56 BIOCHEMISTRY FUNDAMENTALS
and CAC code for histidine (his); GGT/U, GGC, GGA, and GGG code for
glycine (gly); T/UAA, T/UAG, and T/UGA are STOP codons that end the
protein sequence. Each tRNA also has a specifi c charger protein, called an
aminoacyl tRNA synthetase, that binds to a particular tRNA and attaches the
correct amino acid to the acceptor site. The start signal for translation is the
AU/TG codon, coding for methionine (met). Not every protein starts with met,
so that often this amino acid must be removed during post - translational protein
processing. This so - called post - translational modifi cation (PTM) is one of a
huge number of protein modifi cations that take place following protein syn-
thesis. For example, newly synthesized proteins may have certain tyrosine
residues phosphorylated or asparagine residues glycosylated. PTMs are impor-
tant research targets in proteomics, because they have far - reaching implica-
tions for cellular behavior (and misbehavior in disease processes). The
ribosomal large subunit binds to the mRNA and the ribosomal small subunit
and elongation begins. Entering tRNAs with attached amino acids attach to
the A site matching their codon to that on the mRNA. A peptide bond is
formed with the amino acid attached to the tRNA at the P site followed by a
shift of the new peptide to the P site freeing the A site for the next tRNA.
When the ribosome reaches a stop codon, no aminoacyl tRNA binds to the
empty A site, the ribosome breaks into its large and small subunits, and the
newly synthesized protein and mRNA are released.
2.3.4 Genetic Mutations and Site-Directed Mutagenesis,
Two major types of genetic mutation are possible: chromosomal mutations and
point mutations. Chromosomal mutations can alter or shift large sections of
chromosomes (collections of genes), leading to changes in the way their genes
are expressed. In translocations , large DNA segments are interchanged
between two different chromosomes, causing possible changes in gene expres-
sion. Inversions occur when a region of DNA fl ips its orientation with respect
to the rest of the chromosome, also leading to changes in gene expression.
Sometimes large sections of a chromosome may be deleted, leading to loss of
important genes. Chromosome nondisjuction arises when one daughter cell
receives more or less DNA than it should during cell division. Point mutations
arise from single base - pair changes. A nonsense mutation creates a stop codon
where none previously existed, possibly removing essential regions in the
resulting protein — that is, U/TAA (stop) instead of U/TAC (tyr). A missense
mutation changes the mRNA code, placing a different amino acid into the
primary sequence and possibly changing the shape or the metal coordination
sphere of the resulting protein. For example, AAC (asparagine, asn, N) could
be substituted for CAC (histidine, his, N) by a missense point mutation. A silent
mutation codes for the same amino acid in the protein primary sequence; that
is, CAC and CAU/T both code for histidine. Within a gene, insertion of a small
number of bases not divisible by three will result in aframe shift mutation ,