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What are the three types of RNA needed for protein synthesis?
What are the three types of RNA needed for protein synthesis?
Where are rRNAs found?
Where are rRNAs found?
Ribosomes
How much of the total RNA in a cell is made up of rRNA?
How much of the total RNA in a cell is made up of rRNA?
Prokaryotic cells have 23S, 16S, and 5S rRNAs.
Prokaryotic cells have 23S, 16S, and 5S rRNAs.
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Which of these statements about tRNAs is true?
Which of these statements about tRNAs is true?
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A tRNA molecule carries its specific amino acid attached to its 5'-end.
A tRNA molecule carries its specific amino acid attached to its 5'-end.
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What percentage of total RNA in the cell is comprised of mRNA?
What percentage of total RNA in the cell is comprised of mRNA?
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MRNA carries genetic information from nuclear DNA to the cytoplasm.
MRNA carries genetic information from nuclear DNA to the cytoplasm.
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Which type of mRNA is characteristic of prokaryotes?
Which type of mRNA is characteristic of prokaryotes?
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Eukaryotic mRNA is typically polycistronic.
Eukaryotic mRNA is typically polycistronic.
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Eukaryotic mRNA has a 5’ cap and a 3’ poly A tail.
Eukaryotic mRNA has a 5’ cap and a 3’ poly A tail.
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What is the function of the 5' cap on eukaryotic mRNA?
What is the function of the 5' cap on eukaryotic mRNA?
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What is the function of the 3' poly A tail on eukaryotic mRNA?
What is the function of the 3' poly A tail on eukaryotic mRNA?
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What type of enzyme is RNA polymerase?
What type of enzyme is RNA polymerase?
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What is the function of the sigma subunit in prokaryotic RNA polymerase?
What is the function of the sigma subunit in prokaryotic RNA polymerase?
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In which direction is RNA synthesized?
In which direction is RNA synthesized?
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What is the name of the region on DNA where RNA polymerase binds?
What is the name of the region on DNA where RNA polymerase binds?
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What are the two consensus sequences that are recognized by prokaryotic RNA polymerase?
What are the two consensus sequences that are recognized by prokaryotic RNA polymerase?
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The -35 sequence is located 35 bases upstream of the transcription start site.
The -35 sequence is located 35 bases upstream of the transcription start site.
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The Pribnow box is located 10 bases upstream of the transcription start site.
The Pribnow box is located 10 bases upstream of the transcription start site.
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A mutation in either the -10 or -35 sequence can affect transcription of the gene.
A mutation in either the -10 or -35 sequence can affect transcription of the gene.
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The sigma subunit of the prokaryotic RNA polymerase remains attached throughout transcription.
The sigma subunit of the prokaryotic RNA polymerase remains attached throughout transcription.
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RNA polymerase requires a primer for transcription initiation.
RNA polymerase requires a primer for transcription initiation.
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RNA polymerase has a proofreading activity.
RNA polymerase has a proofreading activity.
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DNA polymerase unwinds DNA during transcription.
DNA polymerase unwinds DNA during transcription.
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What is the role of DNA topoisomerases I and II in transcription?
What is the role of DNA topoisomerases I and II in transcription?
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What is the role of the rho (p) factor in transcription?
What is the role of the rho (p) factor in transcription?
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P-independent termination requires the rho (p) factor.
P-independent termination requires the rho (p) factor.
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Rifampin inhibits transcription by binding to the beta subunit of prokaryotic RNA polymerase.
Rifampin inhibits transcription by binding to the beta subunit of prokaryotic RNA polymerase.
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Rifampin affects both prokaryotic and eukaryotic transcription.
Rifampin affects both prokaryotic and eukaryotic transcription.
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What is an operon?
What is an operon?
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The lacz gene codes for beta-galactosidase.
The lacz gene codes for beta-galactosidase.
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The lacy gene encodes for a permease.
The lacy gene encodes for a permease.
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The lacA gene codes for a transacetylase that is essential for lactose metabolism.
The lacA gene codes for a transacetylase that is essential for lactose metabolism.
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Which of the following is NOT a component of the lac operon regulatory region?
Which of the following is NOT a component of the lac operon regulatory region?
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The lac operon is switched on when glucose is present and lactose is absent.
The lac operon is switched on when glucose is present and lactose is absent.
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The repressor protein binds to the operator site.
The repressor protein binds to the operator site.
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The CAP protein binds to the promoter site.
The CAP protein binds to the promoter site.
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When the CAP protein is bound to the promoter site, transcription is inhibited.
When the CAP protein is bound to the promoter site, transcription is inhibited.
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When both glucose and lactose are present, the lac operon is switched on.
When both glucose and lactose are present, the lac operon is switched on.
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Study Notes
RNA Structure & Synthesis (I)
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Learning objectives include describing RNA structure, prokaryotic RNA polymerase properties, steps in RNA synthesis, and transcription from bacterial operons.
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Three types of RNA are needed for protein synthesis: ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA).
Ribosomal RNA (rRNA)
- rRNA is associated with proteins in ribosomes, which are sites for protein synthesis.
- Prokaryotic rRNA types are 23S, 16S, and 5S.
- Eukaryotic rRNA types are 28S, 18S, 5.8S, and 5S.
- Svedberg units (S) are related to molecular weight and shape of the compound.
- rRNA comprises about 80% of total RNA in a cell.
Transfer RNA (tRNA)
- tRNA is the smallest type of RNA molecule (4S).
- There is at least one specific tRNA for each of the 20 common amino acids in proteins.
- tRNA makes up approximately 15% of total RNA in a cell.
- It acts as an adapter molecule, carrying a specific amino acid (covalently attached to its 3' end) to the site of protein synthesis.
- tRNA recognizes the genetic code word on mRNA, specifying amino acid addition to the growing peptide chain.
Messenger RNA (mRNA)
- mRNA comprises about 5% of total RNA in a cell.
- mRNA is the most heterogeneous RNA type, varying in size (500 to 6000 nucleotides) and base sequence.
- mRNA carries genetic information from nuclear DNA to the cytoplasm.
- mRNA can be polycistronic (carrying information from multiple genes), characteristic of prokaryotes, or monocistronic (carrying information from a single gene), characteristic of eukaryotes.
- Eukaryotic mRNA has a poly-A tail on the 3' end, and a "cap" (7-methylguanosine) on the 5' end.
Properties of Prokaryotic RNA Polymerase
- Prokaryotic RNA polymerase synthesizes all RNA, except for short RNA primers needed for DNA replication.
- It is a multisubunit enzyme recognizing a nucleotide sequence (promoter region) at the beginning of a DNA segment to be transcribed.
- It makes a complementary RNA copy of the DNA template strand.
- It recognizes the end of the DNA sequence to be transcribed (terminal region).
Core Enzyme
- The core enzyme is essential for enzyme assembly.
- It contains specific subunits: 2α, β, and β'.
- It is responsible for 5' → 3' RNA polymerase activity and template binding.
- The core enzyme lacks the ability to recognize the promoter region on the DNA template.
- The fifth subunit (Q) shows an unclear in vivo function.
Holoenzyme
- The holoenzyme comprises the core enzyme and the σ subunit ("sigma factor").
- The σ subunit helps RNA polymerase recognize promoter regions on the DNA.
- Different σ factors recognize different groups of genes.
Termination Factor
- Some termination signals in the DNA are recognized by RNA polymerase itself.
- Other termination signals are recognized by termination factors (like the rho factor (ρ) in E. coli).
Steps in RNA Synthesis (Prokaryotes)
- RNA synthesis involves three stages: initiation, elongation, and termination.
- Initiation begins with RNA polymerase binding to a promoter region.
- The promoter region has characteristic consensus sequences recognized by the σ factors of RNA polymerase (like -35 and Pribnow box sequences).
- A mutation in either the -35 or -10 sequence can potentially influence the transcription of the gene.
Elongation
- Holoenzyme recognizes promoter and begins to synthesize a transcript of a DNA sequence (commonly starting with a purine).
- Elongation is marked when the transcript length surpasses ten nucleotides.
- The σ subunit is released during elongation..
- RNA polymerase itself does not require a primer, and does not have proofreading ability.
- RNA polymerase uses ribonucleoside triphosphates, releasing pyrophosphate as each nucleotide is added.
- DNA unwinding caused by RNA polymerase binding results in supercoils that can be alleviated by DNA topoisomerases.
Termination
- Termination occurs when a signal is reached.
- p-dependent termination requires an additional protein (Rho factor).
- p-independent termination relies on sequences in the DNA that cause RNA secondary structure formation (hairpin loop). A formation of a hairpin structure is a common mechanism for termination
Action of Antibiotics
- Some antibiotics inhibit bacterial cell growth by inhibiting RNA synthesis.
- Example: Rifampin inhibits transcription initiation by binding to the β subunit of prokaryotic RNA polymerase and interferes with the formation of the first phosphodiester bond.
Transcription from Bacterial Operons
- In bacteria, structural genes for metabolic pathway proteins are grouped together on a chromosome with regulatory genes.
- These groups of genes often are transcribed into a single mRNA molecule, allowing coordinated gene expression (operons).
- The lactose operon of E. coli is an example of a bacterial operon.
The Lactose Operon (lac Operon)
- The lac operon codes for three enzymes involved in lactose catabolism.
- The lacz gene codes for β-galactosidase, which hydrolyzes lactose into glucose and galactose.
- The lacy gene codes for a permease, facilitating lactose movement into the cell.
- The lacA gene codes for thiogalactoside transacetylase.
Lac Operon Regulation
- Lac operon regulation is controlled by a regulatory portion including a catabolite gene activator protein (CAP) binding site and a promoter (P) region where RNA polymerase binds.
- An operator (O) site is also present.
- The presence or absence of glucose and lactose influences the activity of the repressor protein and CAP.
- Repressor binding to the operator inhibits transcription.
- Allolactose binding to the repressor protein inactivates the repressor and allows transcription.
- CAP, when bound to cAMP, activates transcription when lactose is present and glucose is absent.
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Description
This quiz covers the essential aspects of RNA structure and synthesis, including different types of RNA such as rRNA, tRNA, and mRNA. Participants will learn about the roles of RNA in protein synthesis and the properties of prokaryotic RNA polymerase. Test your knowledge on the steps involved in RNA synthesis and the importance of transcription from bacterial operons.
