Biology Reference
In-Depth Information
genetic code for proteins, the processes involved in DNA replication, and the
changes in DNA that result in mutations.
1.4 The “RNA World” Came First?
It is thought that there could have been an era on early Earth during which RNA
played the role of genetic material and also served as the main agent of catalytic
activity, because RNA can serve as a ribozyme ( DiGiulio 1997, Jeffares et al. 1998,
Poole et al. 1998, Cooper 2000, Eddy 2001, Gesteland et al. 2006, Atkins et al.
2011, Darnell 2011 ). This role implies that enzymatic proteins in the modern world
replaced RNA as the main catalysts. The “RNA organism” is thought to have had a
multiple-copy, double-stranded RNA genome capable of recombination and splic-
ing. The RNA genome was probably fragmented into “chromosomes” ( Jeffares
et al. 1998 ). RNA could have been the first genetic material because it can serve
as a template for self-replication and can catalyze chemical reactions, including
the polymerization of nucleotides ( Johnston et al. 2001 ). It is thought that inter-
actions between RNA and amino acids then evolved into the present-day world
in which DNA is the more stable repository of genetic information. Knowledge of
the number of RNAs and their very diverse functions has increased and an appre-
ciation for the role of RNAs in gene regulation and development is reflected in
many publications. First, however, let's examine the structure and function of DNA
in its role as a stable repository of genetic information.
1.5 The Molecular Structure of DNA
DNA is a long, double-stranded polymeric molecule consisting of individual
monomers that are linked in a series and organized in a helix. Each monomer
is called a nucleotide . Each nucleotide is itself a complex molecule made up of
three components: a sugar, a nitrogenous base, and a phosphoric acid ( Box 1.1 ).
In DNA, the sugar component is a pentose (with five carbon atoms) in a ring
form that is called 2 -deoxyribose ( Figure 1.2 ). The nitrogenous bases are single-
or double-ring structures that are attached to the 1 -carbon of the sugar. The
bases are purines (adenine and guanine) or pyrimidines (thymine and cytosine)
( Figure 1.3 ). When a sugar is joined to a base it is called a nucleoside .
A nucleoside is converted to a nucleotide by the attachment of a phosphoric
acid group to the 5 -carbon of the sugar ring ( Figure 1.4 ). The four nucleotides
that polymerize to form DNA are 2 -deoxyadenosine 5 -triphosphate, 2 -deoxy-
guanosine 5 -triphosphate, 2 -deoxycytidine 5 -triphosphate, and 2 -deoxythymi-
dine 5 -triphosphate ( Figure 1.5 ). These names usually are abbreviated as dATP,
dGTP, dCTP, and dTTP, or shortened further to A, G, C and T, respectively.
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