Molecular Biology Quiz: Protein Synthesis

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Questions and Answers

What does the Shine-Dalgarno sequence do in prokaryotic protein synthesis?

It enhances the affinity of the ribosome for the initiator tRNA.

It pairs with a sequence in ribosomal RNA to help position the ribosome. (correct)

It acts as a promoter for gene transcription.

It provides a stop signal.

Which of the following represents a characteristic of the genetic code in most organisms?

It is linear and cannot overlap.

It exhibits a high level of fidelity with little tolerance for mutations. (correct)

It is unique to each species.

It is completely variable with no patterns.

In which of the following scenarios does mitochondrial DNA differ from the standard genetic code?

AUG codon codes for arginine.

AUA codon codes for isoleucine in regular cells.

UGA codon codes for tryptophan in mitochondria. (correct)

UGA codon codes for leucine.

What is the primary role of release factors during protein synthesis?

To recognize stop codons and terminate protein synthesis. Signup and view all the answers

What establishes the reading frame for translation?

The location of the initiator codon. Signup and view all the answers

What is the significance of splicing in eukaryotic gene expression?

It creates mature RNA by removing introns and linking exons. Signup and view all the answers

What are the typical starting and ending sequences of introns in eukaryotic genes?

UG at the start and AG at the end. Signup and view all the answers

How many introns does the average human gene contain?

8 Signup and view all the answers

Which codon is responsible for initiating protein synthesis?

AUG Signup and view all the answers

What are the three codons that act as stop signals during the translation process?

UAA, UAG, UGA Signup and view all the answers

Which amino acid is encoded by the codon CAU?

Histidine Signup and view all the answers

In the context of the genetic code, what does 'degeneracy' refer to?

Multiple codons can code for the same amino acid. Signup and view all the answers

Which of the following codons specifies proline?

CCU Signup and view all the answers

What is the primary role of ribosomes during translation?

To decode mRNA into a polypeptide chain. Signup and view all the answers

Which amino acid is encoded by the codon AUG apart from being a start signal?

Methionine Signup and view all the answers

What does the process of translation specifically produce?

Polypeptide chains. Signup and view all the answers

What is the function of the 3′–OH group in the mechanism of elongation during RNA synthesis?

It makes a nucleophilic attack on the incoming nucleoside triphosphate. Signup and view all the answers

Which correctly describes the relationship between mRNA and DNA during transcription?

mRNA base composition is complementary to the coding strand of DNA. Signup and view all the answers

What is a promoter site in the context of transcription?

It is a sequence that is recognized and bound by RNA polymerase to initiate transcription. Signup and view all the answers

What kind of structure is formed at the 3′ end of transcribed RNA during prokaryotic transcription termination?

A stem-loop structure followed by U residues. Signup and view all the answers

In which sequence does elongation of RNA transcription occur?

5′ to 3′ direction. Signup and view all the answers

Which statement accurately describes codon bias?

Codon bias can influence the speed and efficiency of protein synthesis in different organisms. Signup and view all the answers

What signal typically marks the beginning of translation in mRNA?

A start codon (AUG). Signup and view all the answers

Which concept describes the redundancy in the genetic code that allows multiple codons to specify the same amino acid?

Codon redundancy. Signup and view all the answers

Study Notes

Stop Codons and Release Factors

Stop codons are recognized by release factors, not tRNA molecules.
Release factors are proteins that bind to stop codons and trigger the release of the newly synthesized polypeptide chain from the ribosome.

Start Signal for Protein Synthesis

Prokaryotes:
- The initiator tRNA carries formylmethionine (fMet), a modified amino acid.
- The initiating AUG codon is preceded by the Shine-Dalgarno sequence, which base pairs with a complementary sequence in ribosomal RNA, ensuring proper initiation.
Eukaryotes:
- The AUG codon nearest the 5' end of the mRNA is the initiator codon.

Reading Frame

The reading frame refers to the order of the three non-overlapping nucleotides in an mRNA sequence.
It is established by the location of the initiator codon, which sets the starting point for translation.

The Genetic Code is Nearly Universal

The genetic code is highly conserved, meaning most organisms use the same code for translating mRNA into proteins.
This universality suggests strong selection pressure against deleterious mutations that would alter the code.
Some exceptions to this universality include:
- Ciliated protozoa: Certain stop codons in other organisms encode amino acids in ciliated protozoa.
- Mitochondria: Mitochondrial DNA utilizes variations in the genetic code, likely due to a distinct set of tRNAs that recognize unique codons.

Distinctive Codons of Human Mitochondria

Human mitochondrial DNA has a few differences in the standard genetic code:
- UGA is a stop codon in the standard code but encodes tryptophan in human mitochondria.
- AUA is a leucine codon in the standard code but encodes methionine in human mitochondria.
- AGA and AGG are arginine codons in the standard code but are stop codons in human mitochondria.

Eukaryotic Genes

Eukaryotic genes are discontinuous, meaning they contain both coding and non-coding regions.
- Exons: Coding regions that are translated into protein sequences.
- Introns: Non-coding regions that are removed during RNA processing.
The average human gene contains 8 introns, with some having over 100.
Intron size varies greatly, ranging from 50 to 10,000 nucleotides.

RNA Processing

Pre-messenger RNA (pre-mRNA) contains both exons and introns.
During RNA processing, introns are spliced out and exons are joined together, forming mature mRNA.
Spliceosomes are complex structures composed of proteins and small nuclear RNAs (snRNAs) responsible for splicing.

Spliceosome Recognition

Spliceosomes recognize specific sequences within introns called splice sites.
Introns typically begin with GU and end with AG preceded by a pyrimidine-rich tract.

RNA Polymerase

RNA polymerases are responsible for transcribing DNA into RNA.
The base composition of newly synthesized RNA is complementary to the DNA template strand.

Transcription Initiation & Termination

Transcription begins at promoter sites, which are regions along DNA that specifically bind RNA polymerase.
- Examples of promoter sites in prokaryotes include the Pribnow box and the -35 region.
- Examples of promoter sites in eukaryotes include the TATA box (Hogness box) and the CAAT box.
Transcription ends at terminator sites, which are regions that signal RNA polymerase to stop transcribing.
- In prokaryotes, termination may be caused by a terminator sequence in the DNA or the action of the protein rho.
- Eukaryotic termination is less well understood.

Transcription Termination in Prokaryotes

Terminator sequences in prokaryotic genes often form a stem-loop structure followed by a sequence of U residues.
- The stem-loop structure is formed by base pairing of self-complementary sequences that are rich in G and C.
- This structure helps to destabilize the RNA polymerase-DNA complex, leading to termination.

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Description

Test your knowledge on protein synthesis, focusing on stop codons, release factors, and the differences between prokaryotic and eukaryotic initiation. Additionally, explore the reading frame and the universality of the genetic code in various organisms.