Chapter 5: The Mitotic Cell Cycle

Question 1

How many chromosomes are present in human cells?

Answer 1

46 chromosomes

Question 2

What is a gene?

Answer 2

Each gene is one unit of inheritance, coding for one polypeptide that is involved in a specific aspect of the functioning of the organism.

Question 3

Describe the structure of a chromosome.

Answer 3

A chromosome is made of two identical structures called chromatids, joined together by a narrow region called the centromere. The centromere can be found anywhere along the length of the chromosome, but the position is characteristic for a particular chromosome.

Question 4

Each chromatid contains one —–

Answer 4

DNA molecule

Question 5

What is DNA?

Answer 5

Deoxyribonucleic acid is the molecule of inheritance and is made up of a series of genes.

Question 6

What is chromatin?

Answer 6

Chromatin is the combination of DNA wound around the outside of specific protein molecules. Chromosomes are made of chromatin.

Question 7

What is mitosis?

Answer 7

Mitosis is nuclear division that produces two genetically identical daughter nuclei, each containing the same number of chromosomes as the parent nucleus.

Question 8

What is the cell cycle?

Answer 8

The cell cycle is a regular sequence of events that takes place between one cell division and the next. It has three stages, namely interphase, nuclear division and cell division.

Question 9

What are the stages of interphase?

Answer 9

G1 phase
S phase
G2 phase

Question 10

What happens during G1 phase?

Answer 10

This is when the cells prepare for growth and DNA synthesis by producing RNA, enzymes and proteins. If there are insufficient growth factors, the cell will not divide or when a signal is not received.

Question 11

What happens during S phase?

Answer 11

When a signal is received during G1 that the cell should divide again, the DNA in the nucleus replicates so that each chromosome consists of two identical chromatids. It is a relatively short phase.

Question 12

What happens during the G2 phase?

Answer 12

The cell continues to grow and the new DNA is checked and errors are repaired. There is a sharp increase in tubulin to make the microtubules for the mitotic spindle.

Question 13

What are the four stages of mitosis?

Answer 13

Prophase, metaphase, anaphase and telophase.

Question 14

What happens in early prophase?

Answer 14

The nuclear envelope is still intact. The centrosomes replicated during interphase can be seen in cytoplasm. Chromosomes begin to become shorter and thicker and appear to have centromeres.

Question 15

What happens in late prophase?

Answer 15

The nuclear envelope breaks up into small vesicles. The chromosomes appear to contain two sister chromatids, each containing one DNA copy. The centrosomes begin to move towards opposite ends of nucleus where they form the mitotic spindle. The nucleolus ‘disappears’ and now forms part of several chromosomes.

Question 16

What happens in metaphase?

Answer 16

In metaphase, the centrosomes reach a pole and begin to organise the production of the microtubules. Chromosomes line up across the equator of the spindles formed from the protein microtubules and are attached by their centromeres to the spindle.

Question 17

What happens in anaphase?

Answer 17

In anaphase, the chromatids begin to move to either ends of the pole, centromeres first, with one of the sister chromatids on either end, pulled by the microtubules.

Question 18

What happens in telophase?

Answer 18

Cytokinesis occurs in telophase. This is when the cell begins to constrict along the centre between the two daughter nuclei formed. The spindles begin to break down. The nuclear envelope is visible around the two daughter nuclei. Nucleoli may form at this point and the chromatids, having reached the poles will replicate during interphase along with the centrosome.

Question 19

What is the role of kinetochores and when are they made? Where are they found?

Answer 19

Each metaphase chromosome has two kinetochores at its centromere, one on each chromatid. These are made of protein molecules which bind to the DNA in the chromatid and also bind to the protein microtubules. They are made during the S phase interphase.

Question 20

What happens in anaphase with reference to kinetochores?

Answer 20

In anaphase, the microtubules attached to a given kinetochore pull the kinetochore, with the rest of its chromatid dragging behind, towards the pole. This is achieved by shortening of the microtubules, both from the pole end and from the kinetochore end.

Question 21

What does the centrosome consist of? ref. proteins

Answer 21

The centrosome consists of a pair of centrioles surrounded by a large number of proteins. It is these proteins that control the production of the microtubules, not the centrioles.

Question 22

What are some biological significance examples of mitosis? (4)

Answer 22

1. Growth: The two daughter cells formed have the same number of chromosomes as the parent cell and are genetically identical, allowing for the growth of multicellular organisms from unicellular zygotes.
2. Replacement of cells and repair of tissues: Cells are constantly dying and are replaced by new cells.
3. Asexual reproduction: In unicellular organisms, cell division results in reproduction. In multicellular organisms, new individual produced bud off from parent.
4. Immune response: The cloning of lymphocytes

Question 23

What are spindle fibres?

Answer 23

Bundles of microtubules called spindle fibres extend from the kinetochores to the poles of the spindle during mitosis.

Question 24

What are telomeres?

Answer 24

They are the ends of chromosomes which contain repeated base sequences which prevents the loss of genes during cell division and to allow continued replication of a cell. The copying enzyme cannot run to the end of the DNA and complete the replication as it stops a little short of the end. Therefore, the main function of the telomeres is to prevent the loss of genes. The enzyme that adds the extra DNA (telomere) is known as telomerase.

Question 25

What are stem cells?

Answer 25

They are cells that can divide repeatedly by mitosis, and can differentiate into specialised cells or remain as stem cells.

Question 26

What are the three types of stem cells?

Answer 26

1. Totipotent: Cells that can divide repeatedly to form any cell in the body. e.g. zygote
2. Pluripotent: Embryonic stem cells that lead to the development of an embryo and later the adult. They are not specialised into the placenta.
3. Multipotent: Adult stem cells that are only able to produce a few types of cells. e.g. stem cells in bone marrow.

Question 27

What is cancer?

Answer 27

It is a mutation that occurs in the genes that control cell division, an oncogene, that results in uncontrollable mitosis. Cancerous cells divide repeatedly and form a tumour, which is an irregular mass of cells.

Question 28

What is a mutation?

Answer 28

A change in any gene is called a mutation.

Question 29

What is a malignant tumour?

Answer 29

These are tumours that spread through the body, invade other tissues and destroy them. These cells break off from their tumour and form secondary growth, known as metastasis.

Question 30

What happens to most mutated cells?

Answer 30

Most mutated cells are either affected in some way that results in their early death or are destroyed by the immune system.