Chapter 9.3 Grade 10 Biology Spring 2011 The Replication of DNA

Bell Ringer DNA is considered to be a relatively stable molecule. What gives DNA this stability, even though the hydrogen bonds between the nitrogen bases are easily broken?

Bell Ringer: Answer The bonds connecting the deoxyribose sugar and phosphate groups, which form the backbone of DNA, are covalently bonded. Covalent bonds are strong and keep strands of nucleotides intact.

Activity Divide into groups Each group will outline one of the three steps of DNA replication Include: Description of step Key terms Diagram that is labeled

Objectives Summarize the process of DNA replication Describe how errors are corrected during DNA replication Compare the number of replication forks in prokaryotic and eukaryotic DNA

Roles of Enzymes in DNA Replication Complementary relationship between sequences of nucleotides Predicted complementary relationship used as basis to make exact copies of DNA One DNA strand serves as a template (pattern) on which the other strand is built

Roles of Enzymes in DNA Replication DNA replication: the process of making a copy of DNA Replication occurs during S phase of the cell cycle, before a cell divides Fig. 9 page 198

Roles of Enzymes in DNA Replication Step #1 in DNA Replication: Double helix unwinds Unwinds with help of enzyme DNA helicase DNA helicase: opens the double helix by breaking the hydrogen bond that link the complementary nitrogen bases between the two strands

Roles of Enzymes in DNA Replication

Roles of Enzymes in DNA Replication Step #1 in DNA Replication: Once strands are separated, proteins attach to each strand to hold them apart Replication Fork: areas where double helix separates Y shape

Roles of Enzymes in DNA Replication Step #2 in DNA Replication: At the replication fork, DNA polymerase enzymes move along each of the DNA strands DNA polymerase: adds nucleotides to the exposed nitrogen bases Follows base-pairing rules Two new double helixes are being formed

Roles of Enzymes in DNA Replication Step #3 in DNA Replication: All DNA is copied Polymerase is signaled to detach Produces 2 DNA molecules Each DNA molecule is composed of a new and an original strand of DNA Nucleotide sequence is identical in both strands and identical to original DNA molecule

Checking for Errors Wrong nucleotides can be added when copying DNA DNA polymerase: has proofreading ability Adds nucleotides to growing strand only if the previous nucleotide is correctly paired to its complementary base If paired wrong, DNA polymerase can backtrack and fix it

DNA Polymerase Proofreading

The Rate of Replication Prokaryotes: Circular DNA molecule Usually has 2 replication forks that begin at single point Replication forks move away from each other until they meet

The Rate of Replication Eukaryotes: Chromosomes contain a single, long strand of DNA Each human chromosome is replicated in about 100 sections that are 100,000 nucleotides long Each section has its own starting point Multiple replication forks Speeds up replication (about 8 hours in humans)

Review Watch this video for a quick review of DNA replication! http://www.youtube.com/watch?v=hfZ8o9D1tus