STUDY THIS Flashcards
Describe the processes by which capping, tailing, and removal of introns from mRNA are achieved co-transcriptionally.
Capping occurs during early transcription, splicing happens as the transcript elongates, and polyadenylation occurs at the 3’ end. Studies show co-localization of transcription and splicing factors using electron microscopy and immunoprecipitation.
Describe the steps (and enzymes involved) in 5’ capping and the functions of this modification.
Steps: (1) Triphosphatase removes phosphate; (2) Guanylyltransferase adds GMP; (3) Methyltransferase methylates the cap. Functions: Protects mRNA from degradation, promotes translation, and aids nuclear export.
Identify the cis-acting sequences in the mRNA and the trans-acting protein complexes that participate in 3’ cleavage and polyadenylation.
Cis-acting: AAUAAA and GU-rich downstream sequences. Trans-acting: CPSF, CstF, and poly-A polymerase. Function: Stabilizes mRNA and enhances translation.
Describe the role of the cis-acting sequences required for RNA splicing and the stepwise assembly of trans-acting “snRNPs” in the spliceosome.
Cis-acting: 5’ splice site, branch point, and 3’ splice site. Assembly: U1 binds 5’ splice site, U2 binds branch point, and U4/U6-U5 tri-snRNP joins to form the active spliceosome.
Explain alternative splicing and connect this to tissue-specific gene expression.
Alternative splicing joins different exon combinations to produce tissue-specific mRNAs, increasing protein diversity and enabling specialized gene expression.
Diagram nuclear structure and relate it to function.
Nuclear envelope protects DNA; lamina provides structural support; nucleolus synthesizes rRNA; nuclear pores mediate transport between the nucleus and cytoplasm.
Define the function of the nucleolus.
The nucleolus synthesizes rRNA and assembles ribosomal subunits for protein synthesis.
Describe and differentiate the mechanism of nuclear transport of small molecules versus large, complex molecules.
Small molecules diffuse passively, while large molecules require active transport via nuclear pore complexes, involving importins/exportins and the Ran GTP/GDP cycle.
Describe the transport proteins and mechanisms governing mRNA and protein nuclear export.
mRNA uses the TREX complex and exportins; proteins use exportins, both driven by Ran GTP/GDP.
Predict how nuclear transport is impacted by mutations in transport proteins or interference with the Ran GTP/Ran GDP cycle.
Mutations disrupt cargo recognition or Ran cycling, leading to impaired import/export and nuclear-cytoplasmic imbalance.
Diagram the common features of eukaryotic mRNAs and relate them to the gene structure.
Eukaryotic mRNAs have a 5’ cap, 5’ UTR, coding sequence, 3’ UTR, and poly-A tail. These correspond to gene regions including the promoter, exons, and terminator.
Explain how the code is read in triplet codons and use the genetic dictionary to determine amino acid sequences.
The genetic code reads three bases (codons) at a time, each specifying one amino acid.
Example: AUG codes for methionine (start).
Explain the evolutionary significance of the universality of the code.
The universal genetic code suggests a shared evolutionary origin and facilitates genetic engineering across species.
Discuss how an amino acid is “activated” by attaching it to a tRNA via aminoacyl-tRNA synthetase.
Aminoacyl-tRNA synthetase links amino acids to their corresponding tRNAs using ATP, forming aminoacyl-tRNA complexes.
Describe the components of two ribosomal subunits and their roles in ribosome assembly.
Small subunit decodes mRNA; large subunit catalyzes peptide bond formation. Both include rRNA and proteins.
Explain the function of the four key ribosomal sites in the eukaryotic 80S ribosome.
A-site: tRNA entry; P-site: peptide bond formation; E-site: tRNA exit; mRNA channel: guides mRNA.
Explain the three steps in translation (initiation, elongation, termination) and the roles of accessory factors.
Initiation: Ribosome assembles on mRNA with help of initiation factors. Elongation: tRNA brings amino acids to A-site, peptide bonds form with elongation factors. Termination: Release factors recognize stop codon, releasing the peptide.
Explain how mutations in DNA sequences can lead to changes at the level of protein.
Mutations alter codons, potentially changing amino acids, leading to altered protein structure and function.
Compare mechanisms that control global translation regulation versus regulation of specific mRNAs.
Global: Phosphorylation of initiation factors reduces translation. Specific: miRNAs or RNA-binding proteins regulate individual mRNA translation.
How do small RNAs (miRNAs) regulate gene expression?
miRNAs bind to complementary mRNA sequences, leading to translational repression or degradation by the RNA-induced silencing complex (RISC).
Predict which step of translation will be inhibited during different types of regulation.
Initiation: Blocked by eIF phosphorylation. Elongation: Inhibited by elongation factor disruption. Termination: Impaired by release factor mutations.
