LECTURE 3 (the molecular basis of mutation 3) Flashcards
What’s deletion?
The loss of base pairs in a gene or a part of a chromosome.
What does a chromosome deletion involves?
It involves the cut of a region that’s not the centromere otherwise it couldn’t be pulled in the chromosomal spindle and it’ll be lost.
What are the two main types of deletions?
Intragenic and multigenic.
describe intragenic deletions
It’s located w/in the gene, it deactivates it and produces a null mutation (the gene won’t be expressed e.g albinism
describe multigenic deletion
not only one gene is missing but several. serious consequences. appears as a result of inbreeding leaving homozygous. can uncover recessive alleles
How can we correct intragenic deletions to the wild type?
it’s impossible, only base pair mutations not involving deletions can be corrected.
How can we detect deletions?
via chromosome mapping, having a look under the microscope and determining the position of the deletion loop or if a recessive allele is expressed (shows dominance) in a heterozygous that implies a deletion
what does the uncovering of recessive alleles imply?
A deletion.
Give two examples of deletions in humans
- Cri du Chat. occurs a the tip of the p arm of chr 5. moonlight face, mental retardation, reach adulthood.
- williams syndrome 1.5Mb deleted in the chr 7 musical abilities, caused by unequal crossover probably
What is pseudo dominance?
When a recessive allele is expressed and so it appears to have dominance
How can we use pseudo dominance to map deletions?
if the deletion location is known pseudo dominance can be used in reverse so we can asses the position of the mutant alleles.
What are duplications?
extra copies of chromosome regions.
What types of duplications can we encounter?
Tandem duplications and insetional duplications
What are tandem duplications?
two identical DNA seq or chr region e.g an extra cur arm or part of the arm attached to a non-homologous chr. They can’t unmask recessive alleles.
What are insertional duplications?
identical DNA seq that are in different section of a chr or even in different chr. they’re essential for evolution bc they get accumulated over several generations.
How can we use microarrays to quantify deletions and duplications?
The DNA of wild type and of mutant are dyed with different fluorescent dyes that emit different wavelength. this are added to cDNA clones and will hybridize and we can compare the abnormal wavelengths to the normal chr one to quantify these types of mutations.
What are transposable elements?
DNA elements that move to new positions in the chr or in a different chr
How are T.E detected?
Through mutations because when they’re inserted they deactivate genes. they have effect on genes if they’re located in the middle of a coding seq.
Which % of human chr are T.E ?
50%
How many classes of T.E are there?
class 1 and class 2
What are T.E class one?
they’re retroelements bc they have an RNA intermediate this RNA intermediate makes the insertion to be permanent
What are class 2 T.E?
they’re simple DNA elements. they were discovered in maize
How are transposons called in euk? What structure do they show?
Bacterial insertion sequences. (IS) They always interrupt a gene inactivating the gene expression. Firstly found in gal operon in E.coli
(IR) abcd transposase dcba (IR)
How many types of bacterial transposons can we find?
- composites
- simple
What are composite bacterial transposons?
composites are a bunch of bacterial genes located between two IS oriented in opposite direction which encode for transposase
why is transposase important in transposons?
Because it catalyzes the mvmt of the whole transposon
What are simple transposons in prok?
They are bacterial genes located between two IR that do not encode for transposase therefore as well as bact genes the simple transposons have to carry their own transposase to allow movement
When do P elements appear?
When labs females are mated with males from natural pop. these P elements were found in P individuals in nature and completely absent in lab individuals
What does the term “hybrid dysgenesis” mean?
Mate females from the labs with males from the nature
(M) F x (P) M? (P) F x (M) M? what do the letters stand for? What’s the result of these crosses?
M are individuals from labs w/o transposons and P are imdv from nature with transposons, the 1st cross gives sterile, chr aberration and high mutation rate offspring whereas the 2nd gives fertile and normal offspring
Characteristics, structure and size of P elements.
3 introns and 4 exons, vary in size (around 3kb). they resemble the simple transposons in bact. Inverted repeats (IR) at each terminus. (32bp). they encode transposase therefore they’re able to jump they’re type II because they do not require RNA intermediate.
Why aren’t P elements present in lab pop?
One hypothesis is that most of the current laboratory strains descended from the original isolates taken from the wild by Morgan and his students almost a century ago. At some point in time be- tween the capture of those original strains and the pres- ent, P elements spread through natural populations but not through laboratory strains
How can we silence T.E?
via the RNAi pathway. dsRNA is chopped by Dicer protein into 21bp ling siena which will bind Ago protein forming RISC (silencing protein) which targets specific mRNA and inactivates it.
Why do we wanna silence T:E
maybe you have a TE in somatic or germ line cells and your progeny is gonna be defective then your organism will activate the RNAi pathway to silence that TE
How many Ago (argonaut families) are there in drosophila?
5 ago 1 ago2 + piwi ago 3 aubergine (aub) Piwi
What is piRNA?
siRNA+ piwi protein
Why is this happening when females from the lab mate with males from nature and not the other way round?
The P element from the male can jump anywhere. if the F has a P element, it’ll need a silencing complex so in the offspring it won’t be expressed. however no P element no need of silencing complex but the M would insert the P element
conclusion: silencing complex are inherited to the offspring from the female oocyte and in dysgenic crosses there’s a lack of blocking system.
What’s the most common T.E in humans?
Alu (it’s a SINE)
Which two types of T.E do we find in humans?
LINEs(moved by retrotransposition with RT code for RT and integrate) and SINEs.(don’t code for any enzyme but use RT encoded by LINEs)
