DNA Structure

The Genetic Material

A. Introductioneukaryotes (animals, plants, some single-celled organisms) have DNA in nucleus

prokaryotes (bacteria) have DNA in cytoplasm

B. Structure of DNADNA (deoxyribonucleic acid) is a

polymer composed of monomers called nucleotides.Each nucleotide consists of three

subunits:five-carbon sugar (deoxyribose)phosphate group (PO4)Nitrogen-containing base

PHOSPHATEGroup

NITROGEN -CONTAINING

BASE

5-CARBON SUGAR

DNA(Polymer)

Made up of

NUCLEOTIDES(monomer)

contain

A nucleotide looks like this

Nitrogen base

5 Carbon sugar

Phosphate group

sugar (deoxyribose) and phosphate (PO4) molecules do not change in each nucleotide so are often referred to as the “backbone”

nitrogen base may be one of four different kinds:Adenine (A)Guanine (G)Thymine (T)Cytosine (C)

pyrimidines

purines

Watson and Crick determined that the DNA molecule consists of two strands twisted around each other into a double helix resembling a double spiral staircase.

• If DNA were to be flattened out, it would look like a ladder.

• 2 strands are held together by hydrogen (H) bonds between pairs of bases.

Sugar-phosphate molecules in the nucleotide are like side rails of the ladder (the backbone)

nitrogen-containing (nitrogenous) bases are similar to steps or rungs of ladder with 1 purine bound to 1 pyrimidine

Nitrogenous bases always pair so that: T pairs with A with 2 hydrogen bondsG pairs with C with 3 hydrogen bonds

C. Base-Pairing Principlebase pairing of A-T and C-G is called complementary because 2 specific bases bond together to make a complete unit.

Specificity of this bonding results in the 2 DNA strands running in opposite directions.

A completed double strand of DNA looks like the picture to the left.

Note that the sugars point in opposite directions.

Each strand is antiparallel to the other strand.

This orientation is crucial in DNA replication.