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MCAT Content / Control Of Gene Expression In Eukaryotes / Post Transcriptional Control Basic Concept Of Splicing Introns Exons

Post-transcriptional control, basic concept of splicing (introns, exons)

Topic: Control Of Gene Expression In Eukaryotes

The transcription produces pre-mRNA that is processed to mRNA. It is processed by three steps that are the addition of a 5′ cap, addition of a 3′ tail and RNA splicing. RNA splicing is the removal of the introns and the addition of the extrons to each other.

In eukaryotic transcription, a pre-mRNA is formed which is processed and converted into a messenger RNA or mRNA. At 5″ end of the pre mRNA, a modified guanine nucleotide is added which is known as a cap. It protects the mRNA and also helps in the attachment of the ribosomes to the mRNA while translation. At 3′ end of the pre mRNA, 100 to 200 adenine nucleotides are added known as poly-A tail. This tail provides stability to the mRNA.

Other than this, RNA splicing is also an important part of mRNA processing. During this, unnecessary or junk sequences of pre mRNA known as introns are removed. This is done by a protein-RNA complex called the spliceosome, which recognizes specific splice donor and acceptor sites in the intron that signal a splice site. The remaining parts that are not removed are known as exons and these are joined to each other by the spliceosome. These result in the formation of final mRNA. Alternative splicing is a process that occurs during gene expression and allows for the production of multiple proteins (protein isoforms) from a single gene coding. Alternative splicing can occur due to the different ways in which an exon can be excluded from or included in the messenger RNA. It can also occur if portions on an exon are excluded/included or if there is an inclusion of introns. For example, if a pre-mRNA has four exons (A, B, C, and D), these can be spliced and translated in a number of different combinations. Exons A, B, and C can be translated together or Exons A, C, and D can be translated. This results in what is called alternative splicing.

Practice Questions


Khan Academy

Reverse transcriptase polymerase chain reaction (RT-PCR) of a UV-dependent gene


MCAT Official Prep (AAMC)

Practice Exam 1 B/B Section Passage 1 Question 5

Section Bank B/B Section Question 31

Section Bank B/B Section Passage 6 Question 43

Section Bank B/B Section Passage 9 Question 68

Sample Test B/B Section Passage 3 Question 13


Key Points

• During the processing of eukaryotic pre mRNA, a modified guanine residue is added as a cap at 5′ end of pre mRNA. It protects mRNA and helps in its binding to ribosomes for translation.

• At 5′ end, a poly-A tail made of 100 to 200 adenine residues is attached that provides strength.

• Introns are intervening sequences within a pre-mRNA molecule that do not code for proteins and are removed during RNA processing by a spliceosome.

• Exons are expressing sequences within a pre-mRNA molecule that are spliced together once introns are removed to form mature mRNA molecules that are translated into proteins.

• Alternative splicing allows for the production of various protein isoforms from one single gene coding.

Key Terms

Intron: a portion of a split gene that is included in pre-RNA transcripts but is removed during RNA processing and rapidly degraded

Exon: a region of a transcribed gene present in the final functional RNA molecule

Spliceosome: a dynamic complex of RNA and protein subunits that removes introns from precursor mRNA

Splice donor and acceptor sites: specific sequences in introns that signal to location of splice sites

Eukaryotic: organisms whose cells have a nucleus enclosed within membranes

5‘ end: the end of DNA with the terminal phosphate group on the 5′ carbon of the deoxyribose

Ribosomes: organelles which produce proteins from mRNA

3‘ end: a terminal hydroxyl (OH) group on the deoxyribose of the 3′ carbon of the deoxyribose

Poly(A) tail: added to an RNA at the end of transcription protects the mRNA molecule from enzymatic degradation

Splicing: the removal of the introns and the addition of the extrons to each other

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