DDT.3

Question 1

Inherited gene mutations vs Acquired/Somatic gene mutations

Answer 1

Inherited gene mutations:
are passed from parent to child through the egg or sperm. These mutations are in every cell in the body.

Acquired (somatic) mutations:
These mutations are acquired at some point in the person’s life, and are more common than inherited mutations. This type of mutation occurs in one cell, and then is passed on to any new cells that are the offspring of that cell.

Question 2

Polymorphism

Answer 2

Presence of two or more alleles within a species; these may be harmful and result in unregulated cellular processes.

Question 3

Ames Test

Answer 3

A chemical is incubated first with a liver extract to allow any metabolic activation to occur; it then is added to several different bacterial cultures designed to detect specific types of mutations.

A positive result in the Ames test shows that a compound has the potential to be carcinogenic.

Question 4

Why do DNA repair enzymes not work with carcinogens?

Answer 4

Most DNA damage is repaired by DNA repair enzymes. However, carcinogens increase with the error rate of a mitotic cell. They may interfere with the normal repair mechanisms of a cell.

Question 5

DNA Lesions

Answer 5

Are sites of damage in the base-pairing or structure of DNA. There are 6 groups of lesions

Question 6

DNA Damage- Abasic Site

Answer 6

A base is missing from the DNA (note that the sugar-phosphate backbone is still intact, just the base is missing). This occurs due to a rise in temperature, a drop in pH, or alkylations on the base, that destabilize the N-glycosidic bonds.

Question 7

DNA Damage- Mismatch

Answer 7

These are caused by replication errors, such as tautomerization, or the spontaneous deamination of cytosime to uracil.

Question 8

DNA Damage- Modified Bases

Answer 8

These lesions are caused by changes to the bases themselves, such as the UV-induced creation of thymine dimers.

Question 9

DNA Damage- Single-stranded breaks

Answer 9

This lesion is a nick in the sugar-phosphate backbone of one strand. This is caused by peroxides, Cu++ion, oxygen radicals, or ionizing radiation.

Question 10

DNA Damage- Interstrand crosslinks

Answer 10

This is where there is an actual covalent linkage between the two strands. DNA replication cannot proceed past this point because helicase can’t melt apart the base-pairs for polymerase. Caused by things such as mitomycin C, cisplatins, and psoralens.

Question 11

DNA Damage- Double-stranded breaks

Answer 11

The most lethal sort of lesion, this is where both strand backbones are broken. This is typically caused by ionizing radiation.

Question 12

Viral DNA Damage

Answer 12

Oncovirus is a virus which can infect a cell and cause tumors.
During viral replication vDNA (viral DNA) can insert and interrupt host gene coding sequences. Cell cycle may be affected. Oncoviruses may upregulate cell cycle for increased viral replication.

An Onco- retrovirus which codes for oncogenes can insert reverse transcribed DNA (cDNA) into the host cell
The host cell will begin to transcribe and translate oncogenes as well as viral proteins.

Question 13

Chromosome Damage (translocation)

Answer 13

Chromosomal translocation can result in regions containing cell cycle genes being moved to another chromosome.
The new loci can result in an upregulation and overexpression of the gene.

Question 14

How many repair enzymes in bacteria and humans?

Answer 14

About 100 kinds of repair enzymes have been discovered in bacteria and 130 in humans

Question 15

What is the purpose of DNA polymerase?

Answer 15

During replication DNA polymerase proofreads each newly added nucleotide against the nucleotide template preventing harmful/lethal mutations (Error rate of only 1 in 109 or 1010 base pairs).

Question 16

General DNA repair mechanism (steps)

Answer 16

1. A nuclease to cut out the damaged DNA
2. A DNA polymerase to add the correct nucleotides
3. DNA ligase to close the breaks in the sugar-phosphate backbone

Question 17

DNA Repair Mechanism: Mismatch repair

Answer 17

Occurs during replication when the wrong nucleotide is placed into the DNA.

Immediately after DNA replication, the template strand has been methylated, but the newly synthesized strand is not methylated yet.

1. Mut proteins bind and cleave mismatched base pairs.
2. Cleavage occurs by exonuclease.
3. The gap is filled by DNA polymerase I.
4. DNA ligase close the breaks in the sugar-phosphate backbone.

Question 18

DNA Repair Mechanism: Nucleotide Excision Repair

Answer 18

1. Endonuclease cleaves open the strand for excision.
2. Proteins (Uvr-A, Uvr-B, Uvr-C) can remove damaged nucleotides (e.g. dimers formed by UV light).
3. The gap is filled by DNA polymerase I.
4. DNA ligase close the breaks in the sugar-phosphate backbone.

Question 19

DNA Repair Mechanism: Base Excision Repair

Answer 19

DNA bases may be modified bydeaminationoralkylation.

1. DNA glycosylase can recognize the AP site and remove its base to form an abasic (AP) site.
2. Then, the AP endonuclease removes the AP site and neighboring nucleotides.
3. The gap is filled by DNA polymerase I.
4. DNA ligase close the breaks in the sugar-phosphate backbone.

Question 20

Other DNA repair mechanisms: If there is no parental strand.

Answer 20

Homologous recombination (not common in humans): A homologous chromosome can align and form the template for the damaged chromosome.

Question 21

Other DNA repair mechanisms: if there is no parental strand or homologous chromosome.

Answer 21

Non-Homologous recombination: A non-homologous chromosome can align and form the template for the damaged chromosome.

Question 22

Other DNA repair mechanisms: Translesion repair.

Answer 22

(SOS response) use of Y-DNA polymerase. These fill in the blanks at random.

Question 23

Why do we need continuous repair?

Answer 23

Many cells that divide very slowly or not at all (e.g., liver and brain cells) rely on there genetic code for weeks or months. Therefore, continuous repair is required to prevent somatic disease.

If repair processes were 100 percent effective, chemicals and radiation would pose no threat to cellular DNA.

However; not all DNA lesions can be repaired effectively (e.g. continuous exposure to carcinogens like cigarette smoke increase the risk of cancer).

Question 24

Xeroderma pigmentosum

Answer 24

Xeroderma pigmentosum individuals have a defect in a nucleotide excision repair enzyme and are susceptible to many skin cancers at an early age because UV light DNA damage cannot be repaired.

Question 25

Oncogenes vs Tumor suppressor genes

Answer 25

Oncogenes: mutations of certain normal genes called proto-oncogenes (the healthy or good type). [Activated]

Tumour suppressor genes: normal genes that slow down cell division, repair DNA mistakes, or tell cells when to die. [Inactivated]

Question 26

Tumor suppressor genes

Answer 26

P53: cell cycle regulator, induces apoptosis.

BRCA 1 & BRCA 2: breast and ovarian cancer gene.

P16: melanoma gene.

Question 27

Proto-oncogenes

Answer 27

C-myc: codes for a transcription factor.

Ras: GTPase proteins.

Question 28

Synergistic mutagens

Answer 28

Oncogenic mutations in the transcription factor p53 and in the small GTPase protein Ras individually have limited effects on promoting cancer. But can cooperate to transform normal cells into cancer cells.

Question 29

Burkitt’s lymphoma

Answer 29

Burkitt’s lymphoma is a solid tumor ofB lymphocytes.

The tumor consists of sheets of a similar in size and shape lymphoid cells with high proliferative activity andapoptoticactivity.
Uncontrolled mitosis of this cell results in acloneof cancer cells, Burkitt’s lymphoma.

The genes for making antibodies are located on chromosomes14(the heavy [H] chains),2(kappa light chains), and22(lambda light chains).

In 90% of the cases of Burkitt’s lymphoma, a reciprocaltranslocation(designated t(8;14) has moved theproto-oncogenec-mycfrom its normal position on chromosome8to a location close to the enhancers of the antibody heavy chain genes on chromosome14.
c-mycnow finds itself in a region of elevated gene transcription,
this overproduction of the c-myc product (a transcription factor essential formitosisof mammalian cells) turns the lymphocyte cancerous.

[8 becomes shorter, 14 becomes longer]

These genes are expressed only in B lymphocytes because only B cells have the necessarytranscription factorsfor thepromotersandenhancersneeded to turn these antibody genes “on”.

Question 30

Why is the risk of translocations involving the heavy chain gene locus is especially high?

Answer 30

Because breaks in its DNA occur naturally during the synthesis of antibodies.

Question 31

Inheritance and cancer genetics

Answer 31

A hereditary mutation is a major factor in about 5%-10% of all cancers.
Certain things make it more likely that an abnormal gene is causing cancers in a family, such as:
- Many cases of an uncommon or rare type of cancer (like kidney cancer)
- Cancers occurring at younger ages than usual (like colon cancer in a 20 year old)
- More than one type of cancer in a single person (like a woman with both breast and ovarian cancer)
- Cancers occurring in both of a pair of organs (both eyes, both kidneys, both breasts)
- More than one childhood cancer in a set of siblings (like sarcoma in both a brother and a sister)

Question 32

Hereditary cancers: Colorectal cancer

Answer 32

Mutation of the APC gene.

Question 33

Hereditary cancers: Papillary renal cancer

Answer 33

Mutations in the gene called MET.