produced. These signals are in the form of specific sequences of bases. The
RNA produced by transcription is known as either heterogeneous nuclear
RNA (hnRNA),premessenger RNA (pre-mRNA)or primary transcript RNA
(ptRNA).
1.6.5 Classification and function of RNA
Ribonucleic acids are classified according to their general role in protein syn-
thesis.
Messenger RNA (mRNA)is believed to be produced from the hnRNA formed
by transcription in the nucleus. The introns are removed and the remaining
exons are spliced together to form a continuous sequence of bases that are
complementary to the gene’s exons. The mRNA now leaves the nucleus via
nuclear pores and carries its message to the ribosome in the cytoplasm. It binds
to the ribosome, where it dictates the order in which the amino acids are
linked to form the structure of the protein This information is carried in the
form of a trinucleotide code known as acodon. The nature of a codon is
indicated by a sequence of letters corresponding to the 5’to 3’order of bases
in the trinucleotide. Some amino acids have more than one codon. The mRNA’s
codon code is known as thegenetic code(Table 1.9) and is used by all living
Table 1.9 The genetic code. The start codon is always AUG. Some codons act as start and stop
signals in protein synthesis. Codons are always written left to right, 5’to 3’
Code Amino acid Code Amino acid Code Amino acid Code Amino acid
UUU Phe CUU Leu AUU Ile GUU Val
UUC Phe CUC Leu AUC Ile GUC Val
UUA Leu CUA Leu AUA Ile GUA Val
UUG Leu CUG Leu AUG Met GUG Val
UCU Ser CCU Pro ACU Thr GCU Ala
UCC Ser CCC Pro ACC Thr GCC Ala
UCA Ser CCA Pro ACA Thr GCA Ala
UCG Ser CCG Pro ACG Thr GCG Ala
UAU Tyr CAU His AAU Asn GAU Asp
UAC Tyr CAC His AAC Asn GAC Asp
UAA Stop CAA Gln AAA Lys GAA Glu
UAG Stop CAG Gln AAG Lys GAG Glu
UGU Cys CGU Arg AGU Ser GGU Gly
UGC Cys CGC Arg AGC Ser GGC Gly
UGA Stop CGA Arg AGA Arg GGA Gly
UGG Trp CGG Arg AGG Arg GGG Gly
NUCLEIC ACIDS 33