teded we study dna for many reasons, e.g., its central importance to all life on earth, medical...
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Why do we study DNA?We study DNA for
many reasons, e.g., its central
importance to all life on Earth,
medical benefits such as cures for diseases,
better food crops.
A HISTORY OF DNA
Discovery of the DNA double helix
A. Frederick Griffith – Discovers that a factor in diseased bacteria can transform harmless bacteria into deadly bacteria (1928)
B. Rosalind Franklin - X-ray photo of DNA.
(1952)
C. Watson and Crick - described the DNA molecule from
Franklin’s X-ray.(1953)
SEE p. 292-293
Watson & Crick proposed…•DNA had specific pairing between the nitrogen bases:
ADENINE – THYMINE
CYTOSINE - GUANINE
•DNA was made of 2 long stands of nucleotides arranged in a specific way called the “Complementary Rule”
Chromosomes and DNA
Our genes are on our chromosomes.
Chromosomes are made up of a chemical called DNA.
DNA Nucleotide
OO=P-O O
Phosphate Group
NNitrogenous base (A, G, C, or T)
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
One Strand of DNA
One strand of DNA is a polymer of nucleotides.
One strand of DNA has many millions of nucleotides.
nucleotide
One Strand of DNAThe backbone of the molecule is alternating phosphate and deoxyribose, a sugar, parts.
The teeth are nitrogenous bases.
phosphate
deoxyribose
bases
The Shape of the Molecule
DNA is a very long polymer.
The basic shape is like a twisted ladder or zipper.
This is called a double helix.
DNA Double Helix
P
P
P
O
O
O
1
23
4
5
5
3
3
5
P
P
PO
O
O
1
2 3
4
5
5
3
5
3
G C
T A
PURINES1. Adenine (A)
2. Guanine (G)
PYRIMIDINES3. Thymine (T)
4. Cytosine (C)
Nitrogenous Bases
T or C
A or G
Adenine must pair with Thymine
Guanine must pair with Cytosine
Their amounts in a given DNA molecule will be about the same.
Chargaff’s Rule
G CT A
BASE-PAIRINGS
CG
H-bonds
T A
Genetic Diversity…Different
arrangements of NUCLEOTIDES in a nucleic acid (DNA) provides the key to DIVERSITY among living organisms.
The Code of Life…The “code” of the chromosome is the
SPECIFIC ORDER that bases occur.
A T C G T A T G C G G…
DNA is wrapped tightly around histones and coiled tightly to form chromosomes
See p. 297
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