Nucleic acids

Nucleic acids

Objectives

At the end of this lecture, student will be able

• Explain the chemistry of nucelosides and nucleotides

• Describe the double helix structure of DNA

• Describe the structure of RNA

• Compare the structural features of DNA and RNA

Content

• Nucleic acids

– Nucleotide

– Nucleoside

• Structure of DNA

• Structure of RNA

Nucleic acids

• Macromolecules formed by repeating units of nucleotides

• Nucleotides – Purine or pyrimidine base linked to sugar phosphate

• In DNA

Purine – adenine or guanine

Pyrimidine – thymine or cytosine linked to deoxy sugar phosphate

• In RNA

Purine – Adenine or guanine

Pyrimidine – Uracil or cytosine linked to ribose sugar phosphate

• Nitrogenous base – Purines and pyrimidines

• Nucleoside – Nitrogenous base + sugar (without phosphate group)

• Nucleotide - Nitrogenous base + Sugar + Phosphate group

• Polymerization of nucleotides by reaction between phosphate groups of one molecule with suger molecule of another – Long polymer – Polynucleotide

Structure of nitrogenous bases

Structure of nucleoside and nucleotide

Nucleosides 

• Bases covalently attached to 1’ – position of pentose sugar ring

• RNA – sugar is ribose

• DNA – sugar is 2’ - deoxy ribose

• -OH group in 2’ position is replaced by a hydrogen

Structure of nucleoside

Nucleotide 

• Nucleoside with one or more phosphate group

• Bound covalently to 3’, 5’ or 2’ position

• If sugar – deoxyribose – deoxynucleotide

• Chemically phosphate esters

Phosphodiester bonds 

• Covalent linkage of a phosphate group between 5’ hydroxyl group of one ribose and 3’ hydroxyl of next

DNA double helix

• 2 separate chain of DNA wound around each other

• Has helical path – results in double helix

• Negatively charged sugar phosphate backbone of the molecule are on outside

• The base of each strand stack one above the other in the center

• Between backbones strands major and minor grooves – helical patch exist

• Strands joined together by hydrogen bonds between bases on opposite strand

• Two strands are complementary

• G pairs with C with 3 hydrogen bonds

• A pairs with T with 2 hydrogen bonds

Structure of DNA 

Ribonucleic acid - RNA 

• RNA occurs as a single stranded molecule

• No helical structure

• Forms globular conformation

• Local regions of helical structures are formed by intramolecular hydrogen bondings

• Bases stack with in single nucleic acid chain

• Occur in regions where one part is complementary to other

• Nuclear RNA are small

• Ribosomal RNA are large

 

DNA vs RNA

Summary

• Nucleic acids are macromolecules formed by repeating units of nucleotides

• Nucleotides consists of purine or pyrimidine base linked to sugar phosphate

• DNA consists of purine (adenine or guanine) and pyrimidine – (thymine or cytosine) linked to deoxy sugar phosphate

• RNA consists of purine (adenine or guanine) and pyrimidine (uracil or cytosine) linked to ribose sugar phosphate

• Nucleosides are combination of nitrogenous base + sugar (without phosphate group)

• Nucleotide are combination of Nitrogenous base + Sugar + Phosphate group

• Polymerization of nucleotides by reaction between phosphate groups of one molecule with suger molecule of another – Long polymer – Polynucleotide