<P> The genome is the total genetic material of an organism and includes both the genes and non-coding sequences . </P> <P> The genome size, and the number of genes it encodes varies widely between organisms . The smallest genomes occur in viruses, and viroids (which act as a single non-coding RNA gene). Conversely, plants can have extremely large genomes, with rice containing> 46,000 protein - coding genes . The total number of protein - coding genes (the Earth's proteome) is estimated to be 5 million sequences . </P> <P> Although the number of base - pairs of DNA in the human genome has been known since the 1960s, the estimated number of genes has changed over time as definitions of genes, and methods of detecting them have been refined . Initial theoretical predictions of the number of human genes were as high as 2,000,000 . Early experimental measures indicated there to be 50,000--100,000 transcribed genes (expressed sequence tags). Subsequently, the sequencing in the Human Genome Project indicated that many of these transcripts were alternative variants of the same genes, and the total number of protein - coding genes was revised down to ~ 20,000 with 13 genes encoded on the mitochondrial genome . With the GENCODE annotation project, that estimate has continued to fall to 19,000 . Of the human genome, only 1--2% consists of protein - coding genes, with the remainder being' noncoding' DNA such as introns, retrotransposons, and noncoding RNAs . Every multicellular organism has all its genes in each cell of its body but not every gene functions in every cell . </P> <P> Essential genes are the set of genes thought to be critical for an organism's survival . This definition assumes the abundant availability of all relevant nutrients and the absence of environmental stress . Only a small portion of an organism's genes are essential . In bacteria, an estimated 250--400 genes are essential for Escherichia coli and Bacillus subtilis, which is less than 10% of their genes . Half of these genes are orthologs in both organisms and are largely involved in protein synthesis . In the budding yeast Saccharomyces cerevisiae the number of essential genes is slightly higher, at 1000 genes (~ 20% of their genes). Although the number is more difficult to measure in higher eukaryotes, mice and humans are estimated to have around 2000 essential genes (~ 10% of their genes). The synthetic organism, Syn 3, has a minimal genome of 473 essential genes and quasi-essential genes (necessary for fast growth), although 149 have unknown function . </P>

What are genes what is their chemical nature give their functions