The Structure of DNA
Ag Biology
What is DNA?
• Deoxyribonucleic Acid
• Structure that stores hereditary material
• By 1950, most scientist accepted genes
were made of DNA but did not know
what it looked like
Watson and Crick’s Model
• James Watson and Francis Crick discovered
that the structure of DNA was important to
determining how DNA serves as the
genetic material.
• They found that DNA is a molecule that is a
double helix
• Two strands twisted around each other
DNA Structure
•
Each strand is made of nucleotides
•
A nucleotides are made of three parts:
1. Phosphate
2. Five-carbon Sugar called deoxyribose
3. Nitrogen base
Nitrogen Bases
•
There are four different kinds nitrogen
bases that make up DNA:
1. Adenine (A)
2. Guanine (G)
3. Thymine (T)
4. Cytosine (C)
Nitrogen Bases
• Adenine and Guanine are
classified as purines (made
of two rings of carbon and
nitrogen atoms)
• Thymine and Cytosine are
classified as pyrimidines
(made of a single ring of
carbon and nitrogen atoms)
Base-paring Rules
• Bases are always paired with
a pyrimidine and purine.
• Adenine is always paired
with Thymine
• Guanine is always paired
with Cytosine
Hydrogen Bonding
• The bases are connected with their pair by
hydrogen bonding which also keeping the
two strands of DNA together.
How did this model come to be?
• Erwin Chargaff
• Biochemist
• In 1949, he observed that the amount of adenine (A)
always equaled the amount of thymine (T) and the
amount of guanine (G) always equaled the amount of
cytosine (C).
• However, the numbers of each varied between organisms
How did this model come to be?
• Maurice Wilkins and Rosalind Franklin
• Scientist from London
• Developed high quality X-ray photographs
of fibers of the DNA molecule
• Pictures suggested that DNA resembled a
tightly coiled helix
How did this model come to be?
• Watson and Crick’s
• From prior discoveries, Watson and Crick
developed a 3-D model using tin and wire
The Replication of DNA
Copying DNA
• In order the cells to divide, DNA
must be copied by the process of
DNA Replication
• Watson and Cricks proposed that
DNA serves as a template for
building other strands.
• Occurs in 3 steps
DNA Replication Step 1
• Double helix unwinds with the help of the
DNA helicases
• DNA helicases are enzymes that break the
hydrogen bonds that link the nitrogen bases.
• Additional proteins are attached to each
strand to keep the strands apart and from
recoiling
• The two ends where the DNA separates are
called replication forks
DNA Replication Step 2
• DNA polymerases (enzymes)
add complementary
nucleotides to each strand at
the replication forks.
• Nucleotides are added according
to the base pairs rule.
DNA Replication Step 3
• Polymerase remain attached until the DNA is
completely copied and the signal for detach
is given.
• Two DNA molecules form that are identical
to the original DNA.
• Each is composed of an original and a new
stand.
• The nucleotide pattern is identical to the original
strands.
Checking for Errors
• Sometimes errors occur during this process when
the wrong nucleotides are added to the strand.
• DNA polymmerases “proofread” by:
• Only being able to add nucleotides when the previous
nucleotide is correctly paired with its base
• Backtracking and removing incorrect nucleotides and
inserting correct ones
• This prevents most errors– only 1 error occurs in a
billion nucleotides. This is called a mutation.
Multiple Replication Forks
• Replication does not begin at one end of the DNA
strand and end at the other.
• Since most eukaryotic cells have one long DNA
strands in each chromosome, it would take too long
to go from one end to the other.
• Human DNA is replicated in about 100 sections
• Each sections has a starting point
• Since multiple forks work at the same time, the entire
chromosome can be replicated in 8 hours.