• 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.

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

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.

Nucleoside: A compound that consists of D-ribose or 2-deoxy-Dribose bonded to a purine or pyrimidine base by a  -Nglycosidic bond.
uracil O
Base
HN b-
D-riboside
5'
HOCH
2
4'
H
H
O
O
H
3' 2'
HO OH
Uridine
1
N
H
1' a b
-N-glycosidic bond anomeric carbon sugar

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.

Deoxythymidine 3 ’ -monophosphate (3 ’ -dTMP),
O
CH
3
HN
5'
HOCH
2 O
O
H
H
O
3'
O
P
O
-
O
-
H
H
N
H
1'

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


GTP is an important store energy. Draw the structure of guanosine triphosphate

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.

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.

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.

Figure 17.4
Threedimensional structure of the
DNA double helix.

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

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.

Figure 17.10 Structure of tRNA.

•
Figure 17.11 The structure of a typical prokaryotic ribosome.


Figure 25.11 cont ’ d



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



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



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

Table 17.3 The Roles of Different Kinds of RNA