what is Gene?

 Gene in the light of present knowledge. So this is the simple definition of gene. Gene as you all know, it is a segment of DNA that codes for a functional protein or functional RNA like tRNA, r RNA or ribozymes that influences phenotype. So this is a simplest definition of gene at present. Now this is a technical definition of gene or exact definition of gene. Gene is a unit of heredity that is located on chromosomes, made up of DNA and regulatory elements, that encodes a functional protein or family of proteins or a functional RNA molecule that influences phenotype.

How we reached this definition? 

As we all know according to Central Dogma of Biology, DNA is transcribed to mRNA, the process of forming RNA from DNA is called as transcription and this information is used to convert this RNA to protein by the protein synthesizing machinery, the ribosome. So according to Central Dogma the information on DNA is transcribed to mRNA and later that is translated to protein. So the definition of gene is a segment of DNA that codes for a functional protein. So after translation this protein undergoes extensive folding and forming a three-dimensional active structure, a functional protein or a polypeptide and this was a first definition of gene. Now we know that gene is not only a DNA segment, this is the eukaryotic gene structure as you can see there are many control elements like enhancers there is a promoter and these regions are not transcribed and within the gene there is intron or non-coding sequences that is not transcribed and that is spliced during RNA processing. 

So gene is a segment of DNA along with many regulatory elements that actually controls the expression of these coding segments. Now we have a slight change in the definition that "Gene is a segment of DNA along with many regulatory elements and non-coding sequences that codes for a functional protein" Gene codes for functional RNA also. As we all know there is r RNA, ribosomal RNA which is not translated, RNA itself is functional whereas tRNA that is involved in translation act as an adaptor molecule. r RNA is a component of ribosome, the protein synthesizing machinery. Then there was an important discovery by Thomas Cech, that is ribozymes or RNAs with catalytic activity. Ribozymes are RNAs with catalytic activity just like proteins. 

Diagram:

Classical examples include spliceosomes in RNA processing, that is the removal of introns during RNA processing, mRNA processing, that introns are removed and exons or coding sequences are joined that is carried out by a ribozyme which is called as spliceosome. Then there is peptidyl transferase, the enzyme that is involved in peptide bond formation during translation between amino acids. Then there is telomerase is involved in telomere replication. So a number of ribozymes are now discovered showing the capacity of RNA to work as enzymes. Now the definition has completely changed so the present definition is "Gene is a unit of heredity, located on chromosomes, made up of DNA and regulatory elements like promoters enhancers etc, that is mixed with non-coding sequences like introns and also junk DNA that is seen between genes that encodes a functional protein or family of proteins or a functional RNA molecule that determines or influences a phenotype". So this is the present definition of gene. 

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