Molecular Biology: Central Dogma and Translation

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Questions and Answers

What determines the reading frame in protein synthesis?

The position of the start codon (correct)

The number of nucleotides in the sequence

The type of amino acids produced

The position of the stop codon

Which statement best describes the genetic code?

It is overlapping and can be ambiguous.

It has no defined start or stop codons.

It is specific, universal, redundant, and non-overlapping. (correct)

It is unique to each species and varies greatly.

What is the result of an insertion or deletion of nucleotides in DNA?

It causes the genetic code to become universal.

It may cause a frame-shift mutation. (correct)

It has no effect on the protein synthesis process.

It leads to the formation of new codons without altering the reading frame.

What is an Open Reading Frame (ORF)?

A set of codons that begins with a start codon and ends with a stop codon. Signup and view all the answers

How can changes in a coding sequence affect health conditions like Huntington's disease?

They may cause codon amplifications that increase amino acid copies in proteins. Signup and view all the answers

What is one method the cell uses to regulate protein activity?

Post-translational modification Signup and view all the answers

Which of the following statements about protein turnover is accurate?

Protein turnover allows for the recycling of amino acids. Signup and view all the answers

What is the primary function of lysosomes in relation to proteins?

Degradation of proteins and other cellular materials Signup and view all the answers

Why are cyclins important for cellular processes?

They control the progression of a cell around the cell cycle. Signup and view all the answers

What role do ribosomal subunits play in protein synthesis?

They are recycled to facilitate ongoing protein production. Signup and view all the answers

What are the three stages of translation?

Initiation, Elongation, Termination Signup and view all the answers

Which molecule serves as the template during protein synthesis?

mRNA Signup and view all the answers

What is the role of ribosomes in the translation process?

They facilitate the translation of mRNA into protein. Signup and view all the answers

How does phosphorylation affect protein function?

It can alter protein localization and activity. Signup and view all the answers

What determines the sequence of amino acids in a protein?

The sequence of nucleotides in mRNA Signup and view all the answers

How many amino acids are commonly used in protein synthesis?

20 Signup and view all the answers

What is a codon in the context of the genetic code?

A sequence of three nucleotides in mRNA Signup and view all the answers

Which factor is NOT a basic requirement for translation?

DNA polymerase Signup and view all the answers

What is the primary function of the ribosome during translation?

To translate the mRNA into amino acids Signup and view all the answers

Which site on the ribosome holds the tRNA with the growing polypeptide chain?

P site Signup and view all the answers

What is required for the initiation of translation?

GTP and initiation factors Signup and view all the answers

During elongation, which step follows the delivery of the aminoacyl-tRNA to the A site?

Peptide bonds are formed between amino acids Signup and view all the answers

What role does the peptidyl transferase centre play in the ribosome?

It catalyzes peptide bond formation Signup and view all the answers

Which of the following describes the direction in which the ribosome moves during translation?

5' to 3' direction Signup and view all the answers

What happens to the uncharged tRNA after it has released its amino acid?

It exits through the E site Signup and view all the answers

What is the function of the decoding centre within the ribosome?

To ensure the correct tRNA matches the mRNA codon Signup and view all the answers

What role does peptidyl transferase play during elongation?

It forms peptide bonds between amino acids. Signup and view all the answers

What happens to the peptidyl chain after a peptide bond is formed?

It moves to the P site. Signup and view all the answers

What occurs when a termination codon arrives at the A site?

A release factor binds to the A site. Signup and view all the answers

Which site in the ribosome does an empty tRNA move to after new aminoacyl tRNA is added to the A site?

E site Signup and view all the answers

What is the order of sites that a tRNA passes through during the elongation phase?

A – P – E Signup and view all the answers

What is a consequence of having multiple codons that code for a single amino acid in the genetic code?

It allows for variations in protein structure and function. Signup and view all the answers

Which characteristic of the genetic code allows it to be interpreted without ambiguity?

It is read continuously without punctuation. Signup and view all the answers

What is an effect of 'frame-shift' mutations on the genetic code?

They cause a change in the reading frame, potentially altering all downstream amino acids. Signup and view all the answers

How do changes in the coding sequence specifically relate to genetic diseases like Huntington's disease?

An increase in codon numbers amplifies the severity of the disease phenotype. Signup and view all the answers

What does the term 'universal' refer to in the context of the genetic code?

It indicates that the same codons code for the same amino acids across all species. Signup and view all the answers

In the elongation phase of protein synthesis, what is the position of the growing peptide chain after a peptide bond is formed?

It moves to the P site. Signup and view all the answers

Which event directly occurs immediately after the binding of the next aminoacyl-tRNA in the A site?

Peptide bond formation takes place. Signup and view all the answers

What triggers the termination of protein synthesis during the translation process?

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

During elongation, what is the function of the E site in the ribosome?

To release uncharged tRNA from the ribosome. Signup and view all the answers

What sequence of sites does a tRNA follow as it participates in elongation?

A – P – E Signup and view all the answers

Which site on the ribosome is responsible for binding incoming aminoacyl tRNA during the translation process?

A site Signup and view all the answers

What is the correct sequence of events during the initiation of translation?

Initial Assembly, Recognition of start codon, Final Assembly Signup and view all the answers

During elongation, which component is responsible for the formation of peptide bonds between amino acids?

Peptidyl transferase centre Signup and view all the answers

Which molecule provides the necessary energy for the translation initiation process?

GTP Signup and view all the answers

Which of the following correctly describes the role of the decoding centre in the ribosome?

Ensures tRNAs with matching anticodons are accepted Signup and view all the answers

What occurs immediately after a peptide bond is formed during elongation?

The ribosome moves three nucleotides in the 3’ direction Signup and view all the answers

Which component is present in the P site during the initiation of translation?

Initiator tRNA Signup and view all the answers

What is the direction of ribosomal movement along the mRNA during the elongation phase?

5’ to 3’ direction Signup and view all the answers

How does a change in the sequence of mRNA bases potentially affect protein function?

It can alter the amino acid sequence, affecting protein function. Signup and view all the answers

What is the purpose of the ribosome during the translation process?

To catalyze the formation of peptide bonds between amino acids. Signup and view all the answers

Which statement accurately describes the role of the genetic code in translation?

It translates sequences of nucleotides into corresponding amino acids. Signup and view all the answers

What is the primary function of release or termination factors in the translation process?

To recognize termination codons and prompt the release of the completed protein. Signup and view all the answers

Which of the following best describes the relationship between codons and the genetic code?

Each codon comprises three nucleotides that correspond to an amino acid. Signup and view all the answers

What role does phosphorylation play in the regulation of protein activity?

It can either activate or deactivate a protein depending on the context. Signup and view all the answers

Which of the following describes a characteristic of activated precursors in translation?

They include aminoacyl-tRNA that is ready for incorporation into a polypeptide. Signup and view all the answers

How does the central dogma of molecular biology relate to protein synthesis?

It outlines the flow of genetic information from DNA to RNA to protein. Signup and view all the answers

What is the role of polyubiquitination in protein degradation?

It tags proteins for recognition by the proteasome. Signup and view all the answers

What is a common modification that can activate or inactivate a protein?

Phosphorylation Signup and view all the answers

Which of the following best describes zymogens?

Inactive precursors that need cleavage to become active. Signup and view all the answers

How do steroid hormone receptors change cellular function?

By entering the nucleus and influencing gene expression. Signup and view all the answers

What is the primary function of the proteasome in the cell?

To cleave and degrade polyubiquitinated proteins. Signup and view all the answers

What is the significance of reversible phosphorylation in cellular signaling?

It allows for quick and dynamic regulation of protein function. Signup and view all the answers

Which amino acids are primarily involved in reversible phosphorylation?

Serine, Threonine, Tyrosine Signup and view all the answers

What is the function of enterokinase in proteolytic activation?

To activate zymogens by cleaving them. Signup and view all the answers

Study Notes

Transmission - Central Dogma

Central Dogma of Molecular Biology describes the flow of genetic information
Proposed by Francis Crick in 1956
DNA to mRNA to protein

Overview of Translation

mRNA is translated into amino acids
Translation occurs in 3 stages:
- Translation: Initiation
- Translation: Elongation
- Translation: Termination

Translation: Protein Synthesis

Requires 3 key components:
- mRNA (template)
- Ribosome (catalyst)
- Aminoacyl-tRNA (activated precursors)

Genetic Code and Protein Sequence

The order of bases in mRNA determines the amino acid sequence of a protein
Variations in mRNA sequence can alter the amino acid sequence and affect the function of the resulting protein

The Genetic Code

Translates nucleotide sequences into amino acid sequences
Serves as the dictionary for protein synthesis
A codon (3 nucleotides) codes for 1 amino acid
There are 64 codons, but only 20 amino acids

Regulation of Protein Activity

Key cellular functions rely on protein activity
Regulating protein activity is crucial

Regulation of Protein Activity: Methods

Protein levels
Protein location
Post-translational modifications

Protein Turnover

A normal cellular process
Housekeeping proteins are damaged and need replacement
Some proteins involved in key processes need to be degraded for tight control
Rapid turnover of proteins allows for change in their levels

Protein Degradation

Lysosomes: Membrane-bound organelles containing degradative enzymes
Lysosomes degrade proteins, other molecules, organelles, and microbes

The Genetic Code: Characteristics

Specific: Each codon codes for a specific amino acid
Universal (almost): Applies to all species
Redundant: Multiple codons can code for the same amino acid
Non-overlapping: Code is read linearly, 3 bases at a time

Huntington’s Disease

Caused by an amplified codon in the Huntingtin gene
Each extra copy of the codon results in an extra copy of the amino acid in the protein
Expansion of the CAG repeat

Ribosome Structure

Composed of ribosomal proteins and rRNA
rRNA forms extensive secondary structures
Ribosomes facilitate the interaction between tRNA and mRNA during translation
A site: Binds incoming aminoacyl-tRNA
P site: Contains growing polypeptide chain
E site: Contains deacylated tRNA
Decoding center: Ensures codon-anticodon matching
Peptidyl transferase center: Catalyzes peptide bond formation

Initiation of Translation

Assembly of components:
- 40s ribosomal subunit
- mRNA
- tRNAmet
- GTP
- Initiation factors
Recognition of the start codon by tRNAmet
Addition of the 60s ribosomal subunit

Elongation

Involves adding amino acids to the carboxyl end of the growing chain
Ribosome moves along mRNA in the 5' to 3' direction
Steps:
- Delivery of aminoacyl-tRNA to the A site
- Peptide bond formation by peptidyl transferase
- Ribosome translocation by 3 nucleotides
- Uncharged tRNA moves to the E site
- A site becomes available for the next tRNA

Termination

Occurs when a stop codon enters the A site
Stop codon is recognized by a release factor
Release factor binds to the A site, causing:
- Protein release
- Ribosome disassembly

Information Flow

The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein
This concept was proposed by Francis Crick in 1956.

Overview of Translation

Translation is the process of converting the information in mRNA into proteins.
Translation has three stages: Initiation, Elongation, and Termination.

Translation: Protein Synthesis

The process requires several components:
- Template: mRNA provides the code for the protein sequence.
- Catalyst: The ribosome, composed of proteins and rRNA, facilitates the translation process.
- Precursors: aminoacyl-tRNA molecules, which are tRNA molecules attached to specific amino acids via energy-rich bonds.
- Release/Termination Factors: These factors help in the release of the polypeptide chain from the ribosome.

The Genetic Code

The sequence of bases in mRNA determines the sequence of amino acids in the protein.
Any variations in the mRNA sequence can affect the protein's amino acid sequence, potentially leading to functional differences.
The genetic code translates nucleic acid sequences into amino acid sequences.
There are 20 amino acids and 4 nucleotides.
A codon, which consists of three nucleotides, codes for a single amino acid.
Enzymes only function under the low pH conditions present inside lysosomes.

Proteasomal Degradation

Proteins can be tagged for degradation by polyubiquitination, where a polyubiquitin chain is attached to the target protein.
Once polyubiquitinated, the protein is recognized by the proteasome.
The proteasome is a multiprotein complex with protease activity. It is found in both the nucleus and cytoplasm.
The proteasome breaks down tagged proteins.

Post-Translational Modifications

Polypeptide chains can be modified after translation to produce mature, functional proteins from immature proteins.
Examples of post-translational modifications include:
- Proteolytic cleavage: This involves the removal of specific amino acid sequences, like the processing of insulin from its pre-pro form or the activation of trypsinogen to trypsin.
- Covalent modifications: These involve the addition of chemical groups to the polypeptide chain:
  - Disulfide bond formation: This stabilizes protein structure.
  - Phosphorylation: This often activates or inactivates proteins.
  - Glycosylation: This typically happens to extracellular proteins.
  - Farnesylation and GPI anchoring: These modifications target proteins to membranes.
- Ligand binding: This can induce conformational changes, expose active sites, release inhibitory subunits, or change the protein's cellular location.

Ligand Binding

Ligand binding changes the protein's shape.
This can lead to an active conformation, the release of inhibitory subunits, or a change in the protein's location within the cell.
For example, steroid hormone receptors bind to their steroid ligands.
This binding causes a conformational change, allowing the receptor to dimerize (form a complex with another similar molecule) and enter the nucleus.

Proteolytic Cleavage

Zymogens are inactive pro-enzymes that are activated by cleavage.
For example, enterokinase activates trypsinogen.
Proteolytic cascades, such as the clotting cascade, involve a series of proteolytic cleavages.

Reversible Phosphorylation

Phosphate groups can be added to serine, threonine, and tyrosine residues.
This process is tightly regulated by kinases, which add phosphate groups, and phosphatases, which remove them.
The ratio of phosphorylated serine, threonine, and tyrosine residues is approximately 100:10:1.
Tyrosine phosphorylation has the most significant effects.

The Genetic Code: Key Properties

Specific: A specific codon always codes for the same amino acid.
Universal: The code applies to all species, demonstrating conservation from early evolution.
Redundant: A given amino acid can be coded for by multiple different codons.
Non-overlapping: The code is read from a fixed starting point as a continuous sequence of bases, taken three at a time.

Huntington's Disease

Huntington's disease is an example of a disease caused by changes in the coding sequence.
Mutations in the Huntington gene can lead to an amplification of a specific codon, resulting in an increased number of copies of a particular amino acid in the protein.

The Ribosome

Ribosomes are complexes of protein and rRNA.
rRNA exhibits extensive secondary structures, similar to tRNA.
Ribosomes bring tRNA and mRNA together to translate the nucleotide sequence of mRNA into the amino acid sequence of a protein.
The ribosome has three sites:
- A site (aminoacyl site): This binds incoming aminoacyl tRNA.
- P site (peptidyl site): This holds the tRNA containing the growing polypeptide chain.
- E site (exit site): This contains deacylated tRNA.
The ribosome also has a decoding center, which ensures that only tRNAs with an anticodon matching the codon are accepted into the A site.
The peptidyl transferase center catalyzes peptide bond formation.

Initiation of Translation

Initiation requires the assembly of several components:
- The 40S ribosomal subunit
- The mRNA to be translated
- The tRNA corresponding to the first codon in the mRNA (tRNAmet)
- GTP, which provides energy
- Initiation factors, which help the ribosome recognize the start of translation.
The initiator tRNA binds to the start codon.
The 60S ribosomal subunit joins the complex to complete the initiation complex.

Elongation

Elongation involves the addition of amino acids to the carboxyl end of the growing polypeptide chain.
The ribosome moves along the mRNA in the 5' to 3' direction.
The steps of elongation are:
- The next required aminoacyl-tRNA is delivered to the A site.
- A peptide bond is formed between adjacent amino acids.
- The ribosome translocates three nucleotides in the 3' direction (one codon) along the mRNA.
- The growing polypeptide chain moves into the P site.
- The uncharged tRNA moves to the E site and is released.
- The A site becomes available for the next aminoacyl-tRNA.
The path of each tRNA through the ribosome is from the A site to the P site to the E site.

Termination

Termination occurs when one of the three termination codons (stop codons) enters the A site.
The stop codon is recognized by a release factor.
The release factor binds to the A site.
This binding leads to:
- The release of the newly synthesized protein from the ribosome.
- The dissociation of the ribosome from the mRNA.

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Description

Test your knowledge on the Central Dogma of Molecular Biology, which describes the flow of genetic information from DNA to mRNA to protein. Explore the details of translation, including its stages and components such as ribosomes and aminoacyl-tRNA. Understand how the genetic code governs protein synthesis and the implications of variations in mRNA sequences on protein function.