22 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”of RNA (ribonucleic acid) and proteins. Genetic messages encoded in
the DNA are transcribed into messenger RNA (mRNA) in the nucleus
and transported to the ribosomes in the cytoplasm, where the nucleo-
tide (or nucleobase) messages (ATGC in DNA, and AUGC in mRNA)
are translated into the amino acid sequence of a protein (Fig. 3.3).
The detail of translation is depicted in Fig. 3.4. Translation is accom-
plished by a cooperative action of mRNA and transfer RNA (tRNA). As
mRNA lines up on the ribosome, each tRNA carries a specific amino
acid matching the genetic code in the mRNA (Table 3.3). The unit of
genetic code is a triplet of nucleotides called codon. There are twenty
kinds of amino acids in a protein, but there are more than sixty codons
Fig. 3.3. Schematic of a cell showing the transcription of messenger RNA (mRNA)
from DNA in the nucleus, the transport of mRNA to the cytoplasm, and the transla-
tion of mRNA into protein on a ribosome (shaded round structure). Transfer RNAs
(tRNAs) are shown carrying amino acids to match the nucleotide sequence on the
mRNA. During translation, the protein chain grows while attached to the tRNA. [US
Natl. Lib. Med.]