BIOB11- Lecture 3- Genome Evolution and Introduction to the Central Dogma of Molecular Genetics
Gene mapping, Figure 10-30
Things that can happen - during unequal crossing over.
Fuzzy chromosome imageSame chromosome but one has suffered an inversion
Some cancers are due to inversions etc. on a chromosome
Mobile genetic elements: Transposons
-transposition means change of position from one place to another.
Barbara McClintock: „controlling elements of maize (corn)‟ Nobel prize in 1983
Experiment: gene C (corn)
A transposable element is inserted into the color gene at make it a colorless kernels.
Can this transposable element now jump out of the gene?
If late then some purple some clear (spot) If it is out early in development then mostly purple
-Larger sectors due to transposon jump early in development
Transposable elements do not have a goal site, they can jump into a random site (called recipient or target
-no need to memorize random (AATTC) sequence.
-the transposon inserts into the recipient DNA
-the ends of the transposon are comprised of inverted repeats (in red)
Inverted repeat – the same sequence from 5‟ to 3‟ but arranged in a inverted order (flipped horizontally and
Direct Repeat- the same sequence from 5‟ to 3‟ in the same direction and order
The transposon has a protein that recognizes the inverted repeats so if it need to jump somewhere else, the
protein recognizes that it need to collect all the DNA between the two inverted repeats.
The inverted repeats are flanked on either side by the direct repeats
Generation of Direct Repeat
1. First find a sequence – target site
2. The enzyme transposase encoded by the transposon which acts on DNA and makes staggered cuts
3. the end structure has a partial single stranded structure hanging
4. The transposon then enters the gap and host enzymes come in and repair the DNA using the
complementary base pairs of the end structures creating a direct repeat sequence.
- this is like a “cut and paste method” In a bacteria, the primary method for a transposon moving is for transposase to catalyze its incision from
one side and its insertion in the other side
Transposase binds to the region of dna and recognizes the sequence and then two molecules of
transposase hold on to the molecule and cut it (staggered cut)
Then it inserts itself and repairs the ends
This Cut and paste method is also known as non replicative transposition because there is no replication.
Foot print of transposon is short repeats on both sides when the transposon was removed – shows some
evolutionary proof of the insertion - possibility
Replicative – called retrotransposons
-first transcription occurs and transcribes the entire transposable element.
-then reverse transcriptase (enzyme) converts a single stranded cDNA to a double stranded cDNA
-then it goes to the recipient site and transposase catalyzes the new DNA
-So now instead of having 1 copy of the transposon in a certain place, now we have two it never left the
donor DNA, just a copy of it went to a different site
-A consequence of transposition is that anytime you are moving from one place to another or inserting to an
important place, there is chance of mutation. A consequence of replicative transposition is that now there is
double the amount of transposon that exists and this is what makes some genomes very large
Half of the sequences of our genome is derived from once active transposable element that had spread themselves around the genome – most of them are now inactive
Genetic and Molecular Mapping of Genes (at different levels)
The “Genetics” approach: examine recombination frequency between two traits (usually visible) to
estimate the distance between two linked genes. –Mendelian
-use the two traits and how often they segregate together or apart to estimate the difference between two
“LINKED” - Linked genes that are on the same chromosomes not different ones one
•Distance between genes on a chromosome proportional to probability that recombination will occur
between them. (see example)
•Centimorgan: a map unit of recombination; 1cM=1% recombination between two linked genes.
The red denotes one parent and the black denotes the other parent red and black are two non-sister
chromatids (imagine that each black and red has a sister chromatid but we only look at the two that ar