Steve Symeonides
2/2/2012
Period 1,4,6,7
Biology
DNA STRUCTURE AND REPLICATION
DNA STRUCTURE
Chapter 8.2 p230-233
Structure of DNA
DNA is a large molecule of a kind called a polymer.
Polymer A polymer is a molecule that is made up of many repeating units or monomers. (like
Monomer a necklace made of many pearls put together- the necklace would be a polymer
made of the pearl monomers).
Nucleotide Structure
The basic unit, or monomer, of DNA is a nucleotide. (box p230)
A nucleotide is made of three chemical groups, a deoxyribose (sugar) a phosphate
group, and a nitrogenous base.
How nucleotides fit
together When nucleotides are put together to form DNA, the sugars and phosphates make up
the long side strands, the nitrogenous bases form the cross pieces between the two
strands. Flattened out it looks like a ladder, with the steps of the ladder being the
paired nitrogenous bases.
(diagram bottom left p230)
There are 4 different nucleotides in DNA, all have the deoxyribose and phosphate,
but there are 4 different nitrogenous bases that can be attached.
Nitrogenous bases
The 4 nitrogenous bases are Thymine, Adenine, Cytosine and Guanine (fig 8.4 p231)
Thymine and Cytosine both are called Pyrimidine bases because they have only one
Purines and pyrimidines ring in their structure. (remember this by seeing that all three have a y in their name).
Adenine and Guanine belong to the Purines, a group of bases that have two chemical
rings in their structure
DISCOVERING THE DOUBLE HELIX
Watson and Crick
The Double Helix shape of DNA (a double sided spiral) was not figured out until 1953,
when James Watson and Francis Crick published a paper describing DNA and showing
a model of its structure (p232).
Watson and Crick’s work on the chemistry of DNA was supported by the work of
scientists including
Chargaff’s Rules
A researcher named Edwin Chargaff analyzed DNA samples and
found that there were the same amounts of Adenine and Thymine in all DNA
samples, there were also the same amounts of Guanine and Cytosine.
Base Pairing Rules
Base pairing rules - that Adenine and Thymine always pair together in DNA and that
Guanine and Cytosine do the same resulted from Chargaff’s rules.
Steve Symeonides
2/2/2012
Period 1,4,6,7
Biology
Franklin used x-ray crystallography, bouncing x-rays off a DNA sample and recording
Rosalind Franklin
X-ray Crystallograhy the results as x-ray photos. The photos, showed and X inside a circular shape- which
she knew meant that the DNA molecule had a double spiral shape.
Franklin’s photos gave Watson and Crick the final information they needed to figure
out the DNA structure.
SUMMARY
ASSIGNMENT
As a summary, write three possible test questions based on
the information above, then answer them in full sentences.
Chapter 8 section 3
DNA REPLICATION
Pages 235 – 238
Why DNA needs to copy
itself.
There must be a complete copy of the DNA genome in every cell
For this to happen the DNA must be copied before a cell divides so that a copy of the
DNA can go into each of the new cells.
The most important thing about DNA is its ability to copy itself, that each side of the
DNA strand can serve as a template (guide) for creating the other side. This is what
Replication happens during replication. Replication is another word for copying, used when
talking about copying of DNA
The job of replication is to make sure that there are perfect copies of all of an
organisms chromosomes, one copy for the original cell, another copy for the new
cell.
How Replication works
Enzymes Replication works because there are a series of enzymes (proteins that help guide
and speed up chemical reactions) that help split the double helix down the middle,
and then help to hook up new, matching nitrogenous bases to the two strands.
DNA polymerase is the most important of the enzymes. It moves along the DNA,
DNA Polymerase helping to bond the new nucleotides to the old DNA strand and also to each other,
connecting up the new DNA strand.
DNA polymerase also proofreads the new DNA, finding any mistakes and fixing the
mis-matched base pairs.
Steve Symeonides
2/2/2012
Period 1,4,6,7
Biology
Step 1 of replication
unzipping
Origins of
replication
Replication starts when an enzyme (helicase) starts to break the bonds between base
pairs (A-T or G-C). This spot, called an Origin of Replication is where the DNA will
start to ‘unzip’ with the two strands separating in both directions from the origin of
replication (origin means starting point). (p236 and 237 #1)
Figure 8.9 on p238 shows how several of the hundreds of origins of replication start,
spread, and eventually join together, speeding up the copying of the DNA by copying
many sections at once.
As the enzyme unzips the DNA, the base pairs are separated and the two sides, or
strands, of DNA move apart and the bases are now available to match up with new
partners.
WHY replication must
happen.
Replication step 2
Making the new strand
DNA polymerase helps put
together the new strand of
nucleotides
Note: Remember that the whole point of replication is that a cell is ready to
divide into two cells – each needing its own DNA. So as a cell gets ready to
divide, it manufactures the enzymes needed for replication AND all of the
nucleotides it will need to build the new DNA strands before replication starts.
Each of the original strands of the unzipped DNA serves as a template (a guide) for
building a new strand.
(The new strand is also called a complementary strand since it is made of
complementary nucleotides(an A for a T, a G for a C)
The enzyme DNA polymerase attaches to the template strand of DNA (see p237 #2)
and moves along the strand, helping the nucleotides floating inside the nucleus
attach to their complementary bases according to the A-T, G-C, base pairing rules.
As the nucleotides match up along the template strand the DNA polymerase bonds
them together, forming a new second strand of DNA along the template strand.
Finishing Replication
Step 1- unzipping, and step 2- attaching new nucleotides, continue,
with the DNA unzipping and each side having a new strand built and attached to it,
until it runs into another section of replicating DNA (which started at a different
Origin of Replication. (see figure 8.9 p 238)
Sister Chromatids
Then copying stops on those sections. When the entire DNA strand, the whole
chromosome, has been copied, the two copies stay tied together. We call these two
copies of the chromosome sister chromatids. They will separate during Mitosis and
one sister chromatid will stay in the old cell and the other go into the new cell.
Mission Accomplished, the chromosomes have all been copied and a complete set
has been put into each new cell’s nucleus.