Biochemistry: Peptidyl Transferase Reaction

Questions and Answers

What role does phosphatidylinositol 3 kinase (PI3K) play in the signaling process?

It signals inositol second messengers through phosphorylation. (correct)

It removes inhibitory proteins from the 40S ribosomal subunit.

It activates mTOR directly to stimulate translation.

It binds to ribosomal proteins to enhance synthesis.

How does AKT contribute to protein synthesis in cells?

By activating ribosomal protein synthesis with mTOR. (correct)

By directly synthesizing ribosomal RNA.

By destabilizing the 7-MeG cap on mRNA.

By inhibiting mRNA decay processes.

Which process occurs when the AKT-mTOR pathway is mutated?

Increased reliance on growth factors for protein synthesis.

Uncontrolled cell growth and proliferation. (correct)

Reduction in ribosome biogenesis.

Immediate decay of all mRNA present in the cell.

What is a consequence of normal mRNA decay?

mRNAs can vary in their stability based on their 3’ UTR regions. Signup and view all the answers

What is the role of exoribonucleases in mRNA decay?

They degrade mRNA either from the 5' or 3' end. Signup and view all the answers

What mechanism allows specific control over mRNA translation and stability?

Interactions involving small noncoding RNAs called microRNAs. Signup and view all the answers

What happens to the 40S ribosomal subunit during protein synthesis in the presence of starvation?

It is inhibited from initiating translation at the 7-MeG cap. Signup and view all the answers

What effect does hyperactivity of the AKT-mTOR pathway have on cellular behavior?

Autonomous growth and independence from external growth signals. Signup and view all the answers

What is the primary function of the peptidyl transferase reaction?

To synthesize proteins based on the genetic code Signup and view all the answers

Which step in translation is most frequently targeted for regulation?

Initiation Signup and view all the answers

How does the 40S ribosomal subunit first interact with mRNA during initiation?

Through the 7-MeG cap at the 5' end Signup and view all the answers

What is the role of the 60S subunit in translation initiation?

To join the 40S subunit and form the 80S ribosome Signup and view all the answers

What is the significance of the 'modified scanning hypothesis'?

It emphasizes the importance of flanking bases for AUG selection. Signup and view all the answers

What crucial role does iron play in the body?

It is essential for the function of proteins like myoglobin and hemoglobin. Signup and view all the answers

What does the control of ferritin synthesis illustrate about translation regulation?

Translation regulation can prevent harmful buildup of substances. Signup and view all the answers

What characterizes the translation initiation process in viral RNAs?

They can utilize internal ribosome entry without a cap. Signup and view all the answers

What is formed after the larger pri-miRNA folds back on itself?

pre-miRNA Signup and view all the answers

What role does the protein argonaute (Ago) play in the miRNA process?

It binds to the final miRNA to regulate gene expression. Signup and view all the answers

How do microRNAs (miRNAs) contribute to gene expression regulation?

By optimizing gene expression and controlling differentiation. Signup and view all the answers

What happens to the passenger strand after miRNA processing by dicer?

It is degraded and discarded. Signup and view all the answers

What type of cell processes the entry of the COVID-19 virus?

Cells in the respiratory tract Signup and view all the answers

Which of the following diseases have been linked to genes encoding microRNAs?

Cancer, metabolic, cardiovascular, and neurological diseases Signup and view all the answers

What is the initial action of the COVID-19 virus once it enters a host cell?

It releases its RNA genome. Signup and view all the answers

What is the role of the viral encoded replicase in the COVID-19 infection process?

It replicates many copies of the viral genome. Signup and view all the answers

What primarily causes the progressive loss of muscle function in DMD?

Death of myocytes and replacement with fat cells Signup and view all the answers

Why are females usually less severely affected by DMD?

They usually have an unmutated copy of the dystrophin gene Signup and view all the answers

What is the size of the dystrophin mRNA encoded by the DMD gene?

2.5 million bases Signup and view all the answers

Where are many DMD-causing mutations located?

Within introns or at splice sites Signup and view all the answers

What is a likely outcome of encountering a premature termination codon?

Creation of a truncated protein Signup and view all the answers

What is the probability of encountering a stop codon due to a translational reading frame-shift mutation?

3 in 64 Signup and view all the answers

What happens to truncated proteins in a cell?

They disrupt protein assemblies and metabolic pathways Signup and view all the answers

What syndrome is an exception that allows females carrying a DMD mutation to be severely affected?

Turner syndrome Signup and view all the answers

What is the primary function of microRNAs in relation to mRNA?

To degrade and inhibit translation of specific mRNA Signup and view all the answers

In which part of the mRNA are miRNA recognition sequences typically located?

3' UTR Signup and view all the answers

What initiates the process of microRNA synthesis?

Transcription from larger precursor transcripts called pri-miRNAs Signup and view all the answers

What is the RNA silencing complex more commonly known as?

RISC Signup and view all the answers

How many microRNAs have been identified in humans since the initial discovery?

Over 2,000 Signup and view all the answers

What common source led to the discovery of human microRNAs?

Comparison with small RNAs in C.elegans Signup and view all the answers

What is the role of the seed sequence in a microRNA?

To serve as a recognition site for mRNA degradation Signup and view all the answers

What can happen once the RISC complex binds to its target mRNA?

Degradation of the mRNA or translational inhibition Signup and view all the answers

Study Notes

Peptidyl Transferase Reaction

Essential biochemical reaction for protein synthesis where protein-encoding sequences in the genome guide the production of proteins.

Translation Initiation

Most regulated step in translation.
40S ribosomal subunit accesses mRNA via the 7-MeG cap or through internal entry to find the start codon (AUG).
Internal entry often occurs with viral RNAs lacking a cap but can also happen with some cellular mRNAs.
The 40S subunit scans mRNA in the 5’ to 3’ direction to locate the AUG start codon.
Joins with 60S subunit to form an 80S ribosome, allowing for protein synthesis.

Scanning Hypothesis

Originally proposed that the 40S subunit selects the first encountered AUG for translation initiation.
Modified scanning hypothesis emphasizes the importance of surrounding bases for proper context of the start codon.

Regulation of Translation: Ferritin Synthesis

Ferritin regulates iron storage to prevent toxicity from free iron.
Growth factor binds a receptor activating phosphatidylinositol 3 kinase (PI3K), triggering downstream signaling pathways.
Activated AKT and mTOR enhance ribosomal protein synthesis and ribosome biogenesis.
AKT-mTOR signaling facilitates 40S ribosomal subunit access at the 7-MeG cap, promoting cap-dependent translation.

mRNA Stability Regulation

mRNA decay occurs when transcripts are no longer needed, varying from hours to longer translations.
Regulatory sites in the 3’ UTR promote instability for mRNAs needing low protein levels.
Exoribonucleases degrade mRNA, either from the 5’ end (decapping) or 3’ end (poly(A) tail removal).

Role of MicroRNAs (miRNAs)

Small noncoding RNAs that regulate gene expression post-transcriptionally.
First discovered in C.elegans; humans have over 2,000 identified miRNAs.
Each miRNA contains a "seed sequence" that binds complementary mRNA sequences, often in the 3’ UTR.
miRNAs inhibit translation or degrade mRNAs through the RNA-induced silencing complex (RISC).

MicroRNA Synthesis Pathways

miRNAs derive from larger precursor transcripts (pri-miRNAs) or from splicing of introns.
Processed in the nucleus and cytoplasm into active miRNA, which binds Argonaute (Ago) proteins to target mRNAs.

Importance of miRNAs

Critical for regulation of cell differentiation, growth, and development; especially relevant in stem cell regulation.
Involved in various diseases, including cancer, with distinct expression profiles for different tumors.

COVID-19 Virus Propagation

The virus targets ACE2 receptors in the respiratory tract via its spike protein, followed by endocytosis.
Releases its single-stranded RNA genome that is translated by the host’s machinery to replicate and package viral genomes.

Dystrophin Gene and Dystrophic Muscle Disorders

Dystrophin gene linked to DMD (Duchenne Muscular Dystrophy), primarily affecting boys due to X-linked inheritance.
Encodes the largest mRNA (over 2.5 million bases) with critical mutations affecting splice sites.
Premature termination codons caused by intron retention or improperly spliced exons lead to truncated, non-functional proteins, disrupting cellular metabolism.

Premature Termination Codons

Only three termination codons: UAA, UGA, UAG, among 64 total codons.
Insertion/deletion mutations yield a 1 in 20 chance of encountering a premature stop codon, leading to truncated proteins that can compete with full-length variants and impair cell function.

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Description

This quiz explores the peptidyl transferase reaction, a crucial biochemical process that facilitates protein synthesis from genomic sequences. It focuses on the initiation stage of translation and its regulatory aspects, particularly the role of the 40S ribosomal subunit. Test your understanding of these fundamental concepts in biochemistry.