DNA RNA as genetic material

1. DNA and RNA are long linear polymers, called nucleic acids,
that carry information in a form that can be passed from
one generation to the next. These macromolecules consist
of a large number of linked nucleotides, each composed of a
sugar, a phosphate, and a base. Sugars linked by phosphates
form a common backbone, whereas the bases vary among
four kinds. Genetic information is stored in the sequence of
bases along a nucleic acid chain. The bases have an
additional special property: they form specific pairs with
one another that are stabilized by hydrogen bonds. The
base pairing results in the formation of a double helix, a
helical structure consisting of two strands. These base
pairs provide a mechanism for copying the genetic
information in an existing nucleic acid chain to form a new
chain. Although RNA probably functioned as the genetic
material very early in evolutionary history, the genes of all
modern cells and many viruses are made of DNA. DNA is
replicated by the action of DNA polymerase enzymes. These
exquisitely specific enzymes copy sequences from nucleic
acid templates with an error rate of less than 1 in 100
million nucleotides.
Genes specify the kinds of proteins that are made by cells,
but DNA is not the direct template for protein synthesis.
Rather, the templates for protein synthesis
are RNA (ribonucleic acid) molecules. In particular, a class
of RNA molecules called messenger RNA (mRNA) are the
information-carrying intermediates in protein synthesis.
Other RNA molecules, such as transfer RNA (tRNA)
and ribosomal RNA (rRNA), are part of the protein-synthesizing
machinery. All forms of cellular RNA are

2. synthesized by RNA polymerases that take instructions
from DNA templates. This process of transcription is
followed by translation, the synthesis of proteins according
to instructions given by mRNA templates. Thus, the flow of
genetic information, or gene expression, in normal cells is:
This flow of information is dependent on the genetic code,
which defines the relation between the sequence of bases
in DNA (or itsmRNA transcript) and the sequence of amino
acids in a protein. The code is nearly the same in all
organisms: a sequence of three bases, called a codon,
specifies an amino acid. Codons in mRNA are read
sequentially by tRNA molecules, which serve as adaptors in
protein synthesis. Protein synthesis takes place on
ribosomes, which are complex assemblies of rRNAs and
more than 50 kinds of proteins.
The last theme to be considered is the interrupted
character of most eukaryotic genes, which are mosaics of
nucleic acid sequences called introns and exons. Both are
transcribed, but introns are cut out of newly
synthesized RNA molecules, leaving mature RNA molecules
with continuous exons. The existence of introns and exons
has crucial implications for the evolution of proteins.