cancer and dna replication

Question 1

Draw and label a pie chart to show the relative amount of time spent in each phase of the cell cycle including the stages of interphase and mitosis, as well as cytokinesis.

Answer 1

Question 2

Explain how cyclins affect control the progression of a cell through the cell cycle.

Answer 2

1) Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches it threshold
2) Cyclins bind to enzymes called cyclin-dependent kinases
3) These kinases then become active and attach phosphate groups to other proteins in the cell
4) The attachment of phosphate triggers the other proteins to become active & carry out tasks (specific to one of the phases of the cell cycle)

Question 3

Outline the roles of the four cyclines involved in control of the cell cycle.

Answer 3

Cyclin A: activates DNA replication inside the nucleus in S phase
Cyclin B: promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis
Cyclin D: Triggers cells to move from the G0 to G1 and from G1 into S-phase
Cyclin E: prepares the cell for DNA replication in S phase

Question 4

Explain what is meant by ‘semi-conservative’ in terms of DNA replication

Answer 4

DNA replication is a semi-conservative process, because when a new double-stranded DNA molecule is formed, it contains one original strand and one newly synthesized strand

• the two strands of the double helix separate, each strand serving as a guide
• new strands are creating by adding nucleotides and linking them together
• because of complementary base pairing, the base sequence on the template strand determines the base sequence of the new strand
• the result is two DNA molecules, both composed of an original strand and a newly synthesized strand, and both identical to the original DNA molecule

Question 5

Outline the three hypotheses that had been proposed for the replication of DNA

Answer 5

1. Conservative Model – An entirely new molecule is synthesised from a DNA template
2. Dispersive Model – New molecules are made of segments of new and old DNA
3. Semi-Conservative Model – Each new molecule consists of one newly synthesised strand and one template strand (this is the currently accepted model)

Question 6

How did Meselson and Stahl’s results support the theory of semi-conservative replication of DNA?

Answer 6

• Meselson and Stahl were able to experimentally test the validity of these three models using radioactive isotopes of nitrogen
• Nitrogen is a key component of DNA and can exist as a heavier 15N or a lighter 14N
• DNA molecules were prepared using the heavier 15N and then induced to replicate in the presence of the lighter 14N
• DNA samples were then separated via centrifugation to determine the composition of DNA in the replicated molecules
• The results after two divisions supported the semi-conservative model of DNA replication
• After one division, DNA molecules were found to contain a mix of 15N and 14N, disproving the conservative model
• After two divisions, some molecules of DNA were found to consist solely of 14N, disproving the dispersive model

Question 7

Differentiate between the leading strand and the lagging strand

Answer 7

When replication begins, the two parent DNA strands are separated. One of these is called the leading strand, and it is replicated continuously in the 3’ to 5’ direction. The other strand is the lagging strand, and it is replicated discontinuously in short sections.

Question 8

Outline the role of helicase

Answer 8

• Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs
• This occurs at specific regions (origins of replication), creating a replication fork of two strands running in antiparallel directions
• The two separated polynucleotide strands will act as templates for the synthesis of new complementary strands

Question 9

Outline the role of DNA gyrase

Answer 9

• DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase
• It does this by relaxing positive supercoils (via negative supercoiling) that would otherwise form during the unwinding of DNA

Question 10

Outline the role of DNA primase

Answer 10

• DNA primase generates a short RNA primer (~10–15 nucleotides) on each of the template strands
• The RNA primer provides an initiation point for DNA polymerase III, which can extend a nucleotide chain but not start one

Question 11

Outline the role of DNA polymerase III

Answer 11

• Free nucleotides align opposite their complementary base partners (A = T ; G = C)
• DNA pol III attaches to the 3’-end of the primer and covalently joins the free nucleotides together in a 5’ → 3’ direction
• As DNA strands are antiparallel, DNA pol III moves in opposite directions on the two strands
• On the leading strand, DNA pol III is moving towards the replication fork and can synthesise continuously
• On the lagging strand, DNA pol III is moving away from the replication fork and synthesises in pieces (Okazaki fragments)

Question 12

Outline the role of DNA polymerase I

Answer 12

• As the lagging strand is synthesised in a series of short fragments, it has multiple RNA primers along its length
• DNA pol I removes the RNA primers from the lagging strand and replaces them with DNA nucleotides

Question 13

Outline the role of single stranded binding proteins

Answer 13

• SSB proteins bind to the DNA strands after they have been separated and prevent the strands from re-annealing
• These proteins also help to prevent the single stranded DNA from being digested by nucleases
• SSB proteins will be dislodged from the strand when a new complementary strand is synthesised by DNA polymerase III

Question 14

Outline the role of DNA primase

Answer 14

• DNA primase generates a short RNA primer (~10–15 nucleotides) on each of the template strands
• The RNA primer provides an initiation point for DNA polymerase III, which can extend a nucleotide chain but not start one

Question 15

Define Okazaki fragment

Answer 15

Okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication.

Question 16

Outline the role of DNA ligase

Answer 16

• DNA ligase joins the Okazaki fragments together to form a continuous strand
• It does this by covalently joining the sugar-phosphate backbones together with a phosphodiester bond

Question 17

What are the two processes required to produce a specific polypeptide using the base sequence of a gene?

Answer 17

1. Transcription
2. Translation

Question 18

List the stages of mitosis in order

Answer 18

(interphase occurs before prophase but is not considered a stage of mitosis)
1. prophase
2. metaphase
3. anaphase
4. telophase
(cytokinesis occurs after telophase but is not considered a stage of mitosis)

Question 19

Supercoiling

Answer 19

• repeated coiling of the DNA molecule to make the chromosome shorter and wider
• histones (protein) in eukaryote chromosomes help with supercoiling and enzymes are also involved

Question 20

Cyclins

Answer 20

• a group of proteins that is used to ensure that tasks are performed at the correct time and that the cell only moves to the next stage of the cycle when appropriate
• bind to enzymes called cyclin-dependent kinases which then become active and attach phosphate groups to other proteins in the cell
• the attachment of phosphate triggers the proteins to become active and carry out tasks specific to one of the phases of the cell cycle
• four main types of cyclin in human cells
• unless the cyclins reach a threshold concentration, the cell does not progress to the next stage of the cell cycle
• thus, cyclins control the cell cycle and ensure that cells divide when new cells are needed, but not at other times

Question 21

Define tumour

Answer 21

abnormal groups of cells that develop at any stage of life in any part of the body

Question 22

Differentiate between a benign tumour and a malignant tumour

Answer 22

• benign tumours are ones in which the cells adhere to each other and do not invade nearby tissues or move to other parts of the body; they are unlikley to cause much harm
• malignant tumours can become detached and move elsewhere in the body and develop into secondary tumours; they are likely to be life-threatening

Question 23

Cancer

Answer 23

diseases due to malignant tumours

Question 24

Define carcinogen

Answer 24

chemicals and agents that can cause cancer

Question 25

Define mutagen

Answer 25

agents that cause gene mutations

Question 26

Define metastasis

Answer 26

• movement of cells from a primary tumour to set up secondary tumours in other parts of the body

Question 27

Define oncogenes

Answer 27

• genes that can become cancer-causing after mutating
• normal cell oncogenes control cell cycle and cell division

Question 28

Explain the development of primary and secondary tumours with reference to: mutagens, oncogenes, and metastasis

Answer 28

• mutagens cause gene mutations in oncogenes
• mutation in oncogenes results in uncontrolled cell division
• uncontrolled cell division forms a tumour
• tumour cell divides repeatedly to form a primary tumour
• metastasis is the movement of cells from a primary tumour to set up secondary tumours in other parts of the body

Question 29

Evaluate the hypothesis that smoking causes cancer.

Answer 29

• there is a positive correlation between smoking cigarettes and death by cancer
• the more cigarettes smoked per day, the higher the death rate due to cancer
• the result of the survey shows increases in death rate due to cancers of the mouth, lungs, larynx, pharynx; since these are the body parts that come into contact with the smoke from the cigaratte, this supports the hypothesis that smoking causes cancer
• however, there is also an increase in death rate due to cancers of the esophagus, stomach, kidney, bladder, pancreas and cervix
• correlation does not equal causation; just because there is a found correlation between smoking and cancer does not mean that one causes the other
• however, it has been found that several substances in cigarette smoke are carcinogens, making it likely that smoking causes cancer

Question 30

Outline the five causes of lung cancer

Answer 30

• Tobacco smoke contains mutagenic chemicals and increase the incidence of lung cancer
• Passive smoking (non-smokers inhale tobacco smoke exhaled by smokers)
• Air pollution (diesel exhaust fumes, nitrogen oxides, smoke from burning coal, wood, or other organic matter)
• Radon gas (radioactive gas that leaks out of certain rocks and accumulates in badly ventilated buildings)
• Asbestos, silica and some other solids if dust or other particles of them are inhaled (construction sites or quarries, mines or factories)

Question 31

Outline the symptoms of lung cancer

Answer 31

• Difficulties with breathing
• Persistent coughing
• Coughing up blood
• Chest pain
• Loss of appetite
• Weight loss
• General fatigue

Question 32

define epidemiology

Answer 32

the branch of medicine which deals with the incidence, distribution, and possible control of diseases and other factors relating to health.

Question 33

Outline the discovery of cyclins including the role of serendipity

Answer 33

Serendipity is a chance event with a positive outcome. Tim Hunt was studying embryonic development when he observed that a protein would increase and decrease in concentration with each cycle of cell division. The discovery of cyclins was serendipitous because Dr. Hunt was not looking for cyclins when he discovered them

Question 34

CDK complexes in controlling the cell cycle

Answer 34

Cyclins bind to enzymes called cyclin-dependent kinases
These kinases then become active and attach phosphate groups to other proteins in the cell.
The attachment of phosphate triggers the other proteins to become active and carry out tasks (specific to one of the phases of the cell cycle).

Question 35

how to determine if a correlation coefficient is significant

Answer 35

The correlation coefficient can range in value from −1 to +1. The larger the absolute value of the coefficient, the stronger the relationship between the variables.

Question 36

role of complementary base pairing in dna replication

Answer 36

Complementary base pairing is very important in the conservation of the base sequence of DNA. This is because adenine always pairs up with thymine and guanine always pairs up with cytosine. As DNA replication is semi-conservative (one old strand an d one new strand make up the new DNA molecules), this complementary base pairing allows the two DNA molecules to be identical to each other as they have the same base sequence. The new strands formed are complementary to their template strands but also identical to the other template. Therefore, complementary base pairing has a big role in the conservation of the base sequence of DNA.