MEISOSIS

Question 1

what is an allele

Answer 1

different forms of the same gene

Question 2

what is a gene

Answer 2

section of DNA that codes for a polypeptide

Question 3

what is a homologous chromosome

Answer 3

chromosomes with the same gene loci

Question 4

what is locus

Answer 4

position of a gene on a chromosome

Question 5

importance of meiosis

Answer 5

involves two nuclear divisions
forms 4 haploid daughter cells
genetically unique
occurs after interphase
haploid gametes but fertilisation will restore the original chromosome number

Question 6

steps of mitosis

Answer 6

interphase
prophase I
metaphase I
anaphase I
telophase I
prophase II
metaphase II
anaphase II
telophase II

Question 7

describe the interphase stage of meiosis

Answer 7

DNA replication
cell replicates each chromosome - forms two chromatids attached via a centromere

Question 8

describe prophase I of meiosis

Answer 8

chromatin condenses
2 centrioles form with mitotic spindles
centrioles begin to migrate to opposite poles of the cell

Question 9

describe metaphase I of meiosis

Answer 9

homologous chromosomes exchange genetic material (crossing over)
point at which this occurs = chiasma / chiasmata
microtubules attach to centromere
homologous chromosomes line up at equator
one pair on either side
independent assortment occurs - arrangement of pairs either side is random

Question 10

describe anaphase I of meiosis

Answer 10

microtubules contract
chromosomes from each pair move to opposite poles
centromeres do not divide so each chromosome is still 2 sister chromatids

Question 11

describe telophase I of meiosis

Answer 11

chromosomes decondense
nuclear membrane reforms around 2 groups of chromosomes
cytoplasm divides
short period of no action occurs (interkinesis)

Question 12

describe prophase II of meiosis

Answer 12

chromatin condenses again
nuclear envelope breaks down
cell only has a haploid number of chromosomes

Question 13

describe metaphase II of meiosis

Answer 13

chromosomes line up at the equator
microtubules from opposite poles attach to each sister chromatid

Question 14

describe anaphase II of meiosis

Answer 14

centromeres divide and both sister chromatids become independent
move to opposite poles of the cell

Question 15

describe telophase II of meiosis

Answer 15

chromosomes decondense
nuclear membrane reforms
cytokinesis occurs
each of the 4 new cells produced are genetically unique

Question 16

describe what happens to chromosomes in meiosis (6)

Answer 16

chromosomes shorten and thicken
chromosomes associate into their homologous pairs
crossing over between chromosomes through the formation of a chiasma
chromosomes join to spindle fibres via their centromeres
whilst at the equator
homologous chromosomes move to opposite poles
pairs of chromatids separated in 2nd division

Question 17

how does meiosis introduce variation

Answer 17

independent segregation
crossing over

Question 18

describe crossing over

Answer 18

1st division
homologous chromosomes associate and adjacent chromatids twist and cross over
sections of DNA is exchanged between the sister chromatids at the chiasmata
tension causes them to break and recombine with another chromatid forming a new combination of alleles (recombination)
leading to gametes having a different combination of alleles on their chromosomes

Question 19

describe independent segregation

Answer 19

1st division
combination of chromosomes on either side of the equator is random
maternal and paternal chromosomes assort independently