6 - cell division

Question 1

What is the interphase?

Answer 1

Referred to as ‘growth period’

3 stages: G1, S, G2

Question 2

What occurs during G1?

Answer 2

Organelles are synthesised and produced => replicated

Cell increases in size

CHROMOSOMES DO NOT REPLICATE IN G1

Question 3

What occurs during the S phase?

Answer 3

DNA replication

Question 4

What occurs during G2?

Answer 4

Cell continues to increase in size
Energy stores increase
Cell checks duplicated chromosomes and repairs error

Question 5

What occurs during G0 and what are reasons for it?

Answer 5

DNA damaged, cell not viable and cannot divide

Permanent cell arrest

Most cells only divide for limited time > become secescent

Cells specialise and can no longer divide - leaves to perform function and does not re-enter the cell cycle

Question 6

Define mitosis

Answer 6

Nuclear division in the mitotic phase of the cell cycle

Question 7

What are chromosomes and chromatids?

Answer 7

Chromosomes - coiled and condensed DNA ready for division

Chromatids - 2 identical copies of a chromosome (DNA) held by a centromere (1 chromosome is converted into 2 chromatids)

1 chromosome > 2 chromatids per chromosome (after replication) > 1 chromatid (after mitotic division)

Question 8

What are the roles of centromeres?

Answer 8

Necessary to ensure chromatids stay together, are precisely manouvred and segregated equally.

Question 9

What occurs during prophase?

Answer 9

1) Chromatin fibres (complex of proteins DNA and RNA) coil and condense to form chromosomes. Nucleolus breaks down and nuclear membrane begins to degrade.

2) Protein microtubules form spindle fibres, which are used to move chromosomes into correct positions

3) Centrioles migrate to opposite poles of the cell - cylindrical bundles of protein which help spindle formation

4) Spindle fibres attach to centromere and begin to move chromosomes

5) Nuclear envelope disintegrates by end of prophase.

Question 10

What occurs during metaphase?

Answer 10

Chromosomes moved by spindle fibres to form plane across the cell

Question 11

What occurs during the anaphase?

Answer 11

Chromatids separate as centromeres holding chromosomes together divide - they are pulled to opposite poles of the cell by shortening the spindle fibres

• The V-shape appears as centromere drags chromatids through liquid cytosol.

Question 12

What occurs during the telophase?

Answer 12

Chromatids reach poles - now called chromosomes

New set of chromosomes assemble at each pole

2 new nuclear envelopes form

Question 13

What occurs during cytokinesis?

Answer 13

Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells.

Question 14

Define haploid.

Answer 14

1/2 the normal chromosome number (1 chromosome of each type)

Question 15

Define diploid.

Answer 15

Normal chromosome number - 2 of each type, one from each parent

Question 16

Define allele.

Answer 16

Gene variants, different versions of the same gene

Question 17

What occurs during ‘Prophase 1’?

Answer 17

Spindle fibres form from centrioles

Nucleolus disappears, and nuclear membrane disintegrates

Homologous chromosomes pair up – now called bivalents (also referred to as a tetrad)

Crossing over (known as recombination) occurs: The exchange of genes between bivalents as chromatids entangle.

Question 18

What occurs during ‘Metaphase 1’?

Answer 18

the homologous pairs of chromosomes align on either side of the equatorial plate.

Question 19

What occurs during ‘Anaphase 1’?

Answer 19

the spindle fibers contract and pull the homologous pairs, each with two chromatids, away from each other and toward each pole of the cell.

Question 20

What occurs during ‘Telophase 1’?

Answer 20

the chromosomes arrive at the cell poles, the mitotic spindle disassembles, and the vesicles that contain fragments of the original nuclear membrane assemble around the two sets of chromosomes.

Question 21

What occurs during ‘Prophase 2’?

Answer 21

no crossing over occurs as there are no homologous chromosomes in pairs (bivalents), instead the spindle reforms, nucleolus disappears, and the nuclear membrane begins to disintegrate.

Question 22

What occurs during ‘Metaphase 2’?

Answer 22

the centromeres of the paired chromatids align along the equatorial plate in both cells.

Question 23

What occurs during ‘Anaphase 2’?

Answer 23

the chromosomes separate at the centromeres.

Question 24

What occurs during ‘Telophase 2’?

Answer 24

nuclear membranes form around each set of chromosomes, and the chromosomes decondense.