Ch. 13 Concept Check & Check Your Understanding

Question 1

An inborn error or metabolism is caused by..

Answer 1

A mutation in a gene that causes an enzyme to be inactive

Question 2

The reason why Beadle and Tatum observed four different categories of mutants that could not grow on media without methionine is because..

Answer 2

Four different enzymes are involved in a pathway for methionine biosynthesis

Question 3

What is the genetic code?

Answer 3

The relationship between a three-base codon sequence and an amino acid

Question 4

The reading frame begins with a ___ and is read ___.

Answer 4

Start codon, in groups of three bases

Question 5

The fourth codon in an mRNA-coding sequence is GGG, which specifies glycine. If we assume that no amino acids are removed from the polypeptide, which of the following statements is correct?

Answer 5

The fourth amino acid from the N-terminus is glycine

Question 6

A type of secondary structure found in proteins is..

Answer 6

• alpha helix

- B sheet

Question 7

Let’s suppose a researcher mixed together nucleotides with the following percentage of bases: 30% G, 30% C, and 40% A. If RNA was made via polynucleotide phosphorylase, what percentage of the codons would be 5’-GGC-3’?

Answer 7

2.7%

Question 8

In the triplet-binding assay of Nuremberg and Leder, an RNA triplet composed of three bases was able to cause the..

Answer 8

Binding of a tRNA carrying the appropriate amino acid

Question 9

If a tRNA has an anticodon with the sequence 5’-CAG-3’, which amino acid does it carry?

Answer 9

Leucine

Question 10

The anticodon of a tRNA is located in the..

Answer 10

Loop of the second stem-loop

Question 11

An enzyme known as ___ attaches an amino acid to the ___ of a tRNA, thereby producing ___.

Answer 11

Aminoacyl-tRNA synthetase

3’ single-stranded region of the acceptor stem

A charged tRNA

Question 12

Each ribosomal subunit is composed of..

Answer 12

• multiple proteins

- rRNA

Question 13

The site(s) on a ribosome where tRNA molecules may be located include the..

Answer 13

• A site
• P site
• E site

Question 14

During the initiation stage of translation in bacteria, which events occur?

Answer 14

• IF1 and IF3 bind to the 30S subunit
• the mRNA binds to the 30S subunit and tRNA binds to the start codon in the mRNA
• IF2 hydrolyzes its GTP and is released; the 50S subunit binds to the 30S subunit

Question 15

The Kozak rules determine..

Answer 15

The choice of the start codon in complex eukaryotes

Question 16

During the peptidyl transferase reaction, the polypeptide, which is attached to a tRNA in the ___, becomes bound via ___ to an amino acid attached to a tRNA in the ___.

Answer 16

P site, a peptide bond, A site

Question 17

A release factor is referred to as a “molecular mimic” because its structure is similar to..

Answer 17

A tRNA

Question 18

The metabolic pathway of phenylalanine breakdown.

Which disease occurs when homogentisic acid oxidase is defective?

Answer 18

Alkaptonuria occurs when homogentisic acid oxidase is defective.

Question 19

An example of an experiment that supported Beadle and Tatum’s one-gene/one-enzyme hypothesis.

What enzymatic function is missing in the strain 2 mutants?

Answer 19

The strain 2 mutants are unable to convert O-acetylhomoserine into cystathionine.

Question 20

The relationships among the DNA coding sequence, mRNA codons, tRNA anticodons, and amino acids in a polypeptide.

Describe the role of DNA in the synthesis of a polypeptide.

Answer 20

The role of DNA is to store the information that specifies the amino acid sequence of a polypeptide. It is a storage role.

Question 21

The amino acids that are incorporated into polypeptides during translation.

Which two amino acids do you think are the least soluble in water?

Answer 21

Tryptophan and phenylalanine are the least soluble in water.

Question 22

Levels of structures formed in proteins.

What type of bonding is responsible for the formation of the two types of secondary structures?

Answer 22

Hydrogen bonding promotes the formation of secondary structures in proteins.

Question 23

The triplet-binding assay.

Explain how the use of radiolabeled amino acids in this procedure helped to reveal the genetic code.

Answer 23

Only one radiolabeled amino acid was found in each sample. If radioactivity was trapped on the filter, this meant that the codon triplet specified that particular amino acid.

Question 24

Recognition between tRNAs and mRNA.

What are the two key functional sites of a tRNA molecule?

Answer 24

A tRNA has an anticodon that recognizes a codon in the mRNA. It also has a 3’ acceptor site where the correct amino acid is attached.

Question 25

Catalytic function of aminoacyl-tRNA synthetase.

What is the difference between a charged tRNA versus an uncharged tRNA?

Answer 25

A charged tRNA has an amino acid attached to it.

Question 26

Wobble position and base-pairing rules.

How do the wobble rules affect the total number of different tRNAs that needed to carry out translation?

Answer 26

The wobble rules allow for a smaller population of tRNAs to recognize all of the possible mRNA codons.

Question 27

Overview of the stages of translation.

Explain how mRNA plays a role in all three stages.

Answer 27

A site in mRNA promotes the binding of the mRNA to the ribosome. The codons are needed during elongation to specify the polypeptide sequence. The stop codon is needed to terminate transcription.

Question 28

The locations of the Shine-Dalgarno sequence and the start codon in bacterial mRNA.

Why does a bacterial mRNA bind specifically to the small ribosomal subunit?

Answer 28

The Shine-Dalgarno sequence in mRNA is complementary to a region in the 16S rRNA within the small ribosomal unit. These complementary sequences hydrogen bond with each other.

Question 29

The elongation stage of translation in bacteria.

What is the role of peptidyl transferase during the elongation stage?

Answer 29

Peptidyl transferase catalyzes the peptide bond formation between amino acids in the growing polypeptide chain.

Question 30

The termination stage of translation in bacteria.

Explain why release factors are called “molecular mimics.”

Answer 30

The structures of release factors, which are proteins, resemble the structures of tRNAs.