Chapter 8: DNA and RNA Section 8-2A: DNA Structure

Animations

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 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