Introduction to Molecular and Cell Biology, Biol. 220 Lecture 13: DNA Replication
	Index CourseInfo LogIn Syllabus References Other Resources

DNA Replication

DNA replication is semiconservative (Fig. 12.1).

Where does DNA begin replication?

DNA replication begins at specific chromosomal sites called origins (Fig. 12.5).
    Prokaryotes have a single origin.
    Eukaryotes have many origins.

How does DNA unwind?

Initiation of DNA replication begins as proteins recognize and bind origins forming an open complex.
    E. coli origin recognition (Fig. 12.7).  = Prokaryote example.

What happens to single stranded dna?

  SV40 origin recognition (Fig. 12.12).  = Eukaryote example.

As the strands are unwound what happens to the rest of the double stranded DNA?

(Demonstration) I need two helicase helpers.

Type I topoisomerases relax DNA by nicking one strand, unwinding, and then closing the duplex DNA (Fig. 12.14).

Type II topoisomerases change DNA topology by breaking and rejoining double-stranded DNA (Fig. 12.16).

Topoisomerase II can also separate concatamers (Fig. 12.16) that form at the end of circular DNA replication (Fig. 12.19).

Linear DNAs have Topoisomerase II attached where DNA binds the protein scaffolding and plays a role in relieving tornsional strain during replication.

How do strands replicate?

Growing replication fork (Fig. 12.9a).

What is the direction of synthesis?

    Bond formation (movie).

DNA Polymerase

Pol I (first purified DNA polymerase, Arthur Kornberg) Prokaryote

• Single protein
• Requires template and primer
• 5' to 3' exonuclease activity combined with 5' to 3' polymerase activity allows it to nick translate which is important in removal of RNA primers.
• Does have 3' to 5' exonuclease activity for PROOF READING

 Pol III Prokaryote

• Very complex enzyme
• In its most active form it is associated with nine other proteins to form the pol III holoenzyme, referring to the fact that the enzyme is composed of several subunits
• Smallest aggregate which retains polymerizing activity is referred to as the Core Enzyme
• Pol III shares with pol I a requirement for a template and a primer
• However, pol III cannot polymerize at a nick. There must be a gap in the sequence .
• Does have 3'-5' exonuclease activity - PROOF READING FUNCTION
•  NO 5'-3'exonuclease activity.

    Leading and Lagging Strand Formation (movie).
    E. coli  (movie).
        Components of replication complex (Fig 12.11).

        Lagging-strand synthesis (Fig. 12.9b).

        Ligase reaction (Fig. 12.9c).

SV40  (Fig. 12.12).  = Eukaryote example.

Properties of DNA Polymerases (Table 12.1).

Replicating the ends of linear DNA (Fig. 12.13) (movie).

Quiz

Created 2004 by CA Rinehart• email CA Rinehart

Index • CourseInfo LogIn • Syllabus • References • Other Resources