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In a eukaryotic cell, a molecule of pre-mRNA is found to have four exons and three introns. Which of the following are possible combinations of the exons, if the order in which they are written is the order in which they will be translated?
I. Exon 1, Exon 2, Exon 3, Exon 4
II. Exon 1, Exon 3, Exon 4
III. Exon 4, Exon 1, Exon 2, Exon 3
This question is asking about alternative splicing. Alternative splicing is a means by which several different proteins can arise from the same pre-mRNA due to the order in which the exons are organized. This typically takes the form of exon skipping. Therefore, both 1 and 2 are potential mature mRNAs that could arise from this pre-mRNA. Option 3 is not an acceptable transcript, however, because alternative splicing maintains the integrity of the genomic order of the exons (i.e. exon 4 will not come before exon 1, 2, or 3).
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__________ are parts of __________ molecules that do not contain information about a protein's primary structure.
After transcription, the resulting RNA molecule must undergo post-transcriptional modification before it becomes mature mRNA. Before these modifications, it is known as heteronuclear RNA (htRNA) or pre-mRNA.
Introns are portions of pre-mRNA molecules that are spliced prior to translation. Unlike exons, introns do not contain information about the structure of the protein. Only after intron splicing is the molecule considered mRNA.
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The primary transcript is much longer than the mRNA that will eventually be translated. This can be explained by which of the following?
Immediately following transcription, the primary transcript will undergo a variety of changes before being translated. One of the largest changes is that a spliceosome complex will remove introns from the primary transcript. Introns are not involved in protein creation, and their removal makes the transcript much shorter. The final mRNA transcript consists of a string of exons, a 5' cap, and a 3' poly-A tail.
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In most cases, introns are spliced out of mature messenger RNA (mRNA) and are not a part of the final translated protein product of a gene. Even though they are not included in the final protein, why are introns important?
These are all reasons that introns are important, despite the fact that they are not included in final proteins. Introns can allow for alternative splicing of exons, in which exons are placed in different orders to create different proteins from one gene. In the gene Dscam in Drosophila, alternative splicing allows for around 38,000 different proteins from one gene sequence. Some introns become non-coding RNAs that control expression of genes. Lastly, it has recently been shown that introns are involved in some special functions like mRNA export - in which mRNA's are moved between the nucleus and other cellular compartments.
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In eukaryotes, which of the following is true about introns and exons?
The primary RNA contains introns and exons because it has not been processed yet, and therefore the introns have not been spliced out. Mature mRNA contains only exons, which are the coding sequences that ultimately get translated. Intron regions are non-coding and are not included in mature transcripts. Note that post-translational modifications such as splicing only occurs in eukaryotes.
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If a gene produces a pre-RNA that is 1200 basepairs long and has the following intron-exon structure:
Exon 1 - 200 bp
Intron 1 - 100 bp
Exon 2 - 50 bp
Intron 2 - 150 bp
Exon 3 - 700 bp
How many basepairs long would we expect the mRNA to be?
This question requires you to know that preRNA contains both intronic and exonic regions, but the intronsget spliced out to produce the mRNA. Therefore, you had to subtract the total intron basepairs (250) from the total length of the preRNA (1200), which gives an mRNA length of 950 basepairs.
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