LECTURE 3 (the molecular basis of mutation 3)

Question 1

What’s deletion?

Answer 1

The loss of base pairs in a gene or a part of a chromosome.

Question 2

What does a chromosome deletion involves?

Answer 2

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.

Question 3

What are the two main types of deletions?

Answer 3

Intragenic and multigenic.

Question 4

describe intragenic deletions

Answer 4

It’s located w/in the gene, it deactivates it and produces a null mutation (the gene won’t be expressed e.g albinism

Question 5

describe multigenic deletion

Answer 5

not only one gene is missing but several. serious consequences. appears as a result of inbreeding leaving homozygous. can uncover recessive alleles

Question 6

How can we correct intragenic deletions to the wild type?

Answer 6

it’s impossible, only base pair mutations not involving deletions can be corrected.

Question 7

How can we detect deletions?

Answer 7

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

Question 8

what does the uncovering of recessive alleles imply?

Answer 8

A deletion.

Question 9

Give two examples of deletions in humans

Answer 9

• 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

Question 10

What is pseudo dominance?

Answer 10

When a recessive allele is expressed and so it appears to have dominance

Question 11

How can we use pseudo dominance to map deletions?

Answer 11

if the deletion location is known pseudo dominance can be used in reverse so we can asses the position of the mutant alleles.

Question 12

What are duplications?

Answer 12

extra copies of chromosome regions.

Question 13

What types of duplications can we encounter?

Answer 13

Tandem duplications and insetional duplications

Question 14

What are tandem duplications?

Answer 14

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.

Question 15

What are insertional duplications?

Answer 15

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.

Question 16

How can we use microarrays to quantify deletions and duplications?

Answer 16

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.

Question 17

What are transposable elements?

Answer 17

DNA elements that move to new positions in the chr or in a different chr

Question 18

How are T.E detected?

Answer 18

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.

Question 19

Which % of human chr are T.E ?

Answer 19

50%

Question 20

How many classes of T.E are there?

Answer 20

class 1 and class 2

Question 21

What are T.E class one?

Answer 21

they’re retroelements bc they have an RNA intermediate this RNA intermediate makes the insertion to be permanent

Question 22

What are class 2 T.E?

Answer 22

they’re simple DNA elements. they were discovered in maize

Question 23

How are transposons called in euk? What structure do they show?

Answer 23

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)

Question 24

How many types of bacterial transposons can we find?

Answer 24

• composites

- simple

Question 25

What are composite bacterial transposons?

Answer 25

composites are a bunch of bacterial genes located between two IS oriented in opposite direction which encode for transposase

Question 26

why is transposase important in transposons?

Answer 26

Because it catalyzes the mvmt of the whole transposon

Question 27

What are simple transposons in prok?

Answer 27

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

Question 28

When do P elements appear?

Answer 28

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

Question 29

What does the term “hybrid dysgenesis” mean?

Answer 29

Mate females from the labs with males from the nature

Question 30

(M) F x (P) M? (P) F x (M) M? what do the letters stand for? What’s the result of these crosses?

Answer 30

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

Question 31

Characteristics, structure and size of P elements.

Answer 31

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.

Question 32

Why aren’t P elements present in lab pop?

Answer 32

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

Question 33

How can we silence T.E?

Answer 33

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.

Question 34

Why do we wanna silence T:E

Answer 34

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

Question 35

How many Ago (argonaut families) are there in drosophila?

Answer 35

5
ago 1
ago2 + piwi
ago 3
aubergine (aub) 
Piwi

Question 36

What is piRNA?

Answer 36

siRNA+ piwi protein

Question 37

Why is this happening when females from the lab mate with males from nature and not the other way round?

Answer 37

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.

Question 38

What’s the most common T.E in humans?

Answer 38

Alu (it’s a SINE)

Question 39

Which two types of T.E do we find in humans?

Answer 39

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)