Clear and orderly summary of Chapter 20 of the book "Genetics: Analysis and Principles, 6th Edition by Robert Brooker". Together with all my other summaries of Genetics I got an 8,5 for this course.
Genetics Summary Chapter 20 – Recombination, immunogenetics and
transpositions
Examine variety of molecular processes in which segments of chromosomal DNA
become rearranged
- Homologous recombination
- Site-specific recombination
- Transposition
20.1 Homologous recombination
Homologous recombination; exchange of identical or similar DNA segments between
homologous chromosomes
It occurs between sister chromatids or between homologous chromosomes
- Sister chromatid exchange; crossing over
between sister chromatids, exchange identical
genetic material. NO new combination of
alleles
- Recombination between homologous
chromosomes during meiosis; YES new
combination of alleles → resulted in genetic
recombination; shuffling genetic material,
creates new combination of alleles
Bacteria are haploid, but they can also undergo homologous recombination.
How happens this?
1. They have more than one copy of chromosome per cell → these can
exchange genetic material via homologous recombination
2. During DNA replication, replicated regions also undergo homologous
recombination
Holliday model for recombination process
Gene conversion; phenomenon in which one
allele is converted (omzetten) to the allele on the
homologous chromosome due to recombination or
DNA repair
Holliday model; model to explain the molecular
mechanism of homologous recombination
Two homologous chromatids are aligned with
each other. → break/nick occurs in one of each
strands → strands invade (binnenvallen) opposite
helices followed by covalent linkage to create
holiday junction; site where unresolved crossover
has occurred. → branch migration; movement of
Holliday junction. → 2 heteroduplex regions are
formed; region of double stranded DNA that
contains one or more mismatches.
, Final 2 steps are called resolution; involved the breakage and rejoining of two DNA
strands to create two separate chromosomes
Holliday junction in two forms
- Boog → strands are broken → nonrecombination with heteroduplex region
- Kruispunt → strands are broken → recombination with heteroduplex region
More recent studies refined the steps of homologous
recombination
Recent studies modified the initiation phase of recombination
- Study, nick in one DNA strand
- Study, break in one DNA strand→ double strand break
model → promotes homologous recombination during
meiosis and DNA repair. Zie Plaatje
Double-strand break → small segment is degraded
(weggehaald), generates single-stranded DNA → D-loop is
formed → gap repair synthesis fills in regions → 2 Holliday
junctions are produced. → end result one nonrecombination
and one recombinant each with a short heteroduplex.
Proteins necessary to facilitate homologous recombination
Several different proteins are involved in homologous
recombination
Crossing over between homologous chromosomes important
during prophase of meiosis I
Gene conversion result from DNA mismatch repair or DNA gap repair
These to mechanisms result in gene conversion during homologous
recombination
- DNA mismatch repair; branch migration → hetroduplexes →
DNA mismatch repair, 4 possible combinations
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