1. DNA Replication
12-3

2. The helical structure of DNA
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
3 end
0.34 nm
5 end
Key features of DNA structure
Partial chemical structure
Space-filling model
The helical structure of DNA

3. 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

4. The parent molecule has two complementary
strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.
The first step in replication is separation of the two DNA strands.
Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.
The nucleotides are connected to form the sugar-phosphate back-
bones of the new strands. Each “daughter” DNA
molecule consists of one parental strand and one new strand.

5. How does all your DNA fit?
DNA double helix
Histone
tails
His-
tones
Linker DNA
(“string”)
Nucleosome
(“bead”)
10 nm
2 nm
Histone H1
Nucleosomes (10 nm fiber)
30 nm
30-nm fiber
300 nm
Loops
Scaffold
Protein scaffold
Looped domains (300-nm fiber)
Metaphase chromosome
700 nm
1,400 nm
How does all your DNA fit?
If you lined up all the DNA in one human cell it would be 2m long!
But a typical human cell is only 10um in diameter (1x10-7 m)
How does it all fit?
Its compacted!

6. 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
DNA Replication: A Closer Look

7. Getting Started: Origins of Replication
Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble”
A eukaryotic chromosome may have hundreds or even thousands of origins of replication
Replication proceeds in both directions from each origin, until the entire molecule is copied
At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating
Getting Started: Origins of Replication

8. Parental (template) strand 0.25 µm Origin of replication
LE 16-12
Parental (template) strand
0.25 µm
Origin of replication
Daughter (new) strand
Bubble
Replication fork
Two daughter DNA molecules
In eukaryotes, DNA replication begins at may sites
along the giant DNA molecule of each chromosome.
In this micrograph, three replication
bubbles are visible along the DNA
of a cultured Chinese hamster cell
(TEM).

9. Elongating a New DNA Strand
Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork
The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells
Elongating a New DNA Strand

10. Other Proteins That Assist DNA Replication
Helicase untwists the double helix and separates the template DNA strands at the replication fork
Single-strand binding protein binds to and stabilizes single-stranded DNA until it can be used as a template
Other Proteins That Assist DNA Replication

11. Overview: The Flow of Genetic Information
How do the As, Cs, Ts, and G’s encode the genetic information?
The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins
Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation
The ribosome is part of the cellular machinery for translation, polypeptide synthesis
Overview: The Flow of Genetic Information