In 1953, Watson and Crick recognize that DNA is a double-helix.

X-ray crystallography image from Franklin that provides clue to DNA structure

The Components and Structure of DNA

• DNA is in the shape of a twisted ladder, called a double helix.

– The sides of the DNA are made up of

1. Deoxyribose (sugar)2. Phosphate group (links the

deoxyribose together)

– The “rungs” of DNA are made up of bases

1. Adenine 2. Cytosine3. Guanine4. Thymine

Nucleotides always pair together.

“Base Pairs”Purines Pyrimidines

Adenine Thymine

Guanine CytosineIn 1949, Chargaff determined that their were equal parts A and T, and equal parts of G and C

Base Pairing

DNA Replication• Helicase “Hacks” the

two strands open at the hydrogen bonds.

• The DNA molecule separates into two strands

• DNA Polymerase “pastes” matching nucleotides on each half of the “unzipped” DNA.

1. Helicase “Hacks” the DNA strands apart by breaking the hydrogen bonds between the nitrogen bases.

2. DNA Polymerase “reads” the nucleotide base sequence and “pastes” the correct nucleotide to the growing strand.

Old StrandOld Strand

New Strands

DNA replication is “Semi-Conservative.” DNA has one old strand and one new strand after replication is complete.

• Amount of DNA varies per organism– Bacteria have ~600,000 base pairs their

genomes. (A genome is an organism’s complete set of DNA.)

– Humans have ~3,000,000,000 base pairs in our genome.

– http://en.wikipedia.org/wiki/Genome

• Each human chromosome contains 50-250 million base pairs.

This single-celled organism, Amoeba dubia, has a larger genome than you do.

Chromosome Structure• DNA is long.

– E. coli bacterium is about 2 μm in length, yet it contains about 1.6 m of DNA.

– A single human cell contains ~1.8m of DNA!

(There is enough DNA in your body to stretch from here to the moon and back 70 times!!)

• Chromosomes are supercoils of DNA– Double-stranded DNA coils around histone

proteins, called chromatin– Chromatin forms coils, and then those coils

form coils again - supercoils

Genes• Genes are the regions of DNA that are

instructions for making proteins (a few make RNA).

• Humans have 20,000-25,000 genes.

• Only about 2% of our DNA is genes– The noncoding regions function to provide

chromosomal structural integrity and to regulate where, when, and in what quantity proteins are made.

• Compare genes: http://en.wikipedia.org/wiki/Gene