Lecture 17 Causes of Mutations

Question 1

Defne a mutation

Answer 1

A Mutation is an alteration in the nucleotide sequence of a DNA molecule

Question 2

State the two ways in which DNA mutations can occur

Answer 2

1) Spontaneous mutations caused by errors in DNA replication
2) Can be caused by mutagens

Question 3

Define and explain Base tautomerism

Answer 3

Occurs when DNA polymerase is tricked due to tautomers.

Question 4

Define tautomer

Answer 4

Isomers of the nucleotide bases. Isomers have slightly different chemical structures.

Question 5

Define tautomeric shift

Answer 5

• Amino adenine isomerizes to form imino adenine.
• Usually A pairs with T but Iminio-adenine base pairs with C.
• So you get an AC base pair which is a mutation.
• Polymerase cannot distinguish between amino-adenine and imino-adenine.

Question 6

List the full names of the 4 bases

Answer 6

• Amino-adenine(AT)
• Keto-thymine (TA)
• Keto-guanine (GC)
• Amino-cytosine (CG)

Question 7

List the full names of the less common tautomers for the 4 DNA bases.

Answer 7

Imino Adenine (pairs with C)
Enol Thymine (pairs with G)
Enol Guanine (pairs with T)
Imino Cytosine (pairs with G)

Question 8

Define mutagens

Answer 8

Mutagens are chemical or environmental agents that cause changes in DNA molecules.

Question 9

How many important ways can mutagens cause mutations to the DNA

Answer 9

2 main pathways by which mutagens cause mutations (either chemical - act as base analogues or environmental - direct damage)

Question 10

State the two ways in which mutagens can cause mutations

Answer 10

Mutagens will either be chemical agents like a de-aminating agent or environmental agents like UV and heat

Question 11

State an important base Analog

Answer 11

5-bromouracil 5bu

5bU is an analog of thymine.

Question 12

How do base analogs cause DNA mutations

Answer 12

• Base analogs mimic the base pairs

- During replication these base analogs can get incoroporated into the DNA

Question 13

Give the name of an important example of a Base Analog

Answer 13

5 BromoUracil (5bU)

Question 14

Of which DNA base is 5-Bromo Uracil an analogue of, and which chemical group is different.

Answer 14

I-t is an analogue of Thymine.

- The Methyl group on Thymine is replaced by a Bromine.

Question 15

How does 5-Bromo Uracil differ from Thymine

Answer 15

The methyl group is replaced by a Bromine.

Question 16

Why does the tautomer 5-B-u not cause mutation?

Answer 16

• Keto-5-Bu will replace T
• Keto 5 Bromo Uracil will base pair with Adenine, as normal.
• Thus if incorporated into the DNA instead of Thymine, this is fine.
• Thus there is no change t

Question 17

Which tautomeric form of 5 Bromo Uracil exhibits mutagenic effects

Answer 17

Enol-5 Bromo Uracil

Question 18

State the names of the two forms of 5- Bromo Uracil

Answer 18

• Keto 5 Bromo Uracil

- Enol 5 Bromo Uracil

Question 19

Why does Enol 5 Bromo Uracil cause mutation while Keto Bromo Uracil does not.

Answer 19

Because Enol 5- Bromo Uracil base pairs with Guanine but Keto 5 Bromo Uracil will base pair with Adenine.

Question 20

Which base pair does Enol 5 Bromo Uracil base pair with

Answer 20

Guanine

Question 21

Which base pair does Keto 5 Bromo Uracil base pair with

Answer 21

Adenine.

Question 22

Nitrous acid and sodium buslphite are ….

Answer 22

Deaminating agents

Question 23

State an example of common chemical mutagen

Answer 23

Deaminating agents

Question 24

What happens when Adenine undergoes deamination

Answer 24

Produces Hypoxanthine.

Question 25

Which base pair does Hypoxanthine form

Answer 25

Hypoxanthine base pairs with cytosine.

Question 26

Apple Tart

Hypoxy Chocolate

Answer 26

Remember this mnemonic

Question 27

Deamination of Cytosine results in ….

Answer 27

Uracil.
Uracil pairs with Adenine.
results in alteration in nucleotide sequence from a G to A.

Question 28

Deamination of Guanine

Answer 28

Xanthine.

Question 29

What does Xanthine do

Answer 29

Blocks DNA replication

Question 30

Summarise the effect of de-amination on

i) Adenine
ii) Cytosine
iii) Guanine

Answer 30

i ) Adenine turns to hypo-xanthanine
ii ) Cytosine turns to Uracil
iii) Guanine turns to Xanthine

Question 31

Which bases are purine bases

Answer 31

Adenine and Guanine

Question 32

Which bases are pyrimidine bases

Answer 32

Cytosine and Thymine and Uracil

Question 33

How many rings in purines

Answer 33

2 ringed bases

Question 34

How many rings in pyrimidines

Answer 34

1 ring

Question 35

What is an alkylating agent

Answer 35

Alkylating agents add alkyl groups

Question 36

Name an example of an alkylating agent

Answer 36

Ethylmethane Sulphonate (EMS)

Question 37

What is Ethlymethane Sulphonate an example of

Answer 37

an Alkylating agent which is a mutagen

Question 38

Name an example of an intercalating agent

Answer 38

Ethidium bromide

Question 39

How does an intercalating agent work to cause mutations

Answer 39

Intercalating agents insert themselves between the base pairs.

Question 40

What does Ethidium bromide do

Answer 40

Inserts itself between base pairs.

Question 41

What does UV light do do DNA

Answer 41

UV radiation causes base dimerization. (eg causes two molecules of the same base to react in an addition reaction forming an adduct.

Question 42

What are the products of UV induced base dimerization of Thymine

Answer 42

• Cyclobutyl dimer

- B 6,4 photoproduct.

Question 43

What is the effect of heat on DNA

Answer 43

• heat causes hydrolysis of bases

- which causes a gap in the DNA.

Question 44

State the hardest type of DNA mutation to repair

Answer 44

When UV Light causes dimerization of Thymine to produce a 6,4 photoproduct.