2. Protein Biosynthesis
The DNA is transcribed to
mRNA which is translated into
protein with the help of
ribosomes.
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3. Nucleotides
• Nucleotides are precursors of the nucleic
acids, deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA).
• The nucleic acids are concerned with the
storage and transfer of genetic information.
• The universal currency of energy, namely ATP,
is a nucleotide derivative.
• Nucleotides are also components of
important coenzymes like NAD+ and FAD, and
metabolic regulators such as cAMP and cGMP.
3
4. Composition of nucleotides
A nucleotide is made up of three
components:
1. Nitrogenous base, (a purine or a
pyrimidine).
2. Pentose sugar, either ribose or
deoxyribose.
3. Phosphate groups esterified to the
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sugar
5. • Nucleoside: Formed when a base combines with
a pentose sugar.
• Nucleotide: Formed when nucleoside is esterified
to phosphate group. It also called nucleoside
monophosphate.
• The nucleic acids (DNA and RNA) are polymers of
nucleoside monophosphates (nucleotide).
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6. Bases present in the nucleic acids
1- Purine bases:
• The purine bases present in RNA and DNA are the
same; adenine and guanine.
• Uric acid is formed as the end product of the
catabolism of other purine bases.
• 2- Pyrimidine bases
• The pyrimidine bases present in nucleic acids
are cytosine, thymine and uracil.
• Cytosine is present in both DNA and RNA.
• Thymine is present in DNA and uracil in RNA.
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15. Structure of DNA
• Deoxyribonucleic acid (DNA) is composed of four
deoxyribonucleotides:
Deoxyadenylate (A) - Deoxyguanylate (G)
Deoxycytidylate (C) - Deoxythymidylate (T)
• These units are combined through 3’ to 5’
phosphodiester bonds to polymerize into a long
chain.
• The nucleotide is formed by a combination of base
+ sugar + phosphoric acid.
• The 3’-hydroxyl of one sugar is combined to the 5’-
hydroxyl of another sugar through a phosphate
group.
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18. • In the DNA, the base sequence is of
paramount importance.
• The genetic information is coded in the
specific sequence of bases; if the base is
altered, the information is also altered.
• The deoxyribose and phosphodiester linkages
are the same in all the repeating nucleotides.
• Therefore, the message will be conveyed,
even if the base sequences alone are
mentioned as e.g.
5’P-Thymine-Cytosine-Adenine-3’OH
or 5’-T-C-A-3’.
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19. Watson-Crick model of DNA structure
1. Right handed double helix:
• DNA consists of two polydeoxy ribonucleotide
chains twisted around one another in a right
handed double helix similar to a spiral staircase.
• The sugar and phosphate groups comprise the
handrail and the bases jutting inside represent the
steps of the staircase. The bases are located
perpendicular to the helix axis, whereas sugars are
nearly at right angles to the axis.
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20. 2- The base pairing rule:
• Always the two strands are complementary to each
other.
• So, the adenine of one strand will pair with thymine of
the opposite strand, while guanine will pair with
cytosine.
• The base pairing (A with T; G with C) is called Chargaff’s
rule, which states that the number of purines is equal
to the number ofpyrimidines.
3- Hydrogen bonding:
• The DNA strands are held together mainly by hydrogen
bonds between the purine and pyrimidine bases.
• There are two hydrogen bonds between A and T while
there are three hydrogen bonds between C and G.
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21. 4- Antiparallel:
• The two strands in a DNA molecule run antiparallel,
which means that one strand runs in the 5’ to 3’
direction, while the other is in the 3’ to 5’ direction.
• This is similar to a road divided into two, each half
carrying traffic in the opposite direction.
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24. Replication of DNA
• During cell division, each daughter cell gets an
exact copy of the genetic information of the
mother cell.
• This process of copying the DNA is known as
DNA replication.
• In the daughter cell, one strand I derived
from the mother cell; while the other strand is
newly synthesized.
• This is called semi-conservative type of DNA
replication.
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26. Steps of Replication
1. Each strand serves as a template or mold, over
which a new complementary strand is synthesized.
2. The base pairing rule is always maintained. The
new strand is joined to the old strand by hydrogen
bonds between base pairs (A with T and G with C).
3. Polymerization of the new strand of DNA is taking
place from 5’ to 3’ direction. This means that the
template is read in the 3’ to 5’ direction. So, the 3’
end of the last nucleotide is free.
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27. 4. Thus, two double strands are produced.
One double strand goes to one daughter
nuclei, and the other to the second
daughter nuclei.
• But each daughter cell gets only one
strand of the parent DNA molecule.
• Old DNA strand is not degraded, but is
conserved for the daughter cell, hence
this is semi-conservative synthesis.
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28. 5. DNA polymerase (DNAP).
This enzyme synthesizes a new
complementary strand of DNA
6. Initiation of DNA replication.
The DNA replication starts with the recognition
of the site of origin of replication.
7. RNA primer is required for DNA synthesis.
An RNA primer, about 100·200 nucleotides
long, is synthesized by the RNA primase.
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29. 8. Elongation of DNA strand.
Under the influence of DNA polymerase, nucleotides
and the one continuously polymerized as they are
sequentially added. The DNA polymerase carries out
the sequential addition of each nucleotide
complementary to the one in the template strand
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31. Inhibitors of DNA replication
• anti-bacterial agents:
Certain compounds will inhibit bacterial
enzymes, but will not affect human cells.
• anti-cancer agents:
Some other components will inhibit human
enzymes, they will arrest new DNA
synthesis, and arrest the cell division.
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