Nucleic Acid Chemistry Dr: Hussein abdelaziz

Objectives

By the end of lecture the student should:

Differentiate between different types of RNA

List differences between DNA and RNA

Mention free nucleotides of biological impotances

3- RNA structure

All RNA molecules are formed in the nucleus from the action of RNA polymerase on DNA template (transcription) transported to cytoplasm There are 3 main types of RNA (All are formed of one strand only) 1- messenger RNA = mRNA 2- transfer RNA = tRNA 3- ribosomal RNA = rRNA There are other types called small nuclear RNAs (snRNAs)(1 % of total RNA), about 30 species include ribozymes

1-mRNA

Single strand formed complementary to template strand of DNA in the nucleus. 5% of total cellular RNA Variable in length Carry genetic codon (a message from DNA) for protein synthesis in the cytoplasm (in ribosomes) The sequence of the bases determines the sequence of the amino acids in the polypeptide chain Each 3 succcessive bases in mRNA are called genetic codon which codes for a specific amino acid 1st codon at 5' end called initiation codon & the last codon at 3' end called termination codon

Template strand

DNA

mRNA

2-tRNA • 15% of total RNA • synthesized from template DNA • The smallest type of RNA (75-85 nucleotides) • It has anticodon arm that recognize genetic codon on mRNA and carry the corrospording AA at 3¯ terminal • When tRNA carries AA → it is said that → AA is activated → t RNA is charged

Common features of tRNA

1. It is arranged in the form of a clover-leaf, presenting 3 loops (D-loop, pseudouridine loop and anticodon loop) & having 2 free ends (3', 5' ends)

2. 3' terminus has the terminal sequence CCA

3. The anticodon loop contains a triplet of nucleotides that can base pair with a codon on mRNA

4. Each codon of mRNA is read by a tRNA anticodon that can specifically base pair with it, then tRNA discharges it’s a.a for incorporation into growing polypeptide chain

5. There are at least 31 tRNA and 20 a.a.

6. The a.a is carried on 3' terminus

3-rRNA

synthesized from template DNA in the nucleolus 80% of total RNA Ribosomes are large ribonucleoproteins found in the cytoplasm ( rRNA + protein ) & they are sites of protein synthesis from mRNA templates

Mammalian Ribosomes has 2 subunits; large subunit (60S) & small subunit (40S) Whole is 80S ribosome

Responsible for protein synthesis from a.a carried by tRNA according to the information provided by mRNA. 1- In ribosomes, mRNA & tRNA interact to synthesize protein molecule 2- Ribosomes contain enzymes responsible for connecting the amino acids together in peptide linkage to form polypeptide chain

Functions of ribosomes

Differences between DNA& RNA

RNA DNA

Cytoplasm mainly

Protein biosynthesis

A, G, C and U

Ribose

One strand

4 types:

mRNA,tRNA,rRNA,

snRNA

Nucleus &

mitochondria

Storage and transfer of

genetic characters

A, G, C and T

Deoxy- ribose

2 strands =double helix

One type but 3 forms

(A,B,Z)

Site

Function

Bases

Sugar

Strands

Types

Function Form Base

Source of energy ADP & ATP Adenine

second messenger 3`,5` cAMP

sulfate donor 3`-phosphoadenosine-

phosphosulfate (PAPS)

methyl donor S-adenosylmethionine (SAM)

activate fatty acids Fatty acyl adenylates

activate amino acids Amino acyl adenylates

energy collection and protein synthesis GDP & GTP Guanine

metabolic regulator and antagonize cAMP 3`,5` cGMP

glycoproteins & mucopolysaccharides synthesis. GDP-Mannose, fucose

Collection of energy & CO2 metabolism IDP & ITP Hypoxanthine

metabolic carrier of glucose, glucuronic acid and galactose. UDP Uracil

carrier of choline, ethanolamine and diglyceride in phospholipid

synthesis.

CDP Cytosine

Synthesis of FMN & FAD riboflavin Vitamin

nucleotides Synthesis of NAD & NADP Niacin

Synthesis of coenzyme A Pantothenic acid

Synthesis of cobamide coenzyme Vitamin B12

Free nucleotides of biological importance