Gene

DNA is made up of four building blocks or nitrogenous bases; adenine (A), guanine (G), cytosine (C), and thymine (T). A, C, G, and T represents the DNA alphabet and different combinations of these letters mean something different. Every gene begins with a specific start sequence and ends with a stop sequence. Therefore, the specific sequences of these four bases determines whether the DNA codes for proteins (coding DNA), the specific protein it encodes, or whether it represents noncoding DNA that does not encode for protein. Non-coding DNA, also called junk DNA accounts for 97% of the genome and despite its name, it serves many purposes including the proper functioning of genes. Each gene can be converted or transcribed into a type of RNA called messenger RNA (mRNA). RNA is very similar to DNA except that instead of thymine as one of its four nitrogenous bases, uracil (U) is substituted. Gene expression can be controlled by proteins called transcription factors, enhancers, and silencers. These regulatory proteins influence whether proteins will be expressed by binding to specific sequences of DNA or through interactions with other DNA binding proteins.

Information is passed from the DNA molecule to the messenger RNA (mRNA) by the pairing of complementary bases in each of the two strands. The mRNA then carries the instructions from the DNA in the nucleus to a ribosome in the cytoplasm. A molecule called transfer RNA (tRNA) transports an amino acid that is designated to match a codon, or a three base pair sequence in the mRNA. Amino acids strung together form a particular protein. The proteins that are produced might, for example, be important for human growth and development or represent important enzymes in physiological pathways.

Knowing the sequence of every gene found in the human genome, made possible in part by The Human Genome Project, will allow scientists to better understand the cause of diseases such as cancer or cystic fibrosis and develop new ways to treat or cure these diseases by characterizing individual genes as well as gene-gene interactions. The rough draft sequence of the human genome was completed and published in February 2001 in both Nature and Science scientific journals and the final sequence is expected to be completed sometime during 2003.