Protein Synthesis and Secretion

In order for a gene to be expressed, it first must be used as a guide, or template, in the production of a complementary strand of messenger RNA.This mRNA is then itself used as a guide to produce a particular type of protein whose sequence of amino acids is determined by the sequence of base triplets (codons) in the mRNA.

When mRNA enters the cytoplasm, it attaches to ribosomes, which appear in the electron microscope as numerous small particles. A ribosome is composed of four molecules of ri-bosomal RNA and eighty-two proteins, arranged to form two

Premrna Mrna

mRNA

■ Figure 3.19 The processing of pre-mRNA into mRNA.

Noncoding regions of the genes, called introns, produce excess bases within the pre-mRNA. These excess bases are removed, and the coding regions of mRNA are spliced together

Ribosomes

Newly synthesized protein

Ribosomes

Newly synthesized protein

Polyribosome
mRNA

■ Figure 3.20 An electron micrograph of a polyribosome. An RNA

strand joins the ribosomes together.

subunits of unequal size. The mRNA passes through a number of ribosomes to form a "string-of-pearls" structure called a polyribosome (or polysome, for short), as shown in figure 3.20. The association of mRNA with ribosomes is needed for the process of genetic translation—the production of specific proteins according to the code contained in the mRNA base sequence.

Each mRNA molecule contains several hundred or more nucleotides, arranged in the sequence determined by complementary base pairing with DNA during transcription (RNA synthesis). Every three bases, or base triplet, is a code word—called a codon—for a specific amino acid. Sample

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Essentials of Human Physiology

Essentials of Human Physiology

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