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DNA structure function

Double helix structure made from 2 chains
of phosphate and deoxyribose (type of sugar). 4 nucleotides form a ladder structure to make a helix and its a stable structure which allows it to be carried from generation to generation. Base pair between is loose hydrogen bonds so they can be unzipped


Turning on a gene

Genes are turned on by transcription factors. Transcription factors tells the DNA to be transcribed into messenger RNA


Gene Expression: Transcription and Tanslation

Transcription: In the nucleus , the gene’s DNA sequence is copied into messenger RNA (mRNA).
Translation: A ribosome attaches to the mRNA and moves along the mRNA, reading each triplet codon (3 bases) and using transfer RNAs (tRNA) to put together the amino acid chain to make a protein.


Cell division : Mitosis and Meiosis

Mitosis – somatic cells (daughter cells identical to parent)Meiosis – gametes (daughter cells contain half the number of chromosomes. Crossing over, allowing genetic diversity)


Mendel's Law

Gene is in one of two forms (known as
alleles) – either tall or dwarf
2 copies of the gene in each parent pea
1 copy is carried to each of the offspring
Height: Tall (T) is dominant
Dwarf (d) is recessive
If the genes are identical (TT or dd): homozygous
If the genes are not identical (Td): heterozygous


Huntington’s Chorea - dominant inheritance

Degeneration of the brain (striatum) leading to progressive deterioration of movement, temperament and cognition.
Autosomal dominant inheritance: single copy will be dominant and lead to the disease (if 1 parent has Huntington’s, 50% of the offspring will develop Huntington’s).


Phenylketonuria – recessive inheritance

Mutation in the PAH gene (phenylalanine hydroxlase)
Enzyme that breaks down dietary phenylalanine
Carrier: 1 in 50
Disease: 1 in 10,000
If both parents are carriers:
25% of offspring will have the disease,
50% will be carriers
Build up of phenylalanine toxic to developing brain
- learning disabilities
- behavioural difficulties
- epilepsy


What are the different chromosomal abnormalities?

Monosomy: single copy of a chromosome.
Spontaneous abortion in early pregnancy.

Trisomy: three copies of a chromosome
Very high rate of spontaneous abortions

Downs syndrome:
trisomy in chromosome 21
(error in first meiotic division)
Chromosome 21 is the only gene where you can actually survive with it and be born with downs syndrome and live a long life


What is Rett syndrome? (X linked – dominant)

Progressive neurodevelopmental disorder almost exclusively affecting females, leading to profound mental impairment.

Rare condition: 1in 10,000

Mutation in the gene MeCP2. This "transcriptional repressor" turns off the expression of unwanted genes during synapse formation.

X – inactivation means that not all cells will express mutated MePC2 gene, therefore variable penetrance, sometimes see milder symptoms.


Fragile X (X-linked)

Mutation in one end of the FMR1 gene (the 5' untranslated region), consisting of an amplification of a CGG repeat (200+ copies; normally between 6 and 40 repeats).

The FMR1 gene encodes the FMR protein, which is thought to shuttle select mRNAs between the cytosol and nucleus.


Genotype v Phenotype

Genotype = genetic information

Phenotype = how it displays…
(Interaction of genotype with environment)


Genomic Imprinting

Faulty gene leads to a condition. Paternal copy of gene switched off so if inherited from father the faulty gene is carried but not active. If then inherited maternally is “switched on” in next generation. Blue genes
maternally imprinted (paternal genes active). Red genes
paternally imprinted (maternal genes active).


Chromosomal Deletion

If deletion on paternal chromosome…
only maternal genes present
normal expression of Red genes
NO expression of Blue genes (Prader-Willi Syndrome)

If deletion on maternal chromosome…
only paternal genes present
normal expression of Blue genes
NO expression of Red genes (Angelman's Syndrome)


Prader-Willi Syndrome

Characterized by intellectual disability, decreased muscle tone, short stature, emotional lability and an insatiable appetite which can lead to life-threatening obesity.


Angelman Syndrome

delayed development, severe language impairment, movement and balance problems, epilepsy, small head, sociable behaviour, frequent smiling)