RNA (Ribonucleic acid) Chapter-2

 

RNA Transcription

1. Overview of RNA Transcription

RNA transcription is a highly regulated process that converts genetic information stored in DNA into RNA. This process is central to cellular functions, enabling the synthesis of proteins and other RNA molecules.

2. Key Components

• DNA Template: The DNA template strand used for transcription is read in the 3' to 5' direction. The RNA polymerase synthesizes RNA in the 5' to 3' direction, meaning that RNA is complementary to the DNA template strand.

• RNA Polymerase: This enzyme has multiple subunits and varies between prokaryotes and eukaryotes:

  • Prokaryotes: RNA polymerase is a multi-subunit complex with a core enzyme made up of two alpha subunits, one beta subunit, one beta-prime subunit, and a sigma factor that binds to the promoter.
  • Eukaryotes: RNA polymerase II, the enzyme responsible for mRNA synthesis, has 12 subunits and requires additional general transcription factors (GTFs) for promoter recognition and initiation.

• Promoter: In prokaryotes, promoters typically have conserved sequences known as the -10 (Pribnow box) and -35 regions. In eukaryotes, the core promoter often includes a TATA box, located approximately 25-30 base pairs upstream of the transcription start site (TSS).

• Terminators:

  • Prokaryotes: Two main types: rho-independent (intrinsic) and rho-dependent. Rho-independent terminators form a hairpin loop followed by a poly-U tract in the RNA that destabilizes the RNA-DNA hybrid.
  • Eukaryotes: Termination often involves cleavage and polyadenylation, where specific sequences (such as the poly-A signal) and associated factors lead to the cleavage of the pre-mRNA and the addition of the poly-A tail.

3. Transcription Process

a. Initiation

1. Promoter Binding:

   • Prokaryotes: Sigma factors recognize specific promoter sequences and facilitate the binding of RNA polymerase.
   • Eukaryotes: General transcription factors (TFIID, TFIIA, TFIIB, etc.) bind to the promoter region and recruit RNA polymerase II.

2. Formation of the Transcription Initiation Complex:

   • Prokaryotes: The RNA polymerase-sigma complex forms a closed complex with DNA, then transitions to an open complex where the DNA is unwound.
   • Eukaryotes: The pre-initiation complex (PIC) forms, including TFIID (which binds the TATA box), and RNA polymerase II is recruited. This process involves the formation of the transcription bubble.

3. DNA Unwinding: The DNA double helix is locally unwound by RNA polymerase and helicase activity, creating a transcription bubble where RNA synthesis occurs.

b. Elongation

1. RNA Synthesis: RNA polymerase synthesizes RNA by adding ribonucleotides complementary to the DNA template strand. The enzyme maintains a transcription bubble, where the RNA-DNA hybrid is about 8-9 base pairs long.

2. Proofreading: RNA polymerase has a rudimentary proofreading mechanism known as pyrophosphorolysis, where misincorporated nucleotides are removed by backtracking and cleaving the RNA.

Reference: https://www.researchgate.net/figure/Transcription-is-a-highly-complex-process-subject-to-numerous-modes-of-regulation-A-A_fig1_315958971

c. Termination

1. Termination Signal:

• Prokaryotes: Rho-dependent termination involves the rho factor protein, which binds to the RNA and moves towards the RNA polymerase, facilitating its release.
• Eukaryotes: The termination process involves cleavage of the pre-mRNA at a specific site and subsequent addition of the poly-A tail. The cleavage is mediated by endonucleases, and the poly-A tail is added by poly-A polymerase.

Reference:https://www.pinterest.com/pin/531002612290782860/

1. RNA Release: After termination, RNA polymerase releases the RNA transcript and dissociates from the DNA.

2. DNA Reformation: DNA re-anneals to restore the double helix structure, and the transcription bubble resolves.

4. Post-Transcriptional Modifications (in Eukaryotes)

1. Capping:

   • 7-Methylguanylate (7mG) Cap: Added to the 5' end of the RNA molecule. The cap is essential for RNA stability, splicing, and translation. The addition involves a 5'-5' triphosphate bridge.

2. Polyadenylation:

   • Poly-A Tail: Added to the 3' end of the RNA after cleavage. This modification increases mRNA stability and facilitates export from the nucleus. The tail is added by poly-A polymerase.

3. Splicing:

• Spliceosome: A complex of small nuclear RNAs (snRNAs) and proteins that carry out splicing. Splicing involves the removal of introns and the joining of exons through a two-step transesterification reaction.
• Alternative Splicing: Allows for the production of multiple protein isoforms from a single gene by differentially including or excluding exons.

Reference: https://www.toppr.com/ask/question/describe-post-transcriptional-processing-of-rna-in-eukaryotes/

5. Types of RNA Produced

• mRNA (Messenger RNA): Encodes the instructions for protein synthesis. Eukaryotic mRNAs have a 5' cap, coding sequence, and a poly-A tail. Prokaryotic mRNAs often lack a poly-A tail.

• rRNA (Ribosomal RNA): Synthesized in the nucleolus and incorporated into ribosomes. rRNA is critical for ribosome structure and function. In eukaryotes, it includes 18S, 5.8S, 28S rRNAs, while prokaryotes have 16S and 23S rRNAs.

• tRNA (Transfer RNA): Adaptor molecules that bring amino acids to the ribosome. Each tRNA has an anticodon that pairs with an mRNA codon and a corresponding amino acid attachment site.

• Other Non-Coding RNAs: Includes microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) involved in gene regulation, chromatin modification, and other cellular processes. miRNAs regulate gene expression by binding to complementary mRNA sequences, while lncRNAs can modulate gene expression through various mechanisms, including chromatin remodeling.

6. Regulation of Transcription

• Transcription Factors: Proteins that regulate transcription by binding to specific DNA sequences. They can be activators (e.g., CREB, which binds to cAMP response elements) or repressors (e.g., REST, which represses neuronal gene expression in non-neuronal tissues).

• Epigenetic Modifications:

  • DNA Methylation: Addition of methyl groups to cytosine residues in DNA, typically leading to gene silencing.
  • Histone Modifications: Post-translational modifications to histone proteins, including acetylation, methylation, and phosphorylation, that affect chromatin structure and gene accessibility.

• Enhancers and Silencers:

  • Enhancers: DNA elements that can be located far from the gene they regulate. They increase transcriptional activity by interacting with the promoter through DNA looping.
  • Silencers: DNA elements that decrease transcriptional activity by binding repressor proteins and inhibiting transcription.

7. Significance

RNA transcription is essential for cellular function and organismal development. It allows for the dynamic regulation of gene expression in response to environmental cues and internal signals. Dysregulation of transcription can lead to diseases such as cancer, genetic disorders, and developmental abnormalities. Understanding the intricacies of transcription helps in the development of therapeutic strategies and provides insights into fundamental biological processes

Please, write your suggetions in comment section.

Follow our blogs for more updates and protocols: https://iammolecularbiologist.blogspot.com/  

Follow us on WhatsApp: https://chat.whatsapp.com/I30OdffWlJhGABXsTmKo95