(29)
11. Beskrive specifik og homolog rekombination, herunder hvornår, hvordan og
hvorfor det sker
Stryer, s.766-8
Devlin, s.186-8,
fig. 4.23
Recombination
– the exchange of genetic material.
Homologous recombination – exchange of genetic material between identical or nearly identical sequences.
Enzymes called recombinases catalyze the exchange of genetic material during recombination. A key intermediate in the process is a cross-like structure, called Holiday junction, formed by the four polynucleotide chains of the 2 chromosomes.
I
will take is stepwise:
1. The process begins with recombinase binding to the DNA substrates and aligns them properly. Single strand breaks are made at homologous positions in strands of same polarity. (fx. both have 5`-3`).
2.
Strands partly unwind and one strand from DNA helix 1 reciprocally base pairs
with the opposite homologous DNA molecule from DNA helix 2, forming a structure
in which two intact strands are joined by two crossed strands. Regions in which
one strand was from DNA molecule 1 and the other one is the homologous strand
from DNA molecule 2 are called heteroduplexes. In the human
genome, where two individuals differ by one base pair in 1000, there is at least
one mismatched base pair per heteroduplex. It can undergo mismatch repair,
converting one allele into another, by gene conversion.
3.
Then the position of the cross-over moves by Branch migration –
there is simultaneous unwinding of the DNA molecule 1 and 2, followed by
rewinding of the two heteroduplexes, but the total number of hydrogen bonds
remains unchanged. This process creates larger heteroduplexes.
4. DNA ligase seals the original nicks of the DNA and all four crossover strands are held on one crossover point.
5.
Then the DNA molecules rotate, so that they form the holiday junction, which is
a cross like structure held by recombinases. There are two resolutions to the
holiday junction, where molecules are separated by a symmetrical cut in either
horizontal or vertical direction.
A horizontal cut will cause no recombination, while a vertical one would cause recombination (if the gene does not lie in the heteroduplex region)
Homologous recombination occurs in mejosis, where two DNA molecules recombine with each other to form new DNA molecules that have segments from both parental strands. This process shuffles the combination of genes before they are passed to the next generation, generating genetical diversity.
Specific recombination
I was not able to find the therm *specific recombination* in the book. As I understand it, (which is not necesserily correct), specific recombination is the same as non-homologous recombination. So, the sequences exchanged between the chromosomes are not homologue.
There are 3 types on non-homologous recombination:
1. Site-specific recombination – a non-homologous recombination, in which specific enzymes catalyse the integration of a sequence into particular sites of DNA.
Fx.
Integration of bacteriophage λ in the bacterial genome
of E. Coli. The λ-integrase
catalyses specific nicking of λ-DNA and of a special
sequence of E.Coli chromosome and the resealing involved. The process is
reversable.
2. Transposition
3. Illegitimate recombination
No homology at all needed. No special
seqeunces.
When genes are introduces in the human genome (with gene therapy) they randomly
insert into the chromosomes probably using the enzymes involved in double strand
repair/recombination. The DNA fragment can insert in a middle of a gene seqeunce
causing muations. This is a major limitation of gene therapy.
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