Transcript DNA Structure_replication.ppt

DNA Structure
Road to Discovery

1944 Oswald Avery’s team
determines that genes are
composed of DNA.
Road to Discovery

In 1949, Erwin Chargaff noticed
the amount of the nitrogen base
adenine and thymine always
equaled each other, and the
amount of nitrogen base cytosine
and guanine always equaled each
other.
In the 1950’s, Maurice
Wilkins and Rosalind
Franklin, working in
London, developed a high
quality X-ray photograph
of strands of DNA.
 These photos resembled
the tightly coiled helix
composed of two or three
chains of nucleotides.
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1953, James Watson
and Francis Crick used
Chargaff’s findings
and Franklin and
Wilkin’s X-ray pictures
to come up with the
model of DNA made
out of tin and wire.
The Double Helix

Watson and Crick
determined that the
DNA molecule is in
the shape of a
double helix, two
strands twisted
around each other,
like a winding
staircase.
Components of DNA

DNA is made up of
nucleotides all linked
together. Each
nucleotide is
composed of 3
things: a phosphate
group, a five carbon
sugar molecule
(deoxyribose), and a
nitrogen base.
The Phosphate Group is always the same.
 The 5 carbon sugar is called deoxyribose,
and that’s how DNA gets it’s name,
Deoxyribonucleic acid. REMEMBER, a lot
of sugar names end in “ose,” like glucose,
lactose, maltose, galactose.
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The nitrogen bases may be any of 4. It
would be adenine, guanine, thymine, or
cytosine.
Nitrogen Bases are grouped into 2’s.

Purines
Bulky, double-ringed.
Either Adenine or
Guanine

Pyrimidines
Smaller, single-ringed.
Either Thymine or
Cytosine.
Draw this in your notes
Base Pairing
Watson and Crick determined that a purine on one
strand of DNA is ALWAYS paired with a
pyrimidine on the opposite strand. They also
determined that it will always be like this:
Purines
Pyrimidines
Adenine
always with
Thymine
Guanine
always with
Cytosine

Complementary Base Pairs

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Adenine forms two weak hydrogen bonds with
Thymine, and Cytosine forms three weak
hydrogen bonds with Guanine.
These hydrogen bonds between the nitrogen
bases keep the two strands of DNA together.
Complementary Base Pairs

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Since the A and T can only bond with each
other and C and G can only bond with each
other, you can always guess what the opposite
strand of DNA will be.
This means they are complementary base pairs.

The sugar and
phosphates make up
the sides of the
DNA staircase (the
backbone), and the
nitrogen bases make
up the steps of the
staircase. The
nitrogen bases are
held together by
weak hydrogen
bonds.
The 5 Carbon Sugar
The 5 Carbon Sugar
Sugar structure: A prime (') notes where a
carbon is located on the sugar.
 Phosphate attaches to the 5' carbon.
 Nitrogen bases attach to the 1' carbon.
 The phosphate of the next nucleotide
attaches to the 3’ carbon.

The 5 Carbon Sugar
DNA is Anti-Parallel
DNA is Anti-Parallel

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Not only does DNA contain complementary base
pairs, but it is also anti-parallel!
Remember how the sugar is 5 carbon, and each carbon
is numbered?
Since only phosphates can attach to either the 5’ or 3’
carbons, and only bases can attach to the 1’ carbon,
the two strands of DNA must run in opposite
directions!
DNA is Anti-Parallel

If I show you ½ of a piece of DNA, can you tell
me the other ½?
We finally learn the reason for
complementary base pairing!
The Reason for Complementary
Base Strands

After the discovery of the double helix, scientists
thought the reason for the complementary bases
was in some way related to making exact copies
of the DNA each time a cell divided.
The Reason for Complementary
Base Strands
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Watson & Crick thought one DNA strand
serves as a template, or pattern, on which the
other strand is built.
They were correct!
S phase of the Cell Cycle
DNA Replication

Before a cell divides, it duplicates its DNA in a
copying process called replication.
DNA Replication
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During DNA replication, the DNA molecule
separates into its two strands, then produces two
new complementary strands following the rules
of base pairing.
How is DNA replicated???
Step 1 of DNA Replication
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The DNA double helix
must unwind or unzip.
An enzyme called DNA
helicase opens the
double helix by
breaking the hydrogen
bonds that link the
nitrogen bases between
the two strands.

Once the strands are broken
apart, proteins move in to
hold them apart.
Step 2 of DNA Replication

Once the strands are separated
at the replication forks,
enzymes known as DNA
polymerases move along each
DNA strand adding
nucleotides to the exposed
nitrogen bases.
The Replication Fork

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ation+Fork
Step 2 of DNA Replication
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As DNA polymerases move along, two new
double helixes are formed.

Notice that there are 2 DNA polymerases, and
only 1 DNA helicase!
Step 3 of DNA Replication

DNA polymerases continue to add nucleotides
until all the DNA has been copied and the
polymerases are signaled to stop.
Step 3 of DNA Replication

DNA replication makes two DNA molecules,
each one with one old strand and one new
strand of DNA. Notice that both molecules of
DNA are the exact same!
DNA Replication Video
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mation__dna_replication__quiz_1_.html
Step 3 of DNA Replication

This process is known as semiconservative
replication, because ½ of the new molecule is
from the old molecule.
Semiconservative Replication
What happens if there is a mistake?
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Errors occur during DNA replication. Thankfully,
DNA polymerases are also capable of “proofreading.”
The DNA polymerase can only add each next
nucleotide if the one before it was the correct one.
If the one before it was wrong, the DNA polymerases
must go back and fix its mistake.
Rate of Replication
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Replication does not begin at one end and end at the
other. DNA replication starts in several places so that it
finishes faster.
Remember, a strand of DNA has about 3 billion
complementary base pairs!!! Our DNA is a little over 3
feet long, in each cell!!! This would take a long time to
make copies of.
Replication Forks