DNA Flashcards
What is the difference between prokaryotes and eukaryotes DNA?
Prokaryotes - has no nuclear membrane. DNA arranged often in a single chromosome
Eukaryotes - DNA is in nucleus and bound to proteins (chromatin complex). Some DNA in mitochondria
Describe a chromosome
Each is made of two identical strands (chromatids) joined in the centre (centromere)
Each has a long arm (q) and a short arm (p) separated by the centromere
What nucleic acid is present in RNA but not DNA
Uracil
What nucleic acid is present in DNA but not RNA
Thymine
Name the pyrimidines
How are they different from purines?
Cytosine
Thymine (DNA)
Uracil (RNA)
Single ringed
Name the purines
How are they different from pyrimidines?
Girls are pure
Adenine
Guanine
Double ring
What is the order of DNA replication?
- DNA opens at replication fork
- Base sequence on each parent strand copied into a complementary daughter strand
- Two parental strands separate in front of the fork
DNA composed as a new and old strand is described as…
Semi-conservative
How fast is DNA replication?
700-1000bp per second
For 6 billion base pairs
What is the role of DNA polymerase?
DNA polymerase reads 3’ to 5’ prints 5’ to 3’
Substrates are deoxyribonucleotides triphosphates
Enzyme stays on the strand, at the the same time extends and proof reads
Add nucleotides to the base
What is the role of helicase?
Opens the strand
What are the roles of single binding proteins (SSB)?
Keep the strand open
What is the role of topoisomerase?
Unwinds the strand (relives supercoiling)
Describe mRNA
Printed as a long linear transcript
Genetic information is transcribed from DNA as a sequence of bases to a ribosome
It has a 5’CAP and a 3’Poly A tail
Describe rRNA
Ribosomal RNA
Combines with proteins to form 80S ribosomes
Describe tRNA
Carry amino acids to ribosomes, and check that they are incorporated in the right position
Each tRNA carries one amino acid
Very small molecules
Anticodon - triplet sequence pairs with mRNA Important for right amino acid for the right triplet
What are the types of variants possible in DNA?
Duplications of genes or part of gene
Deletions (whole gene or some exons)
Variants within the regulatory sequence
Splice site variants
Introduce premature stop codon-nonsense variant
Replace one amino acid in protein with another - mis-sense variant
Expansion of trinucleotide repeats
What does deletions cause?
Disrupts the protein
What does a splice-site variant cause?
Affects the accurate removal of an intron
What does a non-sense variant cause?
Change codon to stop
Out of frame deletion produces a stop codon either at deletion site or further along
RNA detaches from the ribosome and is eliminated
Non sense medicated decay
What does a mid-sense variant cause?
Single base substitution
Changes the type of amino acid in the protein
May or may not be pathogenic
May be a polymorphism of no functional significance
What expansion of a tri-nucleotide repeat causes Huntington’s
CAG
What expansion of tri-nucleotide repeat causes myotonic dystrophy
CTG
What expansion of tri-nucleotide repeat causes fragile X
CGG
Define allelic heterogeneity
Lots of different variants in one gene e.g. cystic fibrosis
Define locus heterogeneity
Variants in different genes give the same clinical conditions e.g. hypertrophic cardiomyopathy
What is loss-of-function variants
Only one allele functioning
Most are recessive
What is gain of function variants
Increased gene dosage
Increased protein activity
What is dominant negative variants
Protein from the variant allele interferes with the protein from the normal allele
What is a diagnostic test
Patient has signs and symptoms suggesting a particular diagnosis
Molecular genetic test will confirm diagnosis
What is predictive testing
Testing health at risk family members for a previously identified familial variant - often dominant
What is a pre-natal test
Performed in pregnancy where an increased risk of a specific condition affecting the foetus
Chorionic villous sample or amniocentesis
What is genetic testing
Testing target population not high risk families
How many bonds are between adenine and thymine
2
How many bonds are between guanine and cytosine
3
How many chromosomes are in the human genome
46 chromosomes
22 pairs and sex chromosomes XY
What is the long arm of a chromosome called
q
What is the short arm of a chromosome called
p
What does Giemsa staining show
G banding
What does quinacrine staining show
Q banding
What is the purpose of G banding and how many bands does it give
Produce a visible karyotype. Chromosome abnormalities and rearrangements in genetic diseases and cancers
G banding gives approx. 400-500 bands per haploid chromosome set
How many mbp does a band have
Approx 6-8 mbp
Where is a telomere located
At the end of the chromosome
Define phenotype
The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment
Define genotype
The genetic constitution of an individual organism
Define vertical transmission
Passage of disease-causing agent (pathogen) from mother to baby during the period immediately before and after birth
Define recurrence risk
Statistic that estimates the probability that a condition present in one or more family members will recur in another relative in the same or future generations
Define variable expression
Range of signs and symptoms that can occur in different people with the same genetic condition
Define reduced penetrance
Genetic trait is expressed in only part of the population
Define sex limitation
Some traits are only in individuals of one sex e.g. where the gene affects an organ only possessed by one of both sexes
Define assortive mating
Tendency for people to choose mates who are similar (positive) or dissimilar (negative) to themselves in characteristics
Define locus heterogeneity
When mutations at multiple genomic loci are capable of producing the same phenotype (e.g. single trait, patterns of traits or disorder), and each individual mutation is sufficient to cause the specific phenotype independently
Define Down Syndrome
Trisomy 21
(extra chromosome 21)
Interrupts normal course of development - causing clinical features of Down syndrome
Define mosaic down syndrome
Have an extra number 21 chromosome in only some of their body’s cells
What is the downs syndrome mutation
Trisomy 21
What are the usual mechanisms for downs syndrome
Non-disjunction of maternal chromosomes
Unbalanced chromosomal material
Mosaicism
What is the strongest risk factor for Down’s syndrome
Maternal age
What is the karyotype for downs syndrome
47 XX + 27
What are the base pairs in DNA
Adenine
Thymine
Guanine
Cytosine
How many bonds does adenine and thymine have
2
How many bonds does cytosine and guanine have
3
What are the pyrimidines
Cytosine
Thymine
Single ring
What are the purines
Guanine
Adenine
Double ring
What happens in G2
Preparation for cell division
What happens in G1
Growth and metabolism
What happens in S phase
DNA replication
Where is G0
Beginning of G1 after mitosis
What direction does DNA polymerase read
3’ to 5’
What direction does DNA polymerase print
5’ to 3’
What direction is the top vs bottom strand of DNA
Top 5’ to 3’
Bottom 3’ to 5’
What is the role of helicase
Opens the strand
What keeps strand open
Single stranded binding proteins (SSB)
What is the role of topoisomerase
Unwinds it (relieves supercoiling)
What enzymes joins okazaki fragments
Ligase
What are the structure of DNA
Phosphate (joined to by 5 or 3 carbon)
Phosphodiester bond to the sugar
Base pair
How many base pairs is one turn of DNA approx to
10
What is the role of p53
DNA damage detected - initiate repair mechanism
Pause cell cycle until repair is carried out - halt cell cycle if DNA not repaired
Apoptosis - command cell to commit suicide if DNA damage not repaired
What are the base pairs of RNA
C
G
U
A
What are the base pairs of DNA
C
G
A
T
Where is DNA found
Nucleus
Mitochondria
Where is RNA found
Cytoplasm
What is the sugar of DNA
Deoxyribose
What type of sugar is RNA
Ribose
What is the role of DNA
Long-term storage of genetic information
What is the role of RNA
Transfer genetic code from the nucleus to the ribosomes to make proteins
What comes first transcription or translation
Transcription
What are the stages of transcription
Initation
Elongation
Termination
Describe initation of transcription
RNA polymerase binds to promoter region on DNA (TATA BOX - TATAAA)
What is the promoter region on DNA
TATA box
TATAAA
Describe elongation transcription
Non-template strand
Template strand - antisense - RNA polymerase active upon
Describe termination transcription
5’ Cap to 3’ poly-A tail
RNA splicing to move introns
mRNA
What are removed to created mRNA
Introns
What is the role of the cap
Protect mRNA strand from being degraded by certain enzymes
What are stages of translation
Initation
Elongation
Termination
Describe initation of translation
Ribosome has small and large subunit
Has 3 active sites E P A
Step 1 - start condon AUG enters p-site where peptide bonds are formed
Describe elongation of translation
2nd tRNA molecule enters A -site
Continues leave via E-site
Describe termination of translation
Stop codon read - non-sense coding
Does not code for amino acid - bind to a-site = release factor
Ribosome disassembles
What is transcription
DNA - mRNA
