Genetics Flashcards
what is DNA composed of?
Deoxyribose Nucleic Acid
Two strands of DNA pair up in an anti-parallel fashion
Made up of nucleic acid bases held on a 2-deoxyribose sugar backbone
The bases are Adenine, Thymine, Guanine and Cytosine
DNA strands associate with proteins and are wound up into chromosomes
how is DNA replicated?
Replicated in 5’-3’ direction by DNA polymerase
what is RNA composed of?
single stranded
Made up of nucleic acid bases held on a ribose sugar backbone
Uracil replaces Thymine
stages of the cell cycle
G1 Phase – cells increase in size and the cell contents are duplicated
S Phase – DNA replication occurs
G2 Phase – cell gets prepared to divide
M Phase: MITOSIS
One diploid parent cell 🡪 results in 2 identical diploid daughter cells
what is meosis?
The process by which gametes (sex cells) are produced
One diploid parent cell 🡪 results in 4 haploid daughter cells
Crossing over of genes occurs
Forms a gamete
what is the structure of a chromosome?
telomere
short arm (p)
centromere
long arm (q)
telomere
how can Chromosomes can be recognised?
Banding pattern
Length
The position of the centromere
how can chromosomes can be analysed?
FISH (specific)
Array CGH (general)
PCR
Next Generation Sequencing (NGS)
what is FISH?
Fluorescent in-situ hybridisation
Can pick up extra or missing material (e.g. trisomy 21)
what is Array CGH?
The first line chromosome test
Can’t detect balanced arrangement
Analyses the whole genome
The patient’s DNA is compared to a reference DNA sample
what is PCR?
Makes lots of copies of one small part of the genome
These can then be sequenced
what is NGS?
Can sequence the whole genome or all known exons
similar to PCR
what is a balanced gene abnormality?
all genes are there, just in different places
what is an unbalanced gene abnormality
has missing or additional material
what is Aneuploidy?
whole extra or missing chromosome
e.g. Downs syndrome (trisomy 21), Edwards syndrome (trisomy 18)
what are insertions and deletions?
can occur when nucleotide base pairs are either added or removed from the strand during replication
This could change the protein
abnormalities that lead to Down syndrome, Miscarriage, Edward syndrome, Turner syndrome and Klinefelter syndrome
Down syndrome - 47 XY + 21 & trisomy 21
Miscarriage - 47 XY + 14b & trisomy 14
Edward syndrome - 47XY + 18 & trisomy 18
Turner syndrome - 45 X
Klinefelter syndrome - 47 XXY
what is a Robertsonian Translocation?
two chromosomes get stuck together
All genetic material is still present so there is no disease
BUT can cause trisomy in offspring (e.g. familial Downs syndrome)
what is reciprocal Translocation?
when segments of material are exchanged between different chromosomes
50% of offspring will have normal chromosomes or a balanced translocation
The remaining 50% will be unbalanced and result in miscarriage or dysmorphic delay
what is polymorphism?
A polymorphism is any variation in the human genome that occurs in >1% of the population
It does not cause disease in its own right but may predispose someone to specific diseases
what is a single nucleotide polymorphism (SNP)?
is an alteration in the DNA sequence
Most have no effect
what are Copy number variations (CNVs)?
extra or missing stretches of DNA
what is a pathogenic variant?
a gene change that causes a disease
what are mendelian disorders?
caused by a change in a single gene and show high penetrance
e.g. autosomal dominant, autosomal recessive, X-linked, mitochondrial
what is penetrance?
the likelihood of having a disease if you have the mutation
e.g. 100% penetrance means you will always get the disease if you have the mutation
what is non-mendelian inheritance?
multi-factorial
e.g. methylation, mosaicism
what is Mosaicism?
different cells have different genetic material
what genes that are involved in cancer?
Oncogenes = make cells divide
Tumour suppressor genes = stop cells dividing
DNA repair genes = repair any damage
Drug metabolism genes = metabolise carcinogens
what is methylation?
Usually occurs on cytosine bases just before guanine bases
Leads to modification of histones that then stop transcription
Therefore, abnormalities in methylation can cause genes to be silenced in cancer
what gene is involved in breast cancer?
BRCA1 – cause a small proportion of familial breast cancer
Caries 80% lifetime risk of breast or ovarian cancer
