?Alternative splicing

Let’s imagine that in our (Polypedilum vanderplanki or any other eukaryotic) organism, all proteins are workers; there are builders, drivers, controllers, doctors etc. Each type of worker has some specific for its work skills, knowledge and abilities, which are coded in a corresponding gene. 

However, any gene contains blocks with very important information/skills for the protein (exons) and blocks that contain additional skills and regulatory patterns for successful protein synthesis (introns). For example, Protein X will be a doctor, so information in exons provides stress-resistance qualities, anatomy knowledge and ability to make fast decisions, while introns help to select the most important exons and store some extra knowledge, for instance, how to make origami or how to not sleep 3 days in a row. Obviously, for being a great doctor, knowledge of anatomy and stress resistance is more vital. This is the moment when splicing comes in handy. Splicing aims to keep the most important blocks (in general exons) from the gene’s copy to provide the best functionality of the protein, so the worker can complete its duties. 

Just keeping the most significant blocks sounds like a good strategy, but the environment is always changing and our Protein X may need some structural/functional modifications to better survival and functioning. Creating and storing one gene for each protein variation is inconvenient, so evolution created alternative splicing. Depending on the situation, proteins with different skill sets (exon & intron combinations) can be created from the same single gene template. Genes become “adaptable” to their environment. 

Imagine 3 doctors: a general practitioner, a dentist, and a gastroenterologist. All of them know human anatomy and are able to make fast decisions, but only gastroenterologists know how to do a colonoscopy. When some variation of the protein (gastroenterologist) has gained or lost some important block with information (knowledge of how to do colonoscopy) the event is called a ‘cassette exon’. But nothing is perfect and sometimes our Protein X can obtain extra helpful or extra useless or extra distracting additional skills which are usually stored in introns. For example, the ability to work several days without sleep, good karaoke skills or bad memory. These alternative splicing events are named ‘intron retention’, or  ‘alternative acceptor splice site’ and ‘alternative donor splice site’ if only part of an intron was included. 

P.S. Those are the major but not the only possible splicing events that genes use for “adapting” to the environment.