Genetics

To understand genes and their biological function in heredity, it is necessary to understand the chemical makeup and structure of DNA. Although some viruses carry their genetic information in the form of ribonucleic acid (RNA), most higher life forms carry genetic information in the form of DNA, the molecule that makes up chromosomes.

The complete DNA molecule is often referred to as the blueprint for life because it carries all the instructions, in the formation of genes, for the growth and functioning of most organisms. This fundamental molecule is similar in appearance to a spiral staircase, which is also called a double helix. The sides of the DNA double helix ladder are made up of alternate sugar and phosphate molecules, like links in a chain. The rungs, or steps, of DNA are made from a combination of four nitrogen-containing bases—two purines (adenine [A] and guanine [G]) and two pyrimidines (cytosine [C] and thymine [T]). The four letters designating these bases (A, G, C, and T) are the alphabet of the genetic code. Each rung of the DNA molecule is contains a combination of two of these letters, one jutting out from each side. In this genetic code, A always combines with T, and C with G to make what is called a base pair. Specific sequences of these base pairs, which are bonded together by atoms of hydrogen, make up the genes.

While a four-letter alphabet may seem rather small for constructing the comprehensive vocabulary that describes and determines the myriad life forms on Earth, the sequences or order of these base pairs are nearly limitless. For example, various sequences or rungs that make up a simple six base gene could be ATCGGC, or TAATCG, or AGCGTA, or ATTACG, and so on. Each one of these combinations has a different meaning. Different sequences provide the code not only for the type of organism, but also for specific traits like brown hair and blue eyes. The more complex an organism, from bacteria to humans, the more rungs or genetic sequences appear on the ladder. The entire genetic makeup of a human, for example, may contain 120 million base pairs, with the average gene unit being 2,000 to 200,000 base pairs long. Except for identical twins, no two humans have exactly the same genetic information.

Genetic information is duplicated during the process of DNA replication, which begins a few hours before the initiation of cell division (mitosis). To produce identical genetic information during mitosis, the hydrogen bonds holding together the two halves of the DNA ladder unzip, in presence of proteins called helicases, to expose single strands of DNA. These old strands act as templates to make new DNA molecules. Replication is initiated by this separation of DNA, and requires short DNA fragments (primers) to start synthesis of a new DNA strand by specific cellular enzymes called DNA polymerases. DNA rarely mutates during replication, as the proofreading and "repair" enzymes make sure that any errors are quickly repaired to protect the accuracy of the genetic information. Once completed, each new half of the DNA ladder has the identical information as the old one. This is achieved by the fact that T always combines with A and C with G, therefore if the template had a sequence ATGCTG the newly made second strand will be TACGAC. When cell mitosis is completed, each new cell contains an exact replica of the DNA.

Cells contain hundreds of different proteins and its functions are dependent on which of the thousands of types of different proteins it contains. Proteins are made up of chains of amino acids. The arrangement of the amino acids to build specific proteins is determined by the basepair sequence contained or encoded in DNA. This genetic information has to be converted to proteins building over half of all solid body tissues and control most biological processes within and among these tissues. This is achieved by using the genetic code, which is a set of 64 triplets of bases (called codons) corresponding to each amino acid and the initiation and termination signals for protein synthesis.

As the sites of protein production lie outside the cell nucleus, the instructions for making them have to be transported out of the nucleus. The messenger that carries these instructions is messenger RNA, or mRNA (a single stranded molecule that has a mirror image of the base pairs on the DNA). mRNA is made in the nucleus during a process called transcription and a single molecule of RNA carries instructions for making only one protein. After being exported out of the nucleus it is transported to ribosomes, which are the protein factories in the cell. In ribosomes the information from mRNA is decoded to produce a protein. This process is called translation. The flow of information is only one way from DNA to RNA and to protein. Therefore characteristics acquired during an organism's life, such as larger muscles or the ability to play the piano, cannot be inherited. However, people may have genes that make it easier for them to acquire these traits through exercise or practice.