Lecture 29 Flashcards
What are highly mutable alleles?
Alleles that commonly mutate partway through develop to cause an obvious phenotypic change
If the mutability is common then there will be many spots, if it occurs early in development then the spots will be large, if it occurs late then the spots will be small
What causes highly mutable alleles?
Segments of DNA known as transposable elements jumpin into the regulatory elements of a gene (if it jumps into the coding region then when it is excised it will leave a mark which can results in a stable mutation)
What was evidence that showed the transposable elements existed in bacteria?
The moving of antibiotic resistance and the characteristic lollipop structure
What are the different types of repeated DNA sequence?
A direct repeat where the sequence is repeated in the same base pair order
A palindrome which is read the same backwards and fowards
Inverted repeats where the base pair order is reversed
How do transposable elements cause the cruciform structure?
They contain inverted repeats at both ends this allows them to base pair with themselves in a hairpin loop fashion to from the lolipop structure
What are the three classes of bacterial transposable elements?
Insertion sequences which are small and have no phenotype
Transposons which are larger and carry bacterial genes
Bacteriophage Mu
Which is a large complex virus
What are the properties of insertion sequences?
Cause polar mutations as they can result in signals for the termination of transcription so all genes downstream in the operon are not transcribed
Contain inverted repeats at the termini which may extend throughout the whole length
The only protein they encode for is the transposase enzyme which allows movement
What are the properties of transposons?
3-10kb long
capable of transposition
Have inverted repeats some of which can act as functional insertion sequences
Contain additional bacterial genes some of which are antibiotic markers
Cause gene disruption which may or may not be polar
What is the transposase enzyme?
An enzyme which is coded for on most transposable elements
Bind at the terminal inverted repeats of transposable elements to mediate movement of the transposable element
Can be random or very specific
What is generated by transposition?
The transposase makes staggered nicks at the site of insertion
Leading to the generation of short direct repeats
These can be characterised by length to group transposable elements into families
What are the two types of transposition?
Replicative where there is a copy produced and inserted elsewhere in the genome but the original remains in its current location
Non-replicative where the original is excised and placed somewhere else in the genome
What are the 10 properties of bacterial transposable elements?
- Short DNA elements (0.2-10kb) able to transpose
- Found in different locations in different genomes
- Encode transposase and sometimes additional genes
- Flanked by inverse repeats (sometimes direct repeats) which is the binding site for transposase
- Generate short direct repeats of characteristic length at insertion site
- Replicative or conservative transpostion
- Insertion site can be random or have sequence specificity
- Excision of conservative elements can be precise, imprecise or lethal
- Frequently cause deletions, inversions and rearrangements
- Transposition frequency is tightly controlled
Why are transposable elements not replicons?
They do not have an origin of replication and therefore replicate passively
How can pyramiding of transposable elements case clinical problems?
Pyramiding of transposable elements occurs when a transposable element inserts itself within another transposable element and can cause bacteria to become resistant to multiple different antibiotics
How are transposable elements used to clone genes?
Can be used as a tag and introduced to pool of host cells on a suicide vector
The mutant with phenotype of interest can then be identified and the transposable element can then be used a probe to clone the surrounding wild type DNA
