00:01
Okay, i wanted to answer this question, let's talk about dna replication, okay? question number one says the only enzyme in the replication process that can form a phosphoryester bond between two unconnected ends of a dna chain with a five phosphate in one and three hydroxide group in the other.
00:19
Okay, so we have to choose practically.
00:21
And the answer here is going to be actually option a that is dna ligase, because imagine this is your double -stranded dna.
00:29
Okay, and this is the origin of the application that has already like unwind these two strands, okay? so the application is going to appear here.
00:39
And let's suppose that the helicases are opening this double -strand molecule in this direction, okay? so if this is a three -end and this is the five end, remember that you're going to have an enzyme called dna polymerate.
00:56
And the dna polymerase is going to polymerize from five to three.
01:00
And also remember that both strands are antiparital.
01:02
It means if this strand goes from three to five, then the second strand should go from five to three.
01:07
So you would have the five end here and the three end here.
01:11
And as your dna polymer is polymerized from five to three, then this is going to be the direction of synthesis of a new strand.
01:18
Okay, like this.
01:18
It means towards the replication of work.
01:21
So practically here, they're going to have only one segment of dna.
01:27
This is because of the dna polymerase and also because they are going in the same direction.
01:32
And this is why this strand here is called the leading strand.
01:35
This is different from here, for this strand here, because you're going to have here the 5 end and the 3 -8.
01:42
Okay, because these two strands have to be ant -parallel.
01:44
So it means that the strand that is synthesized here should go from 3 to 5.
01:53
Okay, and as your dna polymer is synthesized from 5 to 3, then the direction is going to be like in this direction.
02:00
It means in an opposite direction to the application fork.
02:03
So once your dna polymerize ends polymerizing here, it is going to move backwards and polymerize again here, from 5 to 3.
02:12
And then it is going to move backwards again and polymerize from 5 to 3, and so on.
02:18
So this is a discontinuous process, and this is why this strand here is called the lagging strand.
02:23
But at the end, we have to fuse these three strands in order to have a single strand like this...