Biology Reference
In-Depth Information
The question of how the proteins are synthesized led to the discovery of
a network that is orthogonal to this metabolic network at each step of the
latter (see Fig. 1). Each protein is synthesized from amino acids by a complex
machinery, called the ribosome, which consists of protein and a second main
type of macromolecule, i.e. ribosomal ribonucleic acid (rRNA). The diversity
of the proteins stems from the fact that the sequence at which amino acids are
attached to its nascent chain is specified by a specific messenger RNA (mRNA)
molecule. RNA molecules are chains of four types of nucleotide, which are
referred to by a mnemonic of the name of the corresponding 'bases', i.e. as
A, U, G and C. Each of the 64 triplets of such bases corresponds to an amino
acid, with just a few exceptions that deal with the regulation of protein synthesis
itself. Each mRNA molecule is a copy of part of single stranded DNA, i.e. a
very long chain of nucleotides referred to as dA, dT, dG and dC (the 'd's are
often omitted). It occurs in combination with a complementary single stranded
DNA molecule which has a T, A, C or G, respectively, where the other strand
has an A, T, G and C, respectively. This double strandedness makes the DNA a
robust way of storing the information. Damage that can be recognized as such
can be repaired by referring to the sequence of the complementary strand. The
part of the chain that is copied into an mRNA and is ultimately translated into
protein is often called a gene (although this word actually refers to a concept that
predates the discovery of DNA). The copying, which is called transcription, is
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Figure 1
The hierarchical networking of the living cell.
