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Chapter 24
Nucleotides, Nucleic Acids,
and Heredity
The Molecules of Heredity
• Each cell has thousands of different proteins.
• How do cells know which proteins to synthesize out of 100000s
possible amino acid sequences?
• From the end of the 19th century, biologists suspected
that the transmission of hereditary information took place
in the nucleus, more specifically in structures called chromosomes.
• The hereditary information was thought to reside in
genes within the chromosomes.
• Chemical analysis of nuclei showed chromosomes are made up
largely of proteins called histones and nucleic acids.
The Molecules of Heredity
• By the 1940s, it became clear that deoxyribonucleic acids (DNA) carry the
hereditary information.
•
Other work in the 1940s demonstrated that each gene controls the manufacture of
one protein.
work
protein
•
Thus the expression of a gene in terms of an enzyme protein led to the study of
protein synthesis and its control.
Structure of DNA and RNA
(based on Nucleic Acids)
•
Two Kinds in cells:
• ribonucleic acids (RNA)
• deoxyribonucleic acids (DNA)
• RNA & DNA: polymers built from monomers (nucleotides)
•
A nucleotide is composed of:
• 1. a base,
2. a monosaccharide, 3. a phosphate,
(e.g. AMP)
(DNA and
some RNA)
(DNA onl y)
(in RNA only)
1. Purine/Pyrimidine Bases
7
6
1
2
5
N
N H2
N
8
N
3
N9
4
H
Puri ne
O
4
N
N
N HN 5
3
2
N
Adenine (A)
(DNA and RNA)
N
NO
H
N
N
H
PyriGuani
mi dine
ne (G) Cytosine (C)
(DNA and RNA) (DNA and
some RNA)
1
2
5
N
N H2
7
6
N
8
N
3
N9
4
H
Puri ne
Base Pairing:
DNA: A-T;C-G
RNA: A-U;C-G
N
6
N
H 2 N1
N
H
N H2
note:
N
N
N
N
H
Adenine (A
(DNA and RN
Nucleosides (base and sugar)
•
compound that consists of D-ribose or 2-deoxy-D-ribose bonded to a purine
or pyrimidine base by a -N-glycosidic bond.
uracil
HN
-D -ribos ide
1
O
5'
HOCH 2
H
N
O
H
4'
3'
O
H
2'
HO
OH
Urid ine
1'
H
a -N -glycosid ic
bon d
anomeric
carb on
Nucleotides
•
a nucleoside w/ molecule of phosphoric acid esterified with an -OH of
the monosaccharide, most commonly either the 3’ or the 5’-OH.
NH2
N
O
5'
N
O-P-O-CH2
O
N
H
H
1'
O
H 3'
H
HO
OH
Aden os in e 5'-monophosp hate
(5'-A MP)
N
ATP- a nucleotide
In Summary
Nucleoside = Base + Sugar
Nucleotide = Base + Sugar + Phosphoric acid
Nucleic acid = chain of nucleotides
Structure of DNA and RNA
Primary Structure
•sequence is read from the 5’ end to the 3’ end
start
• -bases arranged in various
patterns (like A.A.s for Proteins)
GENE  ((protein))
finish
DNA - 2° Structure
•
the ordered arrangement of nucleic acid strands.
• the double helix model of DNA 2° structure was proposed by James
Watson and Francis Crick in 1953.
Using Chargaff rules: (A-T; C-G)
-X-ray (Franklin, Wilkins)
Watson, Crick and Wilkins
(Nobel Prize 1962)
(R. Franklin, 1920-1958)
•
Double helix: 2° structure of DNA in which two polynucleotide strands
are coiled around each other in a screw-like fashion.
The DNA Double Helix
-Polynucleotide chains run
anti-parallel
-Bases (hydrophobic)
avoid water & stabilize d. helix
w/ H-bonds (below)
Base Pairing
Higher Structure of DNA
• DNA is coiled around proteins called histones.
• Histones are rich in the basic amino acids Lys and Arg,
whose side chains have a positive charge.
• The negatively-charged DNA molecules and positivelycharged histones attract each other and form units
called nucleosomes.
_
+
_
• Nucleosome: a core of eight histone molecules around
which the DNA helix is wrapped.
Chromosomes
Nucleosomes are further condensed into chromatin.
Chromatin fibers are organized into loops, and the loops into the
bands that provide the superstructure of chromosomes.