Polypeptide and Protein Synthesis

POLYPEPTIDE AND PROTEIN SYNTHESIS

Learning Objectives for this Section

DNA is divided into functional units called genes. A gene (def) is a segment of DNA that codes for a functional product (mRNA, tRNA, or rRNA). Since the vast majority of genes are transcribed into mRNA and mRNA is subsequently translated into polypeptides or proteins, most genes code for protein synthesis. The term polypeptide (def) refers to many amino acids (def) connected by peptide bonds (def). While all proteins are polypeptides, not all polypeptides are proteins. In some cases, smaller polypeptides coded for by two or more genes must be joined together to produce a functional protein. In other cases, as will be mentioned below, mRNA carries a transcript of several genes resulting in the synthesis of a large polypeptide that must subsequently be cleaved by enzymes called proteases into two or more smaller functional proteins. For simplicity, we will use the term protein when referring to the end product of transcription and translation.

In this section we will see how the sequence of deoxyribonucleotide bases along one strand of DNA ultimately codes for the amino acid sequence of a particular polypeptide or protein. As mentioned in the Protein Tutorial, the order of deoxyribonucleotide bases in the DNA determines the order of amino acids in the proteins a particular organism is capable of making. The order of amino acids for each protein ultimately determines its final three-dimensional shape. That, in turn, determines the function of that protein. As an example, as seen in the Enzyme Tutorial, the three-dimensional configuration of an enzyme determines with what substrate(s) that enzyme reacts. The shape of an enzyme is determined by its order of amino acids, that is, its primary structure (def) and the primary structure of a protein is coded for by the order of deoxyribonucleotide bases in a gene.

Protein synthesis can be divided into two stages: transcription and translation. In the next two sections we will look at these stages in greater detail.

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