Biology Chapter 17: Transcription and Translation

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Questions and Answers

What is the role of the sigma protein in bacterial transcription?

It breaks apart the DNA double helix.

It provides energy for transcription.

It transcribes genes on its own.

It binds to RNA polymerase to form a holoenzyme. (correct)

What is the significance of the -10 and -35 boxes in bacterial promoters?

They signal the end of transcription.

They have no functional significance.

They are coding regions for proteins.

They determine the orientation and template strand for RNA polymerase. (correct)

In which direction does RNA polymerase move during transcription?

In both directions

Upstream

In reverse order

Downstream (correct)

What happens to the DNA double helix when RNA polymerase initiates transcription?

It is opened to create a transcription bubble. Signup and view all the answers

How long are bacterial promoters typically?

40-50 base pairs Signup and view all the answers

What must occur before transcription can begin in bacterial cells?

Sigma must bind to both the -10 and -35 boxes. Signup and view all the answers

What is the function of the transcription bubble created by RNA polymerase?

It allows the complementary NTPs to pair with DNA bases. Signup and view all the answers

Which of the following statements about the RNA polymerase holoenzyme is true?

It is formed when sigma binds to RNA polymerase. Signup and view all the answers

What is the primary function of RNA polymerase during elongation in bacteria?

It reads the DNA template and adds nucleotides to the 3' end of RNA. Signup and view all the answers

What signal leads to termination of transcription in eukaryotic cells?

Poly(A) signal. Signup and view all the answers

Which of the following describes an important difference between bacterial and eukaryotic transcription?

Eukaryotic transcription involves three RNA polymerases. Signup and view all the answers

What is the result of RNA processing in eukaryotes?

The primary transcript becomes a functional mRNA. Signup and view all the answers

In the discovery of split eukaryotic genes, what was observed during the hybridization of viral DNA and mRNA?

Loops formed along the hybrid molecules indicating noncoding stretches. Signup and view all the answers

Which component is specifically absent in the final mature mRNA after RNA processing?

Introns. Signup and view all the answers

Which of the following is a common feature of prokaryotic transcription?

The transcripts are functional immediately after transcription. Signup and view all the answers

What role do general transcription factors play in eukaryotic transcription?

They recognize and bind to promoters. Signup and view all the answers

What are the two main components that make up small nuclear ribonucleoproteins (snRNPs)?

Small nuclear RNAs and proteins Signup and view all the answers

Which step of RNA splicing involves the binding of snRNPs to specific boundaries?

Binding of snRNPs Signup and view all the answers

What is the role of the 5′ cap added to mRNAs in eukaryotes?

Facilitates the binding of ribosomes and protects from degradation Signup and view all the answers

Which of the following accurately describes the fate of introns after splicing?

They are cut out and degraded Signup and view all the answers

What structural formation does an intron take during its removal?

Lariat structure Signup and view all the answers

What is the primary purpose of adding a poly(A) tail to mRNA?

To prevent degradation and assist in translation Signup and view all the answers

How does RNA splicing contribute to the diversity of proteins produced from a single gene?

By generating different combinations of exons Signup and view all the answers

What is contained in the mature mRNA after splicing and capping?

Exons and untranslated regions (UTRs) Signup and view all the answers

What key role do ribosomes play in translation?

They act as the site for protein synthesis. Signup and view all the answers

How do transcription and translation occur in prokaryotes?

They can occur simultaneously with multiple ribosomes on one mRNA. Signup and view all the answers

What evidence did Britten and colleagues provide to demonstrate that ribosomes are the site of protein synthesis?

Using a pulse-chase experiment with radioactive sulfur. Signup and view all the answers

Which of the following statements is true regarding mRNA processing in eukaryotes?

mRNA is synthesized and processed in the nucleus. Signup and view all the answers

What is the primary function of tRNA during translation?

To bring the correct amino acids to the ribosome. Signup and view all the answers

What determines the amino acid sequence during translation?

The sequence of mRNA codons. Signup and view all the answers

What hypothesis suggests a direct interaction between mRNA codons and amino acids?

That mRNA codons interact directly with amino acids. Signup and view all the answers

What is formed when multiple ribosomes attach to a single mRNA molecule in bacteria?

A polyribosome. Signup and view all the answers

What is the first step in the initiation phase of translation in bacteria?

Binding of the small ribosomal subunit to the Shine-Dalgarno sequence Signup and view all the answers

What does the initiator tRNA carry in bacteria?

Modified methionine (f-Met) Signup and view all the answers

What happens to the tRNA molecules during the elongation phase of translation?

The amino acid from the P-site tRNA is transferred to the A-site tRNA Signup and view all the answers

Which of the following accurately describes the role of ribosomal RNA in translation?

Catalyzes the formation of peptide bonds Signup and view all the answers

During translation initiation, where is the initiator tRNA located once the large ribosomal subunit binds?

P site Signup and view all the answers

What hypothesis does the ribosome's status as a ribozyme support?

RNA world hypothesis Signup and view all the answers

How many steps are involved in the initiation phase of translation in bacteria?

Three steps Signup and view all the answers

What occurs immediately after the initiator tRNA binds to the start codon?

Binding of the large ribosomal subunit Signup and view all the answers

Which RNA polymerase characteristic distinguishes eukaryotes from bacteria?

Eukaryotes utilize three separate RNA polymerases Signup and view all the answers

What processing step is NOT typically performed on RNA in bacteria?

Addition of a 5' cap Signup and view all the answers

What feature of eukaryotic promoters is notably different from bacterial promoters?

Eukaryotic promoters are more variable and larger Signup and view all the answers

Which of the following statements about translation initiation in bacteria is accurate?

It is simpler compared to eukaryotic translation initiation Signup and view all the answers

What occurs during post-translational modification of polypeptides?

Polypeptides become functional through processing Signup and view all the answers

Which RNA processing step is uniquely required for eukaryotic mRNA?

Intron removal Signup and view all the answers

What is a common feature of elongation during translation in both bacteria and eukaryotes?

The elongation process functions the same in both Signup and view all the answers

Which proteins are associated with eukaryotic promoters during transcription?

Various general transcription factors Signup and view all the answers

Study Notes

Chapter 17: Transcription, RNA Processing, and Translation

The information in genes directs the synthesis of RNAs and proteins.
This chapter examines how information in DNA is transcribed into RNA and how that RNA is processed and translated into proteins.

17.1 An Overview of Transcription

RNA polymerases synthesize an RNA version of the instructions stored in DNA.
They use ribonucleoside triphosphates (NTPs).
RNA polymerases match complementary bases to one DNA strand.
One strand of DNA serves as a template (non-template/coding strand matches the mRNA sequence except U for T).

Bacterial and Eukaryotic Transcription Differences

Bacteria have one RNA polymerase, eukaryotes have at least three distinct types (I, II, and III).
RNA polymerases perform template-directed synthesis in the 5' → 3' direction.
Unlike DNA polymerases, RNA polymerase doesn't require a primer to begin transcription.

17.2 RNA Processing in Eukaryotes

In bacteria, transcription produces fully functional RNAs.
In eukaryotes, the initial product of transcription is an immature primary transcript (pre-mRNA) which needs RNA processing.
Primary transcripts must undergo RNA processing before translation.

17.4 The Structure and Function of Transfer RNA

Transfer RNA (tRNA) is the adapter molecule used in translation.
An aminoacyl tRNA is a tRNA molecule linked to its amino acid.
Amino acids are transferred from tRNAs to the growing polypeptide chain.

The Structure of tRNA

tRNA molecules are relatively short (75-95 nucleotides).
They form secondary structures (stem-and-loop) by folding.
A CCA sequence at the 3' end serves as the amino acid binding site.
The loop at the opposite end contains the anticodon - a sequence of three nucleotides that base-pairs with the mRNA codon.

How Are Amino Acids Attached to tRNAs?

ATP is required to attach tRNA to amino acids.
Aminoacyl-tRNA synthetases attach amino acids to tRNAs.
There are 20 amino acids, and each has a specific aminoacyl-tRNA synthetase.

How Many tRNAs Are There?

There are 61 codons but about 40 tRNAs in most cells.
Crick proposed that the anticodon's third position allows for nonstandard base pairing, allowing one tRNA to read more than one codon.

17.5 Ribosome Structure and Function in Translation

Ribosomes contain many proteins and ribosomal RNA (rRNA).
Ribosomes can be separated into two subunits: small (holds mRNA) and large (where peptide bonds form).
During translation, three tRNAs line up within the ribosome.

Ribosome Structure and Function in Translation (more detail)

tRNAs fit into three sites on the ribosome (A, P, and E). Each tRNA is bound to corresponding mRNA codons.
A site (acceptor or aminoacyl): tRNA carries amino acid
P site (peptidyl): holds growing peptide chain
E site (exit): tRNA without amino acid exits the ribosome.
The ribosome synthesizes proteins in a three-step sequence (Arrival of aminoacyl tRNA, Peptide bond formation, Translocation).
Amino acids are always attached to the carboxyl end (C-terminus) of the growing polypeptide.
Translation has three phases: initiation, elongation, and termination.

Initiating Translation

The initiation phase of translation starts near the AUG start codon.
In bacteria, the small ribosomal subunit binds to the ribosome binding site (Shine-Dalgarno sequence) on mRNA.
About 6 bases upstream from the start codon.
Mediated by initiation factors.
The first tRNA is called initiator tRNA (carries a modified methionine, f-Met, in bacteria).
Steps of Translation Initiation (mRNA binds to small ribosomal subunit, Initiator tRNA binds, large ribosomal subunit binds).

Elongation: Extending the Polypeptide

At the start of elongation, the initiator tRNA is in the P site, and the E and A sites are empty.
An aminoacyl tRNA binds to the codon in the A site.
Amino acids at the P and A sites are in the ribosome's active site.
The amino acid on the P-site tRNA is transferred to the amino acid on the A-site tRNA.
This translocation process repeats, moving the ribosome along the mRNA.

Is the Ribosome an Enzyme or a Ribozyme?

The active site of the ribosome is entirely ribosomal RNA.
Ribosomal RNA catalyzes peptide bond formation, showing the ribosome is a ribozyme.
This supports the RNA world hypothesis.

Moving Down the mRNA

Translocation occurs when the ribosome moves one codon toward the 3' end of the mRNA.
Elongation factors help with ribosome movement.
Translocation accomplishes these three things:
- The uncharged tRNA moves to the E-site and exits the ribosome.
- The tRNA attached to the growing protein moves to the P-site.
- The A-site is now available to accept a new aminoacyl tRNA.

Terminating Translation

Termination occurs when the A site encounters a stop codon.
A release factor enters the A site.
It resembles tRNAs, hydrolyzes the bond linking the P-site tRNA to the polypeptide chain, and thus terminates translation.
The newly synthesized polypeptide, tRNAs, and ribosomal subunits separate from the mRNA.

17.3 An Introduction to Translation

The sequence of mRNA bases is translated into an amino acid sequence.
Translation is a complex series of steps carried out by ribosomes, mRNA, and tRNAs.

17.1 An Overview of Transcription: Additional Details

The non-template/coding strand is used for comparison to mRNA
Transcription is the synthesis of RNA from a DNA template.
RNA polymerase opens DNA helix to form a transcription bubble.
NTPs pair with complementary DNA bases for polymerization to initiate.

Gene Expression in Eukaryotes

Eukaryotic gene expression is more complex, involving multiple steps, including RNA processing.
mRNA processing in eukaryotes involves: capping the 5' end, splicing to remove introns, and adding a poly(A) tail at the 3' end
mRNA is processed in the nucleus, and translation occurs in the cytoplasm.
Polypeptides are modified after translation (post-translation) by various processes.

Polypeptide Folding and Chemical Modifications

Folding determines protein shape and function.
Molecular chaperones guide protein folding.
Sugars or lipids may be added to proteins.
Enzymes may add phosphate groups to alter protein activity.

Table Summary (17.1)

Presents a comparison of transcription, RNA processing and translation in prokaryotes and eukaryotes highlighting key differences in the number of RNA polymerases, promoter structure, proteins associated with promoters, and the complexities of RNA processing.

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Description

This quiz covers Chapter 17 of Biology, focusing on the processes of transcription, RNA processing, and translation of proteins. You'll learn about the roles of RNA polymerases, the differences between bacterial and eukaryotic transcription, and the importance of RNA processing in gene expression.