Meiosis (U2, c3/4)

Question 1

How many chromosomes are in the nucleus of a human body cell?

Answer 1

46

Question 2

What may be different in a homologous pair of chromosomes?

Why is this?

Answer 2

The alleles of the genes on each one,

because they are inherited from different parents.

Question 3

What does meiosis produce?

What do these develop into?

Answer 3

Genetically variable haploid cells,

These develop into gametes.

Question 4

What do gametes do during fertilisation?

Answer 4

Fuse their haploid nuclei to produce a diploid cell.

Question 5

What are the four steps in Meiosis I?

Answer 5

Pairing of homologous chromosomes
Crossing over
Alignment of the metaphase plate
Separating homologous chromosomes

Question 6

What are the two steps in Meiosis II?

Answer 6

Alignment on the metaphase plate

Separating sister chromatids

Question 7

What happens during interphase?

Answer 7

Each chromosome in the homogenous pair duplicates so each chromosome is now made up of two sister chromatids.

Question 8

What is a bivalent and how does it form?

Answer 8

A bivalent is what we call the homologous pairs and sister chromatids linked together.
Protein strands form a complex to link them after the homologous chromosomes pair up to align gene by gene.

Question 9

What is a chiasma? (plural = chiasmata)

Answer 9

A chiasma is the point where two homologous chromatids exchange genetic material during crossover in meiosis.

Question 10

Where do chiasmata form (and where do they never form)?

Answer 10

Chiasmata form at random positions between the homologous PAIRS, i.e. they never from between sister chromatids.

Question 11

What is the purpose of “crossing over”?

Answer 11

To shuffle sections of DNA between homologous chromosomes, leading to the recombination of alleles which increases variation in the gametes.

Question 12

When chromosomes align on the metaphase plate:

1) What keeps on holding the sister chromatids together?
2) What holds the homologous chromosomes together?
3) What breaks down?
4) What do the centrosomes do?

Answer 12

1) The centromeres
2) Chiasmata
3) The nuclear membrane
4) Centrosomes send out microtubule, which attach to kinectochores by each centromere. The microtubules form spindle fibres linking across the cell.

Question 13

What is cytokinesis?

Answer 13

The cytoplasmic division of a cell, separating it into two daughter cells.

Question 14

How are the homologous chromosomes separated in Meiosis I? (/sister chromatids in meiosis II?)

Answer 14

The microtubules of the spindle fibres shorten and pull on the kinectochores.

Question 15

At the start of Meiosis II, are the two daughter cells haploid or diploid?

Answer 15

They are haploid, with one copy of each homologous chromosome.

Question 16

What happens to sister chromatids in Meiosis II?

Answer 16

They are separated to each end of the cell - now being called “chromosomes”

Question 17

How many cells does meiosis produce?

Are they haploid or diploid?

Answer 17

Four cells, which are haploid.

Question 18

What are linked genes?

Answer 18

Genes that are located on the same chromosome.

Question 19

What does separating linked genes via crossing over create?

Answer 19

Recombinants - the chromosomes created in this instance.

Question 20

What is a recombinant?

Answer 20

The chromosome created when linked genes are separated during crossing over.

Question 21

How does the alignment of homologous pairs during meiosis I lead to variable gametes being formed?

Answer 21

The orientation of the chromosomes is irrespective of their maternal or paternal origin, meaning there are multiple possible alignments. This leads to variation in the combinations of chromosomes found in the haploid cells at the end of meiosis II.

Question 22

Which three factors produce variation in an organism?

Answer 22

Crossing over
Independent assortment
Sexual reproduction