1. DNA Replication
Accel Bio 2014

2. Overview: What is DNA for?
The purpose of DNA is to store the information necessary to allow cells & organisms to function. This includes storing all the info needed to make all of the proteins in our body.
All of the DNA in a cell, which includes every gene, makes up an organism’s genome.
The human genome is over 3 billion base pairs long!
In our genome, the DNA is organized into structures called chromosomes.

3. What is a chromosome?
A chromosome consists of a long piece of DNA wrapped up with proteins called histones.
When DNA is wrapped with proteins, it is called chromatin.
When a piece of chromatin is folded, coiled, & packed into a small bundle, it’s called a chromosome.
Humans have 46 chromosomes in every body cell.
ANIMATIONS:

4. The Gene The “functional unit” of DNA is the gene.
A GENE is the sequence of DNA that contains the information required to make a specific _polypeptide_, from its first amino acid to its last.
Questions:
How does the structure of DNA allow for its function?
How is DNA copied/replicated (without errors!) when a cell divides?
And, how does the process of making a specific protein from a piece of DNA (a gene) work?

5. DNA Replication: a faithful process
The process of copying a double-stranded DNA molecule is called replication.
Before cells can divide, DNA is replicated, usually without a single error! How??
How difficult is this to do anyways? One way to think about it…
Copying the 3 billion bases in human genome is like typing up a book that is 3 billion letters long.
How many pages would this book be??
Assume average English word = 5 letters
Then, 3 billion bases = 600 million words
Assume 250 words / page (double-spaced 12 pt Times)
Then 600 million words = 2,400,000 PAGES
Could you type this book without any errors in 6-8 hours?

6. DNA Replication: a faithful process
Copying the 3 billion bases in human genome is like typing up a book that is 3 billion letters long.
How many pages would this book be??
Assume average English word = 5 letters
Then, 3 billion bases = 600 million words
Assume 250 words per page (double-spaced 12 pt Times)
Then 600 million words = 2,400,000 PAGES
Could you type this book w/o any errors in 6-8 hrs?
What strategies would help you accomplish this task??

7. DNA Replication
How could we use one strand of DNA to know how to construct the second strand of DNA?
The players: Enzymes like Helicase, Primase, DNA Polymerase, Ligase…
The method: Complementary base pairing

8. DNA Replication is Semi-Conservative
ANIMATION:

9. DNA Replication overview
One side of a replication bubble is called a replication fork (see below).
Below: DNA is copied quickly because there are several replication bubbles that open along a molecule at once. When the bubbles meet, replication is done.

10. Steps of DNA Replication
The enzyme Helicase unwinds & “unzips” the DNA molecule at a particular point (an origin of replication), making two template strands of DNA available.
The enzyme Primase synthesizes (makes) short RNA primers (5’  3’) to start each new DNA strand or fragment. The end of the primer is where DNA Polymerase begins.
The enzyme DNA Polymerase catalyzes the synthesis of the new DNA strands. DNA Polymerase helps the DNA nucleotides (which are readily available in the cell) match up with their complementary base on the template DNA (A & T, C & G).
DNA Polymerase also catalyzes the creation of covalent sugar-phosphate bonds along the backbone of the new DNA strands. (continued on next slide…)

11. Steps of DNA Replication, cont.
DNA Polymerase also catalyzes the creation of covalent sugar-phosphate bonds along the backbone of the new DNA strands.
DNA Polymerase creates the new strands of DNA in one direction only -- from the 5’ end to the 3’ end of the NEW strand. This is because physically, DNA Polymerase can only add on to the 3’ end of the strand it is building.
This results in one strand of DNA (the “leading” strand) being synthesized continuously from 5’ to 3’ while the other strand of DNA (the “lagging” strand) is synthesized from 5’ to 3’ in small pieces called Okazaki fragments.
Why is one strand synthesized in small pieces? Because overall the strand needs to be made in the 3’ to 5’ direction, but DNA Polymerase cannot do this, so it makes several small 5’ to 3’ Okazaki fragments instead. Study the replication pictures to understand this better. Okazaki fragments are then joined together by yet another enzyme -- Ligase.
When synthesis of the two new strands is done, DNA Polymerase and other proofreading enzymes check for errors and repair them. Fantastic!

12. DNA Replication – the details
ANIMATIONS:

13. Rates of DNA Replication
How long does replication take?
In the prokaryotic bacterium E. coli, replication can occur at a rate of 1,000 nucleotides per second.
In comparison, eukaryotic human DNA replicates at a rate of 50 nucleotides per second.
In both cases, replication occurs so quickly because multiple polymerases can synthesize two new strands at the same time by using each unwound strand from the original DNA double helix as a template.

14. DNA Replication Animations
olgenetics/dna-rna2.swf
hill.com/sites/ /student_view0/chapter3/a nimation__dna_replication__quiz_2_.html
n%20Coordination%20Leading%20Lagging%20Stran d%20Synthesis.htm
of-replication-advanced.html

15. A Labeled Replication Fork