DNA replication in 7 easy steps

1. Helicase enzyme breaks hydrogen bonds between bases, unzips and unwinds the helix

A protein that catalyzes chemical reactions.

–Helicase begins to unwind the DNA at the ORIGIN OF REPLICATION (a specific DNA nucleotide sequence)

It’s common to only show one strands sequence of bases, since the other can be inferred.

Helicase enzymes move in both directions from the Point of Origin, forming a REPLICATION BUBBLE. At either end of the replication bubble is a REPLICATION FORK, a Y-shaped region

where the new strands of DNA are elongating.

In prokaryotic cells there is one origin

In eukaryotic cells there are 100’s to 1000’s of origins

1

2

3

4

What does it really look like?

BILL: define terms related to step 1

• Origin of replication

• Helicase

• Single strand binding proteins

• DNA polymerase III

• Enzyme

• 3’ end

• Nucleotides

• Primase

• Primer

• Deoxyribonoucleotides

• Antiparallel

• Replication fork

• Leading strand

• Lagging strand

• Okazaki fragment

• DNA polymerase I

• DNA ligase

2. Single stranded binding proteins hold the DNA strands apart

– Keeps the separated strands apart and stabilize the unwound DNA

single-stranded binding proteins

helicase

replication fork

BILL: define terms related to step 2

• Origin of replication

• Helicase

• Single strand binding proteins

• DNA polymerase III

• Enzyme

• 3’ end

• Nucleotides

• Primase

• Primer

• Deoxyribonoucleotides

• Antiparallel

• Replication fork

• Leading strand

• Lagging strand

• Okazaki fragment

• DNA polymerase I

• DNA ligase

3. RNA nucleotides bind with complementary base

sequences under the direction of the enzyme RNA primase. These RNA nucleotides act as a primer for DNA nucleotides.

– Primers are short segments of RNA, about 10 nucleotides long

– Must have a primer because DNA polymerase can only add nucleotides to another nucleotide

REVIEW BILL:

List three ways RNA is different than DNA.

Primase RNA Primer

DNA

BILL: define terms related to step 3

• Origin of replication

• Helicase

• Single strand binding proteins

• DNA polymerase III

• Enzyme

• 3’ end

• Nucleotides

• Primase

• Primer

• Deoxyribonoucleotides

• Antiparallel

• Replication fork

• Leading strand

• Lagging strand

• Okazaki fragment

• DNA polymerase I

• DNA ligase

4. DNA polymerase III adds DNA nucleoside

triphosphates to the RNA primer sequence in a 5’ to 3’ direction.

Two of the phosphates are stripped

off in the bonding

energy

ATP GTP TTP CTP

Energy of Replication Where does energy for bonding usually come from?

ADP AMP GMP TMP CMP

modified nucleotide

energy

We come with our own energy!

And we leave behind a nucleotide!

Energy of Replication • The nucleotides arrive as nucleosides

– DNA bases with P–P–P • P-P-P = energy for bonding

– DNA bases arrive with their own energy source for bonding

– bonded by enzyme: DNA polymerase III

ATP GTP TTP CTP

• DNA polymerasre III can only add nucleotides to 3 end of a growing DNA strand – need a “starter”

nucleotide to bond to

• strand only grows 53

Adding Bases 3

5

5