AP Biology Unit 6 Gene Expression

​Unit 6 Quizizz 2

Multiple Choice

__________ are parts of __________ molecules that do not contain information about a protein's primary structure.

• Exons . . . mRNA

• Introns . . . mRNA

• Exons . . . pre-mRNA

• Introns . . . pre-mRNA

​After transcription, the resulting RNA molecule must undergo post-transcriptional modification before it becomes mature mRNA. Before these modifications, it is known as heteronuclear RNA (htRNA) or pre-mRNA.

Introns are portions of pre-mRNA molecules that are spliced prior to translation. Unlike exons, introns do not contain information about the structure of the protein. Only after intron splicing is the molecule considered mRNA.

Multiple Choice

Which of these are characteristic of transcription in eukaryotes? 

I.  Transcription can occur at the same time as translation

II.  The resulting RNA molecule is functionally mature

III.  The resulting RNA molecule must undergo splicing and other modifications before it is functional

IV.  Transcription occurs in the cytoplasm

• III only

• I and III

• II only

• I and IV

• I, II, and IV

​Prokaryotic transcription occurs in the cytosol, since prokaryotes lack a nucleus. This allows ribosomes to interact with RNA even while it is still be synthesized.

In contrast, eukaryotic transcription occurs in the nucleus. Once RNA has been synthesized it must be transported from the nucleus to the cytoplasm before it can interact with ribosomes. The newly-synthesized RNA undergoes splicing to remove introns, addition of a 5'-cap, and addition of a poly-A tail before it can exit the nucleus. These modifications help prevent degradation of the RNA. Only after these modifications can the RNA leave the nucleus and becomes functionally active.

Multiple Choice

Which of the following correctly pairs the part of an operator with its function?

• Promoter; contains the genes to be transcribed

• Inducer; decreases RNA polymerases’ transcription of genes

• Corepressor; signals for RNA polymerase to binds and transcribe genes

• Operator; region where the repressor binds

​The operator region is the location where the repressor binds. Other parts of an operon include the promoter (where RNA polymerase binds), and structural genes.

Multiple Choice

Which of the following binds to the repressor of an operon to prevent the repressor from binding to the operator region?

• Activator

• corepressor

• unrepressor

• inducer

​A repressor is a molecule that binds to the operator region of a gene and prevents RNA polymerase from transcribing the genes.

An inducer can bind to a repressor, preventing the repressor from binding to the operator region, and thus allowing RNA polymerase to transcribe the genes.

Multiple Choice

The lac operon contains the lac repressor, which binds to the operator to inhibit transcription. What type of operon is this, and how can transcription of the genes proceed?

• Repressible; a repressor binding to the repressor will allow transcription

• Inducible; an inducer binding to the repressor will allow transcription

• Inducible; an inducer binding to the operator will allow transcription

• Repressible; a repressor binding to the repressor will allow transcription

​The lac operon is inducible, meaning that it is normally turned off or not transcribing genes (due to the repressor binding to the operator region). However, transcription can proceed if an inducer molecule (in the case of the lac operon this will be allolactose) binds to the repressor, preventing the repressor from binding to the operator region and thus allow RNA polymerase to transcribe the genes.

Multiple Choice

The trp operon is normally active or turned “on.” However, when tryptophan is present, it binds to a repressor molecule, activating it, and causing inhibition of transcription of the genes. What type of molecule is tryptophan, and what type of operon is the trp operon?

• inducer; inducible

• corepressor; inducible

• repressor; repressible

• corepressor; repressible

​The trp operon is normally on, meaning that the genes are normally being transcribed by RNA polymerase. Thus, this is a repressible operon. The operon can be turned off or repressed if another molecule (called a corepressor) binds to a repressor and causes the repressor to bind to the operator region (in the case of the trp operon this molecule is tryptophan; it binds to a repressor causing the repressor to bind to the operator region and prevents RNA polymerase from transcribing the genes).

Multiple Choice

The amino acid leucine has several possible codons, due to the degeneracy of the genetic code. This means that there is more than one possible __________ for leucine.

• DNA

• ribosome

• rRNA

• mRNA

• tRNA

​Each tRNA contains the anticodon for a specific mRNA codon and carries the amino acid corresponding to that codon to ribosomes during translation. mRNA is produced by transcription from DNA, and ribosomes translate it into proteins. Multiple codons can code for a single amino acid, and so there can be several tRNA anticodons that could be used for a single amino acid.

Multiple Choice

Which of the following nucleic acids brings an amino acid through the cytosol to a ribosome during translation?

• rRNA

• tRNA

• mRNA

• None of these

​mRNA, tRNA, and rRNA all play a key role in the synthesis of proteins.

tRNA (transfer RNA) is responsible for gathering amino acids in the cytosol and bringing them to the ribosomes when translation is taking place.

mRNA (messenger RNA) is the template for translation. rRNA (ribosomal RNA) is a structural element of the ribosomes.

Multiple Choice

How does a ribosome detect that the correct amino acid is being added during translation?

• Translation factors

• Appropriate base pairing between the mRNA and tRNA

• It doesn’t; correct sequences are checked later by chaperones during protein folding

• Appropriate base pairing between mRNA and rRNA 

​Amino acid sequence is determined by the sequence of codons on mRNA. tRNA is responsible for bringing new amino acids to the ribosome. Interactions between the codons on mRNA and the anticodons on tRNA are what allow the formation of the appropriate peptide bonds.

Chaperones are later used to facilitate the development of protein structure, but are not involved in checking protein sequence.

Multiple Choice

If the enzyme, aminoacyl-tRNA synthetase malfunctions, which of the following processes will be impeded?

• Transcription

• DNA replication

• Translation

• Post-transcriptional RNA processing

​Aminoacyl-tRNA synthetase is responsible for "charging" tRNA with amino acids. During translation, tRNA molecules that are bound to specific amino acids are fed into the ribosome in a specific order that is complementary to the mRNA strand. Once a tRNA is used up, it loses its amino acid. As a result, it must interact with aminoacyl-tRNA synthetase before it can be used again in translation.

A malfunction in aminoacyl-tRNA synthetase would result in a shortage of charged tRNA molecules and a decrease in translation processing.

Multiple Choice

Which of the following choices is the enzyme that adds amino acids to tRNA molecules?

• RNA polymerase

• Aminoacyl-tRNA synthetase

• Synthase

• Primase

​The enzyme aminoacyl-tRNA synthetase joins tRNA molecules with a corresponding amino acid. First, an amino acid is bound to aminoacyl-tRNA synthetase using ATP. Then, the tRNA molecule containing the corresponding anticodon binds to the enzyme. The correct tRNA molecule is identified by aminoacyl-tRNA synthetase by its anticodon sequence and other areas of its structure. Last, the tRNA molecule covalently bonds to the amino acid and is released from aminoacyl-tRNA synthetase.

Multiple Choice

If the codon in the mRNA strand is 5' CAG, what is the anticodon strand sequence?

• 3' GUC

• 5' GTC

• 5' GUC

• 3' GTC

• 3' CUG

​The mRNA strand is synthesized 5' to 3' and contains the codons. tRNA contains the anticodons needed for the corresponding amino acid, and is paired to the codon 3' to 5'.