CONSEQUENCES OF MUTATION

Question 1

Humans appear different because of

Answer 1

normal genetic

variation

Question 2

Variation arises because of

Answer 2

• Mutation
• Other processes (genetic drift, founder effects,
  selection)

Question 3

where is DNA found

Answer 3

in nucleus and mitochondria

Question 4

where is RNA found

Answer 4

in the nucleolus and cytoplasm

Question 5

Central Dogma of Molecular

Biology

Answer 5

DNA= RNA= PROTEIN
replication=transcription= translation

Question 6

Eukaryotic gene structure

Answer 6

1. promotor region = CAAT and TATA box
2. start point of transcription is just after the TATA box
3. initiated codon for protein synthesis= AUG
4. TERMINATer region= UGA, UAA,UAG

Question 7

what is a mutation

Answer 7

Definition: a change in a genomic sequence

• Can be a single base or duplication of entire
  segments or chromosomes
• NOT ALWAYS PATHOGENIC (disease-causing)
• It’s a matter of how RARE it is

Question 8

possible consequences of mutation

Answer 8

1. Silent (no effect)
2. Beneficial (good effect)
3. Deleterious (bad effect)

Question 9

Exogenous (outside of the cell) causes of mutation

Answer 9

• Chemicals
• Radiation
• Pollutants

Question 10

Endogenous (inside of the cell) causes of mutation

Answer 10

• Spontaneous damage
• Replication and recombination errors
• Repair errors

Question 11

Mutation rate – how often do

Do mutations occur?

Answer 11

• In humans and other mammals, uncorrected errors (=
  mutations) occur at the rate of about 1 in every 50
  million (5 x 107) nucleotides added to the chain.
• But with 6 x 109 nucleotides in a human cell, that
  means that each new cell contains some 120 new
  mutations (EXCLUDING exogenous causes of
  mutation).
• Most of these mutations will be in non-coding regions

Question 12

types of mutation

Answer 12

• Point
• Silent
• Missense
• Nonsense
• Static
• Dynamic
• Insertions/Deletions
• In-frame and frameshift
• Splice-site
• Acquired
• Germline

Question 13

acquired mutation= somatic

Answer 13

1. occurs in non-germline tissues
2. can not be inherited
3. e.g mutation in tumor only

Question 14

germline mutation

Answer 14

1. present in the egg and sperm
2. can be inherited
3. cause cancer family syndrome
4. all cells are affected on the offspring

Question 15

static mutation

Answer 15

STATIC variants – do not change when passed to offspring

• Point mutations (single nucleotide substitutions)
• Deletion / Insertion mutations
• Splice site mutations

Question 16

dynamic mutation

Answer 16

DYNAMIC variants – have the potential to change when passed to
offspring
(e.g. triplet repeats)

Question 17

point mutation

Answer 17

• Single nucleotide substitutions (one base is changed
  to another) that occur in less than 1% of the studied
  population
• Reminder: if the substitution is seen in more than 1%
  of the population, it is referred to as a polymorphism
• Point mutations include
• Silent mutations
• Nonsense mutations
• Missense mutations

Question 18

silent mutation- point mutation- static mutation

Answer 18

1. Silent mutations do not change the amino acid coded for

Question 19

missense mutations

Answer 19

2. Missense mutations result in an amino acid change

Question 20

sickle cell disease

Answer 20

• The mutation causing sickle is the
  replacement of ‘A’ by ‘T’ at the
  17th nucleotide of the gene for the
  the beta chain of hemoglobin.

* The mutation changes the codon
GAG (for glutamic acid) to GTG
(which encodes valine). Thus the
6th amino acid in the chain
becomes valine instead of
glutamic acid.

• A classic example of a missense
  mutation.

Question 21

nonsense mutation

Answer 21

3. Nonsense mutations result in a stop codon instead of an amino
   acid.

Question 22

summary of point mutation

Answer 22

• 1. Silent mutation
• (no amino acid change in protein)
• 2. Missense mutation
• (results in an amino acid change to another amino acid)
• 3. Nonsense mutation
• (results in an amino acid change to a STOP codon)

Question 23

impact of silent mutation

Answer 23

• No effect on the amino acid sequence

* Can influence the kinetics of protein folding and RNA splicing

Question 24

impact of a missense mutation

Answer 24

the effect depends on:

• Nature of the amino acid substitution
• Chemistry (e.g. polar to non-polar?)
• Location (conserved or non-conserved region of the gene)
• Size (can the amino acid chain still fold correctly?)

Question 25

impact of nonsense mutation

Answer 25

• Conversion of a codon into a stop codon
• No protein (Nonsense Mediated Decay) or premature
  termination of protein with altered function (truncated)

Question 26

Insertions/Deletions (INDELS)

Answer 26

• Multiples of three → in-frame mutation
• If NOT multiples of three → frameshift
  mutations

Question 27

impact of deletion

Answer 27

• The entire gene eg. a globin gene in a-thalassemia
• Part of the gene eg. p.F508del mutation in CF
• Frameshift or in-frame?

Question 28

impact of insertion

Answer 28

• Insertion of 3 bp or multiples of 3 bp
• Extra amino acids in the protein
• Insertion ≠ 3 bp or ≠ to multiple of 3 bp results in a frameshift during translation
• Altered protein sequence 3’ of the mutation

Frameshift indels generally lead to:

• The possible introduction of a stop codon
• Altered or abolished protein function

Question 29

Splice site mutations

Answer 29

SD – splice
donor
SA – splice
acceptor

~15% of disease-causing mutations in humans affect RNA splicing

Question 30

How does the spliceosome know

where to cut?

Answer 30

• The splice donor (SD) site is a stretch of conserved
  sequence around the beginning of an intron. The first
  two nucleotides of most introns are “GT”
• The splice acceptor (SA) is a stretch of conserved
  sequence around the end of an intron. The last two
  nucleotides of most introns are “AG”
• This is known as the “GT-AG rule”

Question 31

impact of splice site mutation

Answer 31

• Prevention of correct splicing
• Inactivation of
  donor/acceptor splice site
  prevents splicing
• Exon skipping or intron
  retention – difficult to predict