meiosis

Question 1

define diploid

Answer 1

contains homologous chromosome pairs - two complete sets of chromosomes, one from each parent

Question 2

define haploid

Answer 2

containing one chromosome from each homologous pair

Question 3

define gamete

Answer 3

haploid sex cell produced by meiosis in organisms that reproduce sexually

Question 4

define zygote

Answer 4

initial diploid cell formed when two gametes are joined by sexual reproduction

Question 5

define meiosis

Answer 5

nuclear division for the purpose of gamete production
- nucleus divides twice = four haploid cells from one diploid cell

Question 6

define reduction division

Answer 6

where the chromosome number is halved from diploid to haploid (first meiotic division)

Question 7

explain 3 roles of meiosis in life cycles

Answer 7

• production of gametes to allow of sexual reproduction
• ensures organisms produced via sexual reproduction contain correct number of chromosomes (number of chromosomes would double with every round of reproduction otherwise)
• produces genetic variation

Question 8

state 3 ways in which meiosis produces variation

Answer 8

• crossing over during prophase 1
• independent assortment of chromosomes in metaphase 1
• independent assortment of chromatids in metaphase 2

Question 9

suggest the importance of the creation of different allele combinations in populations

Answer 9

• phenotypes will be produced
• reduces chances of a population going extinct - it is more likely an individual will possess advantageous characteristics

Question 10

define homologous chromosomes

Answer 10

matching pair of chromosomes - one inherited from each parent

Question 11

define bivalent

Answer 11

a pair of homologous chromosomes during crossing over

Question 12

define crossing over

Answer 12

exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring

Question 13

define chiasmata

Answer 13

points at which chromatids break and rejoin

Question 14

define recombinant chromatid

Answer 14

chromatid with a combination of DNA from both homologous chromosomes,
- formed by crossing over and chiasmata in meiosis

Question 15

state the stages of meiosis in order

Answer 15

• prophase 1
• metaphase 1
• anaphase 1
• telophase 1
• prophase 2
• metaphase 2
• anaphase 2
• telophase 2

Question 16

what happens at P1?

Answer 16

• chromosomes condense, nuclear envelope disintegrates, nucleolus disappears
• spindle formation begins
• homologous chromosomes pair up, forming bivalents & crossing over occurs

Question 17

what happens at M1?

Answer 17

• homologous pairs assemble along metaphase plate
• independent assortment occurs

Question 18

what happens at A1?

Answer 18

• homologous chromosomes pulled to opposite poles, chromatids stay joined to each other
• entangled sections of DNA on non-sister chromatids from crossing over break off + rejoin at chiasmata = formation of recombinant chromatids

Question 19

what happens at T1?

Answer 19

• chromosomes assemble at each pole, nuclear membrane reforms
• chromosomes uncoil
• cytokinesis occurs, cell divides into two
• reduction of chromosome number from diploid to haploid is complete

Question 20

what happens at P2?

Answer 20

• chromosomes that still consist of two chromatids condense and become visible
• nuclear envelope breaks down, spindle formation begins

Question 21

what happens at M2?

Answer 21

• individual chromosomes assemble on metaphase plate
• due to crossing over, chromatids are not identical - independent assortment occurs

Question 22

what happens at A2?

Answer 22

chromatids of individual chromosomes are pulled to opposite poles after division of centromeres

Question 23

what happens at T2?

Answer 23

• chromatids assemble at poles, uncoil and form chromatin
• nuclear envelope reforms, nucleolus is visible again
• cytokinesis results in division of the cells forming four daughter cells in total
• daughter cells will be haploid and genetically different from each other & from parent cell

Question 24

describe the process of crossing over and how it produces genetic variation

Answer 24

• homologous chromosomes pair up, forming a bivalent
• non-sister chromatids become entangled at chiasmata
• they break off and re-join, sometimes resulting in an exchange of DNA forming recombinant chromatids with different combinations of alleles = genetic variation

Question 25

describe the process of random independent assortment and explain how it produces genetic variation.

Answer 25

• orientation of each homologous pair on metaphase plate is random + independent of any other homologous pair
• maternal or paternal chromosomes can end up facing either pole
• means that daughter cells can end up with any combination of maternal and paternal chromosomes = genetic variation

Question 26

explain how to calculate the total number of possible genetically different gametes that could be produced through independent assortment only.

Answer 26

2(^number of chromosome pairs) - eg. 23 for humans