24 Questions
What is the primary function of introns in eukaryotic genes?
To regulate gene expression
Which codon specifies the amino acid methionine and also acts as a start codon?
AUG
What is the function of the anticodon on a tRNA molecule?
To bind to specific mRNA codons
What is the role of the ribosome in protein synthesis?
To catalyze the chemical reaction that links amino acids
How many codons are there for amino acids?
61
What is the function of the A, P, and E sites on the ribosome?
To provide a set of handy slots for tRNA binding and amino acid delivery
What is the function of the large and small subunits of the ribosome?
To come together around the mRNA and provide a set of handy slots for tRNA binding
What is the role of tRNAs in protein synthesis?
To connect mRNA codons to the amino acids they encode
During the elongation stage of translation, what happens when a new codon is exposed?
A matching tRNA binds to the codon and the amino acid chain is linked to the tRNA
What is the function of tRNA during protein synthesis?
To carry amino acids to the ribosome for protein synthesis
What is the primary function of the 5' cap in eukaryotic mRNA?
To protect the nascent mRNA from degradation and aid in translation initiation
In which stage of translation does the polypeptide chain get released from the ribosome?
Termination
What is the role of the start codon AUG in protein synthesis?
It specifies the first amino acid in the polypeptide chain
What is the main purpose of the RNA-stabilizing proteins that coat the pre-mRNA?
To protect the pre-mRNA from degradation while it is being processed and exported
During elongation, which site of the ribosome does the tRNA move through?
A, P, and E sites
What is the function of the poly-A tail in eukaryotic mRNA?
To protect the mRNA from degradation, aid in export, and bind proteins involved in translation initiation
What is the outcome of the chemical reaction that occurs between the existing polypeptide chain and the amino acid of the tRNA during elongation?
A peptide bond is formed between the amino acids
Why do eukaryotic mRNAs have a longer half-life compared to prokaryotic mRNAs?
Because they undergo additional processing steps that create a more stable molecule
What is the enzyme responsible for adding the poly-A tail to the 3' end of the pre-mRNA?
Poly(A) polymerase (PAP)
What is the consequence of a stop codon entering the ribosome during translation?
The polypeptide chain is released from the ribosome
What is the purpose of removing intervening sequences during eukaryotic mRNA processing?
To specify the appropriate amino acids during protein synthesis
What is the primary function of the 5' cap and 3' poly-A tail in eukaryotic mRNA?
To protect the mRNA from degradation and facilitate translation initiation
Why do eukaryotic pre-mRNAs undergo extensive processing before being translated?
To create a molecule with a longer half-life and facilitate translation
What is the role of the mRNA in protein synthesis?
To provide the genetic code for protein synthesis
Study Notes
Eukaryotic RNA Modifications
- Eukaryotic genes consist of exons (protein-coding sequences) and introns (intervening sequences).
- Introns are removed from pre-mRNA during processing and do not encode functional proteins.
Translation
- There are 61 codons that specify 20 amino acids found in proteins.
- AUG codon specifies methionine and acts as a start codon.
- Three stop codons (UAA, UAG, and UGA) signal the end of protein construction.
- Transfer RNAs (tRNAs) act as molecular bridges, connecting mRNA codons to amino acids.
- Each tRNA has an anticodon that binds to specific mRNA codons and carries the specified amino acid.
- Ribosomes are composed of protein and RNA (rRNA) and provide a site for tRNAs to bind and deliver amino acids.
- Ribosomes have A, P, and E sites where tRNAs can bind and deliver amino acids.
Steps of Translation
Initiation
- The ribosome assembles around the mRNA to be read, and the first tRNA (carrying methionine) binds to the start codon (AUG).
- The initiation complex is required for translation to begin.
Elongation
- The mRNA is read one codon at a time, and the amino acid matching each codon is added to a growing protein chain.
- Each time a new codon is exposed:
- A matching tRNA binds to the codon.
- The existing amino acid chain is linked to the amino acid of the tRNA via a chemical reaction.
- The mRNA is shifted one codon over, exposing a new codon for reading.
- tRNAs move through the A, P, and E sites of the ribosome during elongation.
Termination
- Termination begins when a stop codon (UAG, UAA, or UGA) enters the ribosome.
- The finished polypeptide chain is released, and the polypeptide may still need to fold, undergo processing, or combine with other polypeptides before becoming a functional protein.
Pre-mRNA Processing
- Eukaryotic pre-mRNA undergoes extensive processing before translation.
- The additional steps involved in eukaryotic mRNA maturation create a molecule with a much longer half-life than prokaryotic mRNA.
- Eukaryotic mRNAs last for several hours, whereas E. coli mRNA lasts no more than five seconds.
RNA Stabilization
- Pre-mRNAs are coated in RNA-stabilizing proteins that protect the pre-mRNA from degradation during processing and export out of the nucleus.
5′ Capping
- A 7-methylguanosine cap is added to the 5′ end of the pre-mRNA by a 5′-to-5′ phosphate linkage.
- The cap protects the nascent mRNA from degradation and helps initiate translation by ribosomes.
3′ Poly-A Tail
- An enzyme called poly (A) polymerase (PAP) adds a string of approximately 200 A nucleotides (poly-A tail) to the 3′ end of the pre-mRNA.
- The poly-A tail:
- Protects the mRNA from degradation.
- Aids in the export of the mature mRNA to the cytoplasm.
- Is involved in binding proteins involved in initiating translation.
Learn about eukaryotic RNA modifications, including exon and intron structure, and the process of translation, covering codons, start and stop signals, and transfer RNAs.
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