Nucleic Acid Chemistry Dr: Hussein abdelaziz

Objectives

By the end of lecture the student should:

Understand polymerization of Polynucleotide chain

Illustrate structure of DNA.

Differentiate between different types of RNA

• Nucleotides are linked together by 3`,5` phosphodiester bonds

i.e. pentose sugar of one nucleotide is attached by C 3` to the phosphate of the next nucleotide (by an ester bond), & the latter is attached to 5`C of pentose of next nucleotide

Polynucleotide chain polymerization

Polynucleotide chain polymerization

1) sugar-phosphate linkages form asymmetrical backbone 2) Polarity of chains: 5` end phosphate group & 3` end OH group 3) DNA sequences are written in 5' - 3' direction e.g. GCA 4) polymerization reaction is catalyzed by specific enzymes DNA and RNA polymerases

Characters of polynucleotide chain

2- DNA structure

The body contains trillions of cells

Nucleus (center of cell) contains chromosomes

Chromosomes are threadlike structures composed of DNA molecules + proteins

A gene, a segment of DNA containing the hereditary code

DNA is a long chain of repeating nucleotides

Site: in the nucleus 1% DNA is found in the mitochondria •It forms the main substance of chromosomal material (the chromatin) The chromosomal DNA forms the genes which are the carriers of genetic information

The chromatin

The chromatin is a nucleoprotein made of DNA and basic proteins called histones

DNA primary structure

• A DNA molecule is a polymer of 2 polynucleotides chains

• formed of a large number of deoxyribonucleotides

-The sugar is deoxyribose -The nitrogenous base

is either purine (A & G) or pyrimidine (T & C)

DNA 2ry structure

Watson & Crick model

DNA consists of 2 very long helical, polynucleotide strands twisted around each other forming a right handed double helix (B- form) shaped like a coiled

staircase or ladder

The outside of the ladder is formed by the pentose- phosphate backbones. Purines and pyramidines of each strand are directed inwards toward each other (perpendicular to 2 strands), connected by hydrogen bonds forming the rungs of the ladder& holding the 2 strands together

base pairing rule - Adenine is joined to

thymine by 2 hydrogen bonds (A=T)

- guanine is joined to cytosine by 3 hydrogen

bonds (GΞC) so, sequence of bases in

one strand determines their sequence in other strand.

So the 2 strands are complementary

The 2 strands of the double helix are antiparallel i.e. they run in the opposite direction: One runs in the 5`to 3`direction The other runs in the 3`to 5` direction

Only 1 of 2 strands of DNA is responsible for synthesis of mRNA. It is known as sense (template, non coding) strand The other strand is known as anti sense (non template, coding) strand as it is similar to mRNA

Template strand

Forms of DNA as regards 3D structure

B-Form DNA predominant form It is a right handed double helix

A-Form more compact than B-form but similar to B-form Z-Form DNA completely different from B-form It is a left handed helix and has a zig-zag appearance

o 2- 10 copies oSmall circular o Double-stranded DNA o 1 % of total cellular DNA o Transmitted by maternal non-mendelian inheritance o Codes for only a small fraction of mitochondrial proteins. The rest are coded for by nuclear DNA o All proteins are synthesized in the cytoplasm and transported into the mitochondria

Mitochondrial DNA