I’ve finally finished the paper from yesterday’s topic about X-SCID this
afternoon, and my eyes landed on 2 keywords, DSBR and IL2rg. OK, what are those
things? Easy… I am not going to discuss both of them today. Even 1 topic is
already long enough to be discussed in a day.
For today, I decided to look a bit deeper on the first keyword, DSBR.
So, what is DSBR? It’s an abbreviation of DNA double-strand break
repair. Double-strand break itself reveres to the condition where both strands
of the double helix are severed (or mutated), and DSBR means the effort to
repair this condition. There are (at least…not very sure, haven’t finished
digging up >.<) 3 mechanisms exist to repair the double-strand breaks
(DSBs): non-homologous end joining (NHEJ), microhomology end joining (MMEJ),
and Homologous Recombination.
WAIT!! Have I ever learned these things??As a note, I have a Bachelor of
Biotechnology degree, so AT LEAST I’ve ever read it somewhere. Why I don’t
remember any of it? I mean, during undergraduate, we learned for (at least!!) 5
semesters (equal to 2 and a half years) of DNA technology and all the necessary
things. BUT!!! Why couldn’t I recall any of these?! I even argued some seniors
via twitter whether our lecturers taught us of these or not…
Well, anyway, I dug deeper and got addicted as the papers passed my eyes
page by page.
For now, I will discuss a bit about the “might be the simplest” one first.
I will talk about the Homologous Recombination Repair (HRR). This strategy is
also well-known as might the best explanation for the DNA recombination during
meiosis stage of the cells. Basically, it requires the presence of an identical
(or nearly identical) sequence to be used as the template to repair the break.
This path allows a damages set of DNA (let’s say, chromosome) to be repaired by
using a sister chromatid or a homologous chromosome as their template. Out of
the strategies, this pathway might be the most “error-free” DNA repair
mechanism with an almost 100% guarantee there will be no mistake during
repairing process.
So, let’s imagine it in a simpler way. Let’s say you spoiled a page of a
journal presented in your lab’s journal club today by a cup of coffee. CRAP,
you really need that page!! So, you just borrowed the same journal from your
friend who also attended the club, and copied it. Voila!! You got your spoiled
page back!!
BUT!! This simplicity is not the actual HRR. The experts prefer to call this pathway as Synthesis Dependent Strand Annealing (SDSA) Repair system. The HRR will happen if there is cross-over through the holliday junction. What is
cross-over? As the word says, if the template and the broken DNA cross over
each other, there is a possibility that the DNA strands interlope and switch
over. Now, this is the real combination we are talking about!! The picture will
be not as simple as given above, but now there is a small branch as this one.
Now, the HRR is on the left side and the SDSA is on the right side.
A little extra explanation, is the point (or the place..oh well it's a junction) where the 2 strands of sister chromatids or chromosomes interlope over each other. This holliday junction is the thing which makes the cross-over possible. The cross-over might lead to a "Branch Migration" or a "Double Crossing Over". Now, the picture will get a little more detail into this one.
That's all for this topic. I will discuss another things for the next post. ^^
If you are interested to learn more, I could suggest you to check on
this online article of “Homologous Recombination from a Molecular Perspective”,
or read some papers, like this one as an example.
Happy exploring!!! ^^
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