Cell Division, Diversity and Organisation - Meiosis

Question 1

Define meiosis

Answer 1

Occurs in diploid germ cells to produce haploid gametes. 4 cells are produced that are genetically different and possess only one set of chromosomes.

Question 2

What are homologous chromosomes

Answer 2

Homologous pairs of chromosomes contain the same genes but possibly different alleles

Question 3

Describe interphase during meiosis

Answer 3

• during interphase DNA replicates so each chromosome consists of two sister chromatids
• sister chromatids will contain the same alleles of the same genes ie genetically identical
• non sister chromatids of homologous chromosomes will come gain the same genes but possibly different alleles of those genes

Question 4

Describe prophase 1 (early) during meiosis

Answer 4

• the chromosomes condense, supercoil and become visible
• the nucleolus disappears and the nuclear envelope breaks down
• the centrioles migrate to the poles and form the spindle which is made of microtubules

Question 5

Describe prophase 1 (late) during meiosis

Answer 5

• homologous chromosomes pair up forming a bivalent (one of the chromosomes is maternal and one is paternal)
• recombination occurs. The chromosomes in a bivalent way cross over at points called chiasmata where they may swap sections of DNA/alleles
• recombinant chromatids are formed. The genes exchanged may be different alleles of the same gene, resulting in genetic variation

Question 6

Describe metaphase 1 of meiosis

Answer 6

• the bivalents randomly line up at the equator. This is independent assortment of the homologous pairs of chromosomes
• the spindle fibres attach to them at their centromeres

Question 7

Describe anaphase 1 of meiosis

Answer 7

• the homologous chromosomes of each bivalent are pulled apart by the spindle fibres, towards the poles
• the chiasmata are pulled apart as they seperate
• the centromeres do not divide. Each chromosome consists of two chromatids

Question 8

Describe telophase 1 of meiosis

Answer 8

• in animal cells the nuclear envelope will reform. There may be a short interphase where the chromosomes uncoil a little
• cytokinesis may occur and the cell will split
• most plant cells progress straight to metaphase 2

Question 9

Describe prophase 2 of meiosis

Answer 9

• everything from now on happens in two nuclei
• the chromosomes recondense. Each consists of two chromatids
• the nuclear envelope disintegrates
• the centrioles replicate and the spindle starts to develop perpendicular to the previous division

Question 10

Describe metaphase 2 of meiosis

Answer 10

• the chromosomes randomly line up at the equator. This is independent assortment of chromatids
• they are attached to the spindle fibres at their centromeres

Question 11

Describe anaphase 2 of meiosis

Answer 11

• the chromosomes are pulled apart by the spindle fibres
• the centromeres divide to allow the chromosomes to split
• the chromatids are pulled, centromere leading, to the poles

Question 12

Describe telophase 2 of meiosis

Answer 12

• the chromatids (now called chromosomes) reach the poles and decondense
• the nuclear envelope reforms, forming four seperate nuclei

Question 13

Describe cytokinesis in meiosis

Answer 13

• cytokinesis may now occur and the cells divide
• each cell has one set of chromosomes (half the number of chromosomes as the original parent cell)
• 1 diploid cell—> 4 haploid nuclei

Question 14

what are the causes of variation

Answer 14

• crossing over
• independent assortment of chromosomes
• independent assortment of chromatids
• random fertilisation
• mutation

Question 15

in what stage of meiosis does crossing over take place

Answer 15

prophase 1

Question 16

describe crossing over

Answer 16

• each chromosome in a bivalent contains the same genes at the same loci (places), however they may have different alleles of those genes.
• they break and rejoin at the chiasmata
• this produces a different combination of alleles on each chromatid

Question 17

in what stage of meiosis does independent assortment of chromosomes take place

Answer 17

metaphase 1

Question 18

describe independent assortment of chromosomes

Answer 18

bivalents orientate themselves randomly at the equator, so the maternal and paternal chromosomes could be facing either pole

Question 19

why is it called independent assortment

Answer 19

each bivalent behaves independently of the others. this means that any combination of maternal and paternal chromosomes could end up in each gamete produced

Question 20

what is the formula for the number of combinations for n pairs of chromosomes

Answer 20

2^n

Question 21

in what stage of meiosis does independent assortment of chromatids take place

Answer 21

metaphase 2

Question 22

describe independent assortment of chromatids

Answer 22

chromosomes orientate themselves randomly at the equator, so the chromatids could be facing either pole. this is important because due to crossing over the sister chromatids are no longer identical

Question 23

describe random fertilisation

Answer 23

any one of about 300 million genetically different sperm can fertilise the egg

Question 24

describe mutation

Answer 24

during interphase, when DNA is replicated a mutation could occur. this can lead to new alleles