Nucleic Acids

DNA is composed of fourkinds of nucleotides, eachof which consists of– a five carbon sugar (deoxyribose)

– a phosphate group (PO4-)

– one of four bases – adenine(A), guanine (G), thymine(T), or cytosine (C)

Components of DNA

Thymine and Cytosine

NucleotidesAdenine and

Guanine

Edwin Chargaff, in 1949,noted two critical bits of data

The four kinds of nucleotide bases makingup a DNA molecule differ in relativeamounts from species to species

Rosalind FranklinUsed x-ray diffraction techniques

to produce images of DNA molecules.

She concluded:1. DNA exists as a long, thin

molecule of uniform diameter.2. The structure is highly

repetitive.3. DNA is helical.

Patterns of Base Pairing

Watson and Crick used

numerous sources of data to

build models of DNA.

The bases were hydrogen bonded (a weak bond) together in the

center of the helix.

– T (a pyrimidine) had twohydrogen bonds with A (apurine)

– C (a pyrimidine) had threehydrogen bonds with G (apurine)

Patterns of Base Pairing

The backbone wasmade of chains ofdeoxyribose sugarcovalently bonded (astrong bond) tophosphate groups.

The phosphate groupis bonded to the 3’carbon of onedeoxyribose, and the5’ carbon of another.

Patterns of Base Pairing

Each sugar ofthe backbone iscovalentlybonded to thenitrogenousbase off ofcarbon 1.

Patterns of Base Pairing

There are 10base pairsper turn ofthe helix.

Patterns of Base Pairing

The two sides are anti-parallel,meaning that the sugar andphosphates are running inopposite directions.• Each side ends in a phosphate(5’ end) and a sugar (3’ end)

Since the sides are anti-parallel,one side goes in the 3' to 5'direction, and the other goes inthe 5' to 3' direction.

Patterns of Base Pairing

The base pairing is constant for all species, but the sequence of base pairs in a nucleotide strand is different from one species to the next.

Where is DNA Located?Prokaryotes

bacteria and ArchaeaCircular DNA in

a region calledthe nucleoid

Plasmid

EukaryotesInside the nucleusLinearHighly coiled

Set of eight proteins called histones act as spools to wind the DNA into units called nucleosomes which form additional loops called a chromosome.

Where is DNA Located?

Inside mitochondriaand chloroplastsCircular“Naked” – not

associated withprotein