Molecular Biology
Molecular biology is the field of study concerned with the structural and functional properties of biological systems. Specifically, it attempts to understand the function of molecules within a living system and how they interact. It includes such topics as biophysics, genetics and biochemistry. Researchers in this field have made many significant contributions to modern biology including discovery of the structure of nucleic acids and proteins, the mechanism for heredity, and the processes involved in metabolism. Recently, advances in genetic engineering technology have led to significant discoveries making molecular biology an active area of study for many new scientists.
Molecular biology is a relatively new area of study in the field of biology. While it traces its roots back to the 1850s when Gregor Mendel proposed his laws of inheritance, it actually began in the early 1940s. Around this time, scientists had already figured out that chromosomes contained the hereditary information of the cell. They also knew that chromosomes were composed of DNA and protein. In 1941, George Beadle and Edward Tatum established the relationship between genes and enzymes. They determined that each gene codes for a single protein. Later in 1944, Oswald Avery and his colleagues showed that DNA is the genetic material. This established a molecular basis for heredity and focused the study of molecular biology on determining the structure of DNA. This structure was determined in 1953 by James Watson and Francis Crick. With the structure in hand, scientists worked on new theories to explain how genes express themselves in living organisms.
The study of genetic material has been one of the primary focuses of molecular biology. These investigations have been focused on three main areas including: the composition, organization and structure of the chromosomes which contain DNA, the molecular reactions involved in gene expression, and the control of gene expression.
DNA has been found to be a nearly universal genetic material. All organisms use it except certain viruses that contain a similar molecule called ribonucleic acid (RNA). It is a long polymer made up of monomer units called nucleotides. Nucleotides are composed of three parts including a phosphate group, a five carbon sugar, and a nitrogen containing base. These nucleotides are arranged in a double helix structure which coils around a central axis. The nitrogen bases provide the code which is translated into proteins. Typically, a gene is made up of a sequence of these nucleotides that code for a specific protein. Additional stretches of code influence gene expression.
In prokaryotes, the genetic material is arranged in a single chromosome which contains a circular DNA molecule. This chromosome contains about 4000 genes and each nucleotide is important in gene expression. In eukaryotes, the genes are typically arranged on multiple chromosomes which are made up of linear DNA molecules. The number of chromosomes differs depending on the organism. Human cells contain 23 pairs of chromosomes, each encoding thousands of genes. In both prokaryotic and eukaryotic cells, the DNA is associated with chromosomal proteins.
Since DNA was determined to be the material responsible for heredity, a mechanism for its replication was needed. This mechanism was determined in 1958 by Matthew Meselson and Franklin Stahl. In an experiment, they showed that the replication of DNA followed a semiconservative path. This meant that the two strands of DNA separated and served as templates for two new strands. Each resulting strand was composed of half of the original "parent" strand and half of a new "daughter" strand.
After a model for the replication of DNA was worked out, scientists turned their attention to figuring out how nucleotide sequences were expressed in cells. This led to the discovery in the 1960s that the genetic code is translated into proteins by ribosomes. In this process, DNA is first transcribed into messenger RNA (mRNA). The mRNA then interacts with organelles in the cell called ribosomes. In this process, known as protein synthesis, the nucleotides are translated into amino acids and strung together to create proteins. During this process, three nucleotides are read at once by the ribosome. This triplet then results in the addition of a single amino acid to the growing protein chain.
While investigating a variety of problems in molecular biology, scientists have developed numerous concepts and techniques that have helped solve various biological and biochemical problems. One of these was the use of intact cells and cell-free systems. Since bacterial cultures were simple systems and relatively easy to propagate, they were used for much of the early work in molecular biology. Many of the significant discoveries were made with these cultures. Work with eukaryotic cells has only recently been possible. Molecular biologists also developed cell- free systems in which they could study processes such as DNA synthesis and protein synthesis.
After the structure of DNA was determined and the nature of the genetic code was known, scientists attempted to create maps of where genes were physically located on the chromosomes. At first these maps were crude, only showing relative positions of few genes. A major breakthrough came in 1977 when DNA sequencing became possible. This allowed scientists to find the order of the nucleotide bases on any gene. Work is currently ongoing to sequence the entire human genome. It is hoped that this information will provide invaluable clues in the diagnosis and treatment of genetic disorders.
A variety of new techniques which come under the heading of DNA biotechnology have recently been developed by molecular biologists. Recombinant DNA is one such technology that has had important implications to science, medicine, and industry. In this technique, a certain gene is isolated and then transferred to another organism. This host organism then expresses the gene thereby producing the desired protein. Cloning is another technique that has led to some exciting new developments. In this procedure, the genetic material from one organism is duplicated and a new organism is created. New biotechnologies such as these promise to revolutionize medicine.
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