(4) Chapter 8: Microbial Genetics

What is translation?

The process of synthesizing a protein from the information carried by mRNA.

Where does translation occur?

At ribosomes in the cytoplasm.

What molecule is translated during protein synthesis?

mRNA.

What is the function of ribosomes?

They read mRNA codons and assemble amino acids into proteins.

What are codons?

Three-nucleotide sequences on mRNA that specify an amino acid or a start/stop signal.

How many nucleotides make up a codon?

Three.

What is the genetic code?

The set of rules that determines which codons specify each amino acid.

How many amino acids are used to make proteins?

20.

What is tRNA?

Transfer RNA; it carries specific amino acids to the ribosome during translation.

What is the function of tRNA?

To deliver the correct amino acid based on the mRNA codon.

What is an anticodon?

A three-nucleotide sequence on tRNA that is complementary to an mRNA codon.

Where are codons found?

On mRNA.

Where are anticodons found?

On tRNA.

How do codons and anticodons interact?

They bind by complementary base pairing.

What amino acid is coded by AUG in eukaryotes?

Methionine.

What amino acid is coded by AUG in bacteria?

Formylmethionine (fMet).

What is the start codon?

AUG.

What is the function of the start codon?

It signals the ribosome where translation begins.

Which amino acid is always the first amino acid inserted in bacterial proteins?

Formylmethionine (fMet).

Which amino acid is the first amino acid in eukaryotic proteins?

Methionine.

How does the ribosome know where to begin translation?

It recognizes the AUG start codon.

What happens when the small ribosomal subunit binds mRNA?

It locates the AUG start codon.

What binds to the start codon first?

A tRNA carrying formylmethionine (bacteria) or methionine (eukaryotes).

What happens after the first tRNA binds the start codon?

The large ribosomal subunit joins to form a complete ribosome.

What happens next during translation?

The second codon pairs with its complementary tRNA carrying the correct amino acid.

How is the correct amino acid selected?

By complementary pairing between the codon and anticodon.

What forms the bond between amino acids?

Peptide bonds.

Which molecule catalyzes peptide bond formation?

Ribosomal RNA (rRNA).

Why is rRNA considered unusual?

It is RNA that functions as an enzyme (a ribozyme).

What is a ribozyme?

An RNA molecule that has enzymatic activity.

What type of bond joins amino acids together?

Peptide bonds.

What is a polypeptide?

A chain of amino acids joined by peptide bonds.

What happens after a peptide bond forms?

The first tRNA leaves the ribosome and the ribosome moves to the next codon.

What is translocation?

The movement of the ribosome one codon down the mRNA.

What repeats during elongation?

Codon recognition, peptide bond formation, and ribosome translocation.

What happens when a stop codon is reached?

Translation ends and the completed polypeptide is released.

What are the three stop codons?

UAA, UAG, and UGA.

Do stop codons code for amino acids?

No.

What signals the end of protein synthesis?

A stop codon.

What happens to the ribosome after translation ends?

It dissociates into its subunits.

What happens to the newly synthesized polypeptide after release?

It folds into its functional three-dimensional shape.

What are the levels of protein structure mentioned by your professor?

Primary, secondary, tertiary, and quaternary structure.

What determines the primary structure of a protein?

The sequence of amino acids.

What is meant by the degeneracy of the genetic code?

Most amino acids are specified by more than one codon.

Why is the genetic code called degenerate?

Several different codons can specify the same amino acid.

Why is degeneracy beneficial?

It reduces the harmful effects of some mutations.

How can degeneracy reduce mutation effects?

A base change may still produce the same amino acid.

Which amino acids have only one codon according to the lecture?

Methionine (AUG) and tryptophan (UGG).

How many codons specify most amino acids?

Two to six codons.

How many total codons exist?

64.

How many codons specify amino acids?

61.

How many codons are stop codons?

3.

What language is being translated during translation?

The nucleotide language of mRNA into the amino acid language of proteins.

Why is translation essential?

It produces proteins needed for cell structure and function.

Can multiple ribosomes translate the same mRNA simultaneously?

Yes.

What is a polysome (polyribosome)?

Several ribosomes translating one mRNA molecule at the same time.

Why do bacteria often produce proteins rapidly?

Because transcription and translation occur simultaneously and many ribosomes translate the same mRNA.

What did the electron micrograph shown in lecture demonstrate?

Multiple ribosomes attached to one mRNA producing many copies of the same protein simultaneously.

Which process comes first: transcription or translation?

Transcription.

What is the overall sequence of gene expression?

DNA → Transcription → mRNA → Translation → Protein.

Exam tip: Codon vs. Anticodon

Codon = mRNA; Anticodon = tRNA.

Exam tip: Start codon

AUG starts translation and codes for methionine (or formylmethionine in bacteria).

Exam tip: Stop codons

UAA, UAG, and UGA do not code for amino acids.

Exam tip: Which molecule actually forms peptide bonds?

Ribosomal RNA (rRNA), not a protein enzyme.

Exam tip: Why is rRNA important?

It acts as the catalytic component of the ribosome and forms peptide bonds.

What is the final product of translation?

A polypeptide that folds into a functional protein.