Lecture 12 and 13

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Questions and Answers

Which site on the ribosome is primarily responsible for holding the newly arriving tRNA during elongation?

E site

R site

P site

A site (correct)

Which mechanism primarily controls the expression of genes in the lac operon?

Positive regulation by activators only

Negative regulation by repressors only

Transcriptional enhancement by ribosomes

Repression and activation mechanisms (correct)

During termination of translation, what is primarily responsible for recognizing the stop codon?

Elongation factors

tRNA molecules

Ribosome's A site

Release factors (correct)

In which phase of translation do peptide bonds form between amino acids?

Elongation Signup and view all the answers

Which of the following statements about the ribosome's E, P, and A sites is true?

The E site is where uncharged tRNA exits the ribosome. Signup and view all the answers

How does lactose affect the transcription of the lac operon?

It facilitates the binding of RNA polymerase to the promoter. Signup and view all the answers

What is the role of chaperones in protein synthesis?

To aid in the folding of polypeptides into functional proteins. Signup and view all the answers

What would be the result of a mutation that prevents the ribosome from properly releasing tRNA?

Stalled translation and accumulation of peptides Signup and view all the answers

Which of the following accurately describes the role of transcriptional activators in relation to histone modification?

Transcriptional activators associate with histone acetyltransferases (HATs). Signup and view all the answers

What is the effect of histone deacetylases (HDACs) on transcription?

They lead to the condensation of chromatin, decreasing transcription. Signup and view all the answers

Which of the following statements regarding chromatin states is true?

Euchromatin is associated with active transcription and open chromatin structure. Signup and view all the answers

In cells where Gene X is expressed in the brain but not in muscle, which of the following is most likely true?

Histones associated with Gene X in brain cells show H3K4me modifications. Signup and view all the answers

What modification directly impacts DNA in addition to histone changes?

Methylation of cytosine residues. Signup and view all the answers

What role do tRNAs play in the process of translation?

They serve as adaptors that align amino acids on the mRNA template. Signup and view all the answers

During translation initiation, what is the first interaction that occurs?

The ribosomal small subunit interacts with the mRNA. Signup and view all the answers

What differentiates the initiation of translation between prokaryotic and eukaryotic cells?

Eukaryotic cells require eIFs to recognize mRNA. Signup and view all the answers

What is the end point of translation marked by?

Recognition of a stop codon. Signup and view all the answers

How can translation of mRNAs be regulated?

Through the action of repressor proteins and miRNAs. Signup and view all the answers

What is a characteristic of prokaryotic mRNAs?

They can be polycistronic, allowing multiple proteins to be translated from one mRNA. Signup and view all the answers

What is the role of the large ribosomal subunit in translation?

To participate in peptide bond formation. Signup and view all the answers

What event marks the elongation phase of translation?

The addition of amino acids to the growing peptide chain. Signup and view all the answers

Study Notes

Translation

tRNAs serve as adaptors that align amino acids on the mRNA template during translation.
Ribosomes, made of both structural rRNA and catalytic proteins, catalyze peptide bond formation during translation.
Translation initiation differs in prokaryotes and eukaryotes.
- Prokaryotes lack a nucleus, so translation can begin while genes are transcribed.
- In both prokaryotes and eukaryotes, the small ribosomal subunit and initiator tRNA first interact with mRNA before the large ribosomal subunit joins.
Translation begins at a start codon (AUG) and ends at a stop codon (UAA, UAG, UGA).
Translation can be regulated by repressor proteins and miRNAs.
- Repressor proteins bind to mRNA, preventing translation.
- MicroRNAs can bind to mRNA, leading to degradation or inhibiting translation.
Global translational activity can be regulated by modification of translation initiation factors.

Translation Initiation in Prokaryotes

Shine-Dalgarno sequence: A purine-rich sequence upstream of the start codon that helps align the ribosome.

Translation Initiation in Eukaryotes

eIFs (eukaryotic initiation factors): Proteins that help facilitate the initiation process.
5' cap and 3' poly-A tail: Eukaryotic mRNAs have a 5' cap and 3' poly-A tail that are recognized by eIFs.
Small subunit + eIFs + GTP-bound initiator tRNA: This complex binds to the 5' cap and scans for the start codon.
**Hydrolysis of GTP bound to eIFs:**Releases initiation factors.
Binding of the large ribosomal subunit: Forms the initiation complex.

Translation Elongation

E, P, and A sites: Three sites on the ribosome where tRNA molecules bind and move through during translation.
- A (aminoacyl) site: Where a charged tRNA binds.
- P (peptidyl) site: Where the growing polypeptide chain is attached to the tRNA.
- E (exit) site: Where an uncharged tRNA exists the ribosome.

Translation Termination

Release factors: Proteins that recognize stop codons and trigger the release of the polypeptide chain from the ribosome.
Stop codon: Signals the end of translation.

Functional Proteins

Polypeptides are not automatically functional proteins.
They may require:
- Lipid modification
- Chaperone proteins
- Enzyme activity
- Disulfide bond formation

Gene Regulation in E. coli

Lac operon: A model system for gene regulation in bacteria, controlled by positive and negative mechanisms.
Lac operon is negatively regulated by a repressor protein.
- The repressor protein binds to the operator sequence, blocking RNA polymerase from binding and transcribing the genes.
Lac operon is positively regulated by an activator protein.
- The activator protein binds to the promoter sequence, increasing the affinity of RNA polymerase for the DNA.

DNA Packaging

DNA is packaged into chromatin.
DNA is wrapped around proteins called histones to form nucleosomes, the basic structural unit of chromatin.
Euchromatin: Loosely packed chromatin, typically associated with active genes.
Heterochromatin: Tightly packed chromatin, typically associated with inactive genes.

Histone Modification

Histones can undergo modifications (acetylation, methylation) to promote open euchromatin or closed heterochromatin.
Acetylation: The addition of acetyl groups to histone tails, typically associated with open chromatin.
- Acetyl groups are added by histone acetyltransferases (HATs).
Deacetylation: Removal of acetyl groups from histone tails, typically associated with closed chromatin.
- Acetyl groups are removed by histone deacetylases (HDACs).
Methylation: The addition of methyl groups to histone tails, can either activate or repress gene expression depending on the specific amino acid residue and the number of methyl groups added.
Other factors influence chromatin structure:
- DNA methylation
- Non-histone proteins

Transcriptional Regulation

Transcriptional regulators bind to DNA and can activate or repress gene expression.
Activators:
- Increase transcription by promoting the interaction of RNA polymerase with the promoter.
- Associate with HATs, leading to increased acetylation.
Repressors:
- Decrease transcription by blocking RNA polymerase from binding to the promoter.
- Associate with HDACs, leading to decreased acetylation.

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Description

This quiz focuses on the process of translation initiation in prokaryotes, detailing the roles of tRNAs and ribosomes. It also covers the differences in translation initiation between prokaryotes and eukaryotes, including factors that regulate this process. Challenge your understanding of key concepts and vocabulary associated with translation.