Translesion DNA Synthesis
• Cells bypass lesions encountered at the
replication fork during DNA synthesis and
correct them after replication is finished
• Enzymes used in this mechanism can
correctly synthesize new strand in spite of
lesion on template DNA
Recombinational Repair
• Uses recombination with undamaged DNA
molecule to repair damaged DNA
• Often used to fix damage found in DNA
replication
• One strand of parent DNA needs to have been
undamaged and copied during replication to
provide normal daughter molecule for repair of
damaged strand
• Provides repair mechanism for DS breaks from
chemicals and ionizing radiation
Genetic Recombination
• Important mechanism in DNA repair and increasing
genetic diversity
• Allows for re-assortment of genes into different
combinations
• Can be initiated by DS breaks in the DNA
• Occurs between DNA molecules that share sequence
homology over hundreds of bases
• Doesn’t alter overall arrangement of genes on a
chromosome
• Results from breakage and rejoining of two parental
molecules with different combination from the original
– Complementary strands base pair to each other
– Overlapping SS sections are exchanged during formation of heteroduplex
in the Holliday model
– Heteroduplex separates and recombined regions are on new
chromosomes
– Facilitated by enzymes
DNA Rearrangement
• Recombinational events that lead to
rearrangements in genomic DNA
• Some function in controlling gene expression in
some cell types, others have role in evolution by
contributing to genetic diversity
• First found genes that moved to other locations
in genome to alter expression of nearby genes
in corn in the 1940s
– Idea not widely accepted until 1970s
• Transposable elements (transposons) first identified in
bacteria, make up large part of higher eukaryote genomes
DNA Rearrangements
• Site-specific recombination occurs
between specific DNA sequences
homologous over short stretches of DNA
– Proteins recognize target DNA sequences,
leads to programmed DNA rearrangement
with roles in development and regulation of
gene expression
– Process used in vertebrate immunoglobulin
genes
DNA Rearrangements
• Transposition by DNA intermediates
occurs when DNA sequences move to
another area in genome and have no
requirement for sequence homology
– Transposons move this way
• Insertion sequences move from one
chromosome site without copying DNA
• Other transposons move by replicative mechanism
where DNA is copied and the copy is inserted
elsewhere
DNA Rearrangements
• Transposition by RNA intermediate occurs when
DNA is transcribed to RNA then reverse
transcribed to DNA for insertion
– Retrotransposons are transposable elements that
move via reverse transcription of an RNA
intermediate
– Retroviruses contain RNA genomes in their virus
particles but replicate by the synthesis of a DNA
provirus
• Reverse transcriptase is a DNA polymerase that uses an
RNA template and a primer to synthesize DNA
– Because of the way it functions, direct repeats of hundreds of
nucleotides called LTRs are created at each end of the DNA
DNA Rearrangements
• Other retrotransposons use RNA intermediates
but are not packaged into viral particles for
infection of new cells
– They only move within host cell’s genome
– Retrovirus-like elements or LTR retrotransposons are
like retroviruses but do not produce infectious
particles
– Non-LTR retrotransposons lack LTRs but still
transpose by RNA intermediate
– Other sequences elements transpose by RNA
intermediate but don’t code for reverse transcriptase
Gene Amplification
• Increases copy number of genes in cells
• Result of repeated rounds of DNA replication
• Found as free extrachromosomal molecules or
tandem arrays of sequences within a
chromosome
• Results in increased expression of amplified
gene
• Sometimes developmentally programmed to
support cells
• Abnormal occurrence is associated with cancers
Disclaimer
• This workforce solution was funded by a grant awarded under the
President’s Community-Based Job Training Grants as implemented
by the U.S. Department of Labor’s Employment and Training
Administration. The solution was created by the grantee and does
not necessarily reflect the official position of the U.S. Department of
Labor. The Department of Labor makes no guarantees, warranties,
or assurances of any kind, express or implied, with respect to such
information, including any information on linked sites and including,
but not limited to, accuracy of the information or its completeness,
timeliness, usefulness, adequacy, continued availability, or
ownership. This solution is copyrighted by the institution that
created it. Internal use by an organization and/or personal use by
an individual for non-commercial purposes is permissible. All other
uses require the prior authorization of the copyright owner.