Flashcards in Chromosome Abnormalities, Mutations and Genetic Analysis Deck (37)
What are some types of chromosomal abnormalities?
• Numerical - error in number of chromosomes, gross abnormality
• Structural – error in rearrangement, gross abnormality
• Mutational – error in DNA sequence, minute changes
What is non-disjunction?
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy)
What disorders are caused by autosomal aneuploidy?
• Trisomy 21 (Down syndrome)
• Trisomy 13 (Patau syndrome)
• Trisomy 18 (Edwards syndrome)
Describe the origin of Down syndrome
Trisomy 21 (Down syndrome)
– Incidence: 1 in 650 to 1 in 700
• Increases with advancing maternal age
– Characteristic facial dysmorphologies
– IQ less than 50
– Average life expectancy (50-60 years)
– Alzheimer’s disease in later life
– Chromosomal findings
• Trisomy 21: non-dysjunction (95%), usually maternal origin
• Unbalanced Robertsonian translocation (4%)
• Mosaicism (1%) – differences in karyotypes in different tissues
Describe the origin of Patau syndrome
Trisomy 13 (Patau syndrome)
– Incidence: 1 in 5000
– Multiple dysmorphic features and mental retardation
– About 5% die within first month, very few survive beyond first year
– Non-dysjunction (90%), maternal origin
– Unbalanced Robertsonian translocation (10%)
Describe the origin of Edwards syndrome
Trisomy 18 (Edwards syndrome)
– Incidence: 1 in 3000
– Severe developmental problems; most patients die within first year, many within first month
– Non-disjunction (90%), maternal origin
What diseases are caused by sex chromosome aneuploidy?
• 45,X (Turner syndrome)
• 47,XXY (Klinefelter syndrome)
Describe the origin of Turner syndrome
45,X (Turner syndrome)
– Incidence: 1 in 5000 to 1 in 10000 (liveborn)
– Incidence at conception much greater, about 97% result in spontaneous loss
– Females of short stature and infertile
– Neck webbing and widely spaced nipples
– Intelligence and lifespan is normal
Describe the origin of Klinefelters disease
47,XXY (Klinefelter syndrome)
– Incidence: 1 in 1000
– Tall stature, long limbs
– Male but infertile, small testes, about 50% gynaecomastia (abnormal breast development)
– Mild learning difficulties
What are the two different types of translocation reactions?
Reciprocal: involving breaks in two chromosomes with formation of two new derivative chromosomes
Robertsonian: fusion of two acrocentric chromosomes to form one short chromosome and one long chromosome
What is a balanced translocation?
Balanced translocation – if two simultaneous breaks occur, and repair results in translocation of the genetic material between two chromosome, ∴ all genetic material is still present following reciprocal translocation
What is deletion?
Deletion - a deletion is a mutation (a genetic aberration) in which a part of a chromosome or a sequence of DNA is lost during DNA replication.
What is unbalanced translocation?
Called unbalanced when the exchange of chromosome material is unequal resulting in extra or missing genes
Describe different types of mutations
Coding single mutations
– Silent – synonymous e.g. CGA (Arg) to CGC (Arg)
– Missense - CGA (Arg) to GGA (Gly), incorrect protein
– Nonsense - CGA (Arg) to TGA (Stop), truncated protein
– Frameshift – deletion / insertion, incorrect/faulty/truncated protein
What are the different types of point mutation?
• Purine to purine e.g. A -> G
• Pyrimidine to pyrimidine e.g. C -> T
• Purine to pyrimidine e.g. A -> C
• Pyrimidine to purine e.g. C -> G
Name the purines
adenine and guanine
Name the pyrimidines
cytosine and guanine
What methods are used to detect mutations in the genome?
• Polymerase chain reaction (PCR)
• Gel electrophoresis
• Restriction fragment length polymorphism (RFLP) analysis
• Amplification refractory mutation system (ARMS)
• DNA sequencing
What do you need to carry out PCR?
• Sequence information
• Oligonucleotide primers
• DNA polymerase
What is PCR?
Polymerase chain reaction (PCR) is a technique used in molecular biology to amplify a single copy or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence.
Describe the PCR procedure
Step 1 - denature dsDNA 93-95C
Step 2 -anneal primers at 50-70C
Step 3 - extend new chains at 70-75C
What is gel electrophoresis?
• Separates DNA fragments by size
• Apply an electric field
• DNA is negatively charged
• Separate through agarose gel matrix
• Visualise DNA fragments against standards
What are the advantages of PCR-gel electrophoresis?
• Ease of use
What is ARMS?
The amplification-refractory mutation system (ARMS) is a simple method for detecting any mutation involving single base changes or small deletions. ARMS is based on the use of sequence-specific PCR primers that allow amplification of test DNA only when the target allele is contained within the sample. Following an ARMS reaction the presence or absence of a PCR product is diagnostic for the presence or absence of the target allele.
What are the advantages of ARMS?
• Labelling not required
• Electrophoresis required
What are the limitations of ARMS?
Primer design critical - may miss other unknown mutations associated with a disease (unknown variants)
What are the disadvantages of ARMS?
• Need sequence information
• Limited amplification size
• Limited amounts of product
• Infidelity of DNA replication
Whats the primary function of restriction endonucleases?
• Enzymes from bacterial cells involved in a protective mechanism which degrade DNA of invading viruses
• Recognise specific DNA sequences, usually 4-8 bp
What is RFLP analysis?
In RFLP analysis (restriction fragment length polymorphism), the DNA sample is broken into pieces and digested by restriction enzymes and the resulting restriction fragments are separated according to their lengths by gel electrophoresis.
If a base is missing or has been changed, the fragment wont be recognized and therefore cut
Give an example of a disease that is diagnosed using RFLP analysis
Sickle cell anaemia
In normal Hg gene, MST II cleaves the Hb genet create two bands representing smaller proteins
In sickle cell anaemia, the cleavage point is mutated and therefore isn’t cleaved, producing a band of a larger protein on the gel, suggesting they are affected.
If both bands are present, suggests the individual is a carrier of both the healthy and sickle cell gene.
What are the advantages and disadvantages of RFLP analysis?
• Requires gel electrophoresis
• Not always feasible
Describe the chain termination method (Sanger sequencing) of DNA sequencing
The classical chain-termination method requires a single-stranded DNA template, a DNA primer, a DNA polymerase, normal deoxynucleosidetriphosphates (dNTPs), and modified di-deoxynucleosidetriphosphates (ddNTPs), the latter of which terminate DNA strand elongation. These chain-terminating nucleotides lack a 3'-OH group required for the formation of a phosphodiester bond between two nucleotides, causing DNA polymerase to cease extension of DNA when a modified ddNTP is incorporated. The ddNTPs may be radioactively or fluorescently labeled for detection in automated sequencing machines
What are the advantages and limitations of DNA sequencing?
• Gold standard for mutation detection
• Automation and high throughput
• Expensive equipment
• Poor quality sequence read
• Can be hard to determine between disease causing, disease predisposing and unrelated gene mutations when looking at all mutations in a genome.
What conditions would make the ideal genetic test?
Depends on the abnormality, ideally it would need to be:
• Direct test
• Quick and easy
Describe the origin and diagnosis of disease due to abnormalities in chromosome number (Aneuploidy).
Trisomy 21 (Down’s Syndrome)
• 90% caused by non-disjunction (Chromatid sisters don’t split, and both pairs are carried into one gamete)
• Chances increase with advancing maternal age
• Distinct facial characteristic
• IQ less than 50
Trisomy 13 (Patau Syndrome)
• Usually caused by non-disjunction (90%)
• Multiple dysmorphic features
• Very few survive beyond first year of life
Trisomy 18 (Edwards Syndrome)
• 90% Non-disjunction
• Most die within first year or month of life
X – (Turner Syndrome)
• Short stature and infertile
• Neck webbing and widely spaced nipples
• Normal intelligence and lifespan
• 97% zygotes terminate at conception
XXY (Klinefelter Syndrome)
• Tall stature and long limbs
• Infertile, small testes
• 50% develop breasts (gynaecomastia)
• Mild learning difficulties
What are the different types of chromosomal anomalies?
o Robertsonian Translocation - This occurs in Acrocentric chromosomes (i.e. One arm is shorter than the other). This gives one chromosome with both long arms, while the other has both the short ones.
o Deletions - This occurs when sections of chromosomes just seem to disappear from one of the arms, if not both.
o Inversions - Paracentric (when a section of the DNA in the arm of the chromosome is inverted) or Pericentric (when the DNA around the centromere of the chromosome is inverted)