DNA replication

Version française : Steemit | Busy

The chronicles advance and we deal with deeper subjects with a bit more complexity, if you are still here, is that you can follow!

So, hi there!

Today we discuss DNA replication, I have previously talked about to say that DNA can copy itself identically (however with few errors anyway!). OK, but how does he do that? Has he a copy machine at your fingertips?

However, it’s not the office copy machine!

Before we start, put yourself in mind the double helix of DNA (the twisted ladder), now, two things to know about DNA:

[1] each base (A, C, T, G) is corresponding to a molecule, so the DNA sequence is a big length molecule (which is a molecule chain).
[2] DNA have a sense, eh yes, we deal with a sophisticated technology! Each side of the ladder is called a strand, so DNA is composed of two strands (double-stranded DNA).
Each strand has two edges called 3’ and 5’, this name come from molecule nomenclature but doesn’t matter. Finally, a strand is arranged upside down from each other, it’s mean that if you take an edge of the DNA, you will see 3’ edge of one strand and 5’ edge of the other strand. Do you visualize it?

There will aren’t schemas in this chronicle, the best schemas are which is creating itself in your mind!

OK. The main actor of this mechanism is called DNA polymerase, he will build new strands. He has two important characteristics:

[1] He can’t build DNA from nothing; indeed, he needs a model and a starting point.
[2] He works always in the same sense, from 5‘ to 3’.

The principle of the replication is that of a zipper, as you unzip and you rezip new zippers on each side directly after. In order to obtain two zippers.

So, firstly, we must put things flat, unroll the DNA molecule (try to unzip your zipper if it’s all rolled up!). This the role of the topoisomerase.

Then we must unzip the DNA, this is helicase who manages that. After the unzip, we need to protect every single strand with proteins, which allows bases to not stick up again between them.

Thus, DNA polymerase can start working!

This is the principle! This is like a team which works together for a mission.

But where this process starts? At an edge?

Don’t need to be at an edge, there is one or more place where this can start, we call them the origin of replication (Ori), this is sequences which signal that here is a good place to start.

DnaA protein recognizes and goes on these Ori, it allows the separation of the two DNA strands, enough for the entrance of the helicases, I say helicases because, once the breach opens, two helicases go inside and each start in his direction. Besides this can happens at several places at the same time.

This aperture in the DNA is called the… I don’t know how you say that in English, in French we called that the replication eye for his eye shape.
oiel

Source: https://it.wikipedia.org/wiki/Occhio_di_Horo

And is it a fast process?

Extremely fast! Between 500 and 1000pb per seconds in bacteria but it’s a quite slower in evolved organisms.

You said me that DNA polymerase goes always in the same sense, so, how can it synthesize the two strands?

Very good remark! Indeed, for one of them there is no problem (it’s a continuing synthesis), but for the other one, the DNA polymerase must synthesize the DNA by fragments (it’s called a discontinuing synthesis).

You also said that DNA polymerase needs a model and a starting point, what is that?

Well, simply the model is the strand where it will work, by watching this strand, it adds the corresponding base: if it’s a T it adds an A, if it’s a C it adds a G, … It is active only if sees a 3’ side, place where it adds a base (this is why it goes always in the same sense). And so, his starting point is few bases put on the model strand by his cousin the RNA polymerase.

RNA?

Yes RNA, this will be the next chronicle concerning this surprising technology. In the next chronicle, we will deal with something more concrete: the reproduction!