Protien Synthesis

What is a codon?

A sequence of three nucleotides in mRNA that specifies an amino acid or a stop signal.

How many standard amino acids are used to build proteins?

20.

Why must the genetic code use at least three nucleotides per codon?

Because one or two nucleotides cannot produce enough combinations to specify all 20 standard amino acids.

How many combinations are possible if three nucleotides form a codon?

\(4^3=64\) combinations.

What does it mean that the genetic code is degenerate?

Multiple codons can specify the same amino acid.

What does it mean that the genetic code is non-overlapping?

Each nucleotide belongs to only one codon within a particular reading frame.

ow would the RNA sequence AUGCCAUUU be divided into non-overlapping codons?

AUG | CCA | UUU.

What is the standard start codon?

AUG.Methionine (Met).

What is the function of a start codon?

It helps establish where translation begins and determines the reading frame.

What are the three stop codons?

UAA, UAG, and UGA.

What is the function of a stop codon?

To signal termination of translation.

What is a reading frame?

a nucleotide sequence is divided into consecutive groups of three nucleotides.

Why is the reading frame important?

Because different reading frames can produce different amino acid sequences.

What is the first reading frame of 5′-AUGCCAUUU-3′?

AUG | CCA | UUU.

What is the second reading frame of AUGCCAUUU?

UGC | CAU, with two nucleotides remaining.

What is the third reading frame of AUGCCAUUU?

GCC | AUU, with one nucleotide remaining.

What is an in-frame deletion?

A deletion of a multiple of three nucleotides that preserves the downstream reading frame.

What is an in-frame insertion?

An insertion of a multiple of three nucleotides that preserves the downstream reading frame.

What is a premature stop codon?

A stop codon that appears earlier than the normal translation termination site.

What can happen to the amino acid sequence after a frameshift?

Many downstream amino acids may change, and a premature stop codon may appear.

What is a truncated protein?

A protein that is shorter than normal because translation terminates prematurely or part of the protein is missing.

Why can a frameshift mutation produce a nonfunctional protein?

Because it can alter many amino acids or cause premature termination.

Can an in-frame insertion or deletion still affect protein function?

Yes. Adding or removing amino acids can alter protein structure or function even without a frameshift.

What is the difference between a missense mutation and a frameshift mutation?

A missense mutation changes an amino acid, while a frameshift changes the grouping of downstream codons.

What is the difference between a nonsense mutation and a frameshift mutation?

A nonsense mutation directly creates a stop codon, while a frameshift changes the reading frame and may subsequently create a premature stop codon.

Why are frameshift mutations often more disruptive than synonymous substitutions?

Because frameshifts can change many downstream amino acids, while synonymous substitutions do not change the encoded amino acid.

What is a silent mutation?

A nucleotide substitution that does not change the amino acid encoded by a codon.

What is a missense mutation?

A nucleotide substitution that changes the amino acid encoded by a codon.

What is a nonsense mutation?

A nucleotide substitution that changes an amino-acid-coding codon into a stop codon.

What is the function of mRNA during translation?

To provide codons that determine the amino acid sequence of a protein.

What is mRNA?

Messenger RNA, which carries genetic instructions from DNA to the ribosome.

What is the adapter hypothesis?

The proposal that an intermediate molecule connects mRNA codons to their corresponding amino acids.

What molecule serves as the adapter during translation?

Transfer RNA (tRNA).Because it recognizes an mRNA codon and carries the corresponding amino acid.

What are the two major functions of tRNA?

Recognizing mRNA codons and delivering amino acids to the ribosome.

What part of tRNA recognizes the mRNA codon?

The anticodon.

What part of tRNA carries the amino acid?

The 3′ end.

What is an aminoacyl-tRNA synthetase?

An enzyme that attaches the correct amino acid to its corresponding tRNA.

What is tRNA charging?

The attachment of an amino acid to its corresponding tRNA.

What is an aminoacyl-tRNA?

A tRNA molecule covalently attached to an amino acid.

Why is ATP required for tRNA charging?

It provides energy to activate the amino acid and attach it to the tRNA.

What is the three-dimensional tertiary structure of tRNA?

An L-shaped structure. It positions the anticodon and amino acid attachment site at opposite ends of the molecule.

What are the major structural regions of tRNA?

The acceptor stem, anticodon loop, D loop, T loop, and variable loop.

What are the two major subunits of a ribosome?

A small subunit and a large subunit.

What is the main function of the small ribosomal subunit?

To help decode mRNA through codon-anticodon recognition.

What is the main function of the large ribosomal subunit?

To catalyze peptide bond formation.

What are the three major tRNA binding sites in the ribosome?

A site, P site, and E site.

What is the function of the A site?

To receive incoming charged tRNAs during translation elongation.

What is the function of the P site?

To hold the tRNA carrying the growing polypeptide chain before peptide bond formation.

What is the function of the E site?

To hold the deacylated tRNA before it exits the ribosome.

What are the three major stages of translation?

Initiation, elongation, and termination.

What happens during initiation?

The ribosome assembles on mRNA, recognizes the start codon, and positions the initiator tRNA.

What happens during elongation?

Amino acids are added sequentially to the growing polypeptide.

What happens during termination?

A stop codon is recognized and the completed polypeptide is released.

What are the three main categories of translation factors?

Initiation factors, elongation factors, and release factors.

What is the function of EF-Tu?

It delivers charged tRNAs to the ribosomal A site.

What molecules does EF-Tu bind during tRNA delivery?

GTP and aminoacyl-tRNA.

What is the function of IF1?

It helps organize the bacterial initiation complex and promotes proper initiation.

What is the function of IF2?

It binds GTP and helps recruit the initiator tRNA.

What happens to EF-Tu after successful tRNA delivery?

It hydrolyzes GTP and dissociates from the ribosome.

What is the function of EF-Ts?

It helps EF-Tu release GDP so GTP can bind again.

What is the function of RF3?

It is a GTP-binding protein that helps recycle bacterial release factors.

What is the Shine-Dalgarno sequence?

A bacterial mRNA sequence that helps position the ribosome near the start codon and locate and align with the TIS

Which ribosomal RNA interacts with the Shine-Dalgarno sequence?

16S rRNA.

How do eukaryotic ribosomes usually locate the start codon?

Through 5′ cap-dependent recruitment followed by scanning.

Why can antibiotics selectively inhibit bacterial translation?

Because bacterial translation machinery has structural and functional differences from eukaryotic cytosolic translation machinery.

What are the three major steps of elongation?

Codon recognition, peptide bond formation, and translocation.

What is transpeptidation?

The reaction that transfers the growing polypeptide to the amino acid on the A-site tRNA, forming a peptide bond.

Does peptide bond formation directly require additional ATP or GTP hydrolysis?

No. energy comes from The high-energy ester linkage between the amino acid or growing peptide and its tRNA.

What is translocation?

The movement of the ribosome along mRNA by one codon. A—> P—> E

What triggers translation termination?

A stop codon entering the ribosomal A site.

What recognizes stop codons during translation termination?

Release factors.

What happens when a release factor recognizes a stop codon?

It promotes hydrolysis of the bond between the completed polypeptide and the P-site tRNA.