The organic bases are of two general types: single-ringed pyrimidines
and double-ringed purines. The purines are adenine (A) and guanine
(G). The pyrimidines are cytosine (C), thymine (T), and uracil (U).
Thymine is found primarily in DNA and uracil is found only in RNA. In
each polynucleotide strand of DNA and RNA, adjacent nucleotides are
joined covalently by phosphodiester bonds between the 3' carbon of one
nucleotide and the 5' carbon of the adjacent nucleotide.
Bases in the nucleotides spontaneously form hydrogen bonds in a
highly specific manner. Adenine normally forms two hydrogen bonds
with thymine in a complementary strand of the DNA double helix. Like wise, it can form two hydrogen bonds with U in DNA -RNA hybrids and
in RNA-RNAinteractions. Guanine forms three hydrogen bonds with cytosine.
DNA exists in the uniform shape of a double helix (Figure
2-6), with the complementary chains wound around each other like a spiral
staircase, whereas RNA molecules are synthesized from DNA templates as single strands. The single strand of RNA, however, may fold
back onto itself and form complementary base pairs to make unique secondary
|Figure 2-6 Diagram of double helical DNA.
The two complementary strands of a DNA double helix run in opposite
directions, that is, they are antiparallel. If one chain is read from
the 5' phosphate end, the other would read from the 3' hydroxyl. The double
helix makes a turn every ten base pairs (approximately 3.4 nm). The
paired bases are stacked in the center of the molecule, forming a hydrophobic
core and giving the helix a width of about 2 nm.
There are three classes of RNA based on their functions: (1) transfer
RNAs (tRNAs); (2) messenger RNAs (mRNAs); and (3) ribosomal
RNAs (rRNAs). The tRNAs are the smallest (75-80 nucleotides in
length) and serve to position each amino acid on the ribosome for polymerization
into polypeptide chains. They contain a few unusual bases in
addition to A, C, G, and U. The genetic code that specifies the amino acid
sequences of proteins resides in the DNA sequence, and it becomes transcribed
into complementary ribonucleotide sequences of mRNA, thus the
length and composition of different mRNAs can vary greatly. The rRNAs
are structural components of the ribosomes. There are three classes of
rRNAs in bacteria and four in eukaryotes.
RNA has uracil in place of thymine.
Since A always pairs with T, and G always pairs
with C, the purine:pyrimidine ratio in double
stranded DNA is always 1.