Mutations

Question 1

What does SNPs stand for?

Answer 1

Single nucleotide polymorphism

Happens when single base substituted for another

Question 2

List the types of mutational changes that can occur (4 main)

Answer 2

Base substitution
Deletions
Insertions
Rearrangements
Splice site mutation

Question 3

What is a transitions mutation?

Answer 3

Transition- change of base but still same type of base
Eg purine–> purine
Or Pyrimidine–> Pyrimidine

Question 4

What is a transversion mutation? How does this differ from transition?

Answer 4

A purine base changes to a Pyrimidine or vice versa

Transition stays as the same type (A and G)

Question 5

Which is more common- transition or transversion mutations?

Answer 5

Transition (same type eg still purine)

Question 6

In brief, how can mutations come about?

Answer 6

Spontaneously or induced (due to mutagen)

Question 7

What is the difference between a missense and nonsense mutation?

Answer 7

Missense- results in an amino acid change

Nonsense- results in amino acid change to a stop codon

Question 8

What is a silent mutation? Do they have any effect?

Answer 8

A nucleotide change which causes no change in the amino acid it codes for
These are most often the 3rd nucleotide in a Triplet
- can affect RNA splicing if mutation occurs at a splice site

Question 9

What is a missense mutation? Does it have any effect?

Answer 9

A mutation that results in the substitution of one amino acid for another
Yes- could effect tertiary structure of protein

Question 10

What is a frame shift mutation? How do they come about?

Answer 10

Where 1 or 2 nucleotides have been deleted or inserted and so shifts the reading frame

Question 11

How do frame shift mutations cause problems?

Answer 11

• As its a frame shift every codon will be altered and read differently therefore every amino acid could be different
• Also often new stop codons are found- TF premature termination

Question 12

What is a PTC (premature termination codon)?

Answer 12

Where a frame shift mutation has occurred leading to the formation of a new stop codon

Question 13

What happens to PTC’s?

Answer 13

Degraded by nonsense mediated decay

TF little or no protein is produced as a result

Question 14

List 3 general causes of mutations

Answer 14

During DNA replication
Chemical mutagens
Radiation exposure

Question 15

What changes can occur during DNA replication which can lead to mutations? Describe them

Answer 15

Tautomeric shift- proton changes position briefly forming rare form which affects pairing properties
Strand slippage- on new strand- extra nucleotide added. On template- one less nucleotide

Question 16

List 3 ways chemical mutagen can cause mutations

Answer 16

Directly- nitrous acid changes amino to keto group
- EMS remove purine ring TF RNA polymerase adds in any random base (3/4 times its wrong)

Indirectly- molecule forces itself between bases TF they’re further away and can’t be read

Question 17

What is thymine dimer formation?

Answer 17

UV photons cause adjacent thymines to pair- breaking the DNA double strand shape

Can occur with any Pyrimidine base

Question 18

What are the two purine bases? How many ‘rings’ do they have?

Answer 18

Adenine
Guanine

TWO!!!

Question 19

What are the two Pyrimidine bases? How many ‘rings’ do they have?

Answer 19

Thymine
Cytosine

ONE!!

Question 20

List the ways homozygosity can occur (4)

Answer 20

Mitotic recombination
Point mutation
Deletion (of wild type/normal gene)
Loss of wild type chromosome (v. rare- TF only mutated is present)

Question 21

Who is more likely to develop cancerous cells- someone with or without one inherited mutation? Why?

Answer 21

Someone who already has 1 mutated allele
They are more likely to get the two mutated alleles as they only need one more, whereas someone without needs two mutated alleles before developing cancerous cells.

Question 22

What is the most common form of mutation?

Answer 22

Single base substitution- SNPs

Single nucleotide polymorphism

Question 23

List methods of DNA repair (4)

Say if they occur before or after replication

Answer 23

99%- DNA polymerase fixes error- before
Nucleotide mismatch repair- after
Excision repair- after
Double stranded break repair- after

Question 24

What happens in nucleotide mismatch repair?

Answer 24

An enzyme detects mismatched base in newly synthesised strand
Patch of DNA surrounding error is replaced
Erroneous base replaced with correct one

Question 25

What is the difference between mismatch repair and excision repair?

Answer 25

Similar mechanism to mismatch repair however it is carried out for errors that occurred due to external factors eg oxidation, alkylation etc
Mismatch repair is done for errors as a result of DNA replication

Question 26

How does excision repair work?

Answer 26

Detects erroneous/damaged base and cuts a short strand out (a few bases either side of the damaged base) and makes new bit of strand to replace it with

Question 27

What occurs in double strand break repair?

Answer 27

Both strands of DNA are broken and repaired

Question 28

What 6 characteristics do tumour cells have?

Answer 28

Divide independantly of signals
Divide indefinitely
Stimulate angiogenesis
Ignore anti-growth signals 
Avoid apoptosis
Invade tissues

Question 29

What is angiogenesis?

Answer 29

Formation of new BV’s

Question 30

What do BRCA1/2 genes do?

Answer 30

Code for proteins involved in detecting DNA damage and signalling in cell cycle checkpoints

Question 31

Define oncogenic

Answer 31

Cancer causing

Question 32

How are proto-oncogenes activated?

Answer 32

Often by a change in a key amino acid

Question 33

Name one way you can detect mutations and describe it

Answer 33

SSCP
Single stranded conformation polymorphism

Heat to denature, cool fast- form single strands and then can electrophoresis

Question 34

If a mutation is due to amplification of an exon, how can you detect this?

Answer 34

Multiplex ligation-dependant probe amplification

Probe binds to exon