Download In 1953 Watson and Crick developed a double helix model for DNA

DNA Structure – V3
DNA, Nucleotides and Antiparallel Strands
DNA means deoxyribonucleic acid.
DNA is the molecule in cells that has all the genetic instructions to make
all other cell parts and functions.
In 1953 James Watson and Francis Crick developed a double helix
model for DNA.
DNA consists of 2 strands of nucleic acids that twist around each other.
The basic unit of a nucleic acid is called a nucleotide.
Each nucleotide is made of three parts:
1. a sugar
2. a phosphate group
3. One of 4 nitrogen bases
Adenine (A)
Guanine (G)
Cytosine (C)
Coiled DNA
Thymine (T)
A DNA Nucleotide
The two nucleic acid strands are antiparallel to each other.
That means that one strand is “upside down” compared to the
other.
The 2 nucleic acid strands are held together by hydrogen
bonds between the nitrogen bases.
When the nitrogen bases bond together they follow a specific
pairing rule called complementary base pairing where:
 Adenine - thymine
 Cytosine – guanine
Developing a Model for DNA
Rosalind Franklin had concluded that there were two
antiparallel sugar-phosphate backbones, with the nitrogen
Uncoiled DNA
bases paired in the DNA molecule’s interior.
Watson and Crick then built models of a double helix to match the X-rays and chemistry of DNA.
At first, Watson and Crick thought the nitrogen bases paired like with like (A with A, and so on),
but such pairings did not result in a uniform width of the DNA molecule.
Then Watson and Crick determined that adenine paired only with thymine, and guanine paired
only with cytosine.
Finally, the structure of the molecule carrying genetic traits for all living organisms was figured
out.
DNA Structure – V3
DNA, Nucleotides and Antiparallel Strands
DNA means ____________________________.
DNA is the molecule in cells that has all the ______________________
to make all other cell parts and functions.
In 1953 _________________ and ___________________ developed a
double helix model for DNA.
DNA consists of _____________________________ that
____________ around each other.
The basic unit of a nucleic acid is called a ______________________.
Each nucleotide is made of three parts:
1. A ___________
Coiled DNA
2. A _______________ group
3. One of 4 nitrogen bases
 _______________ (A)
 _______________ (G)
 _______________ (C)
 _______________ (T)
Label the DNA Nucleotide
The two nucleic acid strands are _______________________ to each
other. That means that one strand is “_____________________”
compared to the other.
The 2 nucleic acid strands are held together by ____________ _
_____________ between the nitrogen bases.
When the nitrogen bases bond together they follow a specific pairing
rule called ________________________________________ where:

Adenine – thymine

Cytosine – guanine
Uncoiled DNA
DNA Structure – V3
Developing a Model for DNA
Rosalind Franklin had concluded that there were two antiparallel ____________________
_______________________, with the ________________________ paired in the DNA
molecule’s __________________ based on X-ray crystallography.
Watson and Crick then __________________ of a double helix to match the X-rays and
chemistry of DNA.
At first, Watson and Crick thought the nitrogen bases paired like with like (A with A, and so on),
but such pairings ______________________________________________of the DNA molecule.
Then Watson and Crick determined that adenine paired only with thymine, and guanine paired
only with cytosine. Finally, the structure of the _______________________________________
for all living organisms was figured out.
Homework
Read pgs. 187-190
Do Thought Lab #1-4
Use this diagram to answer the following questions.
1.
Make one circle around all the nucleotides on the
left hand strand.
2.
Write the nitrogen base sequence for the right hand
side starting at the top.
3.
Name the parts of the nucleotide that make up the
“backbone” of a strand of DNA.
4.
Why are the strands anti-parallel?
5.a) How many hydrogen bonds are there between
cytosine and guanine?
b)
How many hydrogen bonds are there between adenine and guanine?
6.
How many nucleotides are in this strand of DNA?
7.
How many nitrogen bases are in one nucleotide?