DNA REPLICATION

DNA Replication Anchor S11.B.2.2 Objectives: You should be able to: Explain the need for replication Describe and model the steps of replication Identify the work of the enzymes of replication Distinguish leading and lagging strands Define “semiconservative”

Vocabulary Replication Semi-conservative Replication fork Leading strand Lagging strand Okazaki fragments Helicase DNA polymerase DNA ligase

Why DNA?

DNA REPLICATION The purpose of replication is to create two identical strands of DNA (chromosomes) prior to cell division. If either of the two new cells being produced lack a chromosome or have too many chromosomes or have an error in the DNA, a mutation will occur.

DNA REPLICATION The replication of DNA is said to be semi-conservative because one half of each newly created molecule is the original strand, and the other half is the newly synthesized strand.

Semi-conservative Replication

Origin of Replication Located by DNA polymerase There are generally several origins on a eukaryotic chromosome because it is linear This speeds the replication process Circular DNA of bacteria is opened at one origin, but replicated in both directions.

Origins of Replication

Origin of Replication in Prokaryotes v. Eukaryotes

DNA REPLICATION DNA is unwound and “unzipped” by helicase. Helicase breaks the hydrogen bonds between the nitrogen bases. The point of separation (“unzipping”) of the DNA molecule is known as the replication fork.

Find the replication fork

DNA REPLICATION DNA polymerase adds nucleotides in a 5’ to 3’ direction along both sides of the unzipped DNA strand.

THE LEADING STRAND The side of the DNA molecule designated as 3’ is the template for the leading strand. It is called leading because nucleotides can be added in a continuous motion toward the replication fork.

THE LAGGING STRAND The side of the DNA molecule designated as 5’ is the template for the lagging strand. Nucleotides cannot be added in a continuous motion because they are being added away from the replication fork.

THE LAGGING STRAND Nucleotides are added in small segments known as Okazaki fragments. Okazaki fragments are joined together by the enzyme called DNA ligase.

DNA REPLICATION Following replication, DNA polymerase “proofreads” the new strands for any mistakes. If base pairs have been mismatched, DNA polymerase corrects the errors.

Replication in Action