Lecture 14b Flashcards
In Barbara McClintock’s experiments with maize, why do the colorless/pigmented sectors occur?
Chromosome 9 in maize is heterozygous for the C, Sh, and Wx genes. Breakage triggered by the Ac genetic factor causes breakage at the Ds site. Breakage causes loss of the WT C, Sh, and Wx genes. Therefore, part of the corn kernel is pigmented and part is colored. Size of colorless sector depends on when during growth of kernel the chromosome break happened.
What is the difference between Ac and Ds?
Ds: Dissociation genetic factor, transposable, located at the site of break, do not encode information necessary for its own movement, they need direction from autonomous elements
Ac: Activator genetic factor, transposable, located anywhere in genome, does not require other elements for its mobility, Ac by itself is unstable (can move on it’s own), Ac can turn into Ds, Ac can insert into C gene and inactivate it.
What is an autonomous element vs nonautonomous element?
Autonomous: require no other elements for their mobility (Ac)
Nonautonomous: do not encode information necessary for their own mobility, they need an autonomous element (Ds)
In Barbara McClintock’s experiments with maize, why do pigmented spots occur?
Unstable phenotype occurs because of jumping of Ds. When Ds maps to the C gene, it knocks out function so the kernel is colorless. However, Ac activates Ds to jump out of the C gene allowing the genotype to revert to the WT and produce color. Each pigmented spot represents a separate jump of Ds.
What are the two types of transposable elements in bacteria (prokaryotes)?
Insertion sequence (IS) elements and Transposons
What are insertion sequence elements?
Segment of DNA about 1000bp long, encodes a protein called transposase which is required for movement, all IS’s begin and end with short inverted repeat sequences, they promote recombination
What are transposons (prok)?
They are an insertion sequence with a variety of genes
What is the difference between simple and composite transposons?
Composite: Variety of genes reside between two nearly identical IS elements oriented in the opposite direction. The IS elements are not capable of transposing, one of the IS elements encodes a functional transposon gene
Simple: Variety of genes flanked by inverted repeats but the repeats don’t encode transposase. Therefore on the genes in the transposon is for transposase.
What are the two mechanisms of transposition in prokaryotes?
Replicative and Conservative
Conservative: original copy moves to a new location
Replicative: new copy made in target location and old copy stays put. Copying requires a cointegrate intermediate –> transposase makes two single strand cuts then two more single strand cuts on target site –> ends joined together –> DNA vopies –> resolvase enzyme makes double stranded breaks and rejoins ends to separate the DNA molecules
What are the two types of transposable elements in eukaryotes?
Retrotransposons and DNA Transposons
What are retrotransposons?
They are very similar to retroviruses but they lack the env gene therefore they cannot leave the cell. They replicate via RNA intermediate then insert back into chromosomal DNA. Retrotransposons are similar to Ty elements in yeast and copia like elements in Drosophila.
What are DNA transposons in eukaryotes?
They are like P elements in Drosophila and Ac elements in corn. They can be used as tags in mutants and as vectors to introduce foreign genes into a chromosome.
Mechanism: The Ac element in maize encodes a transposase that binds its own IRs or those of a Ds element, excising the element, joins the ends, cleaving the target site, and allowing the element to insert elsewhere in the genome.
What are common effects of transposons (in euk and prok)?
- Burden on DNA
- Cause higher incidence of recombination
- Change expression pattern of genes depending on where they insert (coding or regulatory region)
How are transposons a source of evolution?
- Non-homologous recombination drives rearrangement of chromosomes
- Altered expression pattern changes cell response
- Transposons can capture host genes and bring them to new recipients
- transposase genes may be modified and used by host
