Genetics Flashcards
where is the information in DNA held?
it is held in the sequence of the bases which are held on a sugar phosphate backbone
Thymine pairs with
adenine
guanine pairs with
cytosine
DNA is wound into a chromosome by..?
Wrapping around histone proteins
what phase of the cell cycle does DNA replication happen?
S phase
why might DNA be damaged?
the DNA strand breaks, there is chemical cross linking or there is a mismatched base
how can DNA damage be fixed?
by repair mechanisms
What happens when there are defects in the repair mechanisms?
disease
during mitosis, one diploid parent cell turns into…
two identical diploid daughter cells
during meiosis, one diploid parent cell becomes…
4 haploid daughter cell (one of each chromosome to each cell)
when does meiosis occur?
in gamete formation
what is crossing over?
Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis, which results in new allelic combinations in the daughter cells.
how does a gene go from DNA to mRNA
transcription and splicing
how does a gene go from mRNA to protein
translation
what is the amount of protein produced determined by? 4 answers
- rate of transcription
- rate of splicing to mRNA
- half life of mRNA
- rate of processing of polypeptide
what can 3 bases encode for?
1 amino acid or a stop
how is mRNA produced from DNA? Two steps
DNA is transcribed to pre mRNA and then pre mRNA is spliced to mRNA
what is polymorphism?
a variation in the human genome that has a population frequency greater than 1% or a variation that does not cause a disease in its own right
what is a mutation?
a gene change that causes a genetic disorder
why does variation happen?
DNA replication is not perfect
what three things allow a chromosome to be recognised?
banding pattern with specific stains, length and position of centromere
what arm on the chromosome doesn’t really matter?
the short arm
what is the balanced chromosome rearrangement?
all the chromosomal material is present
what is the unbalanced chromosome rearrangement?
extra or missing chromosomal material
term for whole or missing chromosome ?
aneuploidy
term for rearrangement of chromosomes?
translocation
large chromosomal imbalance results in?
a higher chance of miscarriage
notation for male down syndrome
47XY + 21
translocation that has a high chance of miscarriage
trisomy 14
what is the name of the condition trisomy 18?
Edwards syndrome
notation for Edwards syndrome male
47 XY + 18
why is X chromosome aneuploidy better tolerated?
because of X inactivation
what is X inactivation?
the random process of one female X chromosome inactivating
result of reciprocal translocations
50% will have normal chromosomes or balanced translocation but the other 50% will miscarriage or have a dysmorphic delayed child
what do reciprocal translocations have to survive?
meiosis
effects of large translocation
large risk of miscarriage and small risk of a dysmorphic delayed child
effects of a small translocation
small risk of miscarriage but large risk in a having a dysmorphic delayed child
how many bases is too small to see?
5 million
cheapest quickest and most sensitive way to count chromosomes?
molecular techniques - no longer FISH
newest test available for looking at chromosomes?
Array CGH
array genomic hybridisation
benefits of array CGH
allows detection of any size chromosome imbalances
what does array CGH not detect?
balanced rearrangements
what could chromosome changes do?
activate an oncogene or delete a tumour suppressor
what is an oncogene?
a gene which in certain circumstances can transform a cell into a tumour cell
what type of analysis do you use to detect chromosome deletions/duplications for balanced rearrangements?
aCGH
what is PCR?
a process that allows us to select a short stretch of the genome and amplify it
how much of the genome is axons?
only 2-3%
what does the rest of the genome do that isn’t an exon?
regulate genes, space genes out, insulate genes from promotors and add genes
what carries genes?
chromsomes
when can mutations arise during cell division?
at any point in the human life cycle
a genetic change that contributes to cancer
somatic mosaicism for a chromosomal abnormality
what does NGS stand for
Next generation sequencing
what does NGS allow?
large scale sequencing cheaply- whole exam or whole genome
how many base pairs are on the human genome?
about 3 billion
what is a mutation?
a genetic variation that causes a disease
what is a polymorphism?
a genetic variation that is prevalent in the population and not, in itself, disease causing
what does a promotor mutation result in?
no, or reduced transcription/translation
result of a base change
it alters the amino acid sequence therefore a different or non-functioning protein is produced
what is the wild type?
the normal DNA sequence
name for how to describe a mutation
standard nomenclature
what does p. mean?
the change in the peptide (protein) sequence (letters)
what does c. mean?
the change in the mature mRNA sequence (amino acid names)
standard nomenclature for a stop
*
c. for deleting a single base and frameshift (the sixth one and its an A)
c.6delA
what is a trinucleotide repeat expansion?
replication of a trinucleotide
how many polymorphisms are there?
about 3 billion (changes from the ‘normal’
any variant in the wild type may be?
- a mutation
- a variant of uncertain significance
- a polymorphism
what is penetrance?
the likelihood of having a disease if you have a gene mutation
penetrance in autosomal dominance?
100%
what generations is an autosomal dominant disease seen in?
all generations
what causes autosomal recessive diseases?
2 faulty copies of gene to cause disease
what generations is an autosomal recessive disease seen in?
only one generation
x linked disease example
haemophilia
where does the X linked recessive gene lie?
gene fault lies on the X chromosome
who carries the x linked recessive mutation? will they show features of the disease?
Females are carriers and do not show major features of the disease
for an affected X-linked female, how will her offspring be affected?
half of the male children will be affected and half of the females will be carriers
for an affected X-linked male, how will his offspring be affected?
all of his male children will be normal and all of female children will be carriers
why might a female carrier show mild features of an x-linked disease?
x inactivation, half the cells have the working gene on average
what does SNP stand for
single nucleotide polymorphisms
effects of most SNPs
no effect
what does CNVs stand for?
copy number variations
what are CNVs?
extra or missing stretches of DNA
in Mendelian disorders, they have a …….. penetrance and a …… environmental contribution
high penetrance and a small environmental contribution
in multifactorial disease, a genetic change is …….., penetrance for any one mutation is …..
just another risk factor
mutation is low
