bio 3201 unit 2b purple

DNA REPLICATION

DNA Replication: Making a copy of the DNA molecule. As you know, a cell replicates all of its

DNA—its entire genome—in the cell cycle, during S phase of interphase.

A human cell can copy all of its DNA in a few hours, with an error rate of about one per one

billion nucleotide pairs.

DNA Replication is Semi-conservative.

This means that when DNA is copied, each new molecule created contains one strand of

parental DNA and one strand of New DNA.

The Process of DNA Replication

DNA is copied through the following steps.

1. Initiation

2. Elongation & Termination

Followed by Proofreading

Initiation

Replication starts at a specific nucleotide sequence, called the replication origin.

A group of enzymes, called helicases, bind to the DNA at the replication origin. The helicases

cleave and unravel a segment of the double helix. This opening up of a region of DNA creates

two Y-shaped areas at each end of the unwound area. The oval-shaped unwound area is called

a replication bubble. Each Y-shaped area is called a replication fork.

Elongation and Termination

An enzyme called DNA polymerase inserts into the replication bubble and begins to add

nucleotides, one at a time, to create a strand of DNA that is complementary to the existing

strand. The process of joining nucleotides to extend a new strand of DNA is called elongation.

There are two conditions for elongation.

 First, elongation can only take place in the 5´ to 3´ direction.

 Second, a short strand of RNA, known as a primer, must serve as a starting point for the

attachment of new nucleotides.

Therefore, Elongation happens according to the following steps.

a. At the 3’ exposed end of the DNA molecule (called the Leading Strand), DNA

polymerase adds new nucleotides. This occurs in the direction from the 5’ to 3’ end

only (New DNA)! In order for DNA polymerase to know where to begin, a small piece of

RNA Primase called a primer shows it where to begin. Once begun, the addition of

nucleotides continues at a steady pace along the leading strand.

Read 3’-5’!! Build 5’- 3’!!

b. Because DNA polymerase lays down nucleotides from the 3’ to 5’ direction only, the 2 nd

strand of nucleotides are laid down in the opposite direction of the leading strand. This

occurs when short copies of DNA are made in spurts called Okazaki fragments. This

occurs slower than the leading strand and is known as the Lagging strand. In order for

this to happen, RNA primers are needed for each Okazaki fragment.

Another enzyme, DNA Ligase, stitches together the Okazaki fragments to make a complete

strand of DNA.

Proofreading

DNA polymerase has an important proofreading function, as well. After each nucleotide is

added to a new DNA strand, DNA polymerase can recognize whether or not hydrogen bonding

is taking place between the new base and its complement on the original strand. The absence

of hydrogen bonding indicates a mismatch between the bases. When this occurs, DNA

polymerase excises the incorrect base from the new strand and adds the correct base using the

parent strand as a template.

As soon as the newly formed strands are complete, they rewind automatically into their

chemically stable helix structure. Replication proceeds until the new strands are complete and

the two new DNA molecules separate from one another. The completion of the new DNA

strands and the dismantling of the replication machine is called termination.

In a nutshell:

1. Helicase unwinds the helix

2. Primase primes the site for addition of nucleotides

3. DNA Polymerase adds nucleotides in the 5’-3’ direction

4. Ligase pieces together the Okazaki fragments

5. DNA Polymerase proofreads