Download Nucleic acid chemistry lecture 3

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
DNA
mRNA
Template strand
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
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
Common features of
tRNA
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
Functions of ribosomes
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
Differences between DNA& RNA
Site
Function
DNA
RNA
Nucleus &
mitochondria
Storage and transfer of
genetic characters
Cytoplasm mainly
Protein biosynthesis
Bases
A, G, C and T
A, G, C and U
Sugar
Deoxy- ribose
Ribose
Strands
2 strands =double helix
One strand
Types
One type but 3 forms
(A,B,Z)
4 types:
mRNA,tRNA,rRNA,
snRNA
Free nucleotides of biological importance
Base
Form
Function
Adenine
ADP & ATP
Source of energy
3`,5` cAMP
second messenger
3`-phosphoadenosinephosphosulfate (PAPS)
sulfate donor
S-adenosylmethionine (SAM)
methyl donor
Fatty acyl adenylates
activate fatty acids
Amino acyl adenylates
activate amino acids
GDP & GTP
energy collection and protein synthesis
3`,5` cGMP
metabolic regulator and antagonize cAMP
GDP-Mannose, fucose
glycoproteins & mucopolysaccharides synthesis.
Hypoxanthine
IDP & ITP
Collection of energy & CO2 metabolism
Uracil
UDP
metabolic carrier of glucose, glucuronic acid and galactose.
Cytosine
CDP
carrier of choline, ethanolamine and diglyceride in phospholipid
synthesis.
Vitamin
nucleotides
riboflavin
Synthesis of FMN & FAD
Niacin
Synthesis of NAD & NADP
Pantothenic acid
Synthesis of coenzyme A
Vitamin B12
Synthesis of cobamide coenzyme
Guanine