Cell Biology Unit 14 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 occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells
• RNA processing involves modifications to the newly transcribed RNA molecule, such as splicing and addition of a 5' cap and 3' poly-A tail
• Three main types of RNA include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)
• Transcription is tightly regulated to ensure proper gene expression in cells

DNA to RNA: The Basics

• DNA serves as the template for RNA synthesis during transcription
• RNA is synthesized in the 5' to 3' direction, complementary to the DNA template strand
• RNA differs from DNA in several ways:
  • RNA contains the sugar ribose, while DNA contains deoxyribose
  • RNA uses the base uracil (U) instead of thymine (T)
  • RNA is typically single-stranded, while DNA is double-stranded
• The genetic code is read in triplets called codons, each specifying a particular amino acid or stop signal
• The complementary nature of base pairing (A-U and G-C) ensures accurate transcription of genetic information
• RNA molecules are less stable than DNA due to the presence of the 2' hydroxyl group on the ribose sugar

Transcription Machinery

• RNA polymerase is the key enzyme responsible for transcription
• Eukaryotic cells have three types of RNA polymerase: RNA polymerase I, II, and III
  • RNA polymerase I transcribes rRNA genes
  • RNA polymerase II transcribes mRNA and some small nuclear RNA (snRNA) genes
  • RNA polymerase III transcribes tRNA and other small RNA genes
• Prokaryotic cells have a single type of RNA polymerase
• General transcription factors (GTFs) assist RNA polymerase II in initiating transcription in eukaryotes
• Promoter regions upstream of genes contain specific DNA sequences that recruit transcription factors and RNA polymerase
• The TATA box is a common promoter element located approximately 25 base pairs upstream of the transcription start site in many eukaryotic genes

Stages of Transcription

• Transcription can be divided into three main stages: initiation, elongation, and termination
• Initiation involves the assembly of the transcription machinery at the promoter region
  • RNA polymerase binds to the promoter with the help of transcription factors
  • The DNA double helix is unwound, and the template strand is exposed
• Elongation occurs as RNA polymerase moves along the DNA template, synthesizing the RNA transcript
  • Nucleotides are added to the growing RNA chain in a 5' to 3' direction
  • The RNA-DNA hybrid formed during transcription is approximately 8 base pairs long
• Termination happens when RNA polymerase reaches a specific termination signal
  • In prokaryotes, termination is mediated by hairpin loops or Rho-dependent mechanisms
  • In eukaryotes, termination signals are less well-defined and may involve cleavage of the RNA transcript

RNA Processing and Modifications

• Eukaryotic RNA transcripts undergo extensive processing before becoming functional molecules
• 5' capping adds a 7-methylguanosine cap to the 5' end of the transcript, protecting it from degradation and facilitating translation
• 3' polyadenylation adds a poly-A tail (a string of adenine nucleotides) to the 3' end, enhancing stability and translation
• Splicing removes non-coding introns and joins coding exons to form a mature mRNA molecule
  • Splicing is carried out by the spliceosome, a complex of proteins and snRNAs
  • Alternative splicing allows the production of multiple protein isoforms from a single gene
• RNA editing can modify specific nucleotides in the RNA transcript, altering the encoded amino acid sequence (adenosine deamination)

Types of RNA and Their Functions

• Messenger RNA (mRNA) carries the genetic information from DNA to the ribosomes for protein synthesis
  • mRNA molecules are single-stranded and contain a 5' cap, 5' UTR, coding region, 3' UTR, and poly-A tail
• Transfer RNA (tRNA) molecules serve as adapters, bringing specific amino acids to the ribosome during translation
  • 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
  • rRNA molecules combine with ribosomal proteins to form the small and large subunits of the ribosome
• Small nuclear RNAs (snRNAs) are involved in splicing and other RNA processing events
• MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) regulate gene expression through RNA interference (RNAi)

Regulation of Transcription

• Transcriptional regulation is crucial for controlling gene expression and cellular function
• Transcription factors bind to specific DNA sequences (enhancers and silencers) to activate or repress transcription
  • Activators increase transcription by recruiting RNA polymerase and chromatin remodeling complexes
  • Repressors decrease transcription by blocking RNA polymerase or inducing chromatin condensation
• Chromatin structure plays a key role in regulating transcription
  • Histone modifications (acetylation, methylation) can make DNA more or less accessible to transcription factors
  • DNA methylation is associated with gene silencing and is important for genomic imprinting and X-chromosome inactivation
• Hormones and other signaling molecules can influence transcription by modulating the activity of transcription factors (steroid hormone receptors)
• Transcriptional regulation allows cells to respond to environmental cues and developmental signals

Real-World Applications

• Understanding transcription is essential for developing targeted therapies for genetic disorders and diseases
• RNA-based therapies, such as antisense oligonucleotides and small interfering RNAs (siRNAs), can modulate gene expression by targeting specific mRNAs
• Transcriptional profiling using microarrays or RNA sequencing (RNA-seq) can identify gene expression changes associated with diseases or developmental stages
• Induced pluripotent stem cells (iPSCs) are generated by manipulating the transcription of key pluripotency genes (Oct4, Sox2, Klf4, c-Myc)
• CRISPR-Cas9 gene editing relies on guide RNAs (gRNAs) to direct the Cas9 nuclease to specific DNA sequences for targeted modifications
• RNA-based vaccines, such as those developed for COVID-19 (Pfizer-BioNTech and Moderna), deliver mRNA encoding viral antigens to elicit an immune response
• Transcriptional regulation is a key factor in cellular differentiation and the development of multicellular organisms (Hox genes in embryonic development)