Transcription and RNA Processing in Genetics

Transcription: Overview and Mechanism

Definition and Process

Transcription is the process by which DNA is used as a template to synthesize RNA. This is a fundamental step in gene expression, allowing genetic information to be converted into functional products.

• RNA polymerase is the enzyme responsible for transcribing DNA into RNA.

• There are multiple types of RNA polymerases, each with specific roles.

• RNA transcripts are synthesized in the 5' to 3' direction by adding nucleoside triphosphates (NTPs) onto the 3' OH group.

• Each gene is transcribed using only one strand of the double-stranded DNA, known as the template strand.

• Upstream refers to regions of DNA before the gene start site; downstream refers to regions after the gene start site.

• Transcription consists of three main stages: initiation, elongation, and termination. These stages differ slightly between prokaryotes and eukaryotes.

Example: The coding strand and template strand are shown in diagrams, with RNA polymerase synthesizing the RNA strand complementary to the template strand.

Prokaryotic Transcription

Initiation, Elongation, and Termination

Prokaryotic transcription differs from eukaryotic transcription in several key aspects, particularly in promoter structure and termination mechanisms.

• Promoters are DNA sequences that signal the start of transcription. Prokaryotic promoters contain consensus sequences such as the Pribnow box (TATAAT at -10 bp upstream) and a -35 base pair consensus sequence.

• The RNA polymerase holoenzyme binds to the promoter, consisting of a sigma factor and a core enzyme. The sigma factor controls the specificity of RNA polymerase binding.

• During elongation, a transcription bubble of ~18 nucleotides of unwound DNA is formed.

• Termination occurs when RNA polymerase reaches a specific sequence:

  • Rho-dependent terminators require the rho protein to terminate transcription.

  • Rho-independent terminators function without the rho protein, often involving hairpin structures in the RNA.

  • Intrinsic terminators are found in uracil-rich RNA transcripts.

• In prokaryotes, a single terminator may be present at the end of a group of genes, resulting in polycistronic RNA that must be processed into individual genes.

Example: Diagrams illustrate initiation, elongation, and termination, showing the role of the sigma factor and the formation of polycistronic RNA.

Eukaryotic Transcription

Complexity and Regulation

Eukaryotic transcription is more complex than prokaryotic transcription, involving multiple RNA polymerases and regulatory factors.

• Diverse RNA polymerases transcribe different types of RNA:

RNA Polymerase	Transcribes
RNA polymerase I	Ribosomal RNA (rRNA)
RNA polymerase II	Messenger RNA (mRNA)
RNA polymerase III	Transfer RNA (tRNA)

• Transcription initiation requires general transcription factors (GTFs) such as TFIIA, TFIIB, TFIID, and TFIIH.

• The promoter region contains a TATA-box (sequence of Ts and As ~30 bp upstream of start site). TFIID (TATA binding protein) binds this sequence and recruits other GTFs.

• The preinitiation complex includes GTFs and RNA polymerase II at the promoter.

• The carboxyl terminal domain (CTD) of RNA polymerase II controls elongation. Phosphorylation of the CTD by GTFs releases RNA polymerase II from the initiation complex, allowing elongation.

• Termination does not always occur at a specific sequence; RNA polymerase II may transcribe hundreds or thousands of nucleotides past the coding sequence, with RNA processing forming the mature mRNA.

Example: Diagrams show the assembly of the preinitiation complex and the role of the CTD in elongation.

Transcriptional Regulation

Enhancers, Silencers, and Transcription Factors

Transcription is regulated by various elements and factors that can activate or repress gene expression.

• Enhancers are DNA elements that increase transcription rates.

• Silencers are DNA elements that repress transcription.

• Specific transcription factors bind to enhancers or silencers to regulate specific genes.

• These regulatory elements can be located near or far from the transcription start site.

• Cis-acting elements are found on the same chromosome as the gene they regulate.

RNA Processing in Eukaryotes

Modification of Pre-mRNA

After transcription, RNA undergoes several processing steps before translation.

• A 5' cap is added by attaching a 7-methylguanosine molecule, which protects RNA from degradation and is important for translation.

• A 3' polyadenylation tail is added by incorporating 150-200 adenine nucleotides at the end. The polyadenylation signal (AAUAAA) triggers this addition.

Example: Diagrams show the structure of a protein-coding mRNA with 5' cap, coding sequence, and poly-A tail.

RNA Splicing

Removal of Introns

Splicing removes non-coding introns from the pre-mRNA, joining exons to form mature mRNA.

• The spliceosome is a complex of small nuclear RNAs (snRNAs: U1, U2, U4, U5, U6) and proteins, also called the small ribonucleoprotein complex (snRNP).

• The 5' splice site is typically a GU sequence; the 3' splice site is AG (GU-AG rule).

Example: Diagrams illustrate the removal of introns and joining of exons during splicing.

RNA Editing

Post-Transcriptional Modifications

RNA editing is a process that alters nucleotide sequences in RNA after transcription.

• Substitution editing: a nucleotide is changed.

• Insertional editing: a nucleotide is added.

• Deletion editing: a nucleotide is deleted.

• Guide RNAs determine where editing occurs.

Example: Diagrams show guide RNAs directing editing complexes to specific regions of mRNA.

RNA Interference (RNAi)

Post-Transcriptional Gene Regulation

RNA interference is a mechanism for regulating gene expression by targeting RNA transcripts for degradation.

• miRNAs (microRNAs) are single-stranded RNAs that target many different RNA transcripts for degradation. They are processed by Dicer into ~22 nucleotide miRNAs, which are loaded into the RISC complex to target and destroy specific transcripts.

• siRNAs (small interfering RNAs) are double-stranded RNAs that target the degradation of specific transcripts. Dicer processes pre-siRNA into siRNA, which is then loaded into RISC for transcript destruction.

Example: Diagrams show the steps of RNA interference, including Dicer cleavage and RISC-mediated degradation.

Key Terms and Concepts Table

Term	Definition
Transcription	Process of synthesizing RNA from DNA template
RNA polymerase	Enzyme that synthesizes RNA from DNA
Promoter	DNA sequence signaling start of transcription
Enhancer	DNA element increasing transcription rate
Silencer	DNA element repressing transcription
Spliceosome	Complex removing introns from pre-mRNA
miRNA	MicroRNA, regulates gene expression post-transcriptionally
siRNA	Small interfering RNA, targets specific transcripts for degradation

Important Equations and Sequences

• Transcription direction: $5' \rightarrow 3'$

• Pribnow box consensus sequence: $\text{TATAAT}$

• Polyadenylation signal: $\text{AAUAAA}$

Practice Questions

1. Which RNA polymerase is responsible for transcribing mRNA in eukaryotes? a. RNA polymerase I b. RNA polymerase II c. RNA polymerase III d. RNA polymerase

2. Which general transcription factor is involved in the formation of the preinitiation complex? a. TFIIA b. TFIIB c. TFIID d. TFIIH

3. Which modification occurs to the RNA polymerase tail to trigger elongation? a. Methylation b. Acetylation c. Carboxylation d. Phosphorylation

4. Which of the following is a specific transcription factor? a. Silencers b. Specific transcription factors c. Enhancers d. Promoters

5. True or False: siRNAs target a variety of different RNA transcripts for degradation. a. True b. False

6. Which enzyme is responsible for cleaving and processing miRNAs and siRNAs? a. Dicer b. RISC c. Guide RNA enzymes d. RNA polymerase II

7. Which complex is responsible for removing introns from pre-mRNA? a. RNA polymerase III b. The gene promoter c. A gene transcript d. The spliceosome

Additional info: Some diagrams and tables were inferred and expanded for clarity. All key terms and processes were explained in full academic context for Genetics students.