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Brain and Behaviour > Neurogenetics > Flashcards

Neurogenetics Flashcards

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1
Q

What are chromosomes?

A

They are present in every cell of the human Boyd (every living organism = unique genetic makeup).

They carry the genetic DNA that we all have in our bodies. Each chromosome has over 100 million base pairs of DNA

Humans have 23 pairs.

Approximately 23,000 genes on human chromosomes.

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2
Q

How is DNA structured?

A

Double helix structure made from type 2 of phosphate + deoxyribose (type of sugar)

4 nucleotide bases (made up of these chemicals):
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)

Deoxyribonucleic acid

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3
Q

How is our genetics sequenced?

A

We share around 99.9% of our DNA w/ each other

Natural variations in DNA = Single Nucleotide Polymorphisms (SNPs). Around 3.3 million SNPs identified through human genome project sequencing shows, how each of us is different. Diff. variations + combos of these variations.

Unique combination of SNPs = inherit from parents - responsible for the frenetic component that makes us different from each other

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4
Q

How does DNA replicate?

A

Through cell division:

Mitosis = somatic cells (daughter cells identical to parent). How the majority of cells are made.

Meiosis = gametes (daughter cells contain half the no. of chromosomes). Used to make egg + sperm cells.

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5
Q

What is the process of mitosis?

A
  1. Diploid with homologues
  2. Sister chromatids in cell
  3. Cell divides into 2 = diploid
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6
Q

What is the process of meiosis?

A
  1. Diploid with homologues
  2. Sister chromatids in cell
  3. Chiasma = sister chromatids have homologous recombination (cross over between 2 pairs)
  4. Homologues segregate
  5. Sister chomatids segregate = haploid cells that carry only one chromosome (4 unique daughter chromosomes)
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7
Q

Why is meiosis important for genetic inheritance?

A

Homologous recombination or ‘crossing over’ = genetic diversity bc the functions of genes might vary due to mutations - idea of natural selection + evolution. Very slow process.

Offspring all Jahre 50% of each parents genes, but different 50%

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8
Q

How are genes linked to proteins?

A

Genes = long sequences of base pairs in the DNA that encode proteins

Genes are activated by transcription factors through binding to specific sequences on the chromosome which activates a gene to be read + translated into a protein.

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9
Q

How are transcription factors activated?

A

Activated during development/ intracellular signalling cascades from other parts of the cell

This is a way of regulating what proteins are expressed + how this alter the function of the cell

e.g if CREB transcription factor = activated, genes are translated into proteins + build components that are at a synapse + strengthen synapses (+ connections between neurons)

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10
Q

How are genes expressed into proteins?

A

DNA partially unravels = transcription factor can bind to the gene

Transcription = in the nucleus, gene’s DNA sequence is copied into messenger RNA ( through mRNA - photocopies the sequence).

mRNA travels from nucleus to cytoplasm

Translation = ribosome attaches to the mRNA + moves along the mRNA sequence reading each triplet codon (3 bases) + using transfer RNAs (tRNA) to put together the amino acid chain to make a protein

These proteins are the building blocks of cells, or enzymes which change the properties of those cells.

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11
Q

What did Mendel find in plants?

A

Mendel (1865) = inheritance through ‘transmissible units’

Had 2 pea plants = short pea + tall pea - when tried to cross-bread, always got short or tall pea plans = never got medium pea plant.

(P1) Tall + short = all tall pea plant - dominant gene is tall pea. But when cross-fertilised of those generation (F1), short gene reappears in the second gen (F2) 3:1 in offspring.

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12
Q

Using the pea plant example, what is Mendel’s Law (Mendelian Inheritance)?

A

Gene = one of two forms (alleles) - tall/ short

2 copies of the gene in each parent pea

1 copy is carried to each of the offspring

Height = Tall (T) is dominant gene + Short (S) is recessive gene

Dominant inheritance + recessive inheritance

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13
Q

What is genotype?

A

Genetic information

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14
Q

What is phenotype?

A

How is the allele displayed

Interaction of the genotype w/ environment

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15
Q

What is alleles?

A

Variants of a gene

e.g tall vs short alleles of height gene in peas

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16
Q

How can variations affect brain and bhvr?

A

Single gene disorders = recessive + dominant

Gene variations/ mutations = affect function (coding sequence) e.g PKU/ Huntington’s + affect expression (non-coding, regulatory sequences)

Chromosomal abnormalities

X-linked disorders

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17
Q

What is Huntington’s Chorea?

A

Dominant inheritance = degeneration of striatum causing progressive deterioration in mvmnt, temperament + cognition.

Autosomal dom. inheritance = single copy will be dom. + lead to the disease (if 1 parent has it, 50% of offspring will develop it)

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18
Q

What causes Huntington disease?

A

Normal chromosome has 11-34 copies of CAG bases. Huntington’s gene has excess of 40 copies of this base = caused by excessive repeat (:) of CAG bases on Chromosome 4 (single gene disorder)

Disease onset = 35-55, number of : (average 44), early onset (60+ repeats). The more : u have, the earlier the onset.

Unstable triplet + increase risk in later generations

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19
Q

What is Phenylketonuria?

A

Recessive inheritance = mutation in PAH gene (phenylalanine hydrolase) which encodes the
enzyme breaks down dietary phenylalanine.

Build up of phenylalanine = toxic to dev. brain - learning dis., bhvrl diff., epilepsy

PKU screening at birth in UK = prevents symps. bc diet

Same genotype, diff. phenotype (interplay of genes + environment)

20
Q

What are the statistics of having phenylketonuria?

A

Carrier = 1/50, disease = 1/10,000

If both parents are carriers = 25% have it, 50% = carriers.

21
Q

How do chromosomal abnormalities such as trisomy affect brain and bhvr?

A

Trisomy = three copies of chromosome (very high rate of embryonic lethality, why people miscarry early in pregnancy).

Example = Down syndrome = trisomy in chromosome 21 (error in first meiotic division). They carry an extra chromosome.

  • Smaller brain size, frontal lobes + cerebellum
  • Mild-moderate intellectual ability
  • High risk of early onset Alzheimer’s

Symptoms = narrowed down to 20-40 genes on chromosome 21 possibly due to over expression of these genes are causing cells to develop differently.

22
Q

How do chromosomal abnormalities such as monosomy affect brain and bhvr?

A

Single copy of a chromosome = embryonic lethal

A problem during meiosis + fertilisation = reduced gene dosage

23
Q

What are X-linked conditions and how does this affect brain and bhvr?

A

Males = XY, females = XX - major variation in gene dosage between sexes

Y chromosome = v little genes, mostly controlling male sexual function

X chromosome = lots of genes = play vital role in both sexes.

Genes need to ensure cell functions normally with either 1/ 2 X chromosomes. X inactivation in females switches off 1 copy of X chromosome during embryogenesis

X-linked disorder vary in their penetrance according to sex

24
Q

What is the importance of X-inactivation in females

A

X-inactivation ensures that dosage of active genes = maintained in all indvdls

XY cells = no inactivation, XX cells = 1 X

An adult female = mosaic of clones derived from diff. embryonic cells. Within cline, all cells inactivate the same X, but between clones - random. If she carriers X-linked recessive disease = major implications.

25
Q

What is Rett syndrome (x-linked)?

A

Progressive neurodevelopment disorder = almost exclusively affecting females - causes disabilities.

Rare condition = 1/10,000 - spontaneous mutation rathe than inherited

Extreme variability between individuals

26
Q

How is Rett syndrome caused?

A

Mutation in gene MeCP2 = ‘transcriptional repressor’ turns off the expression of unwanted genes during synapse formation

X-inactivation in females = not all cells will express mutated MePC2 gene, variable penetrance = sometimes milder symps.

Affected males = don’t have ‘good copy’ of MePC2 = more severe phenotype, embryonic lethal/ die soon after birth

27
Q

What is fragile X (x-linked) disorder?

A

Most commonly inherited form of learning dis.

Symps. predominantly = males

Mutation in one end of the FMR1 gene (the 5’ untranslated region) consisting of an increase of a CGG repeat (200+ copies; normally between 6-40 repeats) SIMILAR CONCEPT TO HUNTINGTON’S

FMR1 gene encodes FMR protect = thought to shuttle select mRNAa between the cytosol + nucleus

28
Q

What are the stats like for fragile X disorder?

A

Typically affects males = 1/4000, 1/6000 females

Carrier of premutation = 1/259 females + 1/800 male

Milder penetrance in females bc X-inactivation = not always recognised

29
Q

Say a summary of how DNA becomes a protein.

A

DNA to mRNA = transcription

mRNA to Proteins = translation

30
Q

What is epigenetic?

A

How your behaviours and environment can cause changes in the way genes are expressed

31
Q

Names examples of epigenetic mechanisms

A
  • Development in childhood
  • Environmental chemicals
  • Drugs
  • Aging
    -Diet
32
Q

What is DNA methylation?

A

Regulates gene expression through using proteins to activate or inhibiting the binding of transcription factors to DNA

33
Q

How are histones linked to epigenetics?

A

The DNA double helix sequence = coils the chromosomes. When they coil = wrapped around histone proteins.

Histone proteins = depending on how they are wrapped up, it becomes compact, DNA becomes inaccessible + inactive.

Histone modification = the binding of epigenetic factors to histone ‘tails’ alters the extent to which DNA is wrapped around histones + the availability of activated genes in the DNA

34
Q

What is an example of an environmentally activated epigenetic process?

A

Which genes are switched on + off = phenotypic differences (neuron, muscle, etc)

Some are affected by the environment = early development influence on stress resilience/ depression in later life

Maternal care (pup licking) activates serotonin through 5-HT7 receptor (intracellular cascade) to activate transcription factor NGFIA = switches on gene Nr3C1 - expressed Glucocorticoid receptor (GR: regulate you, stress pathway)

Absence of licking = high corticosterone levels, anxiety bc absence of transcription factor binding to this receptor doesn’t get active but methylated.

Stayed the same in adulthood, can’t make GR = loss of feedback in HPA axis, increased stress hormones, increased anxiety/ depression

35
Q

What is transgenerational epigenetics?

A

Disrupted histones in sperm cells = showed altered RNA profile in offspring (+ grand offspring)

Histones can be modified by chemicals e.g smoking/ drinking

36
Q

How do we use gene association studies to study disease?

A

Gene association studies = look for sorting of SNPs in candidate genes

37
Q

How are genome wide association studies (GWAS) used to study disease?

A

Look for what SNPs sort w/ disease state

Functional SNPs vs genetic tags

38
Q

How are genetics linked to Alzheimer’s disease?

A

Mutations in PSEN1, PSEN2, APP, Amyloid precursor protein (on Chromosome 21 - Down’s trisomy), very rare = Alzheimer’s. Causative genes.

Risk gene identified in gene assoc./GWAS of late onset of AD

1 SNP diff. between APOE3 (common allele) + APOE4 (Changes an amino acid in protein).

If you have 1 version of APOE4 = 3x more likely to get Alzheimers or 2 versions = 8-12x more likely

39
Q

How are polygenetic factors (twin studies) used to study disease?

A

Concordance = the degree to which a trait is seen in 2 indvdls

40
Q

What does the genetics of schizophrenia show about polygenic?

A

Gottesman (1991) = high correlation of developing Sz + genetic relationship

Large component, but not purely genetic. GWAS study over 150k (36,000 w/ Sz).

Found 108 genes linked w/ synaptic transmission, glutamate + dopamine (Dopamine D2 receptor) which was linked to people w/ Sz.

41
Q

What are influences of genes on bhvr?

A

Environment = change gene expression (epigenetics)

Genes can alter how we interact + react to a spec. environment

42
Q

What are the difficulties of studying genes + bhvrs?

A
  • Defining genetic + non-genetic factors
  • Understanding the interactions w/ many factors
  • Following steps between gene expression + bhvr
  • Limited experimental control + ethical issues
  • To allow for indvdl diff. bc no spec. combo of genes + experience = replicated exactly
43
Q

How are animal models used to study genetics?

A

Similar genes + biological functions as humans

Conservation bhvr w/ humans

Create inbreed strains of animals that are genetically identically

Control environmental conditions

Manipulate genes = mutate/ remove spec. gene/ insert copy of a human gene

44
Q

Why are mice used to study genetics?

A

Mice = known genome, relatively short life cycle.
Mouse genome = 22,000 genes + 20 chromosomes

99% of mouse genes = homologues in man. Same as us. Similarly organised brain e.g cortex, bhvrl traits common across mammalian species

Lots of background info on biological processes + well-defined bhvrl tests, including models of disease states

Targeted mutagenesis = mutate a spec. gene + look for subtle changes in bhvr. Look of the function of genes in isolations.

45
Q

What genetic tools are used on rodents when studying genetics?

A

Inbred strains = 00.9% identical, but gene differences = diff. in neurobiology +bhvr. Comparison tells us how genetics influence bhvr.

BXD recombinant lines = making 2 mice breed together + continue until you the generation you want. Test the inbred lines

Mutagenesis + Knockouts = what happens in the absence of a spec. gene.
Knock in = introduce spec. mutation, ‘humanised’ mice.
Transgenic mice = reporter constructs to take cells.
Constructs to target cell spec. or time spec. gene manipulations

46
Q

Linking the mouse model to Rett syndrome

A

Mutation in MeCP2 = mouse w/ knockout of MePC2 showed similar symps. to Rett Syndrome

inducible Knocking gene switched off during development + reverts to wildtype (normal) form w/ drug treatment. Neurological deficits = reversed has good applications.

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