During RNA processing, noncoding sequences called are snipped out and the remaining coding segments are spliced together to make a mature messenger RNA.
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Answer:
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Explanation:
For most eukaryotic genes (and some prokaryotic ones), the initial RNA that is transcribed from a gene's DNA template must be processed before it becomes a mature messenger RNA (mRNA) that can direct the synthesis of protein. One of the steps in this processing, called RNA splicing, involves the removal or "splicing out" of certain sequences referred to as intervening sequences, or introns. The final mRNA thus consists of the remaining sequences, called exons, which are connected to one another through the splicing process. RNA splicing was initially discovered in the 1970s, overturning years of thought in the field of gene expressionGene regulation was first studied most thoroughly in relatively simple bacterial systems. Most bacterial RNA transcripts do not undergo splicing; these transcripts are said to be colinear, with DNA directly encoding them. In other words, there is a one-to-one correspondence of bases between the gene and the mRNA transcribed from the gene (excepting 5′ and 3′ noncoding regions). However, in 1977, several groups of researchers who were working with adenoviruses that infect and replicate in mammalian cells obtained some surprising results. These scientists identified a series of RNA molecules that they termed "mosaics," each of which contained sequences from noncontiguous sites in the viral genome (Berget et al., 1977; Chow et al., 1977). These mosaics were found late in viral infection. Studies of early infection revealed long primary RNA transcripts that contained all of the sequences from the late RNAs, as well as what came to be called the intervening sequences (introns).
Subsequent to the adenoviral discovery, introns were found in many other viral and eukaryotic genes, including those for hemoglobin and immunoglobulin (Darnell, 1978). Splicing of RNA transcripts was then observed in several in vitro systems derived from eukaryotic cells, including removal of introns from transfer RNA in yeast cell-free extracts (Knapp et al., 1978). These observations solidified the hypothesis that splicing of large initial transcripts did, in fact, yield the mature mRNA. Other hypotheses proposed that the DNA template in some way looped or assumed a secondary structure that allowed transcription from noncontiguous regions (Darnell, 1978).The biochemical mechanism by which splicing occurs has been studied in a number of systems and is now fairly well characterized. Introns are removed from primary transcripts by cleavage at conserved sequences called splice sites. These sites are found at the 5′ and 3′ ends of introns. Most commonly, the RNA sequence that is removed begins with the dinucleotide GU at its 5′ end, and ends with AG at its 3′ end. These consensus sequences are known to be critical, because changing one of the conserved nucleotides results in inhibition of splicing. Another important sequence occurs at what is called the branch point, located anywhere from 18 to 40 nucleotides upstream from the 3′ end of an intron. The branch point always contains an adenine, but it is otherwise loosely conserved. A typical sequence is YNYYRAY, where Y indicates a pyrimidine, N denotes any nucleotide, R denotes any purine, and A denotes adenine. Rarely, alternate splice site sequences are found that begin with the dinucleotide AU and end with AC; these are spliced through a similar mechanism.
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When an RNA transcript is first made in a eukaryotic cell, it is considered a pre-mRNA and must be processed into a messenger RNA (mRNA).
A 5' cap is added to the beginning of the RNA transcript, and a 3' poly-A tail is added to the end.
In splicing, some sections of the RNA transcript (introns) are removed, and the remaining sections (exons) are stuck back together.
Some genes can be alternatively spliced, leading to the production of different mature mRNA molecules from the same initial transcript.
Introduction
Imagine that you run a book-making factory, and you've just printed up all the pages of your favorite book. Now that you have the pages, is the book ready to go? Well...books usually have front and back covers. So you might want to put those on. Also, were there any blank or messed-up pages made during printing? You should probably check for those and remove them before selling your books, or you might end up with some unhappy customers.