Chapter 5: Molecular biology and genetics

Molecular biology and genetics

Transcription and translation   

Transcription   

The central dogma of molecular biology is now well established, namely, DNA produces RNA, which in turn produces a polypeptide that makes up the protein that provides the cell structure and performs the functions of the cell (Fig 6). The genetic information inherited by each individual is encoded by the sequences of the bases of the DNA in the genome (the genotype), which is translated into proteins and provides the recognizable characteristics of the individual (the phenotype) such as height and weight. For DNA to produce proteins, it must first go through the intermediary step of RNA. DNA, the double-stranded molecule, unwinds to give a single-stranded RNA molecule that serves as the template for protein (Fig 6). This process, since it goes from one nucleic acid to another nucleic acid, is referred to as transcription. This is a key regulatory step in the old process of replicating and maintaining life. There are several aspects in the regulation of transcription, as indicated in Table 3. The process of transcription is initiated by attachment of the enzyme RNA polymerase to specific recognition sites where the DNA is double-stranded but, upon activation by the enzyme, the strands selectively unwind and separate. The binding site for the RNA polymerase 2 is always on the 5′ end of the gene and travels on a single-stranded DNA towards the 3′ end. Messenger RNA in addition to being single-stranded also differs from DNA in that the deoxyribose sugar found in DNA is replaced by ribose. Furthermore, uracil (U) replaces T and, like T, U pairs exclusively with A. The mRNA transcribed from DNA is usually referred to as the primary transcript and is a complementary copy of the DNA. The mRNA exits the nucleus but, prior to transport, undergoes extensive posttranscriptional processing primarily through the three following main events: (1) the addition of the methylated guanosine (for methoguanosine residue) to the 5′ end, referred to as a CAP, which is important for the initiation of translation; (2) addition of a long tail of repeated adenine nucleotides called the polyadenine tail to the 3′ region of the mRNA, which is essential for stability as it passes out into the cytoplasm to serve as a template for protein synthesis; (3) the primary transcript, which contains introns and exons, undergoes a specific splicing process whereby the introns are removed and exons are properly respliced together prior to exit from the nucleus. It is then referred to as the mature mRNA. The exons of the 3′ end do not code for proteins but for signals that terminate translation and direct the addition of the polyadenine tail. The mature mRNA exits the nucleus through nuclear pores and, upon entering the cytoplasm, attaches to ribosomal RNA.

FIG 6. DNA controls cell function; RNA is synthesized from a gene on the DNA template in the nucleus. The protein is then synthesized from RNA to carry out cell function.

Translation   

The process whereby the mRNA codes for a protein is called translation (translates the nucleic acid code of DNA into the amino acid code of protein). Protein translation occurs when a single mRNA moves along the ribosome and is read by tRNA molecules, which bring the amino acid to the chain. Once a polypeptide is formed from the specific mRNA, it may in itself form the protein or combine with other polypeptides to form the mature protein. Once the protein is formed, it has certain amino acid sequences which direct it to its specific compartment in the cell. Secreted protein molecules, for example, contain a hydrophobic tail sequence which directs it to the endoplasmic reticulum membrane. The proteins perform all of the work of the cell and are each synthesized from a unique mRNA. Gene expression refers to the whole process from the formation of the gene to a mature protein. Many proteins undergo chemical modification posttranslationnally, such as glycosylation. This form of posttranslational modification is particularly common with cellular secretory or membrane proteins. It may involve the development of disulfide bonds, proteolytic cleavage of the newly synthesized protein, or the addition of carbohydrate moieties. These posttranslational changes may importantly affect function and subcellular localization of proteins.