Q.What is meant by capping and tailing?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Transcription
Transcription copies the sequence of one DNA strand, the template (non-coding/antisense) strand, into a complementary RNA molecule, catalysed by RNA polymerases, while the other DNA strand, matching the RNA in sequence, is the coding (non-template/sense) strand. Eukaryotes use three RNA polymerases: RNA Pol I makes the large rRNAs (except 5S), RNA Pol II makes the mRNA precursor (hnRNA), and RNA Pol III makes tRNA, 5S rRNA and snRNAs; because each eukaryotic mRNA carries only one gene's coding information, eukaryotic mRNAs are monocistronic. Synthesis always runs 5' to 3', reading a 3'-to-5' template, starting at a promoter that carries regulatory elements (enhancers, which increase transcription, or silencers, which decrease it, bound by transcription factors that guide RNA polymerase since it cannot bind DNA on its own), a TATA/Hogness box about 25 bp upstream of the start site, and the transcription start site itself; a mediator complex relays signals between distant regulatory transcription factors and RNA Pol II; transcri …
Capping adds a modified guanosine cap to the 5' end of the primary transcript; tailing adds a poly(A) tail to its 3' end -- both pre-mRNA processing steps. …
Step 1. Capping modifies the 5' end of the primary transcript (hnRNA) by adding a methylguanosine triphosphate group, protecting the RNA from degradation and assisting removal of the first intron.
Step 2. Tailing (polyadenylation) modifies the 3' end: an endonuclease cleaves the 3' end, then a string of adenine nucleotides (the poly(A) tail) is added. …
Define capping (5' end) and tailing (3' end) separately, noting their chemical …
- CBSE 2025Set ANNUAL1 markMCQQ.When 'hairpin loop' is formed?(a) At the end of RNA synthesis(b) At the initiation phase of RNA synthesis(c) At the end of DNA synthesis(d) At the end of protein synthesis
›Reveal solutionSolution
A hairpin loop forms in the nascent RNA at the termination stage of transcription (Rho-independent termination).
In prokaryotic transcription termination (the Rho-independent/intrinsic mechanism), the DNA template near the end of a gene has an inverted-repeat, GC-rich sequence followed by a stretch of A residues. As RNA polymerase transcribes this region, the newly made RNA (which is complementary, GC-rich and self-complementary) folds back on itself to form a stable hairpin (stem-loop) structure, immediately followed by a run of U residues. This hairpin causes RNA polymerase to pause and destabilises the RN …
- CBSE 2024Set ANNUAL1 markMCQQ.In which phase of Transcription the Rho-protein has some contribution?(a) Initiation phase(b) Extension phase(c) Termination phase(d) None of these
›Reveal solutionSolution
Rho protein mediates Rho-dependent termination, ending transcription.
Transcription termination in bacteria can be Rho-dependent or Rho-independent (intrinsic). In Rho-dependent termination:
- The Rho (p) protein, an ATP-dependent helicase, binds to a specific sequence (rut site) on the newly synthesised mRNA.
- It translocates along the RNA (using energy from ATP hydrolysis) and catches up with RNA polymerase paused at a terminator sequence. …
- CBSE 2024Set ANNUAL1 markMCQQ.A mRNA molecule is produced by :(a) Duplication(b) Replication(c) Translation(d) Transcription
›Reveal solutionSolution
mRNA is produced by transcription -- copying genetic information from a DNA template strand into a complementary RNA strand.
In the central dogma of molecular biology, transcription is the process by which RNA polymerase reads a DNA template strand (3'->5') and synthesises a complementary, antiparallel mRNA strand (5'->3'), carrying the same sequence as the coding/sense strand of DNA (with U instead of T). Translation is a distinct, later process, in which ribosomes read the mRNA codons to assemble a polypeptide. Replication is …
- CBSE 2023Set ANNUAL1 markMCQQ.Which one of the following represents the flow of genetic information ?(a) RNA -> Protein -> DNA(b) DNA -> RNA -> Protein(c) Protein -> RNA -> DNA(d) RNA -> DNA -> Protein
›Reveal solutionSolution
The central dogma states that genetic information normally flows from DNA to RNA to protein.
Working
Proposed by Francis Crick, the central dogma of molecular biology describes the standard directional flow of genetic information within a cell:
- DNA -> RNA (Transcription): the genetic information encoded in a gene's DNA sequence is copied into a complementary messenger RNA (mRNA) molecule by the enzyme RNA polymerase.
- RNA -> Protein (Translation): the mRNA sequence is read by ribosomes (with the help of tRNA) in sets of three nucleotides (codons), and each codon specifies a particular amino acid, which are joined together to build a polypeptide/protein. …
- CBSE 2018Set ANNUAL1 markMCQQ.mRNA is about __________ of the RNA content of the cell.(a) 3 - 5%(b) 10 - 20%(c) 20 - 30%(d) 5 - 10%
›Reveal solutionSolution
mRNA constitutes about 5-10% of a cell's total RNA content.
A cell contains three principal classes of RNA involved in protein synthesis, present in very different proportions. Ribosomal RNA (rRNA) is by far the most abundant, forming roughly 80% of total cellular RNA because it is a structural and catalytic component of the numerous ribosomes in the cell. Transfer RNA (tRNA) makes up most of the remainder, around 10-15%, since many small tRNA molecules are needed to deliver amino acids during translation. Messenger RNA (mRNA), despite being the direct carrier of genetic information from DNA to the ribosome, is present in the smallest amount, only about 5-10% …
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