can be made? DNA sends out a message, in the form ofRNA(ribonucleic acid), describing
howtomaketheprotein. TherearethreetypesofRNAdirectlyinvolvedinproteinsynthesis.
Messenger RNA (mRNA) carries the instructions from the nucleus to the cytoplasm. The
other two forms of RNA, ribosomal RNA (rRNA) and transfer RNA (tRNA) are involved in
theprocessoforderingtheaminoacidstomaketheprotein. Thisprocessiscalledtranslation
and will be discussed below. All three RNAs are nucleic acids and are therefore made of
nucleotides. The RNA nucleotide is very similar to the DNA nucleotide except for the fact
that it contains a different kind of sugar, ribose, and the base uracil (U) replaces the thymine
(T) found in DNA.
mRNA is created in a method very similar to DNA synthesis. mRNA is also made up of
nucleotideunits. Thedoublehelixunwindsandthenucleotidesfollowbasicallythesamebase
paring rules to form the correct sequence in the mRNA. This time, however, U pairs with
each A in the DNA. In this manner, the genetic code is securely passed on to the mRNA. The
process of constructing a mRNA molecule from DNA is known astranscription(Figures
5.17and5.18).
Figure 5.17: Each gene (a) contains triplets of bases (b) that are transcribed into RNA (c).
Every triplet, or codon, encodes for a unique amino acid. ( 20 )
The mRNA is directly involved in the protein synthesis process and tells the ribosome
(Figure5.19) how to assemble a protein. The base code in the mRNA dictates the or-
der of the amino acids in the protein. But because there are only 4 bases in mRNA and
20 different amino acids, one base cannot directly code for one amino acid. The genetic