Week 8 - Genetics Flashcards
DNA - 3 components
- pentose sugar
- phosphate molecule
- Four nitrogenous bases - pyrimidines: Cytosine and thymine
Purines: adenine and guanine
DNA are composed of how many amino acids
20
Frameshift mutation
Insertion or deletion of one or more base pairs
Spontaneous mutation
Mutation that occurs in absence of exposure to known mutagens
Mutagen
An agent known to increase the frequency of mutations
- radiation
- chemicals
Transcription
RNA is synthesised from the DNA template
Results in the formation of messenger RNA (mRNA)
Disjunction
Normal separation of chromosomes during cell division
Nondisjunction
Usually the cause of aneuploidy
Partial trisomy
Only an extra portion of a chromosome is present in each cell
Chromosome mosaics
Trisomies occurring only in some cells of the body
Down syndrome
- Best known e.g of aneuploidy
- Trisomy 21
- Mentally retarded, low nasal bridge, epicentral folds, protruding tongue, poor muscle tone
- Risk increases with maternal age
Sex Chromosome Aneuploidy
- One of the most common is trisomy X. This is a female that has 3 X chromosomes
- Termed ‘metafemales’
Turner Syndrome
Female with only one x chromosome
Characteristics of Turner Syndrome
Absence of ovaries (sterile) short stature Webbing of the neck oedema underdeveloped breasts, wide nipples High number of aborted foetuses X is usually inherited from mother
Klinefelter Syndrome
Individuals with at least 2 X’s and one Y chromosome
Characteristics of Klinefelter syndrome
- Male appearance
- Develop female-like breasts
- Small testes
- Sparse body hair
- Long limbs
Chromosome breakage
If a chromosome breaks, physiological mechanisms will usually repair the break, but the breaks often heal in a way that after the structure of the chromosome
Breakage or loss of DNA
- Cri Du chat syndrome
“cry of the cat”
Low birth weight, mental retardation and microcephaly
Duplication
- Presence of a repeated gene or gene sequence
- Rare occurrence
- Less serious consequences because better to have more genetic material than less (deletion)
- Duplication in the same region as cri du chat causes mental retardation but no physical abnormalities
Inversions
2 breaks on a chromosome
Reversel of the gene order
Translocations
Translocation occurs when two chromosomes break and the segments are rejoined in an abnormal arrangement
Fragile sites
Fragile sites are areas on chromosomes that develop distinctive breaks or gaps when cells are cultured
Generally no apparent relationship to disease
Fragile X Syndrome
Site on the long arm of the X chromosome
Associated with mental retardation; second in occurrence to Down syndrome
Higher incidence in males because they have only one X chromosome
Autosomal Aneuploidy
Partial trisomy – only an extra portion of a chromosome is present in each cell
• Chromosome mosaics – Trisomies occurring only is some cells of the body
• Down syndrome – best known example of aneuploidy – Trisomy 21. Mentally retarded, low nasal bridge, epicanthal folds, protruding tongue, poor muscle tone.
Metafemale
Three X chromosome = sterility, menstrual irregularity and/or mental retardation
Autosomal Dominant & Autosomal Recessive meaning
For a genetic trait to be dominant it must be inherited from only one parent for symptoms to be displayed while an autosomal recessive trait must be inherited from both parents to be dominant. If the trait is recessive but is only inherited from one parent then the child becomes a carrier.
E.g Huntington’s disease is a Autosomal Dominant mutation while Cystic Fibrosis is an autosomal recessive disorder.
Cystic Fibrosis
Is an autosomal recessive disorder. Both parents must have the genetic disposition for CF for a child to have the condition. If only one parent has the disposition for it then the child will not display symptoms of the disease rather they will be a carrier.
