DNA S TRUCTURE

G ENERAL O VERVIEW DNA is short for deoxyribonucleic acid. DNA and RNA ( ribonucleic acid ) are called nucleic acids because they are found in the nucleus. The building blocks of DNA and RNA are nucleotides.

N UCLEOTIDE STRUCTURE Nucleotides are made up of 3 components: 1. Monosaccharide (5 carbon sugar) 2. A nitrogenous base 3. Phosphate group

NUCLEOTIDE ARE USUALLY REPRESENTED LIKE THIS Nitrogen base 5 Carbon sugar Phosphate group

DNA In DNA the 5 carbon sugar is deoxyribose. Notice the numbering of the carbons.

6 P ENTOSE S UGAR Carbons are numbered clockwise 1’ to 5’ CH2 O C1C1 C4C4 C3C3 C2C2 5 Sugar Sugar(deoxyribose) copyright cmassengale

A phosphate group is attached to the sugar molecule and together these 2 molecules are referred to as the backbone of DNA.

There are four bases found in DNA: How do you remember which is a purine and which is a pyrimidine? Pyramid has a diamond shape – diamonds can CUT (cytosine, uracil, thymine) PyrimidinesPurines CytosineAdenine (Uracil in RNA)Guanine Thymine

The bases are joined to the sugar molecule at the 1’ carbon.

Nucleotides in DNA join together to form polynucleotides (long strands of nucleotides) DNA does not exist as a single strand, and back in the 1950’s 2 scientists, Watson and Crick, developed a model of the structure of DNA. According to the Watson-Crick model DNA is composed of 2 strands held together by hydrogen bonds between bases and twisted into a double helix.

The H-bonds in double stranded DNA are between the bases on adjacent chains. This H-bonding is very specific: A IS ALWAYS H- BONDED TO T A T G ALWAYS H- BONDED TO C G C T HIS BASE PAIRING IS SAID TO BE COMPLEMENTARY

In double-stranded DNA the long polynucleotide strands are hydrogen bonded in an antiparallel orientation. Antiparallel means the 5’ end of one chain is paired with the 3’ end of the other.

16 A NTIPARALLEL S TRANDS One strand of DNA goes from 5’ to 3’ (sugars) The other strand is opposite in direction going 3’ to 5’ (sugars) copyright cmassengale

The sum total of an organism’s genetic information is called its genome. Eukaryotic genomes are composed of several large pieces of linear double-stranded(ds) DNA; each piece of ds-DNA is called a chromosome. DNA does not exist as a free floating molecule in the cell it is packaged to fit within the nucleus.

To accomplish this: 1. DNA is wrapped around globular proteins called histones. 2. After being wrapped around histones DNA has the appearance of beads on a string. These beads are called nucleosomes. They are composed of DNA wrapped around an octamer of (a group of 8) histones. 3. DNA becomes tightly packed and is now known as chromatin : closely stacked nucleosomes.