Q2 WK6 D1
11/18/13
DNA: Deoxyribonucleic
Acid
Scientists of DNA
• 1953, James Watson & Francis Crick
were accredited for discovering the
structure of DNA
• Awarded the Nobel Prize for
Physiology and Medicine in 1962;
Watson, Crick and Maurice Wilkins
Scientists of DNA
Rosalind Franklin:
Rosalind refined the technique of X-ray
crystallography, to study DNA and produced “Photo
51”
From photo 51, Rosalind determined that DNA is the
shape of double helix
Watson & Crick used photo 51 and published their
results before Franklin
Rosalind Franklin died in 1985
Location of DNA
• DNA is coiled in chromosomes; chromosomes
are located in the nucleus.
• DNA is the shape of double helix
• Double stranded
• Twisted
• Similar to a spiral staircase
Double Helix looks like…
• If a DNA strand was
straightened, what would it
look like???
Make up of DNA
• DNA is made up of Nucleotides strung together in a certain order.
• Each nucleotide contains…
• Deoxyribose (Sugar)
• Phosphate
• Nitrogen base
•
•
•
•
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
How is DNA organized?
• Sugar alternates with phosphate at the sides
• The rungs (steps) of the DNA molecule are paired nitrogen bases
• The nitrogen bases pair in the following way…
• Adenine ALWAYS pairs with Thymine (A-T)
• Cytosine ALWAYS pairs with Guanine (C-G)
 The rungs are connected at the sugars
Add to notebook check
Nitrogen Base: PRACTICE
1.
2.
3.
4.
5.
6.
7.
8.
Adenine
Cytosine
Thymine
Adenine
Guanine
Guanine
Cytosine
Thymine
-
Write the appropriate pair
with the listed nitrogen
base…
Nitrogen Base: answers
1.
2.
3.
4.
5.
6.
7.
8.
Adenine
Cytosine
Thymine
Adenine
Guanine
Guanine
Cytosine
Thymine
• Thymine
• Guanine
• Adenine
• Thymine
• Cytosine
• Cytosine
• Guanine
• Adenine
DNA structure: REVIEW
• DNA shape = double helix
• Rails = Phosphate & deoxyribose (sugar) alternate
• Rungs= Nitrogen base pairs
• Base Pairing Rule: Adenine – Thymine
Guanine – Cytosine
Purines: Adenine & Guanine; made of 2 carbon rings and nitrogen atoms.
Pyrimidines: Thymine & Cytosine; made of a single ring of carbon and
nitrogen atoms
Q2 WK7 D1
Purines & Pyrimidines
DNA Replication
• During the cell cycle DNA replication occurs the “S” (synthesis) phase
• DNA Replication: The process of making a copy of DNA
• DNA replication results in 2 identical DNA strands
• DNA is copied BEFORE the cell divides
Steps of DNA Replication
• Step 1: Before DNA can replicate, the double helix unwinds.
• An enzyme called “ DNA Helicase” breaks the bonds between the nitrogen
bases.
• Once the strands separate, proteins prevent the strands from making their
double helix shape.
• Areas where the double helix separates are called “Replication forks”.
DNA unwinds
DNA Helicase- Enzyme that opens the double helix
Replication fork- areas where the DNA opens
Tuesday 12/1/2015
Agenda: DNA
 Notes: DNA Replication (Steps 2 and 3)
 Activity: DNA Replication Worksheet
 Homework:
 DNA Replication Worksheet due
Wednesday/Thursday 12/2-12/3/2015
for 15 Points
 GRADEBOOK CLOSES
FRIDAY 12/11/2015!!!
DNA Structure and Replication
Quiz Friday 12/4/2015!!
Steps of DNA Replication
• Step 1: Before DNA can replicate, the double helix unwinds.
• An enzyme called “ DNA Helicase” breaks the bonds between the nitrogen
bases.
• Once the strands separate, proteins prevent the strands from making their
double helix shape.
• Areas where the double helix separates are called “Replication forks”.
DNA unwinds
DNA Helicase- Enzyme that opens the double helix
Replication fork- areas where the DNA opens
Steps of DNA Replication
• Step 2: At the replication fork, an enzyme known as “DNA
polymerase” moves along each of the DNA strands.
• DNA polymerase starts adding nucleotides to the exposed nitrogen
bases; using the base pairing rules.
• End result: 2 new double helices are formed.
DNA polymerase: An enzyme adding nucleotides to
the EXPOSED bases; forming 2 identical copies of DNA

Steps of DNA Replication
• Step 3: Once DNA polymerase adds nucleotides to the exposed bases, it
continues until the polymerase is signaled to detach.
• The 2 new double helices are made up of an old and NEW
strand
• DNA sequence is identical to each other and to the original DNA molecule
• End result: 2 identical DNA
molecules
• Each made up of an OLD and NEW
strand