Chromosome Abnormalities, Mutations and Analysis Flashcards
What type of chromosome abnormalities can occur
Numerical
Structural
Mutational
What percentage of first trimester miscarriages are due to chromosomal abnormalities
50%
What is the main type of chromosomal abnormalities causing first trimester miscarriages
Trisomy
What type of abnormality and syndrome does 47, XX+21
Downs syndrome
Means there are 47 chromosomes with the extra chromosome 21
How can chromosomal abnormalities arise
From non-disjunction which can occur in meiosis 1 or 2
Give examples of autosomal aneuploidy syndromes
Trisomy 21 (Down syndrome) Trisomy 13 (Patau syndrome) Trisomy 18 (Edwards syndome)
Give examples of sex chromosome aneuploidy syndromes
45, X (Turner syndrome)
47, XXY (Klinefelter syndrome)
What is the incidence of trisomy 21
1 in 650 to 1 in 700 (increases with advancing maternal age)
Describe the features of trisomy 21
Characteristic facial dysmorphologies
IQ less than 50
Average life expectancy (50-60 years)
Alzheimer’s disease in later life
What is the incidence of trisomy 13
1 in 5000
Describe the features of trisomy 13 (Patau syndrome)
Multiple dysmorphic features and mental retardation
About 5% die within first month, very few survive beyond first year
What is the incidence of trisomy 18
1 in 3000
Describe the features of trisomy 18 (Edwards syndrome)
Severe developmental problems
Most patients die within first year, many within first month
Describe the features of 45, X (Turner syndrome)
Females of short stature and infertile
Neck webbing and widely spaced nipples
Intelligence and lifespan is normal
Describe the features of 47, XXY (Klinefelter syndrome)
Tall stature, long limbs
Male but infertile, small testes, about 50% gynaecomastia
Mild learning difficulties
What is gynaecomastia
Abnormal development of breast tissue in male
What type of structural abnormalites can occur in chromosomes
Balanced or unbalanced rearrangements Translocations Deletions Insertions Inversions
What type of translocations can occur
Reciprocal
Robertsonian
What is a reciprocal translocation
Involves breaks in two chromosomes with the formation of two new derivative chromosomes
What is a robertsonian translocation
The fusion of two acrocentric chromosomes (the short arms are lost)
The two chromosomes are fused but no genetic information is lost: other translocations occur but do not lead to a viable fetus
What are the outcomes for reciprocal translocation carriers
Partial trisomy and partial monosomy
Partial monosomy and partial trisomy
How can an unbalanced translocation occur
From the combination of a normal set of chromosomes which a set of balanced translocation chromosomes
What can the robertsonian translocation carriers produce
Trisomy 14
Monosomy 14
Monosomy 21
Trisomy 21
What occurs in chromosomal deletions
A break in the chromosome causes genetic material to be deleted
What occurs in a paracentric inversion
A break in the chromosome causes DNA to be inverted (switched upside down)
What type of inversions are there
Paracentric inversion
Pericentric inversion
These are balanced rearrangements
What occurs in a pericentric inversion
A break in the chromosome which includes the centromere causes DNA to be inverted with the centromere (switched upside down)
What kind of genetic mutations can occur
Germline
Somatic - in early development or later
What types of genetic mutations can occur
Non-coding - typically has no effect
Coding
What type of coding mutations are there
Silent – synonymous e.g. CGA (Arg) to CGC (Arg)
Missense e.g. CGA (Arg) to GGA (Gly)
Nonsense e.g. CGA (Arg) to TGA (Stop)
Frameshift – deletion / insertion e.g. CGA (Arg) to CCGA (Pro, then out-of-frame)
What type of point mutations can occur
Transitions
Transversions
Give examples of transition mutations
Purine to purine A > G
Pyrimidine to pyrimidine C > T
Give examples of transversions mutations
Purine to pyrimidine A > C
Pyrimidine to purine C > G
How are mutations named
When labelled above they refer to DNA sequence, below refer to protein sequence
How can mutations be detected
Polymerase chain reaction (PCR)
Gel electrophoresis
Restriction fragment length polymorphism (RFLP) analysis
Amplification refractory mutation system (ARMS)
DNA sequencing
What is required for PCR
Sequence information Oligonucleotide primers DNA Nucleotides DNA polymerase
How does PCR occur
Double stranded DNA is denatured at 93-95oC
Then annealed at 50-70oC
Then extended at 70-75oC
It is then repeated 20-30 times
What does gel electrophoresis do
Separate DNA fragments by size for analysis (e.g. the ones from PCR)
How does gel electrophoresis work
Apply an electric field
DNA is negatively charged
Separate through agarose gel matrix
Visualise DNA fragments
What are the advantages of PCR
Speed
Ease of use
Sensitive
Robust
Where can PCR be applyed
DNA cloning DNA sequencing In vitro mutagenesis Gene identification Gene expression studies Forensic medicine Typing genetic markers Detection of mutations
What are the advantages of ARMS
Cheap
Labelling not required
Electrophoresis required
Primer design critical
What are the disadvantages of ARMS
Need sequence information
Limited amplification size
Limited amounts of product
Infidelity of DNA replication
What are restriction endunucleases
Enzymes from bacterial cells
What do restriction endunucleases do
That have a protective mechanism They degrade DNA of invading viruses Recognise specific DNA sequences Usually 4-8 bp Always cut DNA at the same site
What are the advantages and disadvantages of restriction endunucleases
Advantages: Simple Cheap Non-radioactive Requires gel electrophoresis Disadvantages: Not always feasible
What does DNA sequencing use
Chain termination method (Sanger)
Use of dideoxynucleotides
What are the advantages of DNA sequencing
Gold standard for mutation detection
Automation and high throughput
What are the disadvantages of DNA sequencing
Expensive equipment
Poor quality sequence read (First part of sequence, 15 to 40 bases, Deterioration after 700-900 bases)
What should be considered when determining which genetic mutations detecting method should be used
Direct test Quick and easy Cheap High Sensitivity High Specificity
Where is non-disjunction abnormalities more likely to occur
The maternal gamates
Which chromosomes are acrocentric chromosomes
13 14 15 21 22
When are genetic mutations identified
When they cause gene disruption or are associated with disease
What does PCR do
Amplifies DNA
What is a new type of sequencing becoming more popular
Next generation sequencing - 18 billion bp in 4 days (about 6 human genomes)
