Biology Reference
In-Depth Information
released from the DNA. The initial mRNA product, termed the pre-mRNA,
contains both exons, regions that encode for proteins, and introns, inter-
vening sequences that do not encode protein. In an essential process called
mRNA splicing, the noncoding introns are removed out of the pre-mRNA
to form the mature and functional mRNA (Fig. 2.4).
After the mRNAs are fully processed in the nucleus, they are transported
through the nuclear pores into the cytoplasm. Here the mRNAs are assem-
bled onto ribosomes, the translation machinery that converts the infor-
mation encoded in each mRNA into protein. Each ribosome consists of
two major subunits, one large and one small, and these subunits are made
up of both protein and ribosomal RNA (rRNA) components. The process
of translation, like that of transcription, can be divided into three steps: ini-
tiation, elongation and termination. All three of these processes are cat-
alyzed by the ribosomal proteins, but are dependent on proper rRNA
function.
Translation initiation begins with the binding of the small ribosomal
subunit to a recognition sequence on the mRNA (Fig. 2.5). All 20 amino
acids are encoded by three base sequences of mRNA termed codons. The
translational recognition sequence includes the sequences surrounding the
“start codon,” usually an AUG codon encoding the amino acid methionine.
After binding to the recognition sequence, a methionine-charged tRNA is
brought into place, the large subunit of the ribosome joins the complex, and
translation is initiated. Elongation of the peptide occurs catalytically with
the ribosome moving along the mRNA in the 5
¢ to 3¢ direction. The open
reading frame refers to the subset of nucleotides of the mRNA that are
used as the template to create protein. The ribosome shifts to the next three
bases in the sequence (the next codon) and adds the appropriate amino
acid to the growing peptide. The process of translation is halted when a stop
codon—UAA, UAG, or UGA—is encountered.
FIGURE 2.4. The coding sequence of genes are composed of exons that are sepa-
rated by intervening sequences, introns. The full length of the gene is transcribed
into a primary RNA transcript that undergoes RNA splicing. Introns are removed,
allowing the RNA segments to be joined at the splice junctions to form mRNA.
