Session 3: Mutations

Question 1

What are the three major types of mutation?

Answer 1

Point mutation, Chromosomal mutation, Copy number variation

Question 2

Example of condition caused by point mutation (substitution)?

Answer 2

Sickle cell anemia

Question 3

Example of condition caused by point mutation (deletion)?

Answer 3

Cystic fibrosis

Question 4

Example of expanding trinucleotide repeat conditions?

Answer 4

Fragile X syndrome, Huntington’s disease

Question 5

What is the most common type of point mutation?

Answer 5

Substitution

Question 6

Point mutations are the most ___ type of mutation.

Answer 6

common

Question 7

What are the two major types of point mutation?

Answer 7

1) Transition = a purine (adenine/guanine) is substituted with another purine (or one pyrimidine substituted by another pyrimidine)

2) Transversion = a purine is substituted for a pyrimidine (cytosine/thymine/uracil)

Question 8

What are purines?

Answer 8

PU-AG (purine - adenine and guanine)

Question 9

What are pyrimidines?

Answer 9

(pyrimidine - cytosine, thymine, uracil)

Question 10

Point mutations that occur in DNA protein-encoding sequences can have three effects. What are they?

Answer 10

1) Silent point mutation in protein-encoding sequence: Generation of codon for same amino acid (redundancy)

2) Missense point mutation in protein-encoding sequence: Generation of codon that specifies DIFFERENT amino acid (conservative/non-conservative)

3) Nonsense point mutation in protein-encoding sequence: Generation of STOP codon

Question 11

The K-Ras proto-oncogene is activated by a ___ mutation.

Answer 11

Missense

Question 12

K-Ras G12V details.

Answer 12

• **Missense mutation **
• GGC codon → GTC codon
• Results in **change of amino acid **
• Gly → Val at position 12

Question 13

HbS mutant protein in sickle cell anemia is the result of what type of point substitution mutation?

Answer 13

• Missense mutation
• GAG codon → GTG codon
• Results in change of amino acid in beta-globin gene
• Glutamate → Valine at position 6

Question 14

Insertion or deletion point mutations can be small or large. What can the insertion or deletion of just one base pair lead to?

Answer 14

Frameshift mutation

Question 15

Cystic fibrosis (most common) point mutation?

Answer 15

Deletion

Question 16

Do point mutations ONLY occur in the protein-encoding regions? True or false?

Answer 16

False

Point mutations can also occur in promoter/enhancer site, termination signal, splice donor/acceptor site, ribosome binding site.

Question 17

What are enhancers?

Answer 17

Regulatory elements that specify where and when particular genes are expressed.

Question 18

The ___ gene promoter mutations can be observed in ~30% of primary melanomas.

Answer 18

TERT gene promoter mutation (point mutation)

Question 19

Beta-thalassemia is caused by ___-site mutations (point mutations) found in the genomes of affected patients.

Answer 19

Splice-site mutations

Question 20

Phenylketonuria (PKU) is caused by a single-base mutation in the ___ ___ site of intron 12.

Answer 20

splice donor site

Question 21

What are three causes of mutations in DNA?

Answer 21

1) Errors in DNA replication
2) Chemical damage to DNA = mutagens
3) Radiation = ionizing and UV

Question 22

What is a tautomeric shift?

Answer 22

Error in DNA replication leading to mutation

Tautomeric shifts that modify the pairing of nucleotides can result in base substitutions and mutations.

Question 23

What is slipped-strand mispairing?

Answer 23

Error in DNA replication leading to frameshift mutation

Misalignment of DNA strands during replication of repeated DNA strands leading to genetic rearrangements.

Question 24

Two examples of DNA replication error leading to mutation?

Answer 24

1) Tautomeric shift
2) Replication slippage or slipped-strand mispairing

Question 25

What is a mutagen?

Answer 25

Physical or chemical agent that permanently changes DNA and thus increases the frequency of mutations above natural background level.

Question 26

Give example of two chemical carcinogen mutagens found in tobacco products.

Answer 26

**1) Tobacco-specific nitrosamines (TSNAs) ** 2) Polycyclic aromatic hydrocarbons (PAHs) Cause bulky adducts on DNA

Question 27

How do carcinogens such as TSNAs and PAHs lead to cancer?

Answer 27

1) TSNAs and PAHs cause bulky DNA adducts 2) If not repaired - these DNA adducts lead to mutations 3) Accumulation of mutations lead to cancer

Question 28

Example of another chemical mutagen that can mutate DNA?

Answer 28

Alkylating agents

Question 29

How do alkylating agents exert their mutagen action on DNA?

Answer 29

Alkylating agents work by adding an **alkyl group to the guanine base of DNA** ## Footnote Induce cross-linking between strands of DNA and loss of basic component (purine) or the breaking of the nucleic acid.

Question 30

Name some examples of alkylating agents.

Answer 30

- Nitrogen mustards (chlorambucil and cyclophosphamide) - Cisplatin (anticancer treatments) - Triazines (dacarbazine)

Question 31

What types of radiation can be harmful to DNA?

Answer 31

1) Ionizing radiation (alpha, beta, gamma, neutrons and x-rays) 2) UV radiation

Question 32

What is the direct and indirect effect of ionizing radiation on DNA?

Answer 32

1) **Direct effect** = **double-strand breaks (DSBs), single-strand breaks (SSBs)** - leading to cell death 2) **Indirect effect** = **free radicals & ROS production** which leads to DSBs and SSBs - leading to cell death

Question 33

How does UV radiation harm DNA?

Answer 33

**Formation of pyrimidine dimers** ## Footnote These dimers distort DNA structure - block transcription or replication past the site of damage.

Question 34

What is base-excision repair (BER)?

Answer 34

**Corrects small lesions** that do **not significantly distort DNA helix structure** ## Footnote Damage from oxidation, deamination and alkylation.

Question 35

What is nucleotide-excision repair (NER)?

Answer 35

Recognizes wide variety of damaged bases that DO distort DNA molecule, including UV-induced pyrimidine dimers and bulky groups added to DNA bases as a result of reaction of many carcinogens with DNA ## Footnote Damage from UV radiation and chemical mutagens.

Question 36

What is mismatch repair (MMR)?

Answer 36

Correction of mismatched bases that are incorporated during DNA replication ## Footnote Many are fixed by proofreading ability of DNA polymerase. The ones that are missed are subject to later correction by the mismatch repair system.

Question 37

Familial breast cancer makes up 5-10% of breast cancer. It is associated with the loss of ___ and ___ function.

Answer 37

BRCA1 and BRCA2

Question 38

What other forms of cancer are BRCA1 and BRCA2 mutations ALSO associated with?

Answer 38

Ovarian, Prostate

Question 39

Name two methods which are used for BRCA testing.

Answer 39

1) Next-generation sequencing (NGS) 2) Long-range PCR

Question 40

Name a method used for BRCA testing when the BRCA mutation is NOT known (no proband).

Answer 40

Next-generation sequencing (NGS)

Question 41

Name a method used for BRCA testing when the BRCA mutation is KNOWN (proband).

Answer 41

Long-range PCR