Chapter 17: DNA Replication and Recombination
Concepts | Replication | Recombination
DNA replication and recombination
DNA is reproduced by semiconservative replication
DNA synthesis in bacteria involves three polymerases
as well as other enzymes
Many complex issues must be resolved during DNA replication
The DNA helix must be unwound
Initiation of DNA synthesis requires an RNA primer
Antiparallel strands require continuous and discontinuous DNA synthesis
Concurrent synthesis occurs on the leading and lagging strands
Proofreading and error correction are an integral part of DNA replication
A coherent model summarizes DNA replication
Replication is controlled by a variety of genes
Eukaryotic DNA synthesis is similar to synthesis in prokaryotes but is more
complex
The ends of linear chromosomes are problematic during replication
DNA recombination, like DNA replication, is directed by specific enzymes
Gene conversion is a consequence of DNA recombination
With some exceptions, the process of DNA replication is very
similar in all organisms. DNA synthesis is carried out by DNA
polymerases which require a primer, a template and a ready
supply of precursors (dNTPs). Replication begins at a unique
origin. Replication is semi conservative and proceeds
bi-directional. An RNA primer is used for DNA synthesis.
Synthesis of DNA is continuous on the leading template strand
and discontinuous on the lagging strand. Synthesis proceeds in
the 5' to 3' direction only. And most of all, the DNA is
faithfully replicated. The steps in the process are
demonstrated in the following module.
n a replication bubble that is proceeding bidirectionally there
are two forks, migrating in opposite directions with multiple
events occurring simultaneously. To facilitate the study of this
process, the animation was simplified to depict only a single
replication fork in a two dimensional plane. In actuality, leading
and lagging strands are synthesized simultaneously. In current
models for DNA replication, the lagging strand template loops
around the replication apparatus, or replisome, and is brought
into juxtaposition with the replication fork. Synthesis
progresses until tension precludes further movement, at which
time the newly synthesized strand is released and a new
section of unwound lagging strand template loops around.
Replication proceeds as before, with DNA synthesized 5’ to
3’ along the new section of template.
Generalized homologous recombination occurs between
chromosomes during prophase I of meiosis. Recombination
occurs between identical or nearly identical DNA strands
(nonsister chromatids of homologous chromosomes). General
homologous recombination is also called reciprocal
recombination because equivalent amounts of DNA are
exchanged between recombining molecules. This is not a
requirement, however, in the case of illegitimate recombination
and site-specific recombination.
Recombination is an important force in genetic change. In
different systems, recombination also plays an important role in
the repair of mutations.
Crossing over allows for a greater number of gene
combinations than independent assortment. Gene conversion
as a result of mismatch repair and recombination further
increases the amount of change possible in the genome. The
Holliday model reconciles events at the molecular level with
linkage alteration of phenotypic proportions and cytological
structures observed during crossing over in meiosis.