The Mitotic Cell Cycle

Question 1

How is growth important?

Answer 1

1. The two daughter cells formed have the same number of chromosomes as the parent cell and are genetically identical (clones)
2. This allows growth of multicellular organisms from unicellular zygotes
3. Growth may occur over the entire body, as in animals or be confined to certain regions, as in the meristems (growing points) of plants

Question 2

How is replacement of cells and repair of tissues important?

Answer 2

1. This is possible using mitosis followed by cell division
2. Cells are constantly dying and being replaced by identical cells
3. In the human body’s, for example, cell replacement is particularly rapid in the skin and in the linging of the gut
4. Some animals are able to regenerate whole parts of the body; for example, starfish can regenerate new arms

Question 3

How is asexual reproduction important?

Answer 3

1. Mitosis is the base of asexual reproduction, the production of new individuals of a species by a single parent organism
2. The offspring are genetically identical to the parents
3. Asexual reproduction can take many forms
4. For a unicellular organisms such as Amoeba, cell division inevitably results in reproduction
5. For multicellular organisms, new individuals may be produced which bud off from the parent in various ways
6. Budding is particularly common in plants; it is mist commonly a form of vegetative propagation in which a bud on part of the stem simply grows a new plant. The new plant eventually becomes detached for the parent and lives independently
7. The bud may be part of the stem of an overwintering structure such as a bulb or tuber
8. The ability to generate whole organisms from single cells or small groups of cells is important in biotechnology and genetic engineering and is the basis of cloning

Question 4

How is the immune response important?

Answer 4

1. The cloning of B- and T-lymphocytes during the immune response is dependent on mitosis

Question 5

What are the ends of chromosomes ‘sealed’ by? Describe them

Answer 5

1. The ends of chromosomes are ‘sealed’ by structures called telomeres
2. These are made of DNA with short base sequences that are repeated many times (multiple repeat sequences)
3. In telomeres one strand of the DNA is rich in the base guanine (G) and the other stand is rich in the complementary base cytosine (C)

Question 6

What is the function of telomeres?

Answer 6

1. Their main function is to ensure that when the DNA is replicated, the ends of the molecule are included in the replication and not left out
2. The copying enzyme cannot run to the end of the DNA and complete the replication - it stops a little short of the end
3. If part of the DNA is not copied, that piece of information is lost
4. At each subsequent division, another small section of information would be lost
5. Eventually the loss of vital genes would result in cell death
   - Prevent loss of genes

Question 7

How do you make DNA a bit longer?

Answer 7

1. The solution is to make the DNA a bit longer, by adding some more bases
2. They have no useful information but allow the copying enzyme to complete copying the meaningful DNA
3. As long as extra bases are added during each cell cycle, no vital information will be lost and the cell will be able to continue dividing successful
4. The enzyme that performs this role is called telomeres
5. The extra DNA that it adds is the telomere
6. Therefore the main function of telomeres is therefore to prevent the loss of genes during cell division and to allow continue replication of a cell

Question 8

What happens when cells do not ‘top up’ their telomeres?

Answer 8

1. It has been shown that some cells do not ‘top up’ their telomeres at each division
2. These tend to be fully differentiated (specialised) cells
3. With each division their telomeres get a little shorter until the vital DNA is no longer protected and the cell dies
4. This could be one of the mechanisms of ageing, by which we grow old and die and this of course suggests that by somehow preventing the loss of telomere we might be able to slow down or even prevent the process of ageing

Question 9

What is a stem cell?

Answer 9

• A stem cell is a cell that can divide an unlimited number of times (by mitosis)
• When it divides, each new cell has the potential to remain a stem cell or to develop (differentiate) into a specialised cell such as a blood cell or muscle cell

Question 10

What does potency and totipotent mean?

Answer 10

1. The extent of the power of a stem cell to produce different cell types is a variable and referred to as its potency
2. Stem cells can produce any type of cell described as totipotent

Question 11

What is an example of a totipotent and pluripotent ?

Answer 11

1. The zygote formed by the fusion of a sperm with an egg at fertilisation is totipotent, as are the cells up to the 16 cell stage of development in humans
2. After that, some cells become specialised to form the placenta, while other lose this ability but can for all the cells that will lead to the development of the embryo and later the adult
3. These embryonic stem cells are described as pluripotent

Question 12

What are multipoint stem cells?

Answer 12

1. As tissues, organs and systems develop, cells become more and more specialised
2. There are more than 200 different cell types in an adult human body
3. The more ‘committed’ cells become to particular roles, the more they lose their ability to divide until, in the adult most cells do not divide
4. However, for growth and repair it is essential that small populations of stem cells remain which can produce new cells
5. Adult stem cells have already lost some of the potency associated with embryonic stem cells and are no longer pluripotent
6. They are only able to produce a few types of cell and may be described as multipoint

Question 13

What is an example of a multipoint stem cell?

Answer 13

1. For example, the stem cells found in bone marrow are multipoint
2. They can replicate any number of times, but can produce only blood cells such as red blood cells, monocytes, neutrophils and lymphocytes
3. Mature blood cells have relatively short life span, so the hesitance of these stem cells is essential
4. Fore example, around 250 thousand million (250 billion) red blood cell and 20 billion white blood cells are lost and must be replaced each day

Question 14

What is stem cell therapy?

Answer 14

1. In the adult stem cells are found throughout the body, for example in the bone marrow, skin, gut, heart and brain
2. Research into stem cells has opened up some excited medical applications
3. Stem cell therapy is the introduction of new adult stem cells into damaged tissue to create disease or injury
4. Bone marrow transplantation is the only form of this therapy that has progressed beyond the experimental stage into routine medical practice
   - But in the future it is hoped to be able to treat conditions like diabetes, muscle and nerve damage, and brain disorders such as Parkinson’s and Huntington’s diseases. Experiments with growing are tissue, or even organs form isolated stem cells in the laboratory have also been conducted

Question 15

How is cancer thought to start?

Answer 15

1. Cancer are thought to start when changes occur in the genes that control cell division. A change in any gene is called a mutation
2. The particular term for a muted gene that causes cancer is an oncogene
3. Mutations are not unusual events, and most of the time do not lead to cancer
4. Most mutated cells are either affected some way that results in their early death or are destroyed but the body’s immune system
5. Since most cells can be replaced mutation usually has no harmful effect on the body

Question 16

What is different about cancerous cells?

Answer 16

1. Cancerous cells escape both being destroyed by the immune system and early death , so although the mutation may originally occur in only one cell it is passed on to all that cell’s descendants
2. By the time that it is detected, a typical tumour usually contains about a thousand million cancerous cells

Question 17

What is a carcinogen?

Answer 17

Any agent that causes cancer is called a carcinogen and is described as carcinogenic

Question 18

What is a benign tumour and what is a malignant tumour?

Answer 18

1. Some tumours do not spread from their site of origin, and are known as benign tumours
2. Tumours that spread through the body, invade other tissues and destroy them cause cancer, and these are known as malignant tumours
   - Malignant tumours interfere with the normal functioning of the area where they have started to grow, they may block intestines, lungs or blood vessels
   - Cells can break off and spread through the blood and lymphatic system to other parts of the body to form secondary growths and the spread of cancers in this way is called metastaiss
   - It is the most dangerous characteristic of cancer since it can be very hard to find secondary cancers and remove them

Question 19

Describe Early prophase

Answer 19

1. There is the cytoplasm, cell surface membrane, nucleolus
2. An intact nuclear envelope
3. Centrome with attached kinetochores
4. Centrosomes replicated just before prophase
5. Chromosomes start to appear as the chromatin coils ups becoming shorter and thicker; they are thick enough to become visible when stained

Question 20

Describe Late Prophase

Answer 20

1. Nuclear envelope ‘disappears’ (it breaks up into mall vesicles which are not visible with a light microscope)
2. Nucleolus ‘disappears’ (from part of several chromosomes)
3. Chromosomes are seen to contain of two identical chromatids; each chromatid contains one DNA molecule
4. There are centromeres
5. Centrosome moving to opposite ends of nucleus where they from the poles of the spindle
   - At the end of prophase a spindle is formed

Question 21

Describe metaphase

Answer 21

1. Each centrosome reaches a pole; centrosome help to organise production of the spindle microtubules
2. Spindle (made of microtubules) is present
3. Chromosomes line up across the equator of the spindle; they are attached by their centromeres to the spindle

Question 22

Describe Anaphase

Answer 22

1. Chromatids move to opposite poles, centromeres first, pulled by the microtubules

Question 23

Describe telophase

Answer 23

1. Nucleolus and nucelar envelope re-froming
2. Chromatids have now reached the poles of the spindle; they will now uncoil again (each chromatid contains one DNA molecule, which will replicate itself during interphase before the next division)
3. Remains of a spindle which is breaking down
4. Cytokinesis, and this is the division of the cytoplasm and cell into two by constriction from edges of the cell
5. Cell surface membrane

Question 24

What does the number of chromosomes mean?

Answer 24

The number of chromosomes is characteristic of the species, for example in human cells there are 46 chromosomes and in fruit fly cells ether are only eight

Question 25

Describe the structure of a chromosome

Answer 25

1. The chromosomes in a double structure, is made of two identical structures called chromatids joined together
2. Each chromatid contains one of these DNA copies, and the two chromatids are held together by a narrow region called the centromere, forming a chromosome
   - The centromere can be found anywhere along the length of the chromosome, but the position is characteristic for. a particular chromosome

Question 26

What happens in interphase?

Answer 26

Each DNA molecule makes an identical copy of itself

Question 27

What does each chromatid contain?

Answer 27

1. Each chromatid contains one DNA molecule
2. The fact that the two DNA molecules in sister chromatids, and hence their genes, are identical is the key to precise nuclear division
3. When cells divide, one chromatid goes into one daughter cell and one goes into the other daughter cells making the daughter cells genetically identical
   - The combination of DNA and proteins is called chromatin
   - Chromsomes are made of chromatin

Question 28

Why is DNA coiled?

Answer 28

1. So much information is stored in DNA that it needs to be a very long molecule, although only 2nm wide, the total length of DNA in the 46 chromosomes of an adult human cell is about 1.8 meters, and this has to be packed into a nucleus which is only 6mumeters in diameter
2. In order to prevent the DNA getting tangled up into knots a precise scaffolding made of protein molecules is used
3. The DNA is wound around the outside of these protein molecules
4. Chemically speaking, most of the proteins are basic and are of a type know as histones
5. Because they are basic, they can interact easily with DNA which is acidic

Question 29

How is DNA coiled?

Answer 29

1. Coils can then be coiled up to form ‘supercoils’ and this is called a nucleosome
2. The nucleosome is cylindrical in shape and it is made up of 8 histone molecules
3. DNA is wound around the outside of the cylinder before liking to the next nucleosome
4. The DNA between the nucleosome
   is also held in place by a histone molecule

Question 30

What are the different sages of the cell cycle?

Answer 30

1. Nucelar Division by mitosis
2. Interphase:
   - G1
   - S Phase (DNA replication)
   - G2
   - Therefore interphase consists of G1, S and G2

Question 31

What happens during interphase?

Answer 31

• The cell grows to its normal rise after cell division and carries out its normal functions , synthesising many substances, especially proteins in the process
• At some point during interphase, a signal may be received that the cell should divide again
• If this happens, the DNA in the nucleus replicates so that each chromosome consists of two identical chromatids
• This phase of the cell cycle is called the S phase and S stands for synthesis (of DNA)
• This is a relatively short phase

Question 32

What is the phase after S phase and before cell division called?

Answer 32

• G2 Phase (G for growth)
• During G2, the cell continues to grow and new DNA is checked and any errors are usually repaired
• Preparations are also made to begin the process of division
• For example, there is a sharp increase in production of the protein tubulin which is needed to make microtubules for the mitotic spindle

Question 33

What is the phase after cell division and before the S phase?

Answer 33

• G1 Phase
• During this phase cells make the RNA, enzymes and other proteins need for growth
• At the end of G1, the cell becomes committed to dividing or not dividing

Question 34

What follows after interphase?

Answer 34

• Nucelar division follow interphase
• This may be referred to as the M phase (M for mitosis)
• Growth stops temporarily during mitosis
• After the M phase, when the nucleus has divided into two, the whole clue divides to create two genetically identical cells

Question 35

What is the length of the cell cycle?

Answer 35

1. The length of the cell cycle is variable, depending on environmental conditions and cell type
2. On average, root tip cells of onions divide once every 20 hours; epithelial cells int he human intestine very 1o hours
3. In animal cells, cell division involves construction of the cytoplasm, between two nuclei, a process called cytokinesis. In plant cells, it involves the formation of a new cell wall between the two new nuclei

Question 36

What are the four stages of mitosis?

Answer 36

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Question 37

What are centromeres?

Answer 37

1. The centromere is needed for the separation of chromatids during mitosis
2. It is visible as a constriction and is the site of attachment of spindle microtubules
3. Each metaphase chromosome has two kinetochores at its centromeres, one on each chromatid
4. These are made of protein molecules which bind specifically to the DNA in the centromere and also bind to microtubules
5. Bundles of microtubules called spindle fibres extend from the kinetochores to the poles of the spindle during mitosis
6. Construction of kinetochores begins before nuclear division starts (during the S phase of the cell cycle) and they are lost again afterwards

Question 38

What happens to the microtubules?

Answer 38

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

Question 39

What happens at each pole?

Answer 39

1. The poles of the spindle are where the centrosomes are located, one at each pole. As noted before the centrosome is an organelle found in animal cells that a cis as the microtubule organising centre for construction of the spindle
2. Each centrosomes consists of a pair of centrioles surrounded by a large number of proteins
3. It is these proteins that control production of the microtubules, not the centrioles
4. Plant mitosis occurs without centrosomes
   - About 60-70% of the spindle microtubules that extend from the centrosome are not attached to kinetochores, but have free ends
   - Some of these are microtubules that have not found kinetochores to attach to, since this is a random process
   - Some may be involved in chromatid movement in ways other than that described