DNA Structure & Replication

AP Biology

What is a Nucleotide?

DNA exhibits complementary base pairing

The two strands of DNA are anti-parallel

The Mystery of DNA Replication Revealed by its Structure

• Complementary base pairing reveals a possible copying mechanism for genetic material—Watson & Crick

DNA Replication: Simple View

The Basic Principle: Base Pairing to a Template Strand

• Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication

• In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Animation: DNA Replication Overview

• Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

DNA Replication: A Closer Look

• The copying of DNA is remarkable in its speed and accuracy

• More than a dozen enzymes and other proteins participate in DNA replication

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 16-12b

0.25 µm

Origin of replication Double-stranded DNA molecule

Parental (template) strandDaughter (new) strand

Bubble Replication fork

Two daughter DNA molecules

(b) Origins of replication in eukaryotes

Overview of DNA Replication

Step 1• Replication begins at special sites called origins

of replication, where the two DNA strands are separated by proteins, opening up a replication “bubble”

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Step 2• Replication proceeds in both directions

from each origin, until the entire molecule is copied.

Detailed VIEW of DNA Replication

Enzymes & Proteins involved in DNA Replication

• Helicases: Untwist the double helix at the replication forks

• Single-strand binding protein: Binds to and stabilizes single-stranded DNA until it can be used as a template

• Topoisomerase: Corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands

Elongating a New DNA Strand

• DNA polymerase catalyzes the elongation of DNA at the replication fork

• DNA polymerase adds nucleotides to the growing end of the DNA strand

Antiparallel Elongation• DNA polymerases add nucleotides only to the

free 3end of a growing strand; therefore, a new DNA strand can elongate only in the 5 to 3direction

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

• The “Leading Strand” is the strand of new DNA formed towards the replication fork.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Animation: Leading Strand

Lagging Strand

• The “Lagging Strand” is formed 5’ to 3’, in the direction away from the replication fork.

• The lagging strand does not elongate continuously, instead it is formed in a series of fragments called “Okazaki fragments”

Lagging Strand & Okazaki

Fragments• DNA ligase joins

Okazaki fragments

DNA Replication Summary

• DNA Replication is a semi-conservative process

• Replication differs in the production of the leading and lagging strands and occurs bi-directionally

• http://www.mcb.harvard.edu/losick/images/trombonefinald.swf• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free

/0072437316/120076/micro04.swf::DNA%20Replication%20Fork

DNA Proofreading & Repair

• DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides

• In mismatch repair of DNA, repair enzymes correct errors in base pairing

DNA Proofreading & Repair

• In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA

• Damage shown is a thymine dimer from UV Radiation

Replicating the Ends of DNA Molecules

• Linear (eukaryotic) DNA can’t be completely replicated by DNA polymerase b/c it only adds to the 3’ end of the molecule

• Result: DNA is shortened with each replication

Telomeres To the Rescue

• Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called telomeres

• Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules

Telomerase in Germ Cells

• Telomerase catalyzes the lengthening of telomeres in germ cells

• Telomeres in cancer cells are very short• Some cancer cells have active

telomerase…