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Nucleic Acids
Nucleic acids
DNA and RNA are chemical carriers of a
cell’s genetic information
 Coded in a cell’s DNA is the information
that determines the nature of the cell,
controls cell growth, division
 Nucleic acid derivatives are involved as
phosphorylating agents in biochemical
pathways

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Ribose & deoxyribose
Ribose is a sugar, like glucose, but with only five
carbon atoms in its molecule
Deoxyribose is almost the same but lacks one
oxygen atom
Both molecules may be represented by the
symbol
4
5
The bases
The most common organic bases are
Adenine
(A)
Thymine
(T)
Cytosine
(C)
Guanine
(G)
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Nucleotides
The ribose, the phosphate and
one of the bases
Combine to form a nucleotide
PO4
bases
ribose
Heterocycles in DNA and RNA


Adenine, guanine, cytosine and thymine are in DNA
RNA contains uracil rather than thymine
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Nucleotides and Nucleic Acids

Deoxyribonucleic acid (DNA) and ribonucleic
acid (RNA), are the chemical carriers of genetic
information

Nucleic acids are biopolymers made of
nucleotides, aldopentoses linked to a purine or
pyrimidine and a phosphate

RNA is derived from ribose

DNA is from 2-deoxyribose
◦ (the ' is used to refer to positions on the
sugar portion of a nucleotide)
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Nucleosides in DNA
Base
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Sugar
Deoxyribose
Deoxyribose
Deoxyribose
Deoxyribose
Nucleoside
Adenosine
Guanosine
Cytidine
Thymidine
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Nucleosides in RNA
Base
Adenine (A)
Guanine (G)
Cytosine (C)
Uracil (U)
Sugar
ribose
ribose
ribose
ribose
Nucleoside
Adenosine
Guanosine
Cytidine
Uridine
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Nucleotides
Nucleic acids consist of nucleotides that
have a sugar, nitrogen base, and
phosphate
Base
PO4
Sugar
nucleoside
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Example of a Nucleoside
NH2
N
O
O-
O
P O CH2
-
N
O
O
OH
deoxyctyidine monophosphate (dCMP)
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Nucleotides

In DNA and RNA the
heterocycle is bonded
to C1 of the sugar and
the phosphate is
bonded to C5 (and
connected to 3’ of the
next unit)
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Nucleotides join together in DNA and RNA by
as phosphate between the 5’-on one nucleotide
and the 3 on another
One end of the nucleic acid polymer has a free hydroxyl at C3 (the
3 end), and the other end has a phosphate at C5 (the 5 end).
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Nucleotides in DNA and RNA
DNA
dAMP
dGMP
dCMP
dTMP
RNA
AMP
GMP
CMP
UMP
Deoxyadenosine monophosphate
Deoxyguanosine monophosphate
Deoxycytidine monophosphate
Deoxythymidine monophosphate
adenosine monophosphate
guanosine monophosphate
cytidine monophosphate
uridine monophosphate
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Structure of Nucleic Acids




Polymers of four nucleotides
Linked by alternating sugar-phosphate bonds
RNA: ribose and A, G, C, U
DNA: deoxyribose and A,G,C,T
base
P
sugar
base
P
nucleotide
sugar
base
P
sugar
base
P
sugar
nucleotide nucleotide nucleotide
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Nucleic Acid Structure
NH2
N
CMP
O
O
O-
P O CH2
-
O
N
O
3
NH2
3,5-phosphodiester bond
OH
O
5
O P O CH2
-
N
N
N
O
N
AMP
O
OH
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Base Pairing in DNA: The Watson–
Crick Model




In 1953 Watson and Crick noted that DNA consists of
two polynucleotide strands, running in opposite
directions and coiled around each other in a double
helix
Strands are held together by hydrogen bonds between
specific pairs of bases
Adenine (A) and thymine (T) form strong hydrogen
bonds to each other but not to C or G
Guanine (G) and cytosine (C) form strong hydrogen
bonds to each other but not to A or T
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DNA
DNA stands for deoxyribose nucleic acid
This chemical substance is present in the nucleus
of all cells in all living organisms
DNA controls all the chemical changes which
take place in cells
The kind of cell which is formed, (muscle, blood,
nerve etc) is controlled by DNA
The kind of organism which is produced (buttercup,
giraffe, herring, human etc) is controlled by DNA
DNA molecule
DNA is a very large molecule made up of a long
chain of sub-units
The sub-units are called nucleotides
Each nucleotide is made up of
a sugar called deoxyribose
a phosphate group -PO4 and
an organic base
3
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Nucleotides
PO4
adenine
or
deoxyribose
Thymine
(T)
Cytosine
(C)
Guanine
(G)
Joined nucleotides
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PO4
PO4
PO4
PO4
sugar-phosphate + bases
backbone
A molecule of DNA is
formed by millions of
nucleotides joined
together in a long chain
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In fact, the DNA usually consists of a double
strand of nucleotides
The sugar-phosphate chains are on the outside
and the strands are held together by chemical
bonds between the bases
Hydrogen Bonds in DNA


The G-C base pair involves three H-bonds
The A-T base pair involves two H-bonds
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Complementary Base Pairs
•Two H bonds for A-T
•Three H bonds for G-C
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Replication of DNA


Begins with a partial unwinding of the double helix,
exposing the recognition site on the bases
Activated forms of the complementary nucleotides (A
with T and G with C) associate two new strands begin to
grow
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The Replication Process
Addition takes place 5  3, catalyzed by DNA
polymerase
 Each nucleotide is joined as a 5-nucleoside triphosphate
that adds a nucleotide to the free 3-hydroxyl group of the
growing chain

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Transcription of DNA



RNA contains ribose rather than deoxyribose and uracil
rather than thymine
There are three major kinds of RNA - each of which
serves a specific function
They are much smaller molecules than DNA and are
usually single-stranded
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Messenger RNA (mRNA)


Its sequence is copied from genetic DNA
It travels to ribsosomes, small granular particles in the
cytoplasm of a cell where protein synthesis takes place
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Ribosomal RNA (rRNA)



Ribosomes are a complex of proteins and rRNA
The synthesis of proteins from amino acids and ATP
occurs in the ribosome
The rRNA provides both structure and catalysis
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Transfer RNA (tRNA)
Transports amino acids to the ribosomes where they
are joined together to make proteins
 There is a specific tRNA for each amino acid
 Recognition of the tRNA at the anti-codon
communicates which amino acid is attached

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The Structure of tRNA
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Transcription Process
Several turns of the DNA double helix unwind, exposing
the bases of the two strands
 Ribonucleotides line up in the proper order by hydrogen
bonding to their complementary bases on DNA
 Bonds form in the 5  3 direction,

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Transcription of RNA from DNA




Only one of the two DNA strands is transcribed into
mRNA
The strand that contains the gene is the coding or sense
strand
The strand that gets transcribed is the template or
antisense strand
The RNA molecule produced during transcription is a
copy of the coding strand (with U in place of T)
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Mechanism of Transcription



DNA contains promoter sites that are 10 to 35 base
pairs upstream from the beginning of the coding region
and signal the beginning of a gene
There are other base sequences near the end of the
gene that signal a stop
Genes are not necessarily continuous, beginning gene in
a section of DNA (an exon) and then resume farther
down the chain in another exon, with an intron
between that is removed from the mRNA
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