GENERAL OVERVIEW OF HOMOLOGOUS RECOMBINATION:
In contrast to NHEJ, homologous recombination is a more precise way of repairing DNA
breaks. It uses a template strand to guide the repair process, which helps to ensure that
the repair is accurate. Homologous recombination is more accurate because it can repair
large DNA breaks, such as those caused by radiation or chemotherapy. It's also important
in some normal cellular processes, like meiosis.
Basics:
Homologous recombination starts with a DNA break, just like NHEJ. But in homologous
recombination, there is a second copy of the DNA nearby, called the homologous
sequence. This homologous sequence can be on another chromosome, or it can be a
sister chromatid (two identical copies of the same chromosome). The homologous
sequence provides a template that's used to repair the DNA break. The first step is to line
up the homologous sequence with the broken DNA strand.
Once the homologous sequence has been aligned with the broken DNA strand, enzymes
called helicases and nucleases unwind the DNA strands and make small cuts in the DNA.
This creates small, single-stranded DNA segments called "recombination intermediates."
These intermediates can then be used to copy the information from the homologous
sequence onto the broken DNA strand. This process is called "template-directed repair."
The result is that the broken DNA strand is repaired, using information from the
homologous sequence.
The recombination intermediates are repaired by a process called "single-strand
annealing." In this process, the single-stranded DNA in the recombination intermediates
pair up with each other and form a new DNA strand. This process can sometimes create
a loop of DNA called a "Holliday junction." The Holliday junction (not important for you to
know) is a four-way junction of DNA strands that can be resolved in different ways.
Next, the Holliday junction can be resolved by an enzyme called "DNA helicase." This
enzyme unwinds the DNA and separates the four strands of the junction. Two of the
strands are then cut by an enzyme called "endonuclease," and the DNA is repaired using
information from the remaining strands. This process is called "non-crossover resolution."
The other way that the Holliday junction can be resolved is by "crossover resolution." In
this process, the strands of DNA are exchanged, creating a crossover between the two
DNA molecules.
IN TERMS OF MRN:
The process of homologous recombination involves a number of different proteins. One
of the most important proteins is the MRN complex, which stands for MRE11, Rad50, and
NBS1. The MRN complex recognizes double-stranded breaks in DNA and recruits other
proteins to the site of the break. The MRN complex also helps to coordinate the nucleolytic
processing of the DNA ends. Nucleolytic processing is the process of cutting (through
nucleases) and repairing the DNA ends, which we've already talked about.
Here's the step-by-step process:
1. The MRN complex recognizes the double-stranded break in the DNA.
2. The MRN complex recruits ATM (Ataxia-Telangiectasia mutated) and ATR (ATMand Rad3-related) proteins to the site of the break.
3. The ATM and ATR proteins activate checkpoint proteins, which stop the cell cycle
and prevent DNA replication until the break is repaired.
4. The ATM and ATR proteins also activate two other important proteins: BRCA1
(BReast CAncer gene 1) and BRCA2 (BReast CAncer gene 2). These proteins
help to stabilize the DNA ends and recruit other proteins that are needed for repair.
5. Once the DNA ends have been stabilized, nucleolytic processing begins. This is
done by a number of different enzymes, including Exo1, CtIP, and Mre11.
Next, a number of enzymes called helicases are recruited to the site of the break. These
include the Bloom syndrome protein (BLM), the Werner syndrome protein (WRN), and
the Fanconi anemia proteins (FANCM, FANCJ, and FANCA). These helicases help to
unwind the DNA near the break, and they also help to recruit another set of enzymes,
called polymerases, which are needed to fill in the gaps in the DNA. The red part is just
additional information for you, but not your scope
The final step in nucleolytic processing is the joining of the two DNA strands, a process
called "synapsis." This is done by an enzyme called DNA ligase. Once the DNA has been
synapsed, the cell cycle can resume and the cell can divide. If the DNA repair is not
successful, the cell can undergo a process called "apoptosis," or programmed cell death.
This helps to prevent the spread of potentially harmful mutations.
STRAND INVASION
Strand invasion is an important step in homologous recombination. It involves the Rad52
and Rad54 proteins, which help to locate the homologous sequence on the other DNA
strand. The Rad52 protein binds to the single-stranded DNA near the break, and it then
interacts with the Rad54 protein. The Rad54 protein is a motor protein that helps to scan
the other DNA strand for a homologous sequence.
Now, when the homologous sequence is located, Rad51, another protein, is recruited to
the site. Rad51 helps to pair up the homologous sequences, forming a structure called a
"D-loop." The D-loop then serves as a template for DNA synthesis, which helps to fill in
the gap in the broken DNA strand. Once the DNA has been repaired, the Rad51 protein
dissociates and the D-loop is resolved, allowing the cell cycle to resume.
HOPE ALL MAKES SENSE, I TRIED NARRATING IT.