lecture 5 - Transposons - Dynamic Genomes Flashcards
What are transposons?
natural transposons can be exploited to generate insertional mutants
transposon or transposable element is a discrete DNA sequence element that can move from one location in the genome to another - often under the control of a transposase encoded by the element itself
Sometimes called jumping gene
Dynamic feature
Shown as triangles – indicating it is an insertion
Exploited by transposon mutagenesis
Some transposons have the ability to move themselves – excise that piece of DNA out of the genome and allow it to move to other location in the genome
Selfish gene – appear to have no other function other than moving around the genome
What is the purpose of direct repeats?
integrated transposable elements are flanked by short direct repeat sequences
direct repeats allow transposons to be identified in genome sequencing projects
How do DNA transposons transpose?
DNA transposons use either replicative or conservative transposition
(retro-elements transpose replicatively via an RNA intermediate)
DNA transposons transposing conservatively move to a new position within the the genome
Describe how maize is used as a model system for transposons
variegated pigmentation in maize kernels caused by transposition in somatic cells
a model system for studying transposons
Good genetic model required for evidence of transposable elements
Separated male and female gametes in maize so cross fertilisation can be carried out
Mouse – less control over gametes and less progeny
McClintock - The Ac - Ds transposons in maize: A full-length Ac element contains a functional transposase gene
The Ds element has an internal deletion & does not code for a functional transposase
The transposase recognises the 11 bp inverted repeats (IRs) at either end of the Ac & Ds transposons & catalyses transposition of both Ac and Ds elements
Maize has about 10 different types of Ds element of different sizes with internal deletions of 194 bp to several kb
Ds elements couldn’t move on their own – need second factor
Hypothesised Ac was the factor that allowed Ds to move
What are the consequences of recombination between pairs of transposons?
Recombination between pairs of transposons can result in deletion of segments of the genome (between the transposons) or chromosome breaks
What were the conclusions of McClintock’s experiments?
McClintock discovered that recombination between transposons can also cause chromosomes to break and demonstrated that some regions of the genome are more likely to suffer chromosome breaks than others
McClintock concluded that mobile elements within the chromosomes must be responsible for these breaks because these elements could move to a new position and the location of the chromosome break would move with them
She used the Ac/Ds transposon system to demonstrate this - the Ds elements were stable and did not move in the absence of Ac, but Ds could move in presence of Ac
Even though these experiments were completed in the 1940s and 1950s, was McCintock the first person to realise that genomes are not static and are capable of change
How is the size of the pigment affected by when the mutation occurs in development?
Mutation early in development - large section of pigmentation
Mutation late in development - small section of pigmentation
How is the colour of the maize kernel determined?
Flowering plant seeds are the result of a double fertilisation event - a diploid embryo and a triploid endosperm
One nucleus from the male gametophyte fuses with the egg to form a zygote
The other unites with the diploid cell of the female gametophyte to form a triploid cell that gives rise to endosperm
In maize, the colour of the kernel is determined by the triploid endosperm, not the diploid embryo
What alleles did McClintock use and how was her experiment designed?
Alleles and genes that McClintock worked with:
CI
- dominant allele that prevents pigment from being expressed in the aleurone layer
C - recessive allele that leads to pigment developing in the aleurone layer
Bz - dominant allele that produces purple aleurone pigment
bz - recessive allele that produces a dark brown aleurone pigment
McClintock carefully selected male lines that were homozygous for dominant alleles, and female lines that were homozygous recessive alleles.
Homozygous lines also meant that both alleles are passed onto the embryo and can be analysed in the future generations
McClintock would also know which alleles would be present in the triploid endosperm and in what proportion
McClintock concluded that a chromosome break between Bz and the centromere caused the bronze
sectors. The chromosome break always occurred in the same location. McClintock could confirm the
presence of these breaks by comparing the sizes of the chromosomes.
If Ds transposes to a new location when it is crossed into the same genetic background
as Ac
The position of the chromosome break will also move to the same location as Ds
McClintock could identify these events using a simple screen and could examine the
size of the chromosomes under the microscope to find where the chromosome breaks occurred
Transposon induced chromosome breaks can generate new chromosomes - these new chromosome survived in McClintock’s maize and enabled her to
prove many of her theories
Do transposons have a significant role in genome evolution?
Transposons can initiate recombination events that lead to genome rearrangements
Recombination between a pair of LINE-1 elements 35 MYa is thought to have generated the β-globin gene duplication resulting in Gϒ & Aϒ members of this
family
transposon directed gene expression
mouse gene Slp codes for a protein in the immune response - the
tissue specificity of Slp is conferred by an enhancer located within a
transposon
Describe how Transposons can carry gene segments
Mutator-like transposable elements (MULEs) often contain captured DNA sequences
Pack-MULEs transposons are widespread in rice
Pack-MULEs can collect different gene segments to make new hybrid genes and move these sequences to a new position
