Module 2: Cell Division

Question 1

What is the purpose of the checkpoints in the cell cycle?

Answer 1

• To prevent uncontrolled division that would lead to tumours - To detect and repair damage to DNA

Question 2

Why are the events that control the cell cycle in a specific sequence?

Answer 2

To ensure that: - The cell cycle cannot be reversed - The DNA is only duplicated one during the cell cycle

Question 3

Describe the events within the cell during M phase

Answer 3

• A checkpoint chemical triggers condensation of chromatin - Cell growth stops - Nuclear division: prophase, metaphase, anaphase, telophase - Cytokinesis (cytoplasmic division)

Question 4

G0 phase

Answer 4

• A resting phase - Triggered during early G1 by a checkpoint chemical - Some cells, such as epithelial cells lining the gut, don’t have this phase - In this phase, cells may undergo apoptosis, differentiation or senescence - Some types of cells remain in this phase for a very long time or indefinitely

Question 5

G1 (the growth phase)

Answer 5

• A G1 checkpoint control mechanism ensures that the cell is ready to undergo S phase and begin DNA synthesis - Cells grow - Transcription of genes to make RNA occurs - Organelles duplicate - The P53 gene helps control this phase

Question 6

S phase of interphase

Answer 6

• Every molecule of DNA is replicated - Once the cell has entered this phase it is committed to completing the entire cycle - DNA replicates - When all chromosomes are copied, each one consist of a pair of identical sister chromatids

Question 7

G2 of interphase

Answer 7

• Special chemicals ensure that the cell is ready for mitosis by stimulating proteins that will be involved in making chromosomes condense and in formation of the spindle - Cells grow

Question 8

The cell cycle diagram

Answer 8

Question 9

What is the significance of mitosis in the cell cycle?

Answer 9

1) Asexual reproduction: single-celled protocists divide by mitosis to produce new individuals. Strawberies reproduce asexually by forming new plantlets on the end of stolons.
2) Growth
3) Tissue repair

Question 10

Describe prophase

Answer 10

• The chromosomes have been replicated during S phase and consist of two sister chromatids, now shorten and thicken as the DNA supercoils
• The nuclear envelope breaks down
• The centriole in animal cells divides and two new daughter centrioles move to opposite ends of the cell

Question 11

Describe Metaphase

Answer 11

• The pairs of chromatids attach to the spindle threads at the equator region
• They attach by their centromeres

Question 12

Desrcibe Anaphase

Answer 12

• The centromere of each chromosome splits
• Motor proteins, walking along the tubulin threads, pull each sister chromatid of a pair, in opposite directions, towards opposite poles
• Chromatids are now called chromosomes

Question 13

Describe Telophase

Answer 13

• The separated chromosomes reach the poles
• A new nuclear envelope forms around each set of chromosomes
• The cell now contains two nuclei each genetically identical to the parent cell

Question 14

Describe cytokinesis

Answer 14

Cytokinesis is the division of the cytoplasm of a cell following mitosis

In animal cells, the plasma membrane folds inwards and ‘nips in’ the cytoplasm

In plant cells, an end plate forms where the equator of the spindle was, and new plasma membrane and cellulose cell-wall material are laid down on either side along this end plate

Two new daugther cells are formed. They are genetically identical to each other

Question 15

What is meiosis?

Answer 15

Meiosis is a type of nuclear division that results in the formation of cells containing half teh number of chromosomes of the parent cell

Question 16

What are homologous chromosomes?

Answer 16

Mathcing chromosomes, containing the same genes in the same places. They may contain different alleles for some of the genes.

Question 17

What are the stages of meiosis?

Answer 17

During S phase, each chromosome is duplicated and DNA replicated

In meiosis, chromosomes pair up in their homologous pair

• There are two divisions in meiosis
• In the first division, the stages are prophase 1, metaphase 1, anaphase 1, telophase 1
• In the second division, the stages are prophase 2, metaphase 2, anaphase 2, telophase2
• At the end of the second division, cytokinesis may occur

Question 18

Describe prophase 1

Answer 18

• The chromatin condenses and each chromosome supercoils. In this state they can take up stains and be seen under a light microscope
• The nuclear envelope breaks down, and spindle threads of tubulin protein form from the centriole in animal cells
• Crossing over occurs where non-sister chromatids wrap around each other and may swap sections so that alleles are shuffled

Question 19

Describe metaphase 1

Answer 19

The pairs of homolougous chromosomes attach along the equator of the spindle

Each attaches to a spindle thread by its centromere

The homologous pairs are arranged randomly, with the members of each pair facing opposite poles of the cell. This is known as independent assortement.

Question 20

Describe anaphase 1

Answer 20

• The members of each pair of homologous chromosomes are pulled apart by motor proteins that drag them along the tubuin threads of the spindle
• The centromeres do not divide and each chromosome consist of two chromatids
• The crossed-over areas seperate from each other, resulting in swapped areas of chromosome and allele shuffling

Question 21

Describe telophase 1

Answer 21

• two nuclear envelopes form around each set of chromsomes, and the cell divides by cytokinesis. There is then a short interphase where the chromosomes uncoil
• The nucleus contains half the original number of chromosomes
• Most plant cells go straight from anaphase 1 to prophase 2