Biology Reference
In-Depth Information
Processing of mRNA involves splicing to remove introns; mRNA also is capped
and methylated at the 5
′
end, and most mRNAs are polyadenylated at the 3
′
end. The information in the mRNA then is translated into proteins via protein
synthesis in ribosomes.
A ribosome begins protein synthesis once the 5
′
end of an mRNA is inserted
into it. A lengthening polypeptide chain is produced and, once the 5
′
end of the
mRNA emerges from a ribosome, the mRNA can attach to a second ribosome
and a second identical polypeptide can be synthesized. The assembly of amino
acids into a peptide starts at the amino end and terminates at the carboxyl end.
Amino acids are carried to the ribosome by transfer RNAs (tRNAs). The tRNAs
are held so their anticodons form base pairs with complementary codons of
the mRNA. One tRNA may recognize more than one codon because the genetic
code is degenerate. According to the “wobble hypothesis,” the initial two bases
of the mRNA codon pair according to base-pairing rules, but the third base may
pair with any one of several bases.
Protein synthesis occurs in the ribosomes that are located in the endoplasmic
reticulum of the cytoplasm. Once proteins are produced, they are transported
into the Golgi apparatus where they are processed and transported to their ulti-
mate destination. The protein must be folded and, sometimes, assembled into
multiprotein complexes. Folding may require the assistance of other proteins
called molecular chaperones.
Gene regulation in insects and other eukaryotes is complex, diverse, and the sub-
ject of intensive research. Much of the DNA once labeled as “junk” or noncoding
we now know is transcribed into RNAs that are involved in gene regulation and
development. Protein-coding genes may be amplified or rearranged to increase the
amount of gene product. Protein-coding genes may be regulated by being methyl-
ated. Activator proteins, hormones, enhancers and noncoding RNAs (ncRNAs) affect
transcription. Alternative splicing, alternative promoters, and translational control
are used in gene regulation, as are ncRNAs. Insulators or boundary elements are
naturally occurring DNA sequences that protect genes from position effects, estab-
lishing independent functional domains within the chromosome.
2.2 Introduction
The Central Dogma, that DNA is transcribed into RNA that subsequently is trans-
lated into proteins, describes the process by which information contained in the
protein-coding DNA is made available to the cell and organism (Figure 1.1). Proteins
have many functions in the cell. Structural proteins form part of the framework of
the organism, such as the sclerotin in the exoskeleton of insects. Contractile pro-
teins enable organisms to move. Catalytic proteins, or enzymes, regulate the diverse
