Cells - Cell Division, Cell Cycle and Mitosis

Question 1

What is mitosis?

Answer 1

Mitosis produces two daughter cells that have the same number of chromosomes as the parent cell and each other.

Question 2

What is meiosis?

Answer 2

Meiosis produces four daughter cells, each with half the number of chromosomes of the parent cell.

Question 3

What does mitosis result in?

Answer 3

Mitosis is division of a cell that results in each of the daughter cells having an exact copy of the DNA of the parent cell. Except in the rare event of a mutation, the genetic make-up of the two daughter nuclei is also identical to that of the parent nucleus.

Question 4

What is interphase?

Answer 4

Mitosis is always preceded by a period during which the cell is not dividing. This period is called interphase. It is a period of considerable cellular activity that includes a very important event, the replication of DNA. The two copies of DNA after replication remain joined at a place called the centromere. Although mitosis is continuous process, it can be divided into four stages for convenience.

Question 5

When does the replication of DNA take place?

Answer 5

The replication of DNA takes place during interphase before the nucleus and the cell divide.

Question 6

What is prophase?

Answer 6

In prophase, the chromosomes first become visible, initially as long thin threads, which later shorten and thicken.

Animal cells contain two cylindrical organelles called centrioles, each of which moves to opposite ends (called poles) of the cell. From each of the centrioles, spindle fibres develop, which span the cell from pole to pole. Collectively, these spindle fibres are called the spindle apparatus.

As plant cells lack centrioles but do develop a spindle apparatus, centrioles are clearly not essential to spindle fibre formation. The nucleolus disappears and the nuclear envelope breaks down, leaving the chromosomes free in the cytoplasm of the cell. These chromosomes are drawn towards the equator of the cell by the spindle fibres attached to the centromere.

Question 7

What is metaphase?

Answer 7

By metaphase, the chromosomes are seen to be made up of two chromatids. Each chromatid is an identical copy of DNA from the parent cell. The chromatids are joined by the centromere. It is to this centromere that some microtubules from the poles are attached, and the chromosomes are pulled along the spindle apparatus and arrange themselves across the equator of the cell.

Question 8

What is anaphase?

Answer 8

In anaphase, the centromeres divide into two and the spindle fibres pull the individual chromatids making up the chromosome apart. The chromatids move rapidly to their respective, opposite poles of the cell and we now refer to them as chromosomes.

The energy for the process is provided by mitochondria, which gather around the spindle fibres. If cells are treated with chemicals that destroy the spindle, the chromosomes remain at the equator, unable to reach the poles.

Question 9

What is telephase and cytokinesis (cleavage)?

Answer 9

In this stage, the chromosomes reach their respective poles and become longer and thinner, finally disappearing altogether, leaving only widely spread chromatin. The spindle fibres disintegrate and the nuclear envelope and nucleolus re-form. Finally the cytoplasm divides in a process called cytokinesis.

Question 10

What is the importance of mitosis?

Answer 10

Mitosis is important in organisms as it produces daughter cells that are genetically identical to the parent cells.

Question 11

Why is it essential to make exact copies of existing cells?

Answer 11

• Growth:
  When two haploid cells fuse together to form a diploid cell, it has all the genetic information needed to form the new organism. If the new organism is to resemble its parents, all the cells that grow from this original cell must be genetically identical. Mitosis ensures that this happens.
• Repair:
  If cells are damaged or die it is important that the new cells produced have an identical structure and function to the ones that have been lost.
• Reproduction:
  Single-celled organisms divide by mitosis to give two new organisms. Each new organism is genetically identical to the parent organism.

Question 12

What is the cell cycle?

Answer 12

Only some cells in multicellular organisms retain the ability to divide. Those that do not divide continuously, but undergo a regular cycle of division separated by periods of cell growth. This is known as the cell cycle and has three stages.

Question 13

What are the three stages of the cell cycle?

Answer 13

1. interphase: which occupies most of the cell cycle, and is sometimes known as the resting phase because no division takes place, although it is a period of intense chemical activity
2. nuclear division: when the nucleus divides either into two (mitosis) or four (meiosis)
3. division of the cytoplasm (cytokinesis): which follows nuclear division and is the process by which the cytoplasm divides to produce two new cells (mitosis) or four new cells (meiosis)

Question 14

How long is a complete cell cycle?

Answer 14

The length of a complete cell cycle varies greatly amongst organisms. Typically, a mammalian cell takes about 24 hours to complete a cell cycle, of which about 90% is interphase.

Question 15

What is cancer?

Answer 15

Cancer is a group of diseases (around 200 in total) caused by a growth disorder of cells. It is the result of damage to the genes that regulate mitosis and the cell cycle. This leads to uncontrolled growth and division of cells. As a consequence, a group of abnormal cells, called a tumour, develops and constantly expands in size.

Question 16

Where do tumours develop?

Answer 16

Tumours can develop in any organ of the body, but are most commonly found in the lungs, prostate gland (male), breast and ovaries (female), large intestine, stomach, oesophagus and pancreas.

Question 17

When does a tumour become cancerous?

Answer 17

A tumour becomes cancerous if it changes from benign to malignant.

Benign tumours tend to grow slowly and do not spread. Malignant tumours can grow rapidly, invade and destroy nearby normal tissues, and spread throughout the body.

Question 18

Why do cells divide by mitosis?

Answer 18

Most cells divide by mitosis, either to increase the size of a tissue during development (growth) or to replace dead and worn out cells (repair).

Question 19

How is the rate of mitosis controlled?

Answer 19

The rate of mitosis can be affected by the environment of the cell and by growth factors. It is also controlled by two types of gene.

Question 20

How can a mutation lead to uncontrolled mitosis?

Answer 20

A mutation to one of these genes results in uncontrolled mitosis. The mutant cells so formed are usually structurally and functionally different from normal cells. Most mutated cells die.

However, any that survive are capable of dividing to form clones of themselves and forming tumours.

Question 21

What are malignant and benign tumours?

Answer 21

Malignant tumours grow rapidly, are less compact and are more likely to be life-threatening, as they spread quickly.

Benign tumours grow more slowly, are more compact and are less likely to be life-threatening, as they do not spread.

Question 22

How is cancer treated?

Answer 22

The treatment of cancer often involves killing dividing cells by blocking a part of the cell cycle. In this way the cell cycle is disrupted and cell division, and hence cancer growth, ceases.

Question 23

How do the drugs used to treat cancer (chemotherapy) usually disrupt the cell cycle?

Answer 23

• they prevent DNA from replicating

- they inhibit the metaphase stage of mitosis by interfering with spindle formation

Question 24

What are some problems with chemotherapy?

Answer 24

The problem with using these drugs is that they also disrupt the cell cycle of normal cells. However, the drugs are most effective against rapidly dividing cells. As cancer cells have a particularly fast rate of division, they are damaged to a greater degree than normal cells. Those normal body cells, such as hair-producing cells, that divide rapidly are also vulnerable to damage. This explains the hair loss frequently seen in patients undergoing cancer treatment.

Question 25

How do spindle fibres form?

Answer 25

The centrosomes grow spindles called microtubules made from tubulin (a protein). Some centrosomes have centrioles, but some do not. So although they may assist in making spindle fibres, they aren’t the only thing.

Question 26

What is the centromere?

Answer 26

The centromere is a complex of protein and holds the two sister chromatids together at the centre of a chromosome.

Question 27

What is the kinetochore?

Answer 27

The kinetochore is a complex of proteins associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach at both ends.

The kinetochore “eats” its way along the microtubule.

Question 28

What is a telomere?

Answer 28

Their job is to stop the ends of chromosomes from fraying or sticking to each other. Telomeres also play an important role in making sure our DNA gets copied properly when cells divide.