DNAStructur

e

Cellular composition

DNA is contained in nucleus of cell

Phospho-lipids and proteins combined to form cell membrane

Lipids are fats

Base pairs

A=T – double hydrogen bond

G C – triple hydrogen bond

Nucleotide (3 chemical groups) Sugar – deoxyribose

contains 5 Carbon atoms

Phosphate group A base (A, T, G, C)

DNA is a very large molecule made up of a long chain of sub-units

The sub-units are called nucleotides

Each nucleotide is made up of

a sugar called deoxyribose

a phosphate group -PO4 and

an organic base

DNA molecule

Ribose is a sugar, like glucose, but with only fivecarbon atoms in its molecule

Deoxyribose is almost the same but lacks one oxygen atom

Both molecules may be represented by the symbol

Ribose & deoxyribose

The most common organic bases are

Adenine

(A)

Thymine (T)

Cytosine (C)

Guanine (G)

The bases

The deoxyribose, the phosphateand one of the bases

adenine

deoxyribose

PO4

Combine to form a nucleotide

Nucleotides

A molecule of DNA is formed by millions of nucleotides joined together in a long chain

PO4

PO4

PO4

PO4

sugar-phosphate backbone

+ bases

Joined nucleotides 7

In fact, the DNA usually consists of a double strand of nucleotides

The sugar-phosphate chains are on the outsideand the strands are held together by chemical bonds between the bases

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

2-stranded DNA

The bases always pair up in the same way

Adenine forms a bond with Thymine

and Cytosine bonds with Guanine

Bonding 1

Adenine Thymine

Cytosine Guanine

PO4

PO4

PO4

thymine

PO4

PO4

PO4

PO4

adenine

cytosine

PO4

guanine

Bonding 2

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

Pairing up

The paired strands are coiled into a spiral called

A DOUBLE HELIX

sugar-phosphatechain

bases

THE DOUBLE HELIX

Physical Structure (cont’d)

Chains are anti-parallel (i.e in opposite directions)

Diameter and periodicity are consistent 2.0 nm

10 bases/ turn

3.4 nm/ turn

Width consistent because of pyrimidine/purine pairing

Physical Structure (cont’d)

FORMS OF DNA

Supercoiling

DNA is Dynamic

Like proteins, DNA has 3º structure

Why so many deviations from normal conformation?

Effects on transcription (gene expression)

Enhances responsiveness

May also serve in packaging

NOTE: most cellular DNA exists as protein containing supercoils

Denaturation of DNA Denaturation by heating.

How observed?

A260

For dsDNA,

A260=1.0 for 50 µg/ml

For ssDNA and RNA A260=1.0 for 38 µg/ml

For ss oligos

A260=1.0 for 33 µg/ml

Hyperchromic shift

The T at which ½ the DNA sample is denatured is called the melting temperature (Tm)

A DIY model ofpart of a DNA molecule

http://www.present.udel.edu/biotech/rDNA.htmlhttp://

www.sumanasinc.com/webcontent/anisamples/molecularbiology/plasmidcloning_fla.html

http://www.worldofteaching.com

Accreditation to the following