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Mechanism of Meiotic Recombination  Localization of Rad51 (green) and Dmc1 (red) on meiotic chromosomes |
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Homologous recombination, an
exchange between DNA strands, plays a role in the maintenance
of genome stability and the production of genome diversity. While,
in mitosis, it is required for the repair of DNA damage, it is
for the segregation of homologous chromosome at meiotic division
I. Meiotic recombination is coupled with chromosome morphogenesis
and is under a strict control. Malfunction of the recombination
leads cancer and infertility in human. In order to reveal molecular
mechanism of the recombination, we have been analyzing genes and
proteins involved in the process using molecular, genetical and
biochemical methods.
The major research interests of the Division of Protein Biosynthesis
are focused on 1) mechanism of homologous recombination in eukaryotes
and 2) mechanism of meiotic recombination.
1. Mechanism of Homologous Recombination in
Eukaryotes
Homologous recombination, an exchange of parental DNAs, is
required for repair of DNA damage as well as in the segregation
of homologous chromosome during meiosis. The RAD52 group
genes are involved in the recombination in eukaryotes. We are
currently studying the role of protein/genes in the RAD52
group both in vivo and in vitro. Particularly, Rad51
is a homolog of bacterial RecA. Rad51/RecA forms a right-handed
helical filament on single-stranded (ss) DNA and carries out homology
search and strand exchange. An in vitro Rad51-mediated exchange
reaction is inefficient. It is known that the other factors are
required to promote the Rad51-mediated reaction. Rad52 forms a
ring-like structure on ssDNA and helps the binding of Rad51 to
the DNA.
2. Recombination and chromosome dynamics during meiosis
Meiotic recombination promotes the faithful segregation of homologous chromosomes at meiosis I (MI) by creating physical linkages between the homologs. Recombination produces two types of products: crossovers (COs) and non-crossovers (NCOs). Only COs mature into exchanges between chromosome axes called chiasmata, which together with arm cohesion ensure homolog separation.
Crossing-over during meiosis establishes the physical linkage
of homologs required for their accurate segregation at the first
meiotic division. Two features indicate that meiotic crossing-over
is a highly regulated process. First, crossover assuranceﾓ or
the ﾒobligatory crossoverﾓ describes the observation that each
pair of homologs always obtains at least one crossover despite
the fact that the average per chromosome pair is quite low (typically
1-3 crossovers). Second, adjacent crossovers are more widely spaced
than random expectations, a phenomenon known as crossover interference.
Meiotic recombination is initiated by DNA double-strand breaks
(DSBs) and the total number of DSBs greatly exceeds the final
number of crossovers. Thus, there must be processes that designate
a crossover fate to selected DSBs and then implement that fate
with high efficiency. The remaining majority of DSBs is repaired
as noncrossovers without exchange of chromosome arms.
At the DNA level, differentiation of crossover and noncrossover
pathways can be detected at an early stage. DSBs undergo resection
of the 5ﾕ-strands to yield long 3ﾕ-single-strand tails. DSB-ends
then interact sequentially with a homolog to form two types joint
molecule intermediate. Invasion by one DSB-end produces a Single-End
Invasion (SEI). DNA synthesis and interaction of the second end
then convert the SEI into a double Holliday Junction (dHJ). In
theory, dHJs can be resolved into both crossover and noncrossover
products but available evidence suggests dHJs give rise primarily
or exclusively to crossovers. Moreover, SEIs also appear to be
crossover-specific intermediates Joint molecules that are specific
to the noncrossover pathway have not been identified, perhaps
because they are labile and/or short-lived. It is proposed that
crossover or noncrossover designation occurs very early, at or
before transition from DSBs to SEIs.
Meiotic recombination also mediates homolog pairing, culminating
in the formation of synaptonemal complexes (SCs), which connect
homologs along their entire lengths. Meiotic homologs assemble
a proteinacious core or axis. During the leptotene stage DSBs
form and homolog axes pair, becoming closely associated at sites
of recombination. These axial associations are thought to be sites
where SC polymerization initiates during zygotene. When SCs have
polymerized along the lengths of all homolog pairs, cells enter
the pachytene stage. dHJs are formed and resolved into crossovers
during this stage.
We are interested in molecular mechanisms and their control
of chromosome morphogenesis and chromosome movement as well as
recombination during meiosis
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Mechanism of Meiotic Recombination Localization of Rad51 (green) and Dmc1 (red) on meiotic chromosomes |
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