Genetics Exam Chapter 15

What did Beadle and Tatum use to show that genes encode enzymes?

They used the ascomycete fungus Neurospora to show that genes encode enzymes: “one gene, one enzyme.

What later research showed about genes encoding proteins?

Some genes do not encode enzymes, and other genes encode structural proteins or other protein types: “one gene, one polypeptide.”

What did mutagenesis using X-rays produce in the Beadle and Tatum experiment?

Mutants that could not grow on minimal medium (auxotrophs).

What did they identify in mutants?

Mutations that lacked enzymes needed for synthesis of amino acids.

What did Srb and Horowitz find in their experiments?

They produced many more auxotrophic mutations and found groups of genes that formed pathways.

How many steps were in the arginine pathway?

Three steps from precursor to arginine.

How did they rescue mutants?

With enzyme products: ornithine, citrulline, and arginine.

How many amino acids are found in proteins?

Twenty amino acids.

What common structure is shared by all amino acids?

A central carbon with a hydrogen, carboxyl, amino, and R (radical) groups.

How are amino acids linked in proteins?

By peptide bonds.

What are groups of amino acids based on?

Similar R groups: positively charged, negatively charged, polar uncharged, non-polar aliphatic, and aromatic.

What links amino acids in proteins?

Peptide bonds.

What determines primary structure of a protein?

Its sequence of amino acids.

What causes secondary structure?

Interactions between amino acids causing folding such as the alpha helix.

What produces quaternary structure?

Two or more polypeptide chains associating.

What was first used to determine the genetic code?

Homopolymers of bases (UUU, CCC, AAA; GGG did not work in this experiment).

What was used to decipher additional codons?

Random copolymers of two nucleotides.

How were up to 50 of the 64 codons determined?

Using ribosome-bound tRNAs.

When was the solution completed?

1968

What are the 61 codons that encode amino acids called?

Sense codons.

How many stop codons exist?

Three.

What are codons encoding the same amino acid called?

Synonymous codons.

How many tRNAs are in most organisms?

30–50 tRNAs (not 61).

What are isoaccepting tRNAs?

tRNAs that accept the same amino acid.

What allows recognition of different codons by some tRNAs?

Wobble, or flexibility in base pairing in the third position.

Where does wobble flexibility occur?

Between the first position of the anticodon and the third position of the codon.

What does the slide say about universality?

There are a few organisms that do not use the same genetic code.

What is translation?

The conversion of the genetic information of the mRNA into an amino acid polymer.

How many aminoacyl-tRNA synthetase enzymes exist?

20, one for each amino acid.

Why is specific addition of amino acids to tRNAs important?

It is critical to the fidelity of translation.

What does the small ribosomal subunit recognize?

The Shine-Dalgarno sequence and AUG start codon.

What binds with GTP during initiation?

The methionine tRNA and initiation factors.

What happens when the large ribosomal subunit assembles?

Initiation factors are released and GTP is hydrolyzed.

What are the three ribosome sites?

Aminoacyl (A), peptidyl (P), and exit (E) sites.

Which site does the first met-tRNA occupy?

The P site.

What factor brings the second tRNA to the A site?

Elongation factor EF-Tu-GTP.

What happens when GTP is hydrolyzed during elongation?

EF-Tu-GDP releases, leading to peptide bond formation between the amino acids.

What happens to the first tRNA during elongation?

The first tRNA moves to the E site and is ejected.

Where is the dipeptide-containing tRNA located as elongation continues?

It is in the P site while another round of amino acid addition begins.

What is the tRNA progress during elongation?

Cytoplasm → A site → P site → E site → cytoplasm.

What happens when the ribosome reaches a stop codon?

There is no tRNA that can pair with the stop codon.

What enters the A site during termination?

Release factors enter the A site and RF-3–GTP associates with the complex.

What powers the release of the polypeptide and ribosome components?

GTP hydrolysis.

What is the function of amino acids in tRNA charging?

Building blocks of proteins.

What is the function of tRNAs?

Deliver amino acids to ribosomes.

What do aminoacyl-tRNA synthetases do?

Attach amino acids to tRNAs.

What is the function of ATP in tRNA charging?

Provides energy for binding amino acids to tRNAs.

What is the function of mRNA in initiation?

Carries coding instructions.

What does fMet-tRNAᶠᴹᵉᵗ provide?

Provides the first amino acid in the peptide.

What does the 30S ribosomal subunit do?

Attaches to mRNA.

What is the function of the 50S ribosomal subunit?

Stabilizes tRNAs and amino acids.

What does initiation factor 1 do?

Enhances dissociation of large and small ribosomal subunits.

What does initiation factor 2 do?

Binds GTP and delivers fMet-tRNAᶠᴹᵉᵗ to the initiation codon.

What does initiation factor 3 do?

Binds to the 30S subunit and prevents association with the 50S subunit.

What is the function of the 70S initiation complex?

Functional ribosome with A, P, and E sites where protein synthesis takes place.

What is the role of charged tRNAs?

Bring amino acids to the ribosome and help assemble them in order specified by mRNA.

What does elongation factor Tu do?

Binds GTP and charged tRNA; delivers charged tRNA to the A site.

What does elongation factor Ts do?

Regenerates active elongation factor Tu.

What does elongation factor G do?

Stimulates translocation of ribosome to the next codon.

What is the role of GTP in elongation?

Provides energy.

What is the function of 23S rRNA in the large ribosomal subunit?

Creates peptide bond between amino acids in the A and P sites.

What is the function of release factors 1, 2, and 3?

Bind to the ribosome when a stop codon is reached and terminate translation.