Lecture 3
Molecular Biology ٣٢٥
Sabah Linjawi
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What is nucleic acid?
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A nucleic acid is a macromolecule composed of
chains of monomeric nucleotides
These molecules carry genetic information or form
structures within cells.
The most common nucleic acids are
deoxyribonucleic acid (DNA) and ribonucleic
acid (RNA).
Nucleic acids are universal in living things, as they
are found in all cells and viruses.
Nucleic acids were first discovered by Friedrich
Miescher in 1871.
Molecular Biology ٣٢٥
Sabah Linjawi
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What is nucleic acid?
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The term "nucleic acid" is the generic name
for a family of biopolymers, named for their
role in the cell nucleus.
The monomers from which nucleic acids are
constructed are called nucleotides.
Each nucleotide consists of three
components: a nitrogenous heterocyclic
base, which is either a purine or a pyrimidine;
a pentose sugar; and a phosphate group.
Molecular Biology ٣٢٥
Sabah Linjawi
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Building Blocks - Nucleotides
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Molecular Biology ٣٢٥
Sabah Linjawi
A nucleotide is composed of
three parts: pentose, base
and phosphate group.
In DNA or RNA, a pentose
is associated with only one
phosphate group
but a cellular free
nucleotide (such as ATP)
may contain more than one
phosphate group.
If all phosphate groups are
removed, a nucleotide
becomes a nucleoside
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Pentose
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Molecular Biology ٣٢٥
Sabah Linjawi
The chemical structure of
pentose which contains five
carbon atoms, labeled as
C1' to C5'.
The pentose is called
ribose in RNA and
deoxyribose in DNA
because the DNA's
pentose lacks an oxygen
atom at C2'.
Recalling that RNA stands
for "ribonucleic acid", and
DNA for "deoxyribonucleic
acid".
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Bases
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Molecular Biology ٣٢٥
Sabah Linjawi
There are five different bases, each
is denoted by a single letter as given
in the parenthesis:
Adenine (A), Cytosine (C),
Guanine (G), Thymine (T), and
Uracil (U).
Among them,
A, C, G and T exist in DNA;
A, C, G and U exist in RNA.
Their chemical structures are shown
in the following figure. A and G
contain a pair of fused rings,
classified as purines. C, T, and U
contain only one ring, classified as
pyrimidines.
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Cellular Nucleotides and Nucleosides
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Molecular Biology ٣٢٥
Sabah Linjawi
In cells, a free nucleotide
may contain one, two or
three phosphate groups.
The energy carrier ATP
(adenosine triphosphate)
has three phosphate
groups
ADP (adenosine
diphosphate) has two
AMP (adenosine
monophosphate) has
one.
Their structures are
shown in the following
figure.
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The Nucleic Acid Chain
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Molecular Biology ٣٢٥
Sabah Linjawi
In a nucleic acid chain,
two nucleotides are
linked by a
phosphodiester bond,
which may be formed by
the condensation
reaction similar to the
formation of the peptide
bond.
In cells, such process
has been found in the
ligation between two
nucleic acid fragments.
However, the whole
nucleic acid chain is
usually synthesized by
RNA polymerase or
DNA polymerase.
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DNA Structure
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Molecular Biology ٣٢٥
Sabah Linjawi
In a normal DNA molecule, adenine
(A) is paired with thymine (T)
guanine (G) is paired with cytosine
(C).
The uracil (U) of RNA can also pair
with adenine (A)
U differs from T by only a methyl
group located on the other side of
hydrogen bonding.
A DNA molecule has two strands,
held together by the hydrogen
bonding between their bases
adenine can form two hydrogen
bonds with thymine
cytosine can form three hydrogen
bonds with guanine.
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DNA's B Form, A Form and Z Form
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Molecular Biology ٣٢٥
Sabah Linjawi
In a DNA molecule, the two strands
are not parallel, but intertwined with
each other. Each strand looks like a
helix.
The two strands form a "double
helix" structure, which was first
discovered by James D. Watson
and Francis Crick in 1953.
This structure, also known as the B
form, the helix makes a turn every
3.4 nm
The distance between two
neighboring base pairs is 0.34 nm.
There are about 10 pairs per turn.
The intertwined strands make two
grooves of different widths
The major groove and the minor
groove, which may facilitate binding
with specific proteins.
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DNA's B Form, A Form and Z Form
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Molecular Biology ٣٢٥
Sabah Linjawi
In a solution with higher
salt concentrations or with
alcohol added, the DNA
structure may change to an
A form, which is still righthanded, but every 2.3 nm
makes a turn and there are
11 base pairs per turn.
Another DNA structure is
called the Z form, because
its bases seem to zigzag.
Z DNA is left-handed. One
turn spans 4.6 nm,
comprising 12 base pairs.
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RNA Structure and Function
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Most cellular RNA molecules are single stranded.
They may form secondary structures such as stemloop and hairpin.
Molecular Biology ٣٢٥
Sabah Linjawi
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RNA Structure and Function
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The major role of RNA is to participate in protein
synthesis, which requires three classes of RNA:
messenger RNA (mRNA)
transfer RNA (tRNA)
ribosomal RNA (rRNA)
Other classes of RNA include
Ribozymes
The RNA molecules with catalytic activity.
Small RNA molecules
RNA interference and other functions.
Molecular Biology ٣٢٥
Sabah Linjawi
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The mRNA-ribosome-tRNA complex
formed during protein synthesis
Molecular Biology ٣٢٥
Sabah Linjawi
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References
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Wolfram Saenger, Principles of Nucleic Acid Structure, 1984, Springer-Verlag New York
Inc.
Keith Roberts, Martin Raff, Bruce Alberts, Peter Walter, Julian Lewis and Alexander
Johnson, Molecular Biology of the Cell 4th Edition, Routledge, March, 2002, hardcover,
1616 pages, 7.6 pounds, ISBN 0-8153-3218-1
The Biochemistry of the Nucleic Acids - by Roger Lionel Poulter Adams, John T Knowler 708 pages
Nucleic Acids in Chemistry and Biology - by G Michael Blackburn, ... - 503 pages
The Double Helix: A Personal Account of the ... - by James D Watson, Gunther Siegmund
Stent - 264 pages
Links
Interview with Aaron Klug, Nobel Laureate for structural elucidation of biologically important
nucleic-acid protein complexes provided by the Vega Science Trust.
Nucleic Acid Research Journal
UC Berkeley video lecture on nucleic acids
www.visionlearning.com/library/module_viewer.php?mid
www.answers.com/topic/nucleic-acid
Molecular Biology ٣٢٥
Sabah Linjawi
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