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Summary Chapter 20 - Recombination, Immunogenetics and Transposition
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Chapter 20 of Genetics by Brooker.
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Chapter 20 - recombination, immunogenetics and traposition.20.1 Homologous recombination
Homologous recombination involves an exchange of DNA segments that are similar or
identical in their DNA sequences. Eukaryotic chromosomes that have similar or identical
sequences frequently participate in crossing over during meiosis I and occasionally during
mitosis. Crossing over involves the alignment of pair of homologous chromosomes, followed
by the breakage of two chromatids at analogous locations, and the subsequent exchange of
the corresponding segments.
When such recombination takes place between sister chromatids, the process is called
sister chromatid exchange (SCE). Because sister chromatids are genetically identical, SCE
does not produce a new combination of alleles.
Homologous recombination can also result in genetic recombination, which refers to the
shuffling of genetic material to create a new combination of alleles that differs from the
original.
The holliday model describes a molecular mechanisms for the recombination. Gene
conversion is used to describe the phenomenon in which one allele is converted to the allele
on the homologous chromosome. The holliday model explains the molecular steps that
occur during homologous recombination:
- Two homologous chromatids are aligned with each other.
- A break or nick occurs at identical sites in one strand of each of the two homologous
chromatids.
- The strands invade the opposite helices and base pair with the complementary
strands.
- A holliday junction is created, the cross can migrate in a lateral direction.
- A DNA strand in one helix is swapped for a DNA strand strand in the other helix
(branch migration).
- The swapping of the DNA strand during branch migration may produce a
heteroduplex, a region in the double-stranded DNA that contains base mismatches.
- Resolution: the breakage and rejoining of two DNA strands to create two separate
chromosomes.
More recent models have refined the molecular steps of homologous recombination.
(Meer hoeft niet)
20.2 Immunogenetics
Antibodies, or immunogenetics (Igs) are proteins produced by the immune systems of
vertebrates. Their function is to recognize foreign material, such as viruses and bacteria, and
target that material for destruction. A foreign substance that elicits an immune response is
called an antigen. Antibodies recognize sites within antigens. The recognition between an
antibody and an antigen is very specific.
B cells, a type of lymphocyte, are responsible for producing antibodies within the immune
systems of mammals. Each B cell produces a single type of antibody, and a single individual
can produce millions of B cells.
Antibody diversity is produced by site-specific recombination. Research revealed that
antibodies with different polypeptide sequences are generated by an unusual mechanism in
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