Lecture 14 - Eukaryotic Cell Division - Meiosis

Question 1

Explain what happens in the sexual life cycle

Answer 1

1. Starts with a diploid number of chromosomes 2n. 23 from Mum 23 from dad so 46 in total.
2. Meiosis takes the diploid number of chromosomes and forms to haploid number of chromosomes so each sperm and egg cell has 23 chromosomes each.
3. After fertilisation the fertilised egg will retain 46 chromosomes (diploid number of chromosomes 2n) which forms a diploid zygote.
   4) diploid zygote will then develop and grow by mitosis

Question 2

What is the purpose of meiosis?

Answer 2

To produce gametes (the sperm and eggs) with half of the genetic complement of the parent cells

Question 3

What happens before meiosis starts?

Answer 3

Interphase:
Occurs in diploid cells 2n=46
Inside our body there are already pairs of homologous chromosomes, one from Mum one from Dad. Each of these chromosomes duplicates and form sister chromatids.

Question 4

What happens in meiosis one and what are the stages?

Answer 4

In meiosis I we have homologous chromosomes separate forming haploid cells. n=23

Prophase I
Metaphase I
Anaphase I
Telophase I and cytokinesis

Question 5

What happens in prophase I?

Answer 5

Chromosomes are already duplicated (2 sister chromatids)

Homologous chromosomes align besides and synapse forming chiasmata

Crossing over (between non-sister chromatids) occur at chiasmata which results in chromatids to be a mix from each homologous chromosome

Nuclear envelope fragments

Question 6

What happens in metaphase I?

Answer 6

Pairs of homologous chromosomes are still under synapse (chiasmata) and move to metaphase plate

It is the chiasmata that line up on metaphase plant not kinetochore

Centrosome travels to opposite spindle poles of the cell

Independent assortment

Question 7

What happens in anaphase I?

Answer 7

Recombined (as of crossing over) homologous chromosomes seperate (disjoin)

Sister chromatids remain attached

(In anaphase of mitosis, sister chromatids separate)

Question 8

What happens in telophase I and cytokinesis?

Answer 8

Haploid cells (n=23 chromosomes) with duplicated chromosomes form (it is haploid as only half of genetic information is in each new cell)

1 chromosome = pair of sister chromatids

Cleavage furrow forms

Question 9

What happens in prophase II?

Answer 9

Centrosomes are present, nuclear envelope fragments

Centrosomes seperate forming spindle

Chromatin recondenses

Question 10

What happens in metaphase II?

Answer 10

Chromosomes lineup at metaphase plate lining on centromere (where kinetochore present)

Kinetochore and non-kinetochore microtubules also present

Question 11

What happens in anaphase II?

Answer 11

Two sister chromatids are pulled apart at the centromere by kinetochore microtubule shortening and non-kinetochore microtubules lengthening

Question 12

What happens at telophase II and cytokinesis?

Answer 12

Cleavage furrow forms and haploid cells divide forming daughter haploid cells (genetically diverse gametes)

(Half the number of chromosomes in original parent cell)

Question 13

Compare mitosis to meiosis

Answer 13

Mitosis:
Chromosomes align independently 
No chiasmata
Centromeres on metaphase plate 
Chromatids disjoin
2n -> 2n

Meiosis:
Homologous chromosomes synapse 
Chiasmata
Chiasmata on metaphase plate 
Chromosomes disjoin
2n->n

Question 14

What ways does sexual reproduction produce genetic diversity?

Answer 14

Independent assortment
Crossing over
Random fertilisation of gametes