Eukaryotes share many aspects of protein synthesis with prokaryotes. They show similarities in the operation of the ribosomes, the functions of start and stop codons, and the mass production of proteins by means of polyribosomes. But they do differ in significant ways. The start codon in eukaryotes is also AUG, but it codes for an alternative form of methionine. Another difference is that eukaryotic mRNAs code for just one protein, unlike bacterial mRNAs, which often contain information from several genes in series.
Prokaryotic and eukaryotic cells also differ in location and structure of genes. The presence of DNA in the nucleus of eukaryotic cells means that eukaryotic transcription and translation cannot be simultaneous as it is in prokaryotes. The mRNA transcript must pass through pores in the nuclear membrane and be carried to the ribosomes in the cytoplasm or on the endoplasmic reticulum for translation.
So far we have introduced a simplified definition of a gene that fits well for the majority of prokaryotes. The pattern of their genes is colinear, meaning that their message can be directly read into proteins without any additional processing. Eukaryotic genes differ significantly in this characteristic because they are not colin ear. Located within their genes are one to several intragenic regions, called introns (intervening regions), that do not code for protein. These introns are interspersed between coding regions, called exons (expressed regions), that will be translated into protein (figure 1). To illustrate this we can use words as examples. A short section of colinear prokaryotic gene might read: SAM SAW HIS NEW CAR GET HIT; a eukaryotic gene that codes for the same portion might read SAM SAW SVXF FPL HIS NEW CAR QZWVP GET HIT. The recognizable words are the exons, while the other letters represent the introns.
Fig1. The split gene of eukaryotes. Eukaryotic genes have an additional complication in their transcription/translation. Their expressed sequences, or exons (E), are interrupted at intervals by segments called introns (I) that do not code for proteins. Introns are transcribed but not translated. They are removed by RNA splicing enzymes before translation begins.
This discontinuous genetic structure, also called a split gene, requires further processing before translation. Transcription of the entire gene with both exons and introns occurs first, producing a pre-mRNA. Next, an RNA-protein complex called a spliceosome recognizes the exon-intron junctions and enzymatically cuts through them in a process called RNA splicing. The action of this splicer enzyme loops the introns into lariat-shaped pieces, excises them, and joins the exons end to end. By this means, a strand of mRNA with no introns is produced. This completed mRNA strand can then proceed to the cytoplasm to be translated.
Several different types of introns have been discovered, some of which contain complete genes within their sequence. For example, certain introns have been found to code for an enzyme called reverse transcriptase, which can convert RNA into DNA. Other introns are translated into endonucleases, enzymes that can snip DNA and allow insertions and deletions into the DNA sequence. Some introns have an innate catalytic function and can splice themselves out of an RNA transcript. A great deal of non-protein-coding DNA is proving to be vital for cell function. Human DNA has an average of eight introns per gene, which represents a significant percentage of the DNA found in chromosomes. By selecting only some exons, but not others, for inclusion into the final mRNA, a process called alternative splicing, different versions of a gene may be created. It is likely that future research will discover additional roles of introns in regulating gene expression.
