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Chapter 8:DNA and RNA
Section 8-2A:
DNA Structure
Animations
http://www.johnkyrk.com/DNAanatomy.html
Introductory Questions
If Hershey and Chase are correct and genes are made of DNA, how is the message of a gene coded in a DNA molecule?
How can a molecule carry information?
DNA Structure
DNA = deoxyribonucleic acid Chain of nucleotides (three parts –
phosphate group, nitrogenous base, 5 carbon sugar)
4 different nucleotides, each with a different nitrogenous base – Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
Purines and Pyrimidines
Adenine and Guanine are purines
Cytosine and Thymine are pyrimidines
Chargaff’s Rule
Erwin Chargaff (USA, 1950) Nucleotides in DNA vary from sample to
sample Amount differ, but a pattern is evident –
amounts of adenine and thymine always the same, amounts of cytosine and guanine always the same (Chargaff’s Rule)
Rosalind Franklin
http://en.wikipedia.org/wiki/Rosalind_Franklin
Britain, 1951 Studying DNA molecule
using X-ray diffraction Powerful X-ray beam
aimed at a sample and scattering pattern recorded on film
Pictures give clues to the actual structure of DNA
X-Ray Diffraction
The Watson-Crick Model
James Watson and Francis Crick (USA, 1952)
Already working on DNA structure, saw Franklin’s X-ray diffraction and everything clicked
Saw that DNA was made of two strands twisted around each other
The Watson-Crick Model
Twisting pattern called a helix Built models or DNA structure – sugar-
phosphate backbone, strands wrap around each other in a double helix
Bonds Between Strands
Nitrogenous bases are close together along the center, held together by hydrogen bonds
Bonds form between purines and pyrimidines – between adenine and thymine, guanine and cytosine
Combinations are called base pairs
Bonds Between Strands
The sequence of bases on one strand matches the sequence of bases on the other strand – Chargaff’s Rule
AATGCC
TTACGG
Significance of the Double Helix
1962 – Watson, Crick, and Franklin’s assistant Wilkins won Nobel Prize for discovery of DNA double helix (Franklin died in 1958)
Watson and Crick published their model in a scientific paper only one page long
Also suggested method of DNA copying