Molecular Biology Unit 5 Review: Transcription and RNA Processing

Key Concepts and Definitions

• Transcription converts genetic information stored in DNA into RNA molecules
• RNA polymerase enzymes catalyze the synthesis of RNA from a DNA template
• Promoter regions on the DNA strand signal the start of transcription
• The template strand of DNA is read in the 3' to 5' direction to synthesize RNA in the 5' to 3' direction
• Transcription factors are proteins that regulate the initiation and rate of transcription
• Eukaryotic transcription occurs in the nucleus, while translation takes place in the cytoplasm
• Post-transcriptional modifications, such as splicing and capping, process the primary RNA transcript into mature RNA
  • Splicing removes non-coding introns and joins coding exons together
  • Capping adds a 7-methylguanosine cap to the 5' end of the RNA for stability and translation initiation

DNA to RNA: The Transcription Process

• Transcription begins when RNA polymerase binds to the promoter region of the DNA template strand
• The double-stranded DNA unwinds, allowing RNA polymerase to access the template strand
• RNA polymerase reads the template strand in the 3' to 5' direction and synthesizes a complementary RNA strand in the 5' to 3' direction
  • Adenine (A) on the DNA template pairs with uracil (U) on the RNA strand
  • Thymine (T) on the DNA template pairs with adenine (A) on the RNA strand
  • Guanine (G) on the DNA template pairs with cytosine (C) on the RNA strand
  • Cytosine (C) on the DNA template pairs with guanine (G) on the RNA strand
• The newly synthesized RNA strand grows in the 5' to 3' direction as nucleotides are added
• Transcription continues until RNA polymerase reaches a termination signal on the DNA template
• The primary RNA transcript undergoes post-transcriptional modifications to become a mature RNA molecule

RNA Polymerases and Their Roles

• RNA polymerases are enzymes that catalyze the synthesis of RNA from a DNA template
• In eukaryotes, there are three main types of RNA polymerases: RNA polymerase I, II, and III
  • RNA polymerase I transcribes ribosomal RNA (rRNA) genes
  • RNA polymerase II transcribes protein-coding genes (mRNA) and some non-coding RNAs (snRNA and miRNA)
  • RNA polymerase III transcribes transfer RNA (tRNA) genes and other small RNAs (5S rRNA and U6 snRNA)
• RNA polymerases consist of multiple subunits that work together to initiate, elongate, and terminate transcription
• The largest subunit of RNA polymerase contains the active site for catalyzing RNA synthesis
• RNA polymerases interact with transcription factors and other regulatory proteins to control gene expression

Initiation, Elongation, and Termination

• Transcription occurs in three main stages: initiation, elongation, and termination
• Initiation involves the assembly of the transcription initiation complex at the promoter region
  • Transcription factors and RNA polymerase bind to specific sequences in the promoter region
  • The DNA double helix unwinds, allowing RNA polymerase to access the template strand
• Elongation begins when RNA polymerase starts synthesizing the RNA strand complementary to the DNA template
  • RNA polymerase moves along the DNA template in the 3' to 5' direction, adding nucleotides to the growing RNA strand
  • The RNA-DNA hybrid formed during transcription is approximately 8-9 base pairs long
• Termination occurs when RNA polymerase reaches a specific termination signal on the DNA template
  • In prokaryotes, termination is typically triggered by a hairpin loop structure followed by a series of uracil residues
  • In eukaryotes, termination is more complex and involves additional factors (cleavage and polyadenylation specificity factor, CPSF)

Post-Transcriptional Modifications

• Primary RNA transcripts undergo various post-transcriptional modifications to become mature, functional RNA molecules
• 5' capping adds a 7-methylguanosine cap to the 5' end of the RNA transcript
  • The cap protects the RNA from degradation and facilitates translation initiation
• Splicing removes non-coding introns and joins coding exons together
  • Splicing is carried out by the spliceosome, a complex of small nuclear RNAs (snRNAs) and proteins
  • Alternative splicing allows for the production of multiple protein isoforms from a single gene
• 3' polyadenylation adds a poly(A) tail to the 3' end of the RNA transcript
  • The poly(A) tail enhances RNA stability and facilitates export from the nucleus to the cytoplasm
• RNA editing alters the nucleotide sequence of the RNA transcript
  • Examples include adenosine-to-inosine (A-to-I) editing and cytidine-to-uridine (C-to-U) editing

Types of RNA and Their Functions

• Messenger RNA (mRNA) carries genetic information from DNA to ribosomes for protein synthesis
  • mRNAs contain a 5' cap, coding sequence, and 3' poly(A) tail
• Transfer RNA (tRNA) molecules deliver amino acids to ribosomes during protein synthesis
  • tRNAs have a cloverleaf secondary structure and an anticodon that base-pairs with the mRNA codon
• Ribosomal RNA (rRNA) is a structural and catalytic component of ribosomes
  • rRNAs make up the majority of the ribosome and catalyze peptide bond formation during translation
• Small nuclear RNAs (snRNAs) are involved in splicing and other RNA processing events
  • snRNAs are components of the spliceosome and guide the splicing of pre-mRNA
• MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression post-transcriptionally
  • miRNAs base-pair with target mRNAs to induce degradation or translational repression
• Long non-coding RNAs (lncRNAs) are involved in various cellular processes, such as gene regulation and chromatin remodeling

Regulation of Transcription

• Transcription is tightly regulated to ensure proper gene expression in different cell types and conditions
• Transcription factors are proteins that bind to specific DNA sequences and regulate transcription
  • Activators enhance transcription by recruiting RNA polymerase and other factors to the promoter
  • Repressors inhibit transcription by blocking RNA polymerase binding or inducing chromatin condensation
• Enhancers are distant regulatory elements that increase transcription of target genes
  • Enhancers can be located upstream, downstream, or within introns of the genes they regulate
• Silencers are regulatory elements that decrease transcription of target genes
• Chromatin structure and epigenetic modifications influence transcription
  • Histone modifications (acetylation, methylation) and DNA methylation can alter chromatin accessibility and gene expression
• Hormones, growth factors, and other signaling molecules can modulate transcription by activating or repressing transcription factors

Clinical and Research Applications

• Transcriptional profiling using microarrays or RNA sequencing (RNA-seq) can identify differentially expressed genes in various conditions (disease states, developmental stages)
• Mutations in transcription factors or regulatory elements can lead to genetic disorders and cancer
  • For example, mutations in the TATA-binding protein (TBP) can cause spinocerebellar ataxia type 17 (SCA17)
• Targeting transcription factors or RNA polymerases with small molecules can be a therapeutic strategy for various diseases
  • For instance, the drug triptolide inhibits RNA polymerase II and has anti-tumor and immunosuppressive effects
• RNA interference (RNAi) using small interfering RNAs (siRNAs) or short hairpin RNAs (shRNAs) can silence gene expression by targeting mRNA for degradation
  • RNAi has potential applications in treating viral infections, cancer, and genetic disorders
• RNA-based therapeutics, such as antisense oligonucleotides and mRNA vaccines, leverage the principles of transcription and RNA processing
  • Antisense oligonucleotides can modulate splicing or induce mRNA degradation to treat genetic diseases (Duchenne muscular dystrophy, spinal muscular atrophy)
  • mRNA vaccines deliver mRNA encoding a target antigen to induce an immune response (COVID-19 vaccines)