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Genes are the basic physical and functional units of heredity. They are molecular information that ultimately determine the traits possessed by any organism. A gene is a specific sequence of nucleotide bases on the DNA molecule (Deoxyribonucleic acid). The sequence of nucleotides specifies the information required for constructing proteins, which provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions. Typically the products of several genes are assembled to make a functional protein. Likewise a single gene can be involved with the production of several different proteins.


Main Article: Chromosome

Each DNA molecule contains many genes. The gene is a subunit or "reading frame" of information on long strands of (DNA) called chromosomes. Humans have 46 chromosomes, which range from 1.7 to 8.5cm in length and contains about 1000 genes each. The human genome is estimated to comprise more than 30,000 genes.[1]

Human genes vary widely in length, often extending over thousands of bases, but only about 10% of the genome is known to include the protein- coding sequences (exons) of genes. Interspersed within many genes are intron sequences, which have no coding function. The balance of the genome is thought to consist of other noncoding regions (such as control sequences and intergenic regions), whose functions are obscure.[2]

Primer on Molecular Genetics by the U.S. Department of Energy.

DNA is composed of 4 different nucleotides.

These nucleotides are typically abbreviated with the letters (ATP, CTP, GTP, TTP) or simply (A,C,G,T).


Main Article: Gene Expression
Transcription translation.jpg

The gene is coded molecular information that provides the cell with instructions on how to make specific proteins. The code is specified by the sequence or linear arrangement of the 4 different nucleotides that make-up DNA. An organelle in the cell called a ribosome reads this coded instruction and translates it into a sequence of amino acids that are used to make a protein. This process is known as gene expression.

All living organisms are composed largely of proteins; humans can synthesize at least 100,000 different kinds. Proteins are large, complex molecules made up of long chains of subunits called amino acids. 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 3 bases code for 1 amino acid, the protein coded by an average- sized gene (3000 bp) will contain 1000 amino acids. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins.[2]

Examples of gene codons and the amino acid they represent:
Codon - Amino Acid
TGC = Cysteine
CTG = Leucine
AGT = Serine
GCA = Alanine

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. The cells protein- synthesizing machinery then translates the codons into a string of amino acids that will constitute the protein molecule for which it codes. In the laboratory, the mRNA molecule can be isolated and used as a template to synthesize a complementary DNA (cDNA) strand, which can then be used to locate the corresponding genes on a chromosome map. The utility of this strategy is described in the section on physical mapping.[2]


Main Article: Regulation of gene expression

Regulation of the gene expression system precisely controls the amount of a gene product that is produced and can further modify the product after it is made. This exquisite control requires multiple regulatory input points. One very efficient point occurs at transcription, such that an mRNA is produced only when a gene product is needed. Cells also regulate gene expression by post-transcriptional modification; by allowing only a subset of the mRNAs to go on to translation; or by restricting translation of specific mRNAs to only when the product is needed. At other levels, cells regulate gene expression through DNA folding, chemical modification of the nucleotide bases, and intricate "feedback mechanisms" in which some of the gene's own protein product directs the cell to cease further protein production.[3]

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  1. Molecular Biology of the Cell 3rd edition. 1994. by Bruce Alberts, et. al., Chapter 8 "The Cell Nucleus"
  2. 2.0 2.1 2.2 Primer on Molecular Genetics by the Human Genome Research Project.
  3. What is a Genome by the National Center for Biotechnology Information