Molecular genetics

Molecular genetics is a specialized branch of biology. It investigates how differences in DNA structures or expression create variation among organisms. This field uses an investigative approach to determine the structure and function of genes within a genome. It integrates several disciplines, including biochemistry, cellular biology, and biotechnology. Researchers use these tools to explore gene regulation and the molecular mechanisms of life processes. A primary goal is to identify and study genetic mutations. By linking specific mutations to phenotypes, or observable traits, scientists can search for treatments for genetic diseases.

To understand this field, one must understand the central dogma of molecular biology. This concept describes how genetic information flows through a living system. First, DNA replicates itself to ensure information is passed on. Second, DNA is transcribed into messenger RNA, or mRNA. This transcription occurs within the cell nucleus. Third, the RNA is translated into proteins at the ribosome. This process uses the genetic code, which consists of four nitrogenous bases: adenine, cytosine, thymine, and guanine. In RNA, uracil replaces thymine. The code is redundant, meaning multiple combinations of these base pairs can produce the same amino acid.

DNA is a double-stranded molecule with an antiparallel structure. This means the two strands run in opposite directions. The building blocks of DNA are called nucleotides. Each nucleotide contains a sugar molecule, a phosphate group, and one of the four nitrogenous bases. A single strand is held together by covalent bonds. The two strands are connected by hydrogen bonds between the bases. Adenine always binds with thymine, and cytosine always binds with guanine. This specific pairing allows for semiconservative replication. During cell division, each original strand acts as a template to form a new complementary strand.

The history of this field involves many significant discoveries. In 1869, chemist Johann Friedrich Miescher isolated a molecule he called nuclein from white blood cells. He found it contained hydrogen, oxygen, nitrogen, and phosphorus. Later, Albrecht Kossel identified nuclein as a nucleic acid and named it deoxyribonucleic acid, or DNA. Kossel also isolated the nucleotides: adenine, guanine, thymine, cytosine, and uracil. In the early 1800s, Gregor Mendel discovered principles of inheritance, such as dominant and recessive traits, using pea plants. In 1953, Francis Crick and James Watson derived the 3-D double helix structure. They built upon X-ray crystallography work performed by Rosalind Franklin and Maurice Wilkins.

Other scientists provided essential evidence for the role of DNA. In 1944, Avery, McLeod, and McCarthy showed that DNA could transform bacteria. They found that DNA from a virulent strain could make a harmless strain virulent. In 1950, Erwin Chargaff established rules regarding base composition. He proved that the amount of adenine equals thymine, and guanine equals cytosine. In 1969, Arber and Linn isolated a restriction endonuclease in E. coli. This discovery opened the field of genetic engineering. In 1971, Berg used these enzymes to create the first recombinant DNA molecule. By 1972, Cohen and Boyer created the first recombinant DNA organism by inserting plasmids into E. coli.

Technological advances have greatly expanded the reach of molecular genetics. In the late 1970s, Maxam, Gilbert, and Frederick Sanger developed DNA sequencing techniques. In 1985, Kary Mullis invented the Polymerase Chain Reaction, or PCR. This method uses Taq polymerase to create millions of copies of a specific DNA sequence. These advancements led to the sequencing of the first whole genome, Haemophilus influenzae. This was followed by the Human Genome Project, which finished in 2001. Today, the field of genomics links molecular structures to functional protein expression. Scientists use bioinformatics to compare genes across species using computer databases like NCBI and Ensembl.

Researchers use two main methodologies: forward genetics and reverse genetics. Forward genetics is an unbiased approach to identify genes responsible for a specific phenotype. Scientists generate random mutations using mutagens or transposons. They then screen individuals for the desired trait. They often use model organisms like the nematode C. elegans, the fruit fly Drosophila melanogaster, or the zebrafish Danio rerio.

Reverse genetics works in the opposite direction. It involves making an intentional mutation in a specific gene to see what phenotype results. This helps scientists deduce the original function of that gene. One common method is creating a gene knockout, where a gene is deleted so it is no longer expressed. Mutations can also be missense mutations caused by nucleotide substitution. Other changes include nucleotide additions or deletions that induce a frameshift mutation. These techniques allow scientists to map the specific nature of mutations and understand their impact on life.