ABSTRACT
A genome is the entirety of the genetic material of an organism that is passed along from generation to generation. A genome is composed of thousands to tens of thousands of genes, which are the hereditary units of traits or phenotypes. Genomes are organized in physical units called chromosomes. Chromosomes contain genes at specific locations (called locus). Often, individuals of a population carry minor sequence differences called allelic variants or alleles. Alleles contribute to genetic variability in a population of like
individuals and differing traits like hair color, blood type, and susceptibility for diseases. Genes specify nucleotide sequences of chromosomal DNA that code for proteins and structural RNAs. Genes are not the only information contained in chromosomes. In fact, various amount of noncoding sequence information is found in genomes ranging from a few percent in viruses and bacteria to a whopping 98% in humans. Some of these noncoding sequences appear to be degenerated into long stretches of repeat sequences, which have been labeled junk DNA in the 1970s. Even genes contain noncoding sequences that include regulatory sequences and introns (intervening sequences; 50 to 20,000 nucleotides) fragmenting the protein coding sequences of genes into discontinuous stretches known as exons. While exons often correlate with the coding of functional protein domains, the function of intervening sequences is largely unknown, although they may control alternative splicing and allow exon shuffling, thus contributing to the complexity and evolution of the proteome. Modern genome projects will contribute to a better understanding of their biological roles.
