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)
Thymine (T)
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 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, 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 ______
2. A ______group
3. One of 4 nitrogen bases
· ______ (A)
· ______ (G)
· ______ (C)
· ______ (T)
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
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?