DNA Structure

Watson & Crick & Franklin In 1953, Watson and Crick deduced the structure of DNA by using x-ray diffraction of DNA crystals Watson & Crick only figured out the structure, but relied upon the work of Rosalind Franklin who produce the photographs and crystals Watson & Crick received Nobel prizes, but Franklin didn’t since she had already passed away

Watson & Crick

Franklin

X-Ray Crystallography of DNA

DNA Structure DNA consists of two molecules that are arranged into a ladder-like structure called a Double Helix. A molecule of DNA is made up of millions of tiny subunits called Nucleotides. Each nucleotide consists of: Phosphate group Pentose sugar – in DNA this is Deoxyribose Nitrogenous base

DNA Subunits

Nucleotides Phosphate Pentose Sugar Nitrogenous Base

Nucleotides The phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the “rungs”. There are four types of nitrogenous bases.

Nitrogenous Bases

Nucleotides In 1949, Erwin Chargaff determined that each base will only bond with one other specific base. Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Base-Pairing Rules Straight letters go together! Curvy letters go together!

DNA Structure Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand. The bases on one side of the helix form hydrogen bonds with the complementary base on the other side of the strand

DNA Structure The order of nitrogenous bases on one chain for the DNA molecules is complementary to the order of bases on the opposite chain The order of nitrogenous bases of DNA is called its base sequence AAGCT TTCGA

A C T G G A T C

Why is complementary base pairing important? The hydrogen bonds between the base pairs help hold the two strands of a DNA molecule together The complementary nature of DNA helps explain how DNA replicates before a cell divides. One strand of DNA can serve as a template for making a new complementary strand

What does the abbreviation DNA stand for? DNA stands for deoxyribonucleic acid

Distinguish between purines and pyrimidines. Purines are nitrogenous bases made of two rings of carbon and nitrogen atoms. Pyrimidines are nitrogenous bases made of a single ring of carbon and nitrogen atoms.

What was the significance of Franklin’s X-ray diffraction photographs regarding DNA structure? Their photographs suggested that the DNA molecule resembled a tightly coiled helix and was composed of two or three chains of nucleotides.