Mastering Genetics Chapter 9

Thanks to flexible base pairing, the wobble nucleotides in anticodons can be any of the RNA nucleotides or which modified nucleotide?

I

A tRNA anticodon is 5' GAA 3'. Answer the following questions. Which amino acid does this tRNA carry?

Phenylalanine Note: The tRNA anticodon is 5'-GAA-3'. Considering the wobble hypothesis, that means the anticodon could bind to 3'-CUU-5' or 3'-UUU-5'. If you write these codons in the standard convention from 5' to 3', you have UUC or UUU. Both of these codons encode phenylalanine.

In terms of the polycistronic composition of mRNAs and the presence or absence of Shine-Dalgarno sequences, compare and contrast bacterial, archaeal, and eukaryotic mRNAs.

Some archaeal mRNAs do not contain a 5' UTR and do not contain a Shine-Dalgarno sequence. Bacterial and archaeal mRNAs can be either polycistronic or monocistronic, whereas eukaryotic mRNAs are monocistronic. Bacterial and archaeal mRNAs that contain a 5' UTR also contain Shine-Dalgarno sequences, whereas eukaryotic sequences do not contain Shine-Dalgarno sequences.

Use the table to sort the following ten codons into one of the three bins, according to whether they code for a start codon, an in-sequence amino acid, or a stop codon.

Start: AUG Stop: UAG UAA UGA Amino Acid: UGC CAC GCA ACU AUC AAA Note: Nearly every mRNA gene that codes for a protein begins with the start codon, AUG, and thus begins with a methionine. Nearly every protein-coding sequence ends with one of the three stop codons (UAA, UAG, and UGA), which do not code for amino acids but signal the end of translation.

Which of the following best describes the first step in the formation of the translation initiation complex?

The small ribosomal subunit binds to an mRNA sequence near the 5' end of the transcript

In bacteria, the methionine that initiates the formation of a polypeptide chain differs from subsequently added methionines in that _______.

a formyl group is attached to the initiating methionine Note: This modification is not present on methionine residues added during elongation.

What would you expect to find bound to the stop codon at the A site?

a release factor

What is a charged tRNA?

a tRNA that carries a specific amino acid

A portion of an mRNA attached to a ribosome reads: 5 GACCAUUUUUGA 3 If a tRNA with a histidine amino acid attached is in the P site of the ribosome, a tRNA with which amino acid attached will enter the A site?

phenylalanine

The first base at the 5' end of the anticodon has relaxed base-pairing rules allowing some tRNAs to bind multiple codon sequences. This is called the third base "wobble" hypothesis. How many codons could a tRNA with the anticodon 5'-GAA-3' recognize?

2 Note: The first base at the 5' end of the anticodon has relaxed base-pairing rules allowing some tRNAs to bind multiple codon sequences. This is called the third base "wobble" hypothesis. The 5' nucleotide of this tRNA anticodon is G, a purine (double-ring structure). Base-pairing rules requires a purine-pyrimidine or pyrimidine-purine match. This means that G could pair with either C or U, which are both pyrimidines. Therefore, there are 2 codona that this tRNA could recognize.

Aminoacyl-tRNA synthetases are the enzymes responsible for attaching the correct amino acid to the corresponding tRNA molecule. How many different tRNA synthetase enzymes are typically required to accomplish this task in the cell?

20 Note: There is one tRNA synthetase enzyme for each amino acid. Each enzyme is responsible for charging all iso-accepting tRNAs with the specific corresponding amino acid.

The human β-globin polypeptide contains 146 amino acids. How many mRNA nucleotides are required to encode this polypeptide?

438 Note: (146*3)=438 because 3 nucleotides make up 1 codon, which codes for the amino acid.

Which mRNA below would code for a premature stop codon from the following amino acid sequence? N-Met-Gln-Leu-Arg-Cys-C

5 - AUG CAG UAA GCG UGC UAG - 3

A tRNA anticodon is 5' GAA 3'. Answer the following questions. Which one of the following codons is recognized by this tRNA?

5' UUC 3' Note: Anticodons and codons are present in RNA molecules that base-pair in an anti-parallel orientation just like DNA.

Using mathematical reasoning, a triplet genetic code gives a possible ________ different codons, while a doublet genetic code would yield ________ different codons. Thus, the triplet code accounts for 20 known amino acids and indicates redundancy in the genetic code.

64 16

Which of the following are synonymous codons?

CGA and AGA

Khorana used synthetic mRNAs to determine genetic code possibilities. To do so, he translated synthetic mRNA in vitro in the presence of individual 14C-labeled amino acids. Using the repeating dinucleotide Poly-UG, he identified a polypeptide with which two amino acids alternating?

Cys Val

A tRNA in the P site of the ribosome will enter the ________ site after translocation of the ribosome.

E

Eighteen of the amino acids have two or more synonymous codons. Which two amino acids are the exceptions?

Met Trp

During translation, nucleotide base triplets (codons) in mRNA are read in sequence in the 5' → 3' direction along the mRNA. Amino acids are specified by the string of codons. What amino acid sequence does the following mRNA nucleotide sequence specify? 5′ − AUG GCA AGA AAA − 3′

Met-Ala-Arg-Lys Note: An amino acid sequence is determined by strings of three-letter codons on the mRNA, each of which codes for a specific amino acid or a stop signal. The mRNA is translated in a 5' → 3' direction.

Before a molecule of mRNA can be translated into a protein on the ribosome, the mRNA must first be transcribed from a sequence of DNA. What amino acid sequence does the following DNA template sequence specify? 3′−TACAGAACGGTA−5′

Met-Ser-Cys-His Note: Before mRNA can be translated into an amino acid sequence, the mRNA must first be synthesized from DNA through transcription. Base pairing in mRNA synthesis follows slightly different rules than in DNA synthesis: uracil (U) replaces thymine (T) in pairing with adenine (A). The codons specified by the mRNA are then translated into a string of amino acids.

Given the following mRNA sequence, what is the amino acid sequence for the corresponding polypeptide? 5 - AUG CGA UUU GGG UGC UAG - 3

N Met-Arg-Phe-Gly-Cys C

Identify three pieces of information that supported the triplet hypothesis of genetic code structure.

Single- or double-nucleotide insertions or deletions in the rII gene resulted in a mutant phenotype, whereas three-nucleotide insertions or deletions resulted in a wild-type phenotype. In vitro translation of RNA composed of a dinucleotide repeat produced polypeptides with two alternating amino acids, as expected from a triplet code. At least 20 codons are needed, one for each amino acid. There are only 42, or 16, different doublet nucleotide codons; whereas there are 43, or 64, different triplet codons.

How might a single base insertion into the second codon of the coding sequence of a gene affect the amino acid sequence of a protein encoded by the gene?

The amino acid sequence would be altered.

In the early 1960s, a number of investigators conducted a series of elegant experiments that helped to decipher the genetic code. Which of the following describes a bona fide property of the genetic code?

The genetic code consists of nonoverlapping codons. Note: Experiments by Hans Fraenkel-Conrat in 1960 demonstrated that the genetic code consists of nonoverlapping codons.

What result would you expect if a mutation eliminates one of the four arms of a tRNA?

The tRNA will not be recognized by tRNA synthetase and cannot be charged.

In the unlikely event that a tRNA has been charged with the wrong amino acid, what high-fidelity enzyme is likely to blame?

aminoacyl synthetase

Which step in translation initiation is unique to eukaryotes?

binding of ribosomes to the 5' cap of the mRNA

Translation in bacteria is directly dependent on all of the following associations except _______.

complementary base pairing between mRNA and DNA Note: Transcription, not translation, is dependent on this association.

Experiments by Chapeville and colleagues proved that the genetic code derives its specificity through what interactions?

complementary base pairing between tRNA and mRNA

The Shine-Dalgarno sequence in bacteria ________.

is a purine-rich consensus sequence found in the 5 UTR of the mRNA

What is polycistronic mRNA?

mRNA that encodes multiple different polypeptide products

If you are designing an antibiotic that inhibits peptide bond formation, what enzyme would you target?

peptidyl transferase

Bacteria group their genes such that they share a single promoter and the mRNA transcript synthesizes several different polypeptides. Collectively, these are referred to as ________ mRNAs, which are part of the operon system.

polycistronic

Heinz Fraenkel-Conrat induced point mutations in the genes encoding coat proteins of TMV and looked for changes in the resulting proteins. By examining the effect of single-base changes in the mRNA on the amino acid sequence, Conrat proved that ________.

the genetic code is nonoverlapping

The diagram of a eukaryotic ribosome shown below contains several errors. Examine the diagram carefully, and identify each error.

the terminal amino acid of the peptide is incorrect the ribosome is moving in the wrong direction along the mRNA the amino acid on the tRNA in the A site is incorrect small ribosomal subunit size is incorrect the anticodon sequence of the tRNA in the P site is incorrect the amino acids on the tRNA in the P site are incorrect the mRNA incorrectly contains Ts large ribosomal subunit size is incorrect

In the process of translational initiation in bacteria, where does the initiator tRNA first bind to the ribosome?

to the partial P site on the small ribosomal subunit