DNA Replication Will Fagan IB Biology 2011

3.4 DNA Replication Cells must prepare for doubling the DNA content of a cell through the process of DNA replication. This doubles the quantity and ensures the the copy is exact. This occurs during interphase of the cell cycle.

Interphase During interphase, there is a nuclear membrane which separates nucleoplasm from the cytoplasm. The DNA is in the form of chromatin. There are two very important molecules required for the process of DNA Replication: 1. Enzymes 2. Free nucleotides

Helicase The enzyme that initiates the separation of complementary base pairs is known as helicase. Helicase separates the double helix into two strands by breaking the hydrogen bonds. Helicase begins at a point in or at the end of a DNA molecule and moves one complementary base pair at a time, breaking the hydrogen bonds so the double stranded DNA molecule becomes two strands.

Unzipping The unpaired nucleotides on the single strands are now used as templates to create two double stranded DNA molecules identical to the original.

Formation of Complementary Strands Free floating nucleotides are able to form complementary pairs with the single-stranded nucleotides of the unzipped molecule. This process is not random. A free nucleotide locates on one opened strand at one end and then a second nucleotide comes in to join the first. These two nucleotides must be joined by a covalent bond. This is catalyzed by DNA polymerase.

Formation of Complementary Strands Next, a third nucleotide joins the first two and the process continues in a repetitive way for many nucleotides. Notice that one strand is replicating in the same direction as the helicase is moving and the other strand is replicating in the opposite direction.

Complementary Base Pairing No DNA molecule is ever completely new. Every DNA molecule after replication consists of a strand that was old now paired with a strand that is new. DNA replication is described as semiconservative for this reason.

7.2 Replication HL Summary of Replication –Replication begins at the origin which appears as a bubble because of the separation of the two strands. The unzipping occurs to action of the enzyme helicase on the hydrogen bonds between nucleotides. –At each end of the bubble there is a replication fork. This is where the double stranded DNA opens to provide two parental DNA strands which are the templates necessary to produce the daughter DNA molecules by semiconservative replication.

–The bubbles enlarge in both directions, showing that the replication process is bidirectional. The bubbles eventually fuse with one another to produce two identical daughter DNA molecules.

Elongation of a new DNA strand 1.Primer is produced under the direction of primase at the replication fork. The primer is a short sequence of RNA (5-10 nucleotides). Primase allows joining of RNA nucleotides that match the exposed DNA bases at the point of replication. 2.The enzyme DNA polymerase III then allows the addition of DNA nucleotides in a 5’ to 3’ direction to produce the growing strand. 3.DNA polymerase I removes the primer from the 5’ end and replaces it with DNA nucleotides.

Elongation of a new DNA strand Each nucleotide added to the elongating chain is actually a dNTP. As these molecules are added, two phosphates are lost. This provides the necessary energy for the chemical bonding of nucleotides.

The Antiparallel Strands A DNA molecule is composed of 2 antiparallel strands. One strand is 5’ to 3’ and the other is 3’ to 5’. DNA is only assembled in the 5’ to 3’ direction due to the action of polymerase III. This process is slower for the lagging strand as it is opposite of the 5’ to 3’ strand.

Formation of the Lagging Strand The leading strand is being assembled continuously towards the replication fork. The lagging strand is assembled by way of fragments away from the progressing replication fork. Primer, primase, and DNA polymerase III are required for the lagging strand just as with the leading strand. These three components are necessary for the formation of each fragment. The leading strand is formed continuously, so it requires primase and primer only once.

Fragments The fragments of the lagging strand are called Okazaki fragments, after the Japanese scientist who discovered them. Once the Okazaki fragments are assembled, an enzyme called DNA ligase attaches the sugar=phosphate backbones of the fragments to form a single DNA strand.

Replication Proteins Define: –Helicase –Primase –DNA polymerase III –DNA polymerase I –DNA ligase

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