General Genetics Unit 8 Review: Transcription and RNA Processing

Key Concepts

• Central dogma of molecular biology describes the flow of genetic information from DNA to RNA to proteins
• Transcription converts the genetic information stored in DNA into RNA molecules
• RNA polymerase enzymes catalyze the synthesis of RNA from a DNA template
• 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
• RNA processing modifies the primary transcript to produce mature RNA molecules (mRNA, tRNA, rRNA)
• Alternative splicing allows for the production of multiple protein isoforms from a single gene

DNA to RNA: The Basics

• DNA serves as the template for RNA synthesis during transcription
• RNA is synthesized in the 5' to 3' direction using the antisense strand of DNA as a template
• RNA is a single-stranded molecule composed of ribonucleotides containing the sugar ribose and the base uracil instead of thymine
• Complementary base pairing occurs between DNA and RNA (A-U, G-C)
• The genetic code is the set of rules that defines how the sequence of nucleotides in mRNA is translated into amino acids
  • The genetic code is degenerate, meaning multiple codons can code for the same amino acid
  • The genetic code is nearly universal across all living organisms
• RNA molecules are less stable than DNA due to the presence of the 2' hydroxyl group on the ribose sugar

Transcription Machinery

• RNA polymerases are the enzymes responsible for catalyzing the synthesis of RNA from a DNA template
  • Eukaryotes have three types of RNA polymerases (I, II, and III), each transcribing different classes of RNA
  • RNA polymerase II is responsible for transcribing protein-coding genes (mRNA)
• General transcription factors (GTFs) are proteins that assemble at the promoter and recruit RNA polymerase II to initiate transcription
• The promoter is a region of DNA upstream of the gene that contains binding sites for transcription factors and RNA polymerase
  • The TATA box is a common promoter element located approximately 25 base pairs upstream of the transcription start site
• Enhancers are distant regulatory elements that can increase the rate of transcription by interacting with the promoter through DNA looping
• Transcriptional activators are proteins that bind to enhancers and stimulate transcription by recruiting coactivators and chromatin remodeling complexes

Stages of Transcription

• Initiation begins with the assembly of the transcription initiation complex at the promoter
  • RNA polymerase binds to the promoter and separates the DNA strands to form the transcription bubble
  • The first nucleotide is added to the 3' end of the growing RNA strand, forming a phosphodiester bond with the next nucleotide
• Elongation involves the progressive addition of nucleotides to the growing RNA strand in the 5' to 3' direction
  • RNA polymerase moves along the DNA template, unwinding the double helix and synthesizing the complementary RNA strand
  • The transcription bubble moves with the RNA polymerase, with the DNA strands re-annealing behind the enzyme
• Termination occurs when RNA polymerase reaches a termination signal and releases the newly synthesized RNA molecule
  • In eukaryotes, termination is coupled with polyadenylation, which adds a poly(A) tail to the 3' end of the transcript
  • Rho-dependent termination in prokaryotes involves the Rho protein, which binds to the RNA and causes the dissociation of the RNA polymerase

RNA Processing and Modifications

• Primary transcripts undergo several modifications to produce mature, functional RNA molecules
• 5' capping adds a 7-methylguanosine cap to the 5' end of the transcript, which protects the RNA from degradation and facilitates translation initiation
• Splicing removes introns (non-coding sequences) and joins exons (coding sequences) to produce a continuous coding sequence
  • Splicing is catalyzed by the spliceosome, a large complex of proteins and small nuclear RNAs (snRNAs)
  • Alternative splicing allows for the production of multiple mRNA isoforms from a single gene by selecting different combinations of exons
• Polyadenylation adds a poly(A) tail (a sequence of adenine nucleotides) to the 3' end of the transcript, which enhances stability and facilitates export from the nucleus
• RNA editing is a process that modifies the nucleotide sequence of the RNA molecule, such as the conversion of adenosine to inosine (A-to-I editing)

Types of RNA and Their Functions

• Messenger RNA (mRNA) carries the genetic information from DNA to the ribosomes for protein synthesis
  • mRNA is single-stranded and contains a 5' cap, 5' untranslated region (UTR), coding sequence, 3' UTR, and poly(A) tail
• Transfer RNA (tRNA) is the adapter molecule that delivers amino acids to the ribosome during translation
  • tRNAs have a cloverleaf secondary structure and an anticodon that base-pairs with the corresponding codon on the mRNA
  • Aminoacyl-tRNA synthetases attach the appropriate amino acid to the 3' end of the tRNA
• Ribosomal RNA (rRNA) is a structural and catalytic component of the ribosome
  • rRNAs are synthesized by RNA polymerase I (large subunit) and III (small subunit) in the nucleolus
• Small nuclear RNAs (snRNAs) are involved in splicing and other RNA processing events as part of the spliceosome
• MicroRNAs (miRNAs) are small, non-coding RNAs that regulate gene expression by binding to complementary sequences in target mRNAs and inducing degradation or translational repression

Regulation of Transcription

• Transcriptional regulation controls the rate and timing of gene expression in response to developmental and environmental cues
• Transcription factors are proteins that bind to specific DNA sequences (regulatory elements) and modulate the activity of RNA polymerase
  • Activators enhance transcription by recruiting coactivators and chromatin remodeling complexes
  • Repressors inhibit transcription by blocking the binding of activators or recruiting corepressors
• Chromatin structure plays a crucial role in regulating gene expression
  • Euchromatin is loosely packed and associated with active gene expression
  • Heterochromatin is tightly packed and associated with gene silencing
• Epigenetic modifications, such as DNA methylation and histone modifications, can alter chromatin structure and affect gene expression without changing the DNA sequence
  • DNA methylation typically occurs at CpG dinucleotides and is associated with gene silencing
  • Histone acetylation is associated with active gene expression, while histone deacetylation is associated with gene repression

Real-World Applications and Research

• Understanding transcription and RNA processing is crucial for developing therapies for genetic disorders and diseases
  • Antisense oligonucleotides can be used to target and degrade specific mRNAs or modify splicing patterns (splice-switching oligonucleotides)
  • RNA interference (RNAi) utilizes small interfering RNAs (siRNAs) to silence gene expression by inducing the degradation of complementary mRNAs
• Transcriptomics is the study of the complete set of RNA transcripts in a cell or organism under specific conditions
  • RNA sequencing (RNA-seq) is a high-throughput method for quantifying gene expression and identifying novel transcripts and isoforms
  • Single-cell RNA sequencing (scRNA-seq) allows for the analysis of gene expression in individual cells, revealing cellular heterogeneity and rare cell types
• Synthetic biology and metabolic engineering rely on the manipulation of transcriptional regulatory elements to control gene expression and optimize the production of desired compounds
  • Promoter engineering involves modifying promoter sequences to fine-tune gene expression levels
  • Inducible expression systems allow for the controlled expression of genes in response to specific stimuli (e.g., tetracycline-inducible promoters)
• RNA-based therapies and vaccines have gained attention in recent years
  • mRNA vaccines (e.g., COVID-19 vaccines) deliver mRNA encoding a viral antigen to stimulate an immune response
  • RNA-based gene therapy aims to replace or supplement defective genes by delivering functional mRNA or RNA-guided gene editing components (e.g., CRISPR-Cas9)