BIOL 201 Lecture Notes - Lecture 12: Holliday Junction, Genetically Modified Organism, Chromosomal Crossover

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9 Feb 2013

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The most common form of chromosomal crossover is homologous recombination, where the two
chromosomes involved share very similar sequences. Non-homologous recombination can be
damaging to cells, as it can produce chromosomal translocations and genetic abnormalities. The
recombination reaction is catalyzed by enzymes known as recombinases, such as RAD51. The
first step in recombination is a double-stranded break caused by either an endonuclease or
damage to the DNA. A series of steps catalyzed in part by the recombinase then leads to joining
of the two helices by at least one Holliday junction, in which a segment of a single strand in each
helix is annealed to the complementary strand in the other helix. The Holliday junction is a
tetrahedral junction structure that can be moved along the pair of chromosomes, swapping one
strand for another. The recombination reaction is then halted by cleavage of the junction and re-
ligation of the released DNA.
DNA contains the genetic information that allows all modern living things to function, grow and
reproduce. However, it is unclear how long in the 4-billion-year history of life DNA has
performed this function, as it has been proposed that the earliest forms of life may have used
RNA as their genetic material. RNA may have acted as the central part of early cell
metabolism as it can both transmit genetic information and carry out catalysis as part
of ribozymes. This ancient RNA world where nucleic acid would have been used for both
catalysis and genetics may have influenced the evolution of the current genetic code based on
four nucleotide bases. This would occur, since the number of different bases in such an organism
is a trade-off between a small number of bases increasing replication accuracy and a large
number of bases increasing the catalytic efficiency of ribozymes.
However, there is no direct evidence of ancient genetic systems, as recovery of DNA from most
fossils is impossible. This is because DNA will survive in the environment for less than one
million years and slowly degrades into short fragments in solution. Claims for older DNA have
been made, most notably a report of the isolation of a viable bacterium from a salt crystal 250
million years old, but these claims are controversial.
On 8 August 2011, a report, based on NASA studies with meteorites found on Earth, was
published suggesting building blocks of DNA (adenine, guanine and related organic molecules)
may have been formed extraterrestrially in outer space.
Methods have been developed to purify DNA from organisms, such as phenol-chloroform
extraction, and to manipulate it in the laboratory, such as restriction digests and the polymerase
chain reaction. Modern biology and biochemistry make intensive use of these techniques in
recombinant DNA technology. Recombinant DNA is a man-made DNA sequence that has been
assembled from other DNA sequences. They can be transformed into organisms in the form
of plasmids or in the appropriate format, by using a viral vector. The genetically
modified organisms produced can be used to produce products such as recombinant proteins,
used in medical research, or be grown in agriculture.
Forensic scientists can use DNA in blood, semen, skin, saliva or hair found at a crime scene to
identify a matching DNA of an individual, such as a perpetrator. This process is formally
termed DNA profiling, but may also be called "genetic fingerprinting". In DNA profiling, the
lengths of variable sections of repetitive DNA, such as short tandem repeats and minisatellites,
are compared between people. This method is usually an extremely reliable technique for
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