Download Nucleic acid chemistry lecture 2

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
Polynucleotide chain polymerization
• 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
Characters of polynucleotide chain
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
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 pentosephosphate 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 Bform
It is a left handed
helix and has a zigzag appearance
Mitochondrial DNA
o 2- 10 copies
oSmall circular
o Double-stranded DNA
o 1 % of total cellular DNA
o Transmitted by maternal nonmendelian 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