The Genetic Material

DNA can be found in the nucleus of eukaryotic (animals, plants, some single celled organisms) cells, and in the cytoplasm of prokaryotes (bacteria).

The Structure of DNA DNA (DEOXYRIBONUCLEIC ACID) is a polymer composed of monomers called NUCLEOTIDES. Each nucleotide consists of three subunits: 1. A five-carbon sugar 2. A phosphate group 3. A nitrogen containing base

PHOSPHATE Group 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

The sugar and phosphate molecules do not change in each nucleotide but the nitrogen base may be one of four different kinds. The 4 bases in DNA are: ADENINE (A) GUANINE (G) THYMINE (T) CYTOSINE (C)

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

If DNA were to be flattened out it would look like a ladder. The 2 strands are held together by hydrogen bonds between the pairs of bases.

The sugar-phosphate molecules in the nucleotide are like the 2 side rails of the ladder while the nitrogen- containing bases are similar to the steps or rungs of the ladder. In DNA these nitrogen bases always pair so that : T pairs with A and G pairs with C

thymine - adenine pair interacts through two hydrogen bonds T=A cytosine-guanine pair interacts through three hydrogen bonds C G The base pairs have a “specific fit” for each other- like Cinderella’s foot and shoe!

Base-Pairing Principle The base pairing is called complementary because 2 specific bases bond together to make a complete unit. hydrogen bonds connect the 2 bases. The 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 important in DNA replication.