Molecular Biology

Molecular biology is the study of biological molecules and the molecular basis of structure and function in living organisms.

Molecular biology is an interdisciplinary approach to understanding biological functions and regulation at the level of molecules such as nucleic acids, proteins, and carbohydrates. Following the rapid advances in biological science brought about by the development and advancement of the Watson-Crick model of DNA (deoxyribonucleic acid) during the 1950s and 1960s, molecular biologists studied gene structure and function in increasing detail. In addition to advances in understanding genetic machinery and its regulation, molecular biologists continue to make fundamental and powerful discoveries regarding the structure and function of cells and of the mechanisms of genetic transmission. The continued study of these processes by molecular biologists and the advancement of molecular biological techniques requires integration of knowledge derived from physics, chemistry, mathematics, genetics, biochemistry, cell biology and other scientific fields.

Molecular biology also involves organic chemistry, physics, and biophysical chemistry as it deals with the physicochemical structure of macromolecules (nucleic acids, proteins, lipids, and carbohydrates) and their interactions. Genetic materials including DNA in most of the living forms or RNA (ribonucleic acid) in all plant viruses and in some animal viruses remain the subjects of intense study.

In 1945, William Astbury coined the term "molecular biology" referring to the study of the chemical and physical structure of biological macromolecules (large sized molecules). There was and still is a strong belief that all forms of life have uniformity in biological processes. The pioneer findings in prokaryotes (a simple or primitive cell type, e.g., bacteria and blue green alga) are extended to eukaryotes (a complex or well developed cell type, e.g., animal and plant cells).

The complete set of instructions for making an organism (i.e., the complete set of genes) is called its genome. It contains the master blueprint for all cellular structures and activities for the lifetime of the cell or organism. The human genome consists of tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein molecules organized into structures called chromosomes. In humans, as in other higher organisms, a DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder whose sides, made of sugar and phosphate molecules, are connected by rungs of nitrogen-containing chemicals called bases. Each strand is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base. Four different bases are present in DNA adenine (A), thymine (T), cytosine (C), and guanine (G). The particular order of the bases arranged along the sugar-phosphate backbone is called the DNA sequence; the sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits.

Each time a cell divides into two daughter cells, its full genome is duplicated; for humans and other complex organisms, this duplication occurs in the nucleus. During cell division the DNA molecule unwinds and the weak bonds between the base pairs break, allowing the strands to separate. Each strand directs the synthesis of a complementary new strand, with free nucleotides matching up with their complementary bases on each of the separated strands. Strict base-pairing rules are adhered to. Adenine will pair only with thymine (an A-T pair) and cytosine with guanine (a C-G pair). Each daughter cell receives one old and one new DNA strand. The cells adherence to these base-pairing rules ensures that the new strand is an exact copy of the old one. This minimizes the incidence of errors (mutations) that may greatly affect the resulting organism or its offspring.

Each DNA molecule contains many genes, the basic physical and functional units of heredity. A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing proteins, which provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions.

The central dogma of molecular biology is that DNA is copied to make mRNA (messenger RNA) and mRNA is used as the template to make proteins. Formation of RNA is called transcription and formation of protein is called translation. Transcription and translation processes are regulated at various stages and the regulation steps are unique to prokaryotes and eukaryotes. DNA regulation determines what type and amount of mRNA should be transcribed, and this subsequently determines the type and amount of protein. This process is the bottom line for growth and morphogenesis.

All living organisms are composed largely of proteins, the product of genes. Humans can synthesize at least 100,000 different kinds. Proteins are large, complex molecules made up of long chains of subunits called amino acids. The protein-coding instructions from the genes are transmitted indirectly through messenger ribonucleic acid (mRNA), a transient intermediary molecule similar to a single strand of DNA. For the information within a gene to be expressed, a complementary RNA strand is produced (a process called transcription) from the DNA template in the nucleus. This mRNA is moved from the nucleus to the cellular cytoplasm, where it serves as the template for protein synthesis.

Twenty different kinds of amino acids are usually found in proteins. Within the gene, each specific sequence of three DNA bases (codons) directs the cells protein-synthesizing machinery to add specific amino acids. For example, the base sequence ATG codes for the amino acid methionine. Since three bases code for one amino acid, the protein coded by an average-sized gene (3,000 bp) will contain 1,000 amino acids. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins.

Molecular biology also deals with: (1) the processes of DNA replication (making an exact copy of DNA) and DNA repair; (2) mutations (sudden alterations in nitrogen containing bases of DNA), their effects, and the agents that cause mutations (e.g., ultra-violet rays and chemicals); and (3) mechanisms and rearrangement and exchange of genetic materials via small segments of DNA such as plasmids, transposable elements, insertion sequences, and transposons to obtain recombinant DNA (DNA with recombined or exchanged nitrogenous bases).

Genetic engineering is an offshoot of molecular biology. Several biochemical, microbial, and molecular biological techniques are combined to obtain desirable DNA sequences in larger quantities, which may be subsequently used to manufacture proteins in larger quantities (e.g. insulin production).

Advances in molecular biology have led to significant discoveries about such changes in cell function and behavior as the development of higher organisms, the immunologic response, cancer and cell evolution. It has contributed tremendously to applications in the field of medicine, forensic science, biotechnology and biomedical industries.