DNA Structure

The Chemical Composition of DNA

• DNA is made of 3 different components: a deoxyribose sugar, a phosphate group, and a nitrogenous base

• Deoxyribose Sugar – sugar molecule containing 5 carbons that lose the OH (hydroxyl group) on its 2’ carbon

• Phosphate group – group of 4 oxygen atoms surrounding a central phosphorus atom found in the backbone of DNA

• Nitrogenous base – an alkaline, cyclic molecule containing nitrogen

• The source of variation in DNA is found in the nitrogenous bases.

• 4 nitrogenous bases exist:Purines: Adenine (A) and Guanine (G) – both are double-ringed structuresPyrimidines: Thymine (T) and Cytosine(C) – single-ringed structures

Structure of Nucleotides

• Each of DNA’s 4 NT’s comprises a deoxyribose sugar attached to a phosphate group and a nitrogenous base

• The nitrogenous base is attached to the 1’ C of the sugar by a glycosyl bond – a bond between a sugar and another organic molecule by way of an intervening nitrogen or oxygen atom.

• The phosphate group is attached to a 5’ C by an ester bond phosphodiester bond

• RNA is usually single-stranded and generally does not form double helices.

• The carbon atoms in the five-carbon sugar are numbered clockwise, starting with the carbon atom to the right of the oxygen. The first one is known as the one prime (1’).

• In DNA, the nitrogenous base is attached to the 1’ carbon by a glycosyl bond

Building a Model for DNA Structure• Many scientists were working on the structure:

Linus Pauling (California), Rosalind Franklin and Maurice Wilkins (London), and James Watson and Francis Crick (Cambridge University).

• Franklin and Maurice were using X-ray diffraction analysis of DNA to determine its structure.

• The X-rays are deflected by the atoms in the molecule, producing a pattern of lighter and darker lines on photographic film. The pattern that is produced is analyzed and the 3-D structure is determined using mathematics.

• The pattern revealed that DNA has the shape of a helix or corkscrew, is about 2nm in diameter, and has a complete helical turn every 3.4nm (1nm = 10-9m).

Animations

HHMI's BioInteractive - Watson constructing base pair models

DNA Structure: The Double Helix

• DNA consists of 2 antiparallel strands of nucleotides.

• Antiparallel – parallel, but running in opposite directions; the 5’end of one strand of DNA aligns with the 3’end of the other strand in a double helix.

• Complimentary Base Pairing – pairing of the nitrogenous base of one strand of DNA with the nitrogenous base of another strand; AT & GC

• Proposed by Erwin Chargaff (1949)

HHMI's BioInteractive - Building blocks of DNA

• Purines are nitrogenous bases that have 2 rings of carbon: adenine and guanine

• Pyrimidines are nitrogenous bases that have only 1 ring of carbon: thymine and cytosine.

• One purine must combine with one pyrimidine for the complementary base pairing, otherwise the DNA structure would look “wonky”.

• Nucleotides are 0.34nm apart• To complete one turn of the helix, you need

10 nucleotides

• The bases of the two DNA strands are bound by hydrogen bonds.

• Hydrogen bonds on their own are relatively weak, but multiple hydrogen bonds stronger.

• There are 2 hydrogen bonds between A and T, and 3 hydrogen bonds between G and C. This is why A cannot bind with C and G cannot bind with T.

• The 2 strands of DNA run anti-parallel. One strand runs in the 5’ to 3’ direction, the other runs n the 3’ to 5’ direction.

• The 3’ end terminates with the hydroxyl group of the deoxyribose sugar. The 5’ end terminates with a phosphate group.

• 5’ – ATGCCGTTA – 3’• 3’ – TACGGCAAT – 5’

HHMI's BioInteractive - DNA packaging

DNA structure