change

Question 6

what’s necessary for sexual reproduction to occur

Answer 6

• in many organisms, the body cells are diploid
• they must produce haploid gametes

Question 7

why are haploid gametes required for sexual reproduction

Answer 7

when 2 gamete nuclei fuse during fertilisation, a diploid zygote is produced & the normal chromosomes number is maintained

Question 8

where does meiosis occur

Answer 8

• in diploid germ cells to produce haploid gametes

Question 9

where are the diploid cells undergoing meiosis found

Answer 9

specialised organisms called gonads - testes/ovaries

Question 10

how many chromosomes are in your body cells

Answer 10

46

Question 11

what are homologous chromosomes & what do they contain

Answer 11

• matching pairs of chromosomes
• one maternal & one paternal –> same genes at same places on chromosome

Question 12

what occurred prior to meiosis

Answer 12

• the S phase of interphase:
  
  • each chromosome was duplicated as its DNA replicated
  • each chromosome now consists of 2 sister chromatids

Question 13

how many divisions in meiosis/stages

Answer 13

2
- 4 stages in each (8 total)

Question 14

stages in 1st meiotic division

Answer 14

prophase 1
metaphase 1
anaphase 1
telophase 1

Question 15

what may the cell enter before the 2nd meiotic division

Answer 15

short interphase

Question 16

stages in 2nd meiotic division

Answer 16

prophase 2
metaphase 2
anaphase 2
telophase 2

Question 17

what does the 2nd meiotic division take place in

Answer 17

a plane at right angles to meiosis 1

Question 18

what may occur at the end of the 2nd division

Answer 18

cytokinesis

Question 19

events during stage 1 - prophase 1

Answer 19

• chromatin condenses & each chromosome supercoils
• nuclear envelope breaks down
• spindle threads of tubulin protein form from centriole (animal cells)
• chromosomes come together in homologous pairs
• each member of pair consists of 2 chromatids
• crossing over occurs –> non-sister chromatids wrap around each other & may swap sections = alleles shuffled

Question 20

events during stage 2 - metaphase 1

Answer 20

• pairs of homologous chromosomes (crossed over state) attach along equator of spindle
• each attaches to spindle thread by centromere
• homologous pairs arranged randomly (members of each pair face opposite poles) = independent assortment
• way the line up determines how they’ll segregate independently when pulling apart in anaphase

Question 21

events during stage 3 - anaphase 1

Answer 21

• members of each pair of homologous chromosomes pulled apart by motor proteins (drag along tubulin threads of spindle)
• centromeres don’t divide & each chromosome = 2 chromatids
• crossed-over areas separate = swapped areas of chromosome & allele shuffling

Question 22

events during stage 4 - telophase 1

Answer 22

• (most animal cells) 2 new nuclear envelopes form around each set of chromosomes & cell divides (cytokinesis) –> then, short interphase & chromosomes uncoil
• each new nucleus contains half original no. of chromosomes but each chromosomes = 2 chromatids
• (most plant cells) cell goes straight from anaphase 1 to prophase 2

Question 23

events during stage 1 - prophase 2

Answer 23

• (if nuclear envelopes reformed) break down again
• chromosomes coil & condense –> each one = 2 chromatids
• chromatids of each chromosome are not identical (due to crossing over)
• spindles form

Question 24

events during stage 2 - metaphase 2

Answer 24

• chromosomes attach (centromere) to equator of spindle
• chromatids of each chromosome = randomly arranged
• way they’re arranged determines how chromatids separate in anaphase

Question 25

events during stage 3 - anaphase 2

Answer 25

• centromeres divide
• chromatids of each chromosome pulled apart by motor proteins –> drag along tubulin threads of spindle to opposite poles
• chromatid = randomly segregated

Question 26

events during stage 4 - telophase 2

Answer 26

• nuclear envelope form around each of 4 haploid nuclei
• (animals) 2 cells now divide = 4 haploid cells
• (plants) tetrad of 4 haploid cells formed

Question 27

how does meiosis cause genetic variation

Answer 27

• crossing over (prophase 1) shuffles alleles
• independent assortment of chromosomes (anaphase 1) = random distribution of maternal/paternal chromosomes of each pair
• independent assortment of chromosomes (anaphase 2) = further random distribution of genetic material
• haploid gametes produced = can undergo random fusion with gametes derived from another organism of same species