Download Chapter 17 Nucleotides, Nucleic Acids, and Heredity

Chapter 17
Nucleotides, Nucleic Acids, and
Heredity
The Molecules of Heredity
• Each cell of our bodies contains thousands of different
proteins.
• How do cells know which proteins to synthesize out of the
extremely large number of 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.
• 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.
• Thus the expression of a gene in terms of an enzyme protein
led to the study of protein synthesis and its control.
Nucleic Acids
There are two kinds of nucleic acids in cells:
• Ribonucleic acids (RNA)
• Located elsewhere in the nucleus and even outside the
nucleus (cytoplasm)
• Deoxyribonucleic acids (DNA)
• Present in the chromosomes of the nucleic of eukaryotic
cells
Both RNA and DNA are polymers built from monomers called
nucleotides. A nucleotide is composed of:
• A base, a monosaccharide, and a phosphate
Purine/Pyrimidine Bases
For DNA, the bases are A, G ,C and T
For RNA, the bases are A, G, C and U
•
Figure 17.1 The five principal bases of DNA and RNA.
Nucleosides
Nucleoside: A compound that consists of D-ribose or 2-deoxy-Dribose bonded to a purine or pyrimidine base by a -Nglycosidic bond.
uracil
N
O
H
H
1
O
5'
4'
Base
HN
b-D-riboside
HOCH2
O
H
2'
3'
HO
OH
Uridine
sugar
1'
H
a b-N-glycosidic
bond
anomeric
carbon
Nucleotides
Nucleotide: A nucleoside in
which a molecule of
phosphoric acid is
esterified with an -OH of
the monosaccharide,
most commonly either at
the 3’ or the 5’-OH.
Anhydride
Ester
AMP
ADP
ATP
Nucleotides
Deoxythymidine 3’-monophosphate (3’-dTMP),
O
CH3
HN
5'
HOCH2
O
O
H
H
N
H
3'
H
O
O
1'
P
O-
H
O-
Nucleotides
Adenosine 5’triphosphate (ATP)
serves as a
common currency
into which energy
gained from food is
converted and
stored.
Table 17.1 The Eight Nucleosides and Eight Nucleotides in DNA and RNA
DNA—Primary (1°) Structure
For nucleic acids, primary structure is the sequence of
nucleotides, beginning with the nucleotide that has the free 5’
terminus.
◦ The strand is read from the 5’end to the 3’end.
◦ Thus, the sequence AGT means that adenine (A) is the base
at the 5’ terminus and thymine (T) is the base at the 3’
terminus.
Structure of DNA and RNA
Figure 17.2
Schematic diagram of a nucleic
acid molecule. The four bases of
each nucleic acid are arranged
in various specific sequences.
The base sequence is read from
the 5’ end to the 3’ end.
DNA—2° Structure
Secondary 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.
Double helix: A type of 2° structure of DNA in which two
polynucleotide strands are coiled around each other in a
screw-like fashion.
THE DNA Double Helix
Figure 17.4
Threedimensional
structure of the
DNA double
helix.
Base Pairing
Figure 17.5 A and T
pair by forming two
hydrogen bonds. G
and C pair by forming
three hydrogen bonds.
 The complemetary base pairs
Superstructure of Chromosomes
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 one another and form units
called nucleosomes.
Nucleosome: A core of eight histone molecules around
which the DNA helix is wrapped.
• Nucleosomes are further condensed into chromatin.
• Chromatin fibers are organized into loops, and the loops
into the bands that provide the superstructure of
chromosomes.
Superstructure of Chromosomes
•
Figure 17.8
Superstructure of Chromosomes

Figure 17.8 cont’d
Superstructure of Chromosomes

Figure 25.8 cont’d
Superstructure of Chromosomes

Figure 25.8 cont’d
DNA and RNA
The three differences in structure between DNA and RNA
are:
• DNA bases are A, G, C, and T; the RNA bases are A, G, C,
and U.
• the sugar in DNA is 2-deoxy-D-ribose; in RNA it is D-ribose.
• DNA is always double stranded; there are several kinds of
RNA, all of which are single-stranded.
Information Transfer
Different Classes of RNA


Messenger RNA( mRNA): produced in the process called
transcription and they carry the genetic information from the
DNA in the nucleus directly to the cytoplasm.
◦ Containing average 750 nucleotides
◦ Not-long lived
Transfer RNA (tRNA): transport amino acid to the site of
protein synthesis in ribosomes
◦ 74-93 nucleotides per chain
◦ Contains cytosine, guanine, adenine, uracil and amodified
nucleotide called 1-methylguanosine
Different Classes of RNA


Ribosomal RNA (rRNA) Ribosomes: small spherical
bodies located in the cells but outside the nuclei, contain
rRNA
◦ Consists of about 35% protein and 65% ribosomal RNA
Small Nuclear RNA (snRNA): found in the nucleus of
eukaryotic cells.
◦ 100-200 nucleotides long, neither subunit tRNA or rRNA
◦ To help with the processing of the initial mRNA
transcribed from DNA into a mature form
Different Classes of RNA


Micro RNA (miRNA): another type of small RNA
◦ 20-22 nucleotides long
◦ Important in the timing of an organism’s development.
◦ They inhibit translation of mRNA into protein and
promote the degredation of mRNA
Small Interfering RNA (siRNA): eliminate expression of
an undersirable gene, such as one that causes uncontrolled
cell growth or one that came from a virus
◦ Has been used to protect mouse liver from hepatitis and to
help clear infected liver cells of the disease