cell cycle and mitosis

Question 1

What are the 3 main stages of the cell cycle?

Answer 1

The 3 main stages are:

1. Interphase (G1, S and G2)
2. Nuclear division
3. Cell division (cytokinesis)

Question 2

Why are divisions of the nucleus very precise?

Answer 2

This ensures the correct distribution of chromosomes between the daughter cells (genetic stability)

Question 3

_______ (thread-like structures made out of DNA and protein, coiled and compact) are visible at the time the nucleus divides, at other times, they appear as a diffuse network (long, thin, uncoiled threads) called ______.

Answer 3

Chromosomes (thread-like structures made out of DNA and protein) are visible at the time the nucleus divides, at other times, they appear as a diffuse network called chromatin.

Question 4

• The nucleus contains the ________ of the cell, and the ________ contain the coded instructions for the organization and activities of cells and for the whole organism.
• Chromosomes hold the hereditary factors, ______.

Answer 4

• The nucleus contains the chromosomes of the cell, and the chromosomes contain the coded instructions for the organization and activities of cells and for the whole organism.
• Chromosomes hold the hereditary factors, genes.

Question 5

What are some features of chromosomes?

Answer 5

1. They are of fixed length and with a centromere somewhere along their length
2. Chromosomes occur in pairs called homologous pairs, one of each pair came originally from each parent
3. Chromosomes are copied (DNA replication) before nuclear division occurs, forming 2 identical chromatids attached at the centromere till separation and become chromosomes again

Question 6

Mitosis is essential for…

Answer 6

It is essential in growth and development of the embryo, repair of damaged/worn out cells, and asexual reproduction (genetically identical)
It is also essential for genetic stability. Each daughter cell produced by mitosis has a full set of chromosomes which are identical to those of the parent cell.

Question 7

What happens during mitosis? What are the 4 phases of mitosis?

Answer 7

Chromosomes are separated, accurately and precisely distributed to two daughter nuclei. It is a continuous process with these 4 phases: prophase, metaphase, anaphase, telophase.

Question 8

What happens during prophase?

Answer 8

Chromosomes increasingly shorten and thicken by supercoiling to become 2 chromatids held together at the centromere. The nuclear envelope breaks down and the centrosome divides, the 2 centrioles replicate to form two centrosomes.

Question 9

What happens during metaphase?

Answer 9

Two centrosome move to opposite ends of the cell. Microtubules of the cytoplasm start to form a spindle radiating out from the centrioles. Each pair of chromatids is attached to a microtubule of the spindle and is arranged at the equator of the spindle.

Question 10

What happens during anaphase?

Answer 10

Centromeres separate, spindle fibres shorten, chromatids are pulled by centromeres to opposite poles (once separated, they are referred to as chromosomes)

Question 11

What happens during telophase?

Answer 11

Nuclear envelope reforms around both groups of chromosomes at opposite ends of the cell. Chromosomes uncoil and become chromatin. Nucleolus reforms in each nucleus.

Question 12

What is the DNA content and chromosome number of the daughter cells after mitosis?

Answer 12

Chromosome number: same as parent cell

DNA content: half of the parent cell (before mitosis)

Question 13

What is cytokinesis?

Answer 13

It is the division of the cytoplasm that follows interphase, where cell organelles become evenly distributed between the daughter cells. (In animal cells, cytokinesis occurs by a process known as cleavage)
In-tucking of the cell surface membrane at the equator of the spindle, “pinching” the cytoplasm in half

Question 14

How does cancer arise?

Answer 14

Cancer arises when the cell cycle operates without its normal controls. This causes the rate of cell multiplication to be faster than the rate of cell death, and a tumour is formed. (DNA molecules of chromosome are damaged which result in a mutation, mutations build up in DNA of body cells and cause cancer)

Question 15

What are common carcinogens known to increase mutation rates + the likelihood of cancer?

Answer 15

• Ionising radiation (X-rays and radiation: gamma rays etc) from various radioactive sources
• Non-ionising radiations (UV light) which are less penetrating than ionising radiation
• Chemicals (e.g. chemicals that are carcinogens are present in tobacco smoke