meosis

Question 1

Explain how chromosomes halved during meiosis

Answer 1

Homologous chromosomes pair and one of each pair go to opposite poles

Question 2

Describe how a cell divides by meiosis

Answer 2

In interphase, DNA replicates → 2 copies of each chromosome (sister chromatids), joined by a centromere
1. Meiosis I (first nuclear division) separates homologous chromosomes
○ Chromosomes arrange into homologous pairs
○ Crossing over between homologous chromosomes
○ Independent segregation of homologous chromosomes
2. Meiosis II (second nuclear division) separates chromatid

Question 3

Explain how crossing over creates genetic variation

Answer 3

● Homologous pairs of chromosomes associate / form a bivalent
● Chiasmata form (point of contact between (non-sister) chromatids)
● Alleles / (equal) lengths of (non-sister) chromatids exchanged between chromosomes
● Creating new combinations of (maternal & paternal) alleles on chromosomes

Question 4

Other than mutation and meiosis, explain how genetic variation within a
species is increased

Answer 4

Random fertilisation / fusion of gametes
● Creating new allele combinations / new maternal and paternal chromosome combinations

Question 5

xplain the importance of meiosis

Answer 5

Two divisions creates haploid gametes
● So diploid number is restored at fertilisation → chromosome number maintained between generations
● Independent segregation and crossing over creates genetic variation

Question 6

Describe how mutations in the number of chromosomes arise

Answer 6

Spontaneously by chromosome non-disjunction during meiosis
● Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis
● So some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)