DNA Replication Pre-AP Biology

Back to last class… Draw & label a nucleotide Draw a deoxyribose & label the 5’ & 3’ ends Draw a DNA strand that is at least 5 nucleotides long—label the 5’ & 3’ ends of each strand as well as each part of the nucleotides

Why do we need to replicate DNA? In order to grow, reproduce, and repair, an organism’s cells must divide Before a cell can divide, it must copy its DNA so that both new cells end up with the exact same information

Describe what you see happening in the diagram…

Describe what you see happening in the diagram…

Overview of DNA Replication

The model for DNA replication is known as the semiconservative model because half of the parental molecule is maintained (conserved) in each daughter molecule.

Did you know??? New nucleotides are added at the rate of … 50 per second in mammals 500 per second in bacteria

Hmmm… How does complementary base pairing make DNA replication possible? When an adenine is exposed, a thymine is added. When a cytosine is exposed, a guanine is added.

DNA Replication During replication, each strand serves as a template to make a new DNA molecule Steps 1. DNA is “unzipped” 2. Free floating nucleotides attach by complementary base pairing (Each strand is a complement of one of the original parent strands) 3. Results in the formation of two DNA molecules, identical to the original strand

Origins of Replication Where DNA replication begins Replication proceeds in both directions (bi-directional) Creates “bubbles” There are many origins of replication, and there can be thousands of bubbles along a chromosome at once

3’ and 5’ ends Remember that… the sugar’s 3’ carbon is attached to hydroxyl (-OH) the sugar’s 5’ carbon is attached to phosphate The two strands run in opposite directions= antiparallel

Important Enzymes Helicase- unwinds the DNA strands Topoisomerase- prevents tangling of DNA

Important Enzymes Primase- lays down a short piece of RNA primer DNA polymerase- adds new DNA nucleotides and replaces RNA with DNA

Important Enzymes DNA polymerase can only add new nucleotides to the 3’ end of the strand (NOT to the 5’ end) A daughter strand of DNA can only grow 5’-3’. One of the daughter strands is made continuously- leading strand

Important Enzymes The other daughter strand (the lagging strand) is made in small pieces, called Okazaki fragments These fragments must be glued together, using an enzyme called DNA ligase.

Summary of DNA Replication

DNA Replication in Action… Animation DNA Replication Song

Why do we need to copy our DNA? Before our cells can divide, they need to make a copy of DNA. Every cell needs instructions! Proper replication of DNA ensures that all body cells in an organism have the same genetic information.