D2.1 Cell and Nuclear Division

Question 1

Define cytokinesis.

Answer 1

splitting of cytoplasm

Question 2

Distinguish between mitosis and cytokinesis.

Answer 2

Mitosis is division of the nucleus; cytokinesis is division of the cytoplasm

Question 3

Compare and contrast cytokinesis in plant and animal cells.

Answer 3

In an animal cell, a ring of contractile actin and myosin proteins pinches a cell membrane together to split the cytoplasm. In a plant cell, vesicles assemble sections of membrane and cell wall to achieve splitting.

Question 4

State two examples where unequal cytokinesis occurs.

Answer 4

Budding and human oogenesis.

Question 5

State the role of mitosis.

Answer 5

Mitosis maintains the chromosome number of cells.

Question 6

State the role of meiosis.

Answer 6

Meiosis halves the chromosome number and generates genetic diversity.

Question 7

What must occur prior to both mitosis and meiosis?

Answer 7

DNA replication

Question 8

Outline the role of histones in the condensation of DNA.

Answer 8

Histones package DNA into structures called nucleosomes
Each nucleosome consists of a strand of DNA coiled around eight histone proteins. Condensation occurs by the repeated coiling of the DNA molecule (supercoiling).

Question 9

How are chromosomes moved during cell division?

Answer 9

Microtubules and microtubule motors are responsible for the movement of chromosomes during cell division.
Microtubules are tubulin fibres that form part of the cytoskeleton of the cell. They are able to lengthen and shorten in order to enable chromosome movement.

Question 10

List the phases of mitosis.

Answer 10

Prophase
Metaphase
Anaphase
Telophase

Question 11

Outline key processes in prophase.

Answer 11

Chromosomes condense
The two centrosomes move towards opposite poles
Spindle fibres begin to emerge from the centrosomes
The nuclear envelope breaks down into small vesicles
The nucleolus disappears

Question 12

Outline key processes in metaphase

Answer 12

Centrosomes reach opposite poles.
Spindle fibres continue to extend from centrosomes and reach the chromosomes and attach to the centromeres.
Chromosomes line up at the equator of the spindle.

Question 13

Outline key processes in anaphase

Answer 13

The sister chromatids separate at the centromere (the centromere divides in two).
Spindle fibres (protein microtubules) begin to shorten.
The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres.

Question 14

Outline key processes in telophase.

Answer 14

Chromosomes arrive at opposite poles and begin to decondense.
Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes.
The spindle fibres break down
New nucleoli form within each nucleus.

Question 15

Define the term ‘diploid’

Answer 15

contains two copies of every chromosome (one maternal copy and one paternal copy)

Question 16

Define the term ‘haploid’

Answer 16

possessing only one copy of each chromosome

Question 17

What is the result of meiosis I?

Answer 17

the homologous pairs of chromosomes are split up, to produce two haploid (n) nuclei

Question 18

What is the result of meiosis II?

Answer 18

the chromatids that make up each chromosome separate to produce four haploid (n) nuclei

Question 19

What is a bivalent?

Answer 19

A pair of homologous chromosomes

Question 20

What is a chiasmata?

Answer 20

The crossing points where there can be an exchange of genetic material (alleles) between non-sister chromatids in the bivalent

Question 21

What is crossing over?

Answer 21

This exchange of genetic material between non-sister chromatids

Question 22

Why is meiosis I known as reduction division?

Answer 22

Because in meiosis I the chromosome number halves, from 2n to n, in the first division of meiosis (meiosis I), not the second division (meiosis II)

Question 23

What are recombinant chromosomes?

Answer 23

The result of crossing over, where there is a new combination of alleles on the two chromosomes following crossing over in prophase I

Question 24

Why is meiosis important for a sexual life cycle?

Answer 24

The halving of the chromosome number during meiosis is very important for a sexual life cycle as it allows for the fusion of gametes.

Question 25

What is a zygote?

Answer 25

A fertilised egg cell, the result of the fusion of the nuclei of two gametes

Question 26

When can non-disjunction occur in meiosis?

Answer 26

In anaphase I and anaphase II

Question 27

What happens if non-disjunction occurs?

Answer 27

If the abnormal gametes are fertilized, then a chromosome abnormality occurs as the diploid cell (zygote) will have the incorrect number of chromosomes

Question 28

Down Syndrome is caused by trisomy of what chromosome number?

Answer 28

Chromosome #21

Question 29

How many chromosomes would be seen in cells of individuals with Down Syndrome?

Answer 29

Individuals with this syndrome have a total of 47 chromosomes in their cells as they have three copies of chromosome 21.

Question 30

Outline the link between maternal age and Down Syndrome.

Answer 30

The risk of chromosomal abnormalities increases significantly with age. The age of the mother is particularly important in the case of Down Syndrome as non-disjunction is more likely to happen in older ova.

Question 31

What is random orientation?

Answer 31

The random alignment and positioning of homologous or sister chromatids during meiosis.

Question 32

What formula can be used to calculate the number of possible combinations produced at the end of meiosis I?

Answer 32

2 to the power of n can be used to calculate the number of possible combinations produced at the end of meiosis I, where ‘n’ is the haploid number.

Human cells have a diploid number of 46, and therefore a haploid number of 23. The number of possible outcomes after meiosis I is 2 to the power of 23 = 8 388 608.

Question 33

What processes in meiosis generate genetic diversity?

Answer 33

1. Random orientation of bivalents
2. crossing over