DNA Replication

A little background information about the structure of the DNA:

The DNA consists of nucleotides. A nucleotide consists of a phosphate group, the deoxyribose sugar and the nitrogenous base. There are four nitrogenous bases in the DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

As you can see on the picture below, the Nucleotides are bound together by hydrogen bonds (I am going to make another video and/or blogpost about hydrogen bonds:)).

Let’s talk about the numbers on the picture. What does 5′ and 3′ mean? Well, a deoxyribose molecule contains carbon, right? Here is how a deoxyribose molecule looks like: 

(There is a carbon at 1′, 2′,3′,4′ and 5′!)

The process of replication:

Let’s move on and talk about the DNA replication. DNA replication is semiconservative. The double helix unwinds and the  new nucleotides are added on each strand. However, it doesn’t happen in the same way, because the strands are running in the opposite directions. The strand that is running (chemically mean) in the 5′ to 3′ direction is called the leading strand, and the strand that runs in the 3′ to 5′ direction is called the lagging strand. (see the picture above)

When DNA replication starts, an enzyme called helicase separates the double helix (more specifically: it separates the hydrogen bonds between the nucleotides). The single strand binding proteins hold each strand in place. Let’s see how the nucleotides are added on each of these strands:

(look at the picture below while you read :))

The leading strand: 

An enzyme called DNA polymerase III runs down the leading strand and adds on the new nucleotides.

The lagging strand: 

Since new nucleotides can only be added on the 3′ end, the DNA polymerase III cannot just add new nucleotides on, like on the leading strand. First, an enzyme called RNA primase adds an RNA primer (a little bit of RNA). Then, the DNA polymerase III can start adding on new nucleotides. The process repeats over and over. The DNA fragments between the RNA primers are called the Okazaki fragments. The DNA polymerase I replaces the RNA primers with DNA, and the DNA ligase links the Okazaki fragments. 

Please check out my video about the DNA replication HERE :)

Also, it's very helpful (at least in my opinion :)) to watch animations about this topic. Maybe I will be able to make animations in the future (I will do my best :)).

Greetings!