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describe DNA structure
double helix made from two chains of phosphate and deoxyribose (type of sugar)
4 nucleotide bases: adenine (A), thymine (T), Cytosine (C), Guanine (G)
what are single nucleotide polymorphisms (SNPs)?
the natural variations in our DNA
what type of cells does mitosis make?
somatic cells (daughter cells identical to parent)
what type of cells does meiosis make?
gametes (daughter cells containing half the number of chromosomes)
how does genetic inheritance work?
occurs via meiosis
homologous recombination or ‘crossing over’
allows genetic diversity
offspring all share 50% of each parents genes but a different 50% (i.e. a different combination)
how many genes are on the human chromosome?
approx. 23,000
what are genes?
long sequences of base pairs in the DNA that encode proteins
how do genes work?
they are turned on by transcription factors
transcription factors are activated during development or by intracellular signalling cascades from other parts of the cell
how is messenger RNA (mRNA) made?
the transcription factor binds to the sequence of DNA and activates a process of transcription
what is transcription?
the way in which a gene’s DNA sequence is copied into mRNA
what is translation?
when a ribosome attaches to the mRNA and moves along it reading each triplet codon (3 bases) and using transfer RNAs (tRNA) to put together the amino acid chain to make a protein
what is Mendel’s law/Mendelian inheritance?
inheritance occurs via ‘transmissible units’
we all inherit one copy of each gene from each parent and so have two total
what is a genotype?
genetic information
what is a phenotype?
how the genotype displays (interaction of genotype with environment)
what are alleles?
variants of a gene
what are the genetic variations that can affect the brain and behaviour?
single gene disorders (dominant or recessive)
gene variations/mutations (affect function - coding sequence; affect expression - non-coding, regulatory sequences)
chromosomal abnormalities
X-linked conditions
what is Huntington’s Chorea?
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
single gene disorder
what is Phenylketonuria?
mutation in the PAH gene (phenylalanine hydroxylase - enzyme that breaks down dietary phenylalanine)
recessive inheritance
what are the types of chromosomal abnormalities?
monosomy - single copy of a chromosome (embryonic lethal)
trisomy - three copies of a chromosome (high rate of embryonic lethality)
what causes Downs Syndrome?
trisomy in chromosome 21
what causes major variation in gene dosage between the sexes?
males have XY chromosomes and females have XX chromosomes
y chromosome has very few genes; mostly controls male sexual function
x chromosome - has lots of genes that are important in both sexes
what is X inactivation?
in females, one copy of the X chromosome is switched off during embryogenesis
this ensures that the dosage of active genes is maintained in all individuals
what is Rett syndrome?
X-linked disease
progressive neurodevelopmental disorder almost exclusively affecting females leading to profound disabilities (autism-spectrum disorder)
caused by a mutation in the gene MeCP2
what is fragile X?
most common inherited form of learning disability
X-linked: symptoms predominantly in males
mutation in one end of the FMR1 gene
what is epigenetics?
the study of changes in organisms caused by modification of gene expression (phenotype) rather than alteration of the genetic code itself (genotype)
how does the environment interact with epigenetics?
some epigenetic modifications can be affected by the environment
early developmental influence on stress resilience/depression in later life
transgenerational epigenetics
environmental influence on parents can affect offspring
e.g. disrupted histones in sperm cells - showed altered RNA profile in offspring and grand-offspring
what do gene association studies do?
look for sorting of SNPs in candidate genes
genome wide association studies (GWAS) look for what SNPs sort with disease state
what is Alzheimer’s disease?
most common form of dementia
age-related disease
which genes are associated with Alzheimer’s disease?
mutations in amyloid precursor protein or PSEN1/PSEN2/APP genes are causative for AD but are rare
APOE4 - changes expression of amino acid (one SNP different from APOE3, a common allele)
what is concordance?
the degree to which a trait is seen in 2 individuals
relationship between genes and environment
environment can change gene expression (epigenetics)
genes can alter how we interact and react to a particular environment
benefits of animal models
similarity of genes and biological function with humans
conservation of behaviour with humans
create inbreed strains of animals that are genetically identical
control environmental conditions
manipulate genes - mutate or remove particular gene or insert copy of a human gene
why are mice used for animal models?
known genome (22,00 genes, 20 chromosome pairs)
99% of genes have homologues in humans
similarly organised brain and behavioural traits common across mammalian species
lots of background info on biological processes and well-defined behavioural tests including models of disease states
what is targeted mutagenesis?
mutate a particular gene and look for subtle changes in behaviour
types of genetically engineered mouse models
knockout mice - what happens in the absence of a specific gene
knock-in mice - introduce a specific mutation; ‘humanised’ mice
transgenic mice - reporter constructs to tag cells; constructs to target cell-specific or time-specific gene manipulations
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