DNA-Directed Protein Synthesis
Deoxyribonucleic acid (DNA) directs
the synthesis of the many thousands of proteins that are contained in the
different cells of the body. Although some of the proteins are structural
proteins, the majority are enzymes that catalyze the different chemical
reactions in the cell. Because DNA is located in the cell’s nucleus and protein
synthesis takes place in the cytoplasm, a second type of nucleic
acid—ribonucleic acid (RNA)—participates in the actual assembly of the
proteins.
There are three types of RNA:
messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) that
participate in (1) the transcription of the DNA instructions for protein
synthesis and (2) the translation of those instructions into the assembly of
the polypeptides that make up the various proteins.
The genetic code is a triplet of
four bases (adenine [A], thymine [T], guanine [G], and cytosine [C], with
thymine in DNA being replaced with uracil [U] in RNA) that control the sequence
of amino acids in a protein molecule that is being synthesized. The triplet RNA
code is called a codon.
Transcription
Transcription involves copying the
genetic code containing the instructions for protein synthesis from DNA to a
complementary strand of mRNA. Transcription is initiated by an enzyme called
RNA polymerase, which binds to a promoter site on DNA. Many other proteins,
including transcription factors, function to increase or decrease
transcriptional activity of the genes. After mRNA has been transcribed,
it detaches from DNA and is processed by cutting, removing introns, and splicing
the exon RNA sequences to produce a variety of mRNA molecules from a single
gene. Once mRNA has been processed, it diffuses through
the nuclear pores into the cytoplasm, where it controls
protein synthesis.
Translation
The process of translation involves
taking the instructions transcribed
from DNA to mRNA and transferring them to the rRNA of ribosomes located in the cytoplasm. When the mRNA carrying the instructions for a particular
protein comes in contact with a
ribosome, it binds to a small subunit of the rRNA. It then travels through the ribosome where the transcribed instructions are
communicated to the tRNA, which
delivers and transfers the correct amino acid to its proper position on the growing peptide chain. There are 20 types of tRNA, one for each of the
20 different types of amino acid.
Each type of tRNA carries an anticodon complementary to the mRNA codon calling for the amino acid carried by the tRNA, and it is the
recognition of the mRNA codon by
the tRNA anticodon that ensures the proper
sequence of amino acids in a synthesized protein. In order to be functional, the newly
synthesized protein must be folded
into its functional form, modified further, and then routed to its final position in the cell.
