Eukaryotic Transcription and Translation Quiz

Chapter 17: Transcription, RNA Processing, and Translation

• This chapter covers the processes of transcription, RNA processing, and translation.
• These processes are crucial for cells to synthesize proteins, which perform various roles within the cell.
• The building blocks for cell proteins come from instructions encoded within the genome.
• Cells use these instructions to create proteins: first transcribing DNA to RNA, then translating mRNA into a protein.

RNA Polymerases and Transcription

• RNA polymerases synthesize an RNA version of DNA's instruction set.
• They use ribonucleoside triphosphates (NTPs).
• They match complementary bases with one DNA strand forming the template strand.
• Only one DNA strand is the template, the non-template strand matches the mRNA sequence (with Uracil instead of Thymine).
• Transcription happens in the 5' to 3' direction.
• Bacteria have a single RNA polymerase, eukaryotes have multiple types (I, II, and III).
• RNA polymerases do not require a primer to start transcription.

Bacterial Transcription Initiation

• Bacterial RNA polymerase's initiation capability is dependent on a sigma protein.
• This joins to the RNA polymerase to create a holoenzyme.
• Holoenzymes find promoter sites, regions containing sequences (-35 box and -10 box) where transcription begins.
• Sigma recognizes and binds to the promoter region.
• This process opens the DNA helix.
• Once initiation is complete, the sigma protein detaches and the core enzyme continues transcribing..
• Promoters are relatively short sequences of DNA (40-50 base pairs). The -10 box is about 10 bases upstream of the transcription start site and a -35 box is located about 35 bases from the transcription start site.
  • Downstream refers to the same direction as RNA polymerase movement, upstream the opposite direction.

Events Inside the Holoenzyme (Detailed)

• Transcription begins when sigma binds to -35 and -10 boxes.
• Sigma's binding orientation determines which DNA strand is the template.
• Once the DNA helix is open, the template strand is threaded through the RNA polymerase's active site.
• Incoming NTPs enter the channel in the active site, pairing with complementary DNA bases and polymerization begins.

Elongation and Termination in Bacteria

• During elongation, RNA polymerase reads the DNA template, adding nucleotides to the 3' end of the RNA.
• Termination occurs when RNA polymerase transcribes a transcription-termination signal that causes a hairpin loop to form on the RNA.
• The hairpin structure causes the RNA to separate from RNA polymerase, halting transcription.

Transcription in Eukaryotes

• Eukaryotic transcription has three RNA polymerases.
• Eukaryotic promoters are more diverse than in bacteria, often including a TATA box.
• General transcription factors (instead of sigma proteins) recognize promoters.
• Instead of hairpins, termination often involves a poly(A) signal.
• The RNA downstream of the poly(A) signal is cut creating a mature mRNA molecule.
• Transcription happens in the cell's nucleus, whereas translation happens in the cytoplasm.

RNA Processing

• Primary RNA transcripts (pre-mRNAs) in eukaryotes are not immediately functional.
• They undergo processing before becoming a functional mRNA.
• Introns (non-coding regions) are removed through a process called splicing.
• The spliceosome (a complex of small nuclear ribonucleoproteins, snRNPs) catalyzes the removal process.
• A 5' cap and a 3' poly(A) tail are added to the mature mRNA, protecting it and providing binding sites necessary for the ribosome to accurately recognize it and initiate translation.
• Pre-mRNA is made in the cell's nucleus; after processing, mature mRNA travels to the cytoplasm for translation.

Translation: Overview

• The process of translation converts the sequence of mRNA bases into an amino acid sequence.
• Ribosomes are the crucial sites for protein synthesis, where the mRNA codons are translated.
• Ribosomes and tRNAs (with their amino acids) cooperate to accomplish this process.
• Ribosomes are made of proteins and rRNA (ribosomal RNA).
• Each ribosome is made from two subunits.

Ribosome Structure and Function

• tRNAs fit into three sites in the ribosome to align with mRNA codons: the A (acceptor, aminoacyl), P (peptidyl), and E (exit) sites.
• The A site receives the aminoacyl tRNA carrying an amino acid.
• The P site holds the tRNA with the growing polypeptide chain.
• The E site holds the tRNA that will exit.
• The ribosome synthesizes proteins in a three-step sequence.

Translation Initiation

• Initiation in bacteria involves the binding of the small ribosomal subunit to the mRNA's ribosome binding site.
• The initial tRNA (initiator tRNA) carries a modified methionine (Methionine but often called f-Met).
  • It binds to a start codon (AUG)
• The large ribosomal subunit binds completing the ribosome.
• Translation initiation is a three-step process.

Translation Elongation

• Elongation begins when a charged aminoacyl tRNA binds to the codon in the A site.
• Peptide bond formation joins the amino acid on the P-site tRNA to the growing polypeptide chain on the A-site tRNA.
• Translocation (movement of the ribosome) moves the tRNA-polypeptide complex to the P site. The uncharged tRNA is moved to the E site and exits. The A-site is ready to receive the next aminoacyl-tRNA matching the next codon).

Translation Termination

• Termination occurs when the ribosome reaches a stop codon.
• A release factor (a protein) enters the A site.
• This release factor hydrolyzes the bond linking the final tRNA and the polypeptide chain.
• The polypeptide is released, and the ribosomal subunits separate.

Table 17.1 (Summary of Differences in Bacterial and Eukaryotic Transcription and Translation)

• This table summarizes the differences in the processes of transcription and translation between bacteria and eukaryotes in terms of RNA polymerase types, promoter structures, protein organization and RNA processing steps.

Post-Translational Modifications

• Most proteins go through post-translational modifications to become functional.
• These modifications occur after the initial translation.
• These processes can involve protein folding, addition of sugar or lipid groups, or adding phosphate groups.
• These modifications may happen at various locations within the cell.

Web Activities

• These are activities related to concepts in the chapters. They do not provide information that needs summarization for study notes.

Test your knowledge on the processes of transcription and translation in eukaryotic cells. This quiz covers key concepts such as the roles of RNA polymerase, introns, polyribosomes, and the distinctions between bacterial and eukaryotic translation. Ideal for students studying molecular biology and genetics.