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mutation and genetic variations Flashcards

1
Q

define gene

A

a DNA segment with a nucleotide sequence encoding an RNA product that is either directly functional or encodes protein

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2
Q

define allele

A

one of at least 2 possible DNA sequences at a locus. wild-type is the normal common version & the other alleles are mutants

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3
Q

define locus

A

the location of a gene on a chromosome

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4
Q

define phenotype

A

describes the outward appearance of an organism for a given characteristic. results from interactions between the genotype & external environment

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5
Q

define genotype

A

describes a certain set of alleles at a locus

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6
Q

define homozygous

A

two identical alleles at a given locus

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7
Q

define heterozygous

A

two different alleles at a given locus

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8
Q

define diploid

A

46 chromosomes/23 pairs –> from father & mother in somatic cells

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9
Q

define haploid

A

23 chromosomes in sex cells

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10
Q

how can mutations occur

A
  • DNA replication via homologus recombination during meiosis
  • environmental exposure
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11
Q

define euploid

A

a somatic cell that contains 46 chromosomes or a gamete with 23 chromosomes

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12
Q

define polyploid

A

the presence of a complete set of extra chromosomes in a cell

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13
Q

define aneuploid

A

cells that contain missing or additional individual chromosomes

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14
Q

most common cause aneuploidy

A

= nondisjunction = failure of chromosomes to properly seperate during meiosis I or II = result in gamete that lack chromosome or have 2 copies = monosomy or trisomy zygote

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15
Q

two types of structural chromosome abnormalities x2

A
  1. unbalanced = rearrangement cause a gain or loss of chromosomal material
  2. balanced = the rearrangement does not produce loss/gain of material
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16
Q

rearrangements that alter the structure of chromosomes x3

A
  1. deletions or duplications
  2. inversions
  3. translocation
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17
Q

what does deletion of chromosome result in

A

= loss of genetic material, if not centromere it will be lost during mitosis

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18
Q

define duplication chromosome

A

extra copy of DNA region in chromosome

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19
Q

how does deletion & duplication of chromosomes occur

A

when homologous chromosomes fail to line up properly during meiosis, resulting in an unequal exchange of genetic material during homologous recombination

= during crossing over which occurs during prophase I

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20
Q

define inversion & what they may cause

A

when two breaks occur in a single chromosome & the intervening DNA rotates 180 degrees

  • may cause misalignment of homologous chromosomes during meiosis = duplication & deletion of genetic material
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21
Q

define paracentric inversion

A

includes a centromere

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22
Q

define pericentric inversion

A

doesn’t include a centromere

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23
Q

define translocation

A

= result from chromosomal breakage & the exchange of chromosomes segments from usually different chromosomes

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24
Q

define reciprocal translocations

A

= where breaks occur in 2 different chromosomes & material mutually exchanged = results in derivative chromosomes = affects offspring which may have a loss of chromosome or duplication

25
Q

list single gene mutations x3

A
  1. substitution
  2. insertion
  3. deletion
26
Q

structural effects of single gene mutations on protein x5

A
  1. silent polymorphism
  2. missense mutation
  3. frameshift mutation
  4. nonsense mutation
27
Q

define triple or trinucleotide repeat expansions

A

= typically involve cytosine/guanine-rich trinucelotides (CGG, CCG, CAG, CTG) that can involve a few copies to several thousand repeats

= over 28 repeats = unstable = change in successive generations = repeat expands = symptoms of disorder become apparent at earlier age e.g., huntingsons disease = earlier onset & more severe symptoms cause more repeats

28
Q

mutations in splice sites - when can mutations alter the amino acid sequence

A
  • introns remain in the processed mRNA transcript

- exons are spliced out from the processed mRNA transcript

29
Q

what are the functional effects of mutation on protein

A
  1. gain of a function mutation produces novel or excess protein product e.g., increased enzymatic activity is an example of gain of a function
  2. loss of function mutation reduces or eliminated protein product e.g., haploinsufficiency occurs when 50% of gene function results in an abnormal phenotype
  3. dominant negative mutation (allele 2) produces abnormal protein product the interferes with normal protein produced by allele 1
30
Q

how do we know if mutation pathogenic

A

use data bases to know if identified variant is previously associated with disease = most of time has neutral affect

31
Q

list 5 types of DNA damage

A
  1. depurination
  2. deamination
  3. UV radiation
  4. alkylation
  5. adducts
32
Q

define depurination DNA damage

A

when the guanine or adenine purine bases are lost from nucleotides without breaking the DNA phosphodiester backbone.

= can lead to loss of nucelotide pair or the incorporation of an incorrect nucelotide during DNA replication

=When DNA replication machinery arrives at the missing purine, it will skip to the next complete nucleotide resulting in a single nucleotide deletion.

33
Q

define deamination DNA damage

A

is the spontaneous loss of an amino group from a cytosine to produce the base of a uracil. Similar to depurination it does not break the phosphodiester backbone of the DNA.

= of cytosine results in substitution of an adenine when the mutated strand is replicated

34
Q

define DNA damage from UV radiation

A

from sunlight can damage DNA by promoting the formation of covalent bonds between adjacent pyrimidine bases to form dimers. Thymine-Thymine dimers are a common example.

35
Q

define Alkylation DNA damage

A

is the addition of methyl or ethyl groups to the DNA bases

36
Q

define adducts DNA damage

A

are formed when chemical groups (carcinogens) bind to DNA segments.

37
Q

most common result from DNA damage

A

DNA replication machinery will place an incorrect nucleotide across from the missing base, resulting in a substitution mutation

38
Q

4 common steps in DNA repair mechanisms

A
  1. Detection: the damaged section of DNA is recognised
  2. Excision: nucleases remove the damaged base/s
  3. Polymerisation: DNA polymerase replaces the nucleotides using the other strand as a template
  4. Ligation: DNA ligase seals the nicks in the phosphodiester backbone
39
Q

non-homologous end joining

A

= not as accurate
= cause genes to get mixed up or moved around
= used when assisstant DNA is not available

40
Q

non-homologous end joining

A

= not as accurate
= cause genes to get mixed up or moved around
= used when assistant DNA is not available

41
Q

role of the mismatch repair pathway

A

= repairs mismatched bases produced during DNA replication that have been missed by DNA polymerase proofreading = strand specific (a repair pathway for newly replicated DNA)

42
Q

steps in the mismatch repair pathway

A
  1. specialised proteins detect the mismatch on the daughter DNA strand & bind to this DNA strand
  2. mismatch repair machinery searches for the nearest GATC sequence to the mismatch, where it cuts the DNA strand.
  3. Exonucleases will degrade the newly synthesised DNA strand all the way back to the mismatched base.
  4. DNA polymerase re-synthesises the DNA sequence before the break is sealed by DNA ligase.
43
Q

transient niks

A

potentially the mechanisms for distinguishing between two strands

44
Q

in what way can mismatch repair be inefficient

A

= if the GATC site is many base pairs away from mismatch site = loss of long stretch of DNA = inefficient needs to be re-synthesised

= mutation rates 100 times higher than normal

45
Q

example of cancer caused by mutation in human homologs of the MutS and MutL genes which are needed in the detection of mismatched bases.

A

Hereditary no polyposis colorectal cancer (HNPCC)

46
Q

define base excision repair

A

= corrects DNA damage to a single nucleotide caused by oxidation, deamination, or alkylation

47
Q

steps in base excision repair x5

A
  1. DNA glycosylase recognises and cleaves the bond between deoxyribose and the modified or mismatched DNA base, leaving an abasic site
  2. An enzyme called AP (apurinic/apyrimidinic) endonuclease creates a nick in the strand of DNA
  3. phosphodiesterase removes the deoxyribose sugar from the affected strand leaving a gap
  4. DNA polymerase then replaces the missing nucleotide using the complementary DNA strand as a template
  5. DNA ligase then completes the repair by sealing the gaps in the DNA backbone
48
Q

define abasic

A

nucleotide without a base

49
Q

what are DNA glycosylases

A

group of enzymes that recognise and remove damaged bases or cytotoxic bases

50
Q

define nucleotide excision repair

A

repairs DNA lesions that cause distortion of the DNA helix

51
Q

steps of nucleotide excision repair

A
  1. specific proteins recognise helix distorting lesions & recruit other proteins which make incisions on either side of the lesion releasing a single-stranded DNA fragment containing the damaged DNA and surrounding nucleotides
  2. DNA polymerases then synthesises the missing segment using the complementary strand as a template
  3. DNA ligase seals the break
52
Q

defects in nucleotide excision repair

A

several autosomal recessive disorders:

  1. Xeroderma pigmentosum, Cockayne syndrome
  2. UV-sensitive syndrome.

Xeroderma pigmentosum = hypersensitivity to sunlight and predisposition to skin cancer. Eight proteins encoded by genes mutated in Xeroderma pigmentosum take part in NER.

53
Q

what causes double-strand DNA breaks

A

ionizing radiation or drugs or arise spontaneously after replication

54
Q

risk from double-strand DNA breaks

A

carry the risk of losing chromosomal material or causing chromosomal translocations

55
Q

what are the 2 major pathways to repair double-strand breaks & most common occurrence

A

non-homologous end joining (NHEJ) = G1 & early S phase

homologous recombination = late S & G2 phase

56
Q

define non homologous end joining

A

= no requirement for homology

= rapidly sticking the broken ends back together, before the DNA fragments drift apart and are lost

57
Q

result of non homologous end joining

A

loss of nucleotides during the cleaning process = could impact coding region = loss gene function

= could cause chromosomal translocations if the error with rejoining unrelated DNA

58
Q

define homologous recombination

A

uses the undamaged sister chromatid as a template for the accurate repair of double-strand breaks = often occurs shortly after DNA replication but before the cell has divided, when the sister chromatids are still located close together

59
Q

steps in homologous recombination

A
  1. nuclease digests the 5’ ends of the two broken strands.
  2. 3’ end of one of the broken strands will then “invade” the unbroken homologous sister chromatid and search for the complementary sequence
  3. it will then use the complementary strand as a template to elongate past the point of where the break occurred.

When this has happened the newly repaired strand will then base pair with its original DNA strand partner. The other broken strand will then use this newly repaired strand as a template to elongate and complete its repair.

hc (11 decks)

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