G3 Flashcards
Why is deleted fragment of a chromosome lost in cell divsion?
because it does not contain a centromere so cannot be pulled to poles
What is an intragenic deletion and its affects and how it can be distinguished from a point mutation
deletion within a gene
has the same affect as a null mutation
distinguished from point as doesnt revert to wildtype
what is an example of a viable homozygous null mutation
human albinism
- if homozygous null mutation is viable than deletion will be too
what is a multigenic deletion and what affect does it have on a homozgote or heterozygote
several genes are missing - serious consequences
if homozygous by inbreeding, lethal
may be lethal for heterozygote
what can a multigenic deletion uncover
deleterious recessive alleles, allowing single copies to be expressed
what can lethality of large heterozygous mutations be explained by
gene imbalance and expression of deleterious recessives
where can deletion loops be found, what are they and what can they tell us
polytene chromosomes in drosophillia melangaster
failure of normal homologs to pair creates a deletion loop
deletions can be allocated to specific location by determining position of loop
what is pseudodominance and what can we detect from this
when recessive allles appear to have dominance due to deletion in other homolog
- can detect deletions from this
What is deletion mapping
when each deletion is paired with each mutant under test and the phenotype observed to see if the mutation is pseudodominant - able to poinpoint location of mutant
if pseudodominant deletion and mutation in same region
if deletetion location of know can be applied in the reverse to map assess position of mutant alles
what is cri du chat syndrom
caused by heterozygous deletion at tip of short arm chromosome 5, low fatalty rates
What are tandem duplications
2 adjacent identical dna seqs or chromosome segments can be seen in drosophilia polytene chromosome - rare in humans
what is an exammple of humans homoxygous for duplication
case unknown
what are insertional duplications
identical dna seqs of chromosomes segements that are in different locations of a chromosome or in different chromosomes - KEY TO EVOLUTION
Describe Class 1 transposons
reteroelements
move by retrotransposition
high copy number possible as several RNAs can be transcribed froma s ingle class 1 element.
permenant transposition
describe class 2 transposons
named DNA elelmetns
move from one site to another - excise from donor site
if insertion of a gene has created a mutation, excision leads to the reversion of orginal mutation
give an example of class 1 and class 2 transposons
class 1 = alu element class 2- discovered in maize leads to spotted kernels due to their excision from genes expressing pigmentq
what are mendels wrinkled peas an example of
a wildtype round R allele interuptted by a transposon. `
how much of human mouse and drosophilia genome are transposons
n
human and dros = 50%
mouse =70%
What are bacterial insertion sequences?
prokaryotic transposons
segements of bacterial DNA that move from one position toa different position on the same chromosome
block expression of other genes in same operon if these genes are downstream from the promoter
What are the two types of bacterial insertion sequences
complex/composite and simple
describe complex/composite transposons
contain several genes between 2 IS orientated at opposite sides of bacterial genes.
enzyme transposase encoded by IS element catalysed movement.
IS element that makes up composite transposons cannot transpose on their own
give an example of a complex transposon
Tn10 carries a gene that confers resistance to tetracycline and is flanked by two IS10 elements in opposite orientations.
Describe simple transpososons
flanked by IR sequences which are short and dont encode transposase enzyme
tranposase is encoded within genes in addition to bacterial genes.
how were P elements in drosophilia discovered
found in natural but not lab grown
occured in last 70 years
Describe RNA interferance/silencing
dsRNA are processed by DICER, into siRNAs of 21 bps, resulting siRNAs bind to argonaut (Ago) protein to form silenecing complex
siRNA Ago cleaves the target transcript - silencing the gene
main way of silencing for tranposable elements in germ line
What are the 5 members of the argonaut family in Drosophilia?
Ago1, Ago2, Ago4, ad the PIWI subfamily - Piwi and Aubergine (Aub)
What do PIWI family members do
associate with piRNAs
what are piRNAs
PIWI interacting RNA
in dros = 23-30nt long ssRNA that mainly original from transposable elements
What is an M cytotype
What is a P cytotype
M =lab stocks
P= natural stocks
what happens when you cross M female with P male
offspring show different phenotypes
sterility - high mutation and high freq of chromosomal aberration = dysgenic offspring
what happens when you cross P female with M male
non dysgenic offspring produced
What are wild females of D.melanogaster thought to contain that lab grown do not?
repressor that prevents transposase transcription found in cytoplasm of oocyte.
which reduces disruption to genome caused by P elements allowing fertile progeny
What is hybrid dysgenesis and what is it marked by
dygenesis of offsping produced by M mother and P father. marked by temp depepndent sterilty and elevated mutation rates as well as increased chromosome rearragment and recombination
What part does Aub play in P female mating of d. melanogaster
aub is the PIWI protein that identifies Ptranposon mRNA in early embryo and cuase aub dependent destabilisation.
what are lines
retrotransposons that move by reverse transcription
what are sines, give an example
dont produce own reverse transcription so rely on sines. examples is ALU
Why does tranpostion of P element only occur in germline cells
because splicing event needed to make transposase mRNA doesnst occur in somatic cells
