Genes

Ageneis the molecular unit of heredityof a living organism. It is used extensively by the scientific community as a name given to some stretches of deoxyribonucleic acids(DNA) and ribonucleic acids(RNA) that code for a polypeptideor for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains. Genes hold the information to build and maintain an organism's cellsand pass genetic traitsto offspring. All organisms have genes corresponding to various biological traits, some of which are instantly visible, such as eye coloror number of limbs, and some of which are not, such as blood type, increased risk for specific diseases, or the thousands of basic biochemicalprocesses that comprise life. The wordgeneis derived from the Greekwordgenesismeaning "birth", orgenosmeaning "origin" (see pangenesis). The chemical structure of a four-base fragment of a DNA double helix. A modern working definition of a gene is "a locatable regionof genomicsequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions, and or other functional sequence regions". [ 1 ] [ 2 ]Colloquial usage of the termgene(e.g. "good genes", "hair color gene") may actually refer to an allele: ageneis the basic instruction— a sequence of nucleic acids (DNA or, in the case of certain virusesRNA), while analleleis one variant of that gene. Thus, when the mainstream press refers to "having" a "gene" for a specific trait, this is customarily inaccurate. In most cases, all people would have a gene for the trait in question, although certain people will have a specific allele of that gene, which results in the trait variant. Further, genes code for proteins, which might result in identifiable traits, but it is the gene ( genotype), not the trait ( phenotype), which is inherited. Big genes are a class of genes whose nuclear transcriptspans 500 kb (1kb = 1,000 base pairs) or more of chromosomal DNA. The largest of the big genes is the gene for dystrophin, which spans 2.3 Mb. Many big genes have modestly sized mRNAs; the exons encodingthese RNAstypically encompass about 1% of the total chromosomal gene region in which they occur.