 You know the coding sequence of a gene is a series of three codons of nucleotides and these series of codons they specify the sequence of amino acids in its polypeptide product. So the sequence of a gene which is in the form of codons that is translated into the sequence of amino acids. It is generally assumed that the coding sequence is contiguous that is the codon for one amino acid is immediately adjacent to the codon for the next amino acid and in this way the the codons of all the amino acids they are adjacent to each other without any space in between. This is true in the vast majority of cases in bacteria and their phages but it is rarely so for eukaryotic genes. So in prokaryotic organisms such as bacteria and phages the sequence is contiguous that is all the codons are adjacent to each other but in eukaryotes it is different. In those cases the coding sequence is interrupted by stretches of known coding sequences so the full sequence of gene that is interrupted by some known coding sequences. Many eukaryotic genes are thus mosaics consisting of blocks of coding sequences separated from each other by the blocks of known coding sequences. So if we see a gene eukaryotic gene it will consist of a small sequence of coding sequence and then a small or longer known coding sequence then coding sequence then maybe a long known coding sequence so in this way a eukaryotic gene is made up of. The coding sequences are called axons and the known coding or intervening sequences are called introns. So a gene is composed of axons and introns. Once transcribed into an RNA transcript the introns must be removed and the axons must be joined together to create the correct mRNA of that gene. Means correct means the mRNA with correct amino acid sequence in the protein. So this is you can see the structure of eukaryotic gene so in this gene this is whole gene and this is strand of DNA after transcription this is primary transcript or it is also called pre-mRNA and you can see this contains this green portion this is axon and then this yellow portion this is intron then this is axon 2 then intron 2 then axon 3 so this small stretch contains different axons and introns and after splicing it becomes in this way and this long stretch is only composed of axons there is no intron and then this is transcribed in a protein. This is the structure of gene of eukaryotes. The number of introns found within a gene varies enormously from one in the case of most yeast genes and few human genes to 50 in the case of chicken pro alpha 2 collagen gene and to as many as 363 in the case of titan gene of humans. The sizes of the egg zone and introns they also vary indeed introns are very often much longer than the axons. Thus for example axons are typically on the order of 150 nucleotides whereas introns although they too can be short maybe 150 or even shorter than 150 but they can be as long as 800 000 nucleotides or 800 kb. As another example the mammalian gene for the enzyme dihydrofolate reductase is more than 31 kb long and within it are dispersed six axons that corresponds to only 2 kb of the MRNA. So the gene is 31 kb and the actual MRNA is only 2 kb. So in this case the coding portion of the gene is 10% or even less of its total length. Like the uninterrupted genes of prokaryotes the split genes of eukaryotes are transcribed into a single RNA copy of the entire gene. The primary transcript that contains introns as well as axons because the length and number of introns in the primary transcript can be very long indeed. As already mentioned the primary transcript of intron containing genes must have their introns removed before they can be translated into proteins. So the introns must be removed from the pre MRNA. The process of intron removal is called RNA splicing. It converts the pre MRNA into mature MRNA that only contains axons. RNA splicing must occur with great precision to avoid the loss or addition of even a single nucleotide at the sites at which two axons join. So at this joining point there must not be even loss of a single nucleotide. The triplet nucleotide codons of MRNA are translated in a fixed reading frame that is set by first codon. Why the removal of even a single codon is that important because all the codons are set in a reading frame and that frame starts from the first codon. And if the one nucleotide is missed at any point this will disturb all the reading frame. Some pre MRNAs can be spliced in more than one way. Thus MRNAs containing different selections of axons can be generated from given pre MRNA. Alternative splicing strategy enables a gene to give rise to more than one polypeptide product. These alternative products are called isoforms. It is estimated that 90% or more of the protein-coding genes in the human genome are spliced in alternative ways to generate more than one isoforms.