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Organismal genetics > L14-18 > Flashcards

L14-18 Flashcards

1
Q

why study drosophila malanogaster

A
  • rapid life cycle (10 days)
  • easy to obtain mutants
  • ~100 years of accumulated knowlage and tools
  • inexpensive
  • sequenced genome
    -drosophila has XY sex determination
  • small number of chromosomes
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2
Q

what/when is the drosophila malanogaster not undergo meiotic recombination

A
  • chromosome 4
  • males
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3
Q

what is a hypomorphic mutation (example)

A
  • partial loss of function
  • white apricot mutation causes orange coloured fly eyes
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4
Q

what is an amorphic mutation (example)

A
  • complete loss of function
  • white mutation in flies leads to no pigmentation
  • notch causes wing abnormalities, needs two copies of wild type for wild type phenotype (dosage effect)
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5
Q

what is an antimorphic mutation

A
  • when duplications are put in there is still not a complete wild type phenotype
  • negative activity
  • e.g. ebony, interferes with wild type protein function via interaction with mutant protein and wild type protein (implies wild type is a polymer protein
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6
Q

how does an antimorphic mutation work

A
  • anitmorphic alleles act as dominant negatives by competing with and reducing the activity of wild type gene products
  • nonsense mutations results in expression of trucated protien, competes with wild type for binding to substrate 1 but cannot bind substrate 2. overexpress a dominant negative muatnt form
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7
Q

what is a hypermorphic mutation

A
  • gain of function
  • elipse isgain of function mutation in the drosophila EGF receptor which results in increased singalling, dominant, icreased activity however disrupts eye, gain of function as deficiency restores eye
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8
Q

what types of mutagenesis can occur

A
  • alkylating agent (induces point mutation, very efficient)
  • radiation (induces chromosomal rearangement, deletions of varying sizes, inversions, translocations, less efficient mutagen, easier to map location than point mutations)
  • pelements (transposons, induce insertion mutaions, low frequency and non random, very easy to map insertion site, large and growing collections of single P-element, insertion lines available
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9
Q

how do alkylating agents work

A

EMS replaces hydrogen bond in guanine, alkylates oxygen, changes the hydorgen bonding pattern from a 4 bond to a 2 bond and changes the partner to which this base will interact with in the double helix

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10
Q

what is the most used transposon in drosphila

A

p element

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11
Q

what is the cellular blastoderm cell differentialted into

A
  • pole cells (primordial germ cells)
  • 6000 somatic cells
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12
Q

what are the problems of genetic screening

A
  • most mutations are recessive
  • many mutations are lethal so you can only recognise the phenotype when the fly is already dead
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13
Q

how to screen recessive lethal mutations and maintain fly stock

A
  • hit male flies with a mutagen
  • cross to wild type females (diploid progeny will be heterozygous)
  • cross individual f1 males to wild type females
  • self cross f2 progeny and identify those carrying lethal mutations (cannot destinguish +/+ from +/-
  • to distinguish we use a mutant marker in mutant, i.e. eye/ body colour
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14
Q

what is needed to identify a lethal recessive mutation during recombination

A

a balancer chromosome
- heavily mutated chromosome from female f1
- chromosome cannot lineup with partner due to structural differences
- supresses genetic recombination

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15
Q

what is an example of a balancer chromosome in action

A
  • a CyO (carries Cy,cn mutations)
  • screen will end up with a with a-/CyO, Cy,cn (where a- is lethal)
  • crossing them together
  • creates two lethal a-/a- and CyO/CyO
  • one viable Cy phenotype a-/Cyo,Cy/cn
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16
Q

what are the maternal effect mutations

A

zygotic mutants
- gap genes
- pair rule genes
- segment polarity genes
- homeotic genes

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17
Q

what is an example of maternal effect gene

A

bicoid
- female bicoid appears normal (bicoid-/bicoid-)
- male is missing head and thoracic structures (bicoid-/+)
- if some anterior/posterior cytoplasm is replased then tail/head will form
- bicoid gene is from mother

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18
Q

what occurs to the drosophila egg cells during formation

A
  • germ line has 16 eggs but only 1 becomes the oocyte, the rest are nurse cells
  • nurse cells form cytoplasmic bridges that feed in mRNA and the bicoid gene
  • diffusion of bicoid mRNA is restricted by binding to the cytoskeleton, (causes concentration gradient at anterior end)
  • nuclei along A-P exis exposed to varyung bicoid conc.
  • bicoid is a transcription factor, able to read position due to diff in conc of biciod
  • in humans nucleation in first cell
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19
Q

what are gap gene mutants

A

several segments of embryo are missing , missing segements are where the GAP genes are expressed

20
Q

what happens if gap genes are mutated

A

loss of several segments and regions in the embryo

21
Q

what is the function of gap genes

A
  • give course, initial sub-devision of the embryo,
  • encode transcription factors regulating gene expression,
  • give original A-P
  • bicoid gradient binds to enhancers that regulate GAP ecpression, different conc enables expression of different gap genes
22
Q

what are pair rule mutants

A
  • missing alternate segments (either odd or even)
  • expressed in narrow stripes in the segment affected by mutation
  • encode transcription factor proteins
  • integration of singnals regulated by the different gap genes controls expression of pair rule genes
  • croeates spatial seperation 4 cells wide
  • Second even skipped strip has control regions that respond to certain thresholds of bicoid and other GAP genes called hunchback and a gap gene giant and kruppel, (combine to make a precise even skip)
23
Q

what are segment polarity mutants

A
  • mutants missing parts of segments
  • expressed in narrow stripes down to single cell width
  • expression regulated by pair rule genes
  • frequently encode signalling pathway components
24
Q

what mutations occur due to segment polarity mutants

A
  • expressed in a narrow region however are required for the making of the naked cuticle part of the repeating segmental pattern
  • mutation of this causes a continously spiky fly
25
Q

what is the homologous of segment polarity genes in humans

A

associated with cancer
- cyclopia (fused eye), the region of which hedgehod is expressed disapears

26
Q

what are homeotic mutations

A
  • do not alter number of segments but alter identity of segments
  • are selector genes ( initiate programs of gene expression which conger identity of segments
27
Q

what is the function of homeotic genes

A
  • evolved from gene duplication
  • all encode transcription factors
  • contain homeobox domian (a 60 amino acid helix turn helix class DNA binding domain)
  • binds to DNA allowing transcription factor activity of the genes
28
Q

what genes regulate expression of hemeotic genes

A

gap genes and pair rule genes
- determine position and number of segments and together determine which combinations of hemeatic genes are expressed in what segment

29
Q

what is a loss of function hemeotic mutation

A

identity change to more anterior segment

30
Q

what is a gain of function hemeotic mutation

A

identity change to more posterior segment

31
Q

why study the fly eye

A
  • Array of around 800 ommatidia
  • Highly ordered packing of ommatidia
  • Easily identify mutations that disrupt ordered packing of ommatidia
  • Eye is not required for viability of the fly (Maintain stocks of fly that are lacking a functional eye)
32
Q

how are flies lacking the R7 in the ommatidia identified

A

do not move to UV light

33
Q

what are enhancers screens

A

assumption: if two mutation are in genes within the same pathway then the phenotype resulting from the combination of the mutants will have a greater phenotype than expected from the simple addition of their phenotypes i.e. a synergistic interactio

34
Q

what are supressor screen

A

mutation of one gene in a pathway supresses phenotypic connsequences of a mutation in another pathway component

35
Q

what is a synergistic phenotypic change

A

when the mutation of two phenotype mutants exceeds the phenotypic reaction two the addition of the two phenotypes

36
Q

what is an example of conditional mutants

A

use of temperature sensitive allele of sevenless
- perform screen at temperature where there is just enough sevenless activity to support R7 differentiation, identified 3 genes, Ras, son of sevenless, downstream of receptor kinase

37
Q

what is RAS

A
  • a GTPase enzyme
  • transmits signals from receptors
  • has two conformations (GTP signal on and GDP singla off)
  • when GTP is hydrolysed it switches to GDP which turns it off
38
Q

what is the son of sevenless mutation

A
  • functions to activate Ras,
  • biochemically shown to act as a GTP/GDP exhange factor
  • promotes replacemtn of GTP for GDP
39
Q

what is downstream of receptor kinase

A
  • SH3 (binds to proline rich protein sequence, in sos
    -SH2 (binds phosphorylated tyrosine to sevenless
  • may be a liker protein linking the receptor to RAS
40
Q

how is a sepressor screen for negative regulators of a signal tested

A
  • perform screen at temp where not enough sevenless to support R7 differentiation
  • identified GTPase activating protein (GAP) (stimulates GTPase activity and promotes conversion of RAS to its inactive GDP form)
41
Q

what is the human homologe of Ras

A

an oncogene with a mutation that locks GTP on, so the signal is perminantly on

42
Q

how does downstream of receptor kinase function

A

as an adaptor protein between RTK and sos allowing recruitment and activation of Ras

43
Q

how can epistasis be used to determin gene function in the same pathway and theri function order

A
  • need two mutations with opposite phenotypes
  • hypermorphic (GOF) mutation of B (receptor(=) resutlts in increased signal (C) in absence of ligand (A)
44
Q

what is mosaicism

A
  • dominant mutation in early developement
  • could arive through chimerism (cells from a different origin(absorbed cells from a non-edenticle twin that failed to develop)
45
Q

how does mosaicism arive from transposons

A
  • maize mutations caused by insertion of transposable element
  • detected by pigment phenotype

Organismal genetics (4 decks)

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