Which of the following is a characteristic of RNA splicing in Eukaryotes?

A) It involves removal of introns from a gene sequence followed by transcription and subsequent splicing of exons. B) Exon/intron boundaries are typically characterized by a 5′ GU splice junction and a 3′ AG splice junction. C) After splicing occurs, the U1, U2, U5, U6 snRNP complex removes remaining exons for degradation. D) It involves recognition of sequence-specific intron/exon boundary sites by cytoplasmic proteins.

The correct answer is B) Exon/intron boundaries are typically characterized by a 5′ GU splice junction and a 3′ AG splice junction.

Diagram showing the basic steps of RNA splicing
Diagram showing the basic steps of RNA splicing (BCSteve [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)])

Protein synthesis involves two stages in eukaryotic cells, first transcription then translation. Transcription takes place in the nucleus of the cell and involves the formation of messenger RNA.

Enzymes are involved in separating the DNA double-strand so that the coding strand can act as a template for making messenger RNA. The enzyme RNA polymerase is involved in this process in which free RNA nucleotides are brought in and aligned opposite the complementary nitrogen bases on the DNA strand.

The initial RNA that is formed is precursor mRNA or pre-mRNA. This RNA strand contains both non-coding sequences of bases known as introns and coding sequences known as exons.

RNA splicing occurs at specific intron and exon locations, namely at 5’ GU and 3’ AG splice junctions, which ensures that the introns are removed and only the coding bases are left. The final strand that is formed at the end of the splicing is the mature mRNA transcript that will then pass out of the nucleus.

In the cytoplasm, the mRNA then binds to a ribosome which is the site for the translation part of polypeptide synthesis. Transfer RNA (tRNA) molecules in the cytoplasm attach to amino acids which are then carried to the ribosome to where the mRNA is.

The tRNA molecules line up by complementary base pairing with the mRNA molecules. In this way, the correct sequence of amino acids is ensured. The amino acids then bond together to form a polypeptide chain.

Transcription of the DNA code

The first important step of protein synthesis that occurs in eukaryotic cells is called transcription. This is the method in which the genetic code of the DNA is copied and a messenger RNA molecule is generated.

Several enzymes are important in the process which has various steps that occur in a specific sequence. In the first step the double helix of the DNA unwinds, and the bases between the adjacent strands separate.

The two strands of DNA separate by the activity of enzymes. There is a non-coding and a coding strand and it is the coding DNA strand that is copied.

This strand is the template for the formation of mRNA. The RNA polymerase enzyme is involved in this stage of the process. The enzyme catalyzes the reaction in which free ribonucleic acid nucleotides are brought in and lined up opposite the corresponding bases of the DNA.

The transcription begins at a sequence of bases that is called the promoter and continues until an ending sequence, a terminator is reached.

The initial mRNA transcript is known as pre-mRNA and it consists of both non-coding sequences known as introns and coding sections known as exons.

Since there are non-coding sections it means that this initial RNA strand has to be modified before the next stage of protein synthesis can take place.

RNA splicing

The non-coding sequences have to be removed or cut out of the strand in a process that is called RNA splicing. There are specific conserved cleaving sites that occur at the ends of the introns on both the 5’and 3’ end.

The splicing process usually occurs at specific junctions known as the 5′ GT and at the 3′ AG dinucleotides of the intron. Another important section is found about 40 bases up from the 3’ end and is called a branch point that always contains an adenine base.

The splicing process is catalyzed by small nuclear ribonucleoproteins (snRNPs) which act on the pre-mRNA strand which is first cleaved at the 5’ part of the intron. The snRNP that is known as U1 then attaches to the corresponding complementary sequence on the intron.

The 5’ end of the intron then pairs downstream and the 3’ end is also cut. Other snRNPs bond and bring about further splicing of the pre-mRNA transcript strand between the two ends of an intron.

This makes a loop structure and it is the combination of all the snRNPs which forms what is called a spliceosome. The intron is then cut out and the remaining exons then come together and the spliceosome falls apart and snRNPs are released.

The process of splicing has to be precise since the reading frame may be altered if there is a mistake. This would have potentially severe consequences for the later translation of the mRNA.

Translation of the code

The mature mRNA transcript leaves the nucleus via the nuclear pores and moves to a ribosome in the cytoplasm. It is at the ribosome that the translation of the code on the RNA takes place.

The tRNA molecules in the cytoplasm circulate and attach to amino acids. Each tRNA carries a triplet base on one end that codes for a specific amino acid.

An enzyme catalyzes the reaction in which the amino acid attaches to the end of the tRNA that is opposite the anticodon.

Each tRNA lines up opposite a triplet of bases (a codon) on the mRNA that is complementary to the anticodon that it is carrying.

Once all the tRNA have assembled then the amino acids they are carrying are lined up in the correct order for making a polypeptide.

Eventually, peptide bonds form between each pair of amino acids to produce the primary structure of the protein, the polypeptide chain.

References

  1. S Walker (2008). RNA Splicing: Introns, Exons and Spliceosome. Retrieved from nature.com.
  2. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. USA: Philadelphia, Saunders College Publishing.
  3. CL Will, R Lührmann (2011).  Spliceosome structure and function Cold Spring Harbor Perspectives in Biology.
  4. Y Wan, K Chatterjee (2019). RNA. Retrieved from Encyclopedia Britannica.
  5. H Lodish, A Berk, Sl Zipurksy, et al. (2000). Molecular Cell Biology. USA: New York, W.H. Freeman Publishers

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