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13 - Mutations Flashcards

1
Q

What is a mutation and what is mutagenesis?

A

Mutation - A change in the nucleotide sequence

Mutagenesis - Process of mutation generation

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2
Q

What is deanimation mutation?

A

Removal of an amino group from a molecule causing a change in base

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3
Q

What are some DNA level mutations and how do they occur?

A

- Exogenous sources (radiation, free radicals etc)

- Endogenous sources (transposable elements, replication defects)

- Spontaneously

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4
Q

Why does spontaneous deanimation always result in a mutation?

A
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5
Q

What are transposons?

A
  • Sit within chromosome
  • Greater length than one gene
  • Can move as discrete unit and insert themselves into another location (transposition)
  • Issue if inserted into genes as can cause non-functional polypeptide, inactive/activate gene

- Bigger the gene bigger the risk of transposition, e.g DMD

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6
Q

What is a SNiP?

A

Single nucleotide polymorphism

  • Where one nucleotide is different to the rest of the population
  • Anonymous SNP, coding SNP, non-coding SNP (not in gene)
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7
Q

What are micro mutations?

A
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8
Q

What are the two types of substitution?

A

Transition - Same base for same base

Transversion - Change to different base (pyramidine to purine)

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9
Q

What can a single nucleotide change result in?

A
  • No effect
  • Change polypeptide length
  • Change amount of product
  • Change gene product
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10
Q

What is the genetics behind sickle cell anaemia?

A
  • Autosomal recessive
  • Base substitution in codon 7 of HBB, changing 6th AA
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11
Q

What is a non-sense mutation?

A

Mutation causes a change in the stop codon so it is not recognised so polypeptide longer than should be

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12
Q

What is a missense mutation?

A

Mutation that results in a different amino acid being coded for

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13
Q

What is a synonymous mutation?

A

Substitution, same amino acid coded for. (could be due to wobble)

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14
Q

What is a frameshift mutation?

A

When there is a change in the number of nucleotides, not in the multiple of 3, so all the amino acids downstream are different

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15
Q

Why might a deletion of three bases not cause any affect, and why may it cause a drastic effect?

A
  • No frameshift so only one amino acid missing
  • If amino acid usually located in active site will cause non-functional protein but if not, not much difference to overall structure of protein
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16
Q

What is haemoglobin wayne and haemoglobin CS?

A
  • Normal alpha Hb: 141 aa
  • alpha wayne: 147 aa (deletion of A at 139 causing frameshift so stop codon not read)
  • alpha cs: 172aa (substitution in 142 stop codon so not read)

NON-SENSE MUTATIONS

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17
Q

How could a mutation lead to a change in the amount of a gene product?

A
  • Mutation in regulatory sequence
  • Alter promoter activity
  • Prevent mRNA splicing (splicing sites change)
  • Reduce mRNA stability (mutations in tails and caps)
  • Earlier start codon
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18
Q

What are haemoglobin lyon and leiden?

A
  • Leiden: Loss of Glu at 6
  • Lyon: Loss of Lys and Val at 17/18
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19
Q

What are the types of chromosome mutation?

A
  • Translocation (reciprocal/robertsonian)
  • Inversion (para/pericentric)
  • Duplication
  • Substitution
  • Deletion
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20
Q

What are the two types of chromosome inversion?

A

Paracentric: on one side of the centromere, within one arm

Pericentric: inversion occurs over the centromere

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21
Q

What happens during chromosome substitution (insertion)?

A

One part of a chromosome is moved from one chromosome to another

22
Q

What are the two types of protein translocation?

A

Reciprocal: Swapping of genetic material

Robertsonian: Breaking at the centromere on two acrocentric chromosomes. Lose the p arms, two q-arms fuse. Individual has 45 chromosomes

23
Q

Apart from the main chromosome mutations, what are three other structural mutations that can occur?

A

- Ring chromosomes (end break and fuse to ring)

- Isochromosomes (two p together, abnormal centromere split)

- Marker chromosomes (small fragements of chromosomes)

24
Q

What is polyploidy?

A

- more than two sets of homologous chromosomes (3n=69)

  • usually due to polyspermy
  • triploidy occurs in 2-3% of pregnancies and causes 15% of miscarriages
25
What is aneuploidy?
- Abnormal number of chromosomes - Due to non-disjunction in meiosis - Not always lethal like polyploidy - Doesn't involve more than two sets of homologous chromosomes
26
What are some examples of monosomy and trisomy?
**_Mono:_** _-_Turners Syndrome 45,X **_Tri:_** **-** Down Syndrome 47,+21 - Patau 47,+13 - Edwards 47,+18 - Klinefelter Syndrome 47,XXY
27
Does chromosome non-disjunction only occur in some chromosomes?
No, all of them. Only some are visible as they are the only ones viable for life, rest can't live
28
What is Turner's syndrome?
- Lack of X chromosome - Only affects female - Only known monosomy
29
What is Patau syndrome?
- Trisomy 13 - Mostly miscarry/still birth. 90% die in first year
30
What is Edward's syndrome?
- Trisomy 18 - Life span: 5-15 days - Maternal meiosis II error
31
What is Klinefelter syndrome?
- Trisomy XXY - Male with two X's - Infertile
32
What is Down syndrome?
- Trisomy 21 - Early alzheimers, hypotonia, heart defects, facial features distinct, increased risk of leukemia **- Critical region:** 21q22 (don't have to have full chromosome duplication to have downs)
33
What is mosaicism?
- When chromosome non-disjunction occurs in mitosis. Results in two or more cell lines - Occurs after first mitotic division and can save trisomy, reducing the number of cells in the body with trisomy
34
What are balanced and unbalance chromosomal abnormalities?
**Balanced -** No genetic information is lost or gained and the phenotype is normal **Unbalanced -** Abnormal chromosome has led to abnormal phenotype
35
What is balanced translocation?
**Reciprocal Translocation** **-** 2 chromosomes clean broken and material is swapped. - Leads to issues in meiosis as quadrivalents are formed - During meiosis balanced and unbalanced gametes can be produced depending on segregation
36
When can reciprocal translocation be an issue?
- Philadelphia Chromosome - Fusion of BCR-ABL gene on chromosome 22, kinase signally gene constantly on so cell uncontrollably divides - Associated with leukemia
37
What is the issue with balanced translocation?
- In meiosis a quadrivalent is formed rather than bivalent so different types of segregation * 3:1 non-disjunction * Alternate segregation * (Non-homologous) Adjacent 1 segregation * (Homologous) Adjacent 2 segregation
38
What are the consequences of different types of segregation when meiosis occurs with chromosomes that have undergone translocation?
39
If a couple with one being a carrier for reciprocal translocation, how would you work out the risk that they would have a child with an imbalance?
- Analyse couples genes to see which chromosomes have been affected by translocation - Look up if anyone else has had the same translocation before and quote the risks - Work out based on the knowledge known
40
What happens during meiosis with a person with a Robertsonian translocation and what happens to their offspring?
- Person has 45 chromosomes - Trivalent formed at meiosis - Leads to risk of aneuploidy - Females with 45 at higher risk than males (oogenesis pauses in meiosis and trivalent very unstable)
41
Why can people appear phenotypically normal but be infertile or have abnormal children?
- They have balanced translocation which is not picked up on microarray - Could have two 21's fused so end up with trisomy and downs - Infertile as odd number of chromosomes
42
What is cytogenetics and why is it carried out?
Study of the genetic make up of cells through visualisation and analysis of chromosomes **- Better clinical management** (e.g hormones for XXY) **- Pre-natal diagnosis** **- Accurate diagnosis** (identify syndrome, account for pregnancy loss) **- Assess future reproductive risks** (affected baby or risk of unalive birth)
43
How can cytogenetics be carried out?
- Karyotyping - FISH - Microarray hybridisation - DNA sequencing
44
What are some referral reasons for cytogenetic testing?
- Delayed sexual development - Pregnancy loss - Prenatal diagnosis - Birth defects - Infertility - Leukemia - Tumours - Prognostic info for specific translocations
45
What is a protooncogene and an oncogene?
**Protooncogene -** Gene that has the potential to become an oncogene if there are mutations **Oncogene -** Gene that has the potential to cause cancer
46
Can a mutation in bases very upstream cause a change in the gene product?
Yes! Could affect a regulatory sequence that normally stimulates an effector to start transcription, therefore more or less gene expression
47
What is a gain of function mutation?
A mutation that leads to a new or enhanced function of a protein. Almost always dominant
48
What is nitrous acid and how is it formed?
- Sodium nitrate is a food additive in smoked meats - When gets into stomach it reacts with HCl to form nitrous acid - Nitrous acid is a mutagen which causes deanimation of C --\> U
49
What is the relationship between sickle-cell trait and malaria?
- If you have sickle cell trait normally immune to malaria - Malaria is caused by a parasite that normally replicates within red blood cells - If red blood cells lysing then parasite killed before it can get to replicate
50
Can you test for Down Syndrome whilst a mother is pregnant?
Yes NIPT. Can take a blood test from the mother at 9-10 weeks as their blood will contain some fetal DNA, not all the fetal chromosomes but some

MCBG (20 decks)

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