Steps in DNA Replication

What is DNA?
Deoxyribonucleic acid, abbreviated DNA, is a macro molecule containing genetic information which characterizes each organism this information is used in the development and maintenance of the organism. Smaller segments of DNA are known as Genes. DNA is wrapped around proteins forming longer structures known as chromosome having an ‘X-like’ figure. DNA consists of two strands that are in opposite directions often called DNA double helix. In biological terms DNA contains what is known as a backbone made of phosphate groups and sugar groups. Attached to each phosphate-sugar complex (specifically the sugar) is a base/nucleobase forming what is known as a nucleotide. These bases are what are responsible for genetic coding.

DNA
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Shortly before cell division DNA needs to be duplicated so that the resulting daughter cells, after the cell has divided, contain equal amounts of DNA. The process whereby the DNA duplicates to produce new ones is known as DNA Replication.

Here are the major steps involved in DNA replication:

Step 1
The protein Helicase splits the double stranded DNA molecule forming two single stranded templates. (These templates are what will be the guide for the formation of the new strands; that is each strand will be copied to produce new ones. In the end there will be two DNA double helix, each made from one of the old strand and a new strand.)

Helicase unwinds DNA Double Helix

Step 2
After the DNA double strand has been split special binding proteins known as Single-Stranded DNA Binding proteins (SSB) then come along and attach to the recently separated DNA strands preventing the strands from reannealing. Without these proteins the strands could easily reform the double helix.

SSB prevents DNA Reannealing

Step 3
DNA polymerase comes along and attaches itself to one strand, as it moves along this strand it joins incoming nucleotides together continuously in the 5' to 3' direction forming a new strand. Because the newly formed strand is continuous, it is called the leading strand. However the second strand allows for nucleotides to be synthesized in a discontinuous pattern, the newly formed strand is impeded and is therefore called the lagging strand. In order for replication to begin on the second strand RNA primase comes along and synthesizes RNA primers. A different DNA polymerase then comes along and joins the incoming nucleotides also in the 5' to 3' direction (but is moving in a different direction from the continuous strand; please refer to diagram below). This DNA polymerase however synthesizes shorter discontinuous strands and stops where it meets another RNA primer; the polymerase will then go back up to another newly formed RNA primer and again start to synthesize more nucleotides.

Action of DNA Polymerase

Step 4
The enzyme RNAse H then comes and removes the recently synthesized RNA primers on the lagging strand. This leaves small gaps in what should be the newly synthesized strand. Because this isn’t a continuous strand but more like fragments we would call them Okazaki fragments. (Note: The leading strand also had to have RNA primers to begin replicating, however this didn’t pose any problems because the type of DNA polymerase used caused the strand to be continuous.)

Okazaki Fragments

Step 5
DNA Ligase then comes and fills the short gaps between the Okazaki fragments to form the new COMPLETED strand (lagging strand). The replication process is now complete and we have two new double stranded DNA.

Two newly formed DNA

References:

• DNA Replication Animation ~ wiley.com
• DNA Structure ~  chemguide.co.uk