Concept of the gene:

Through the year of 1960s, the gene was clearly described as the region of DNA which gives rise to a single polypeptide or to a single RNA for genes whose final product is RNA not protein, example for. ribosomal RNA genes. The existence of operons in prokaryotes did  not challenge this concept because, although various  clustered genes produced a single polycistronic  mRNA,  one could still recognized  single DNA regions as genes  based on the distinct polypeptides they encoded. The concept even  accommodated the discovery in which several  protein- coding genes in eukaryotes comprise coding regions (exons) separated through noncoding sequences (introns) since, again, only one polypeptide was encoded through this region of DNA. More recently, moreover, other mechanisms have come to light in eukaryotic  cells that can lead to a variety of polypeptides being produced  from a single DNA sequence;  for  instance  alternative RNA splicing substitute polyadenylation sites and RNA editing. Nevertheless,  in every of these cases the protein products are closely related through sequence  and all are derived from the similar single region of DNA. Therefore the original  definition perhaps requires tweaking to indicate in which a protein-coding gene is a region of DNA which encodes a single polypeptide or a group of closely- related polypeptides but otherwise the definition is intact. The substitute scenario to regard a single DNA sequence which gives rise to say 10 closely- related polypeptides  through post-transcriptional processing as representing  10 genes would certainly not fit in with accepted practice.

The genome of an organism encompasses all of the genes of in which organism. Even in a bacterial cell like as Escherichia coli (E.  Coli), the amount of DNA needed is substantial 4.6 million base pairs and so that DNA must be packaged. In the eukaryotic cell the problem is even greater. The typical human cell, for instance, contains about 1000 times more DNA than an E. coli cell. The rest of this Topic defines how DNA is packaged in both eukaryotes and prokaryotes.