14.2 Translation: initiation, elongation, and termination
4 min read•august 7, 2024
Translation is the process of turning into proteins. It's like following a recipe to make a dish. The is the kitchen, and the mRNA is the recipe. , , and are the steps in this cooking process.
Initiation sets the stage by assembling the ingredients. Elongation is where the real cooking happens, adding amino acids one by one. Termination is like plating the dish, releasing the finished protein. This process is crucial for making all the proteins our cells need.
Ribosome Structure and Sites
Ribosome Composition and Function
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- Ribosome consists of two subunits (small and large) made up of (rRNA) and proteins
- Ribosomes are the sites of protein synthesis where mRNA is translated into polypeptide chains
- Ribosomes contain three main sites: , , and
- Ribosomes catalyze the formation of peptide bonds between amino acids using peptidyl transferase
Ribosomal Sites and Their Roles
- P site (peptidyl site) holds the carrying the growing polypeptide chain
- A site (aminoacyl site) is where the incoming aminoacyl-tRNA binds, delivering the next amino acid to be added to the polypeptide chain
- E site (exit site) is where the deacylated tRNA exits the ribosome after donating its amino acid to the growing polypeptide chain
- Peptidyl transferase is an rRNA component of the large ribosomal subunit that catalyzes the formation of peptide bonds between the amino acids in the P and A sites
Translation Initiation
mRNA and Initiation Factors
- (mRNA) is the template for protein synthesis, carrying the genetic information from DNA to the ribosomes
- (IFs) are proteins that assist in the assembly of the initiation complex, which includes the small ribosomal subunit, mRNA, and initiator tRNA
- Initiation factors help recognize the (AUG) and promote the binding of the initiator tRNA to the P site of the ribosome
Initiation Complex Formation
- Small ribosomal subunit binds to the 5' end of the mRNA and scans for the start (AUG)
- Start codon is recognized by the of the initiator tRNA (usually tRNA-Met) carrying the amino acid methionine
- Kozak consensus sequence is a nucleotide sequence surrounding the start codon that enhances the efficiency of translation initiation in eukaryotes (consists of a purine at position -3 and a G at position +4 relative to the A of the AUG codon)
- Once the start codon is recognized, the large ribosomal subunit joins the complex, forming the complete ribosome ready for elongation
Elongation and Termination
Elongation Process
- (EFs) are proteins that assist in the elongation phase of translation, ensuring the accurate and efficient addition of amino acids to the growing polypeptide chain
- Aminoacyl-tRNA, carrying a specific amino acid, enters the A site of the ribosome, guided by the codon-anticodon base pairing with the mRNA
- Peptide bond formation occurs between the amino acid in the P site and the incoming amino acid in the A site, catalyzed by the peptidyl transferase
- The ribosome then translocates, moving the tRNA in the A site to the P site, and the deacylated tRNA in the P site to the E site, exposing the next codon for the next aminoacyl-tRNA to bind
Termination of Translation
- (RFs) are proteins that recognize the stop codons (UAA, UAG, or UGA) in the mRNA and trigger the release of the completed polypeptide chain from the ribosome
- When a is encountered in the A site, release factors bind and promote the hydrolysis of the peptidyl-tRNA bond, releasing the polypeptide chain
- The ribosome then dissociates into its subunits, which can be recycled for another round of translation
Polysome Formation
Polysome Structure and Function
- Polysome (or polyribosome) is a complex of multiple ribosomes simultaneously translating the same mRNA molecule
- Multiple ribosomes can bind to a single mRNA, allowing for the simultaneous translation of multiple copies of the same protein
- Polysomes increase the efficiency of protein synthesis by enabling the production of multiple polypeptide chains from a single mRNA template
Factors Affecting Polysome Formation
- mRNA stability plays a role in polysome formation, as more stable mRNAs can support a higher number of ribosomes and thus form larger polysomes
- Translation efficiency is influenced by factors such as the presence of secondary structures in the mRNA, the availability of tRNAs, and the activity of translation factors
- Highly expressed genes often have a higher number of ribosomes per mRNA (higher polysome density), reflecting the increased demand for the encoded proteins (globin mRNA in reticulocytes)
Key Terms to Review (24)
A site: The A site, or aminoacyl site, is one of the three key binding sites on the ribosome during protein synthesis. It is specifically where incoming aminoacyl-tRNA molecules bind to the ribosome, delivering their associated amino acids for incorporation into the growing polypeptide chain. This process is crucial for translation, as it ensures that the correct amino acid is added based on the codon sequence in the mRNA.
Anticodon: An anticodon is a sequence of three nucleotides in a transfer RNA (tRNA) molecule that is complementary to a corresponding codon in messenger RNA (mRNA). This interaction ensures that the correct amino acid is added during protein synthesis by matching the tRNA to the mRNA codon. The specificity of this pairing plays a crucial role in translating the genetic code into functional proteins.
ATP: ATP, or adenosine triphosphate, is a nucleotide that serves as the primary energy carrier in all living cells. It plays a crucial role in cellular processes by providing the energy needed for various biochemical reactions, including metabolism, muscle contraction, and the synthesis of macromolecules.
Codon: A codon is a sequence of three nucleotides in mRNA that corresponds to a specific amino acid or a stop signal during protein synthesis. Each codon is essential for translating the genetic information encoded in DNA into proteins, which are critical for cellular function and structure. Codons are integral to the genetic code, determining how the sequence of nucleotides is read and interpreted by the cellular machinery involved in translation.
E site: The e site, or exit site, is a critical location on the ribosome where the uncharged tRNA exits after delivering its amino acid during protein synthesis. This site plays a significant role in the process of translation, specifically during the termination phase, ensuring that the ribosome efficiently releases the tRNA that has completed its function. The proper functioning of the e site is essential for maintaining the flow of translation and allowing for the next tRNA to enter the ribosome.
Elongation: Elongation is the process in which a polypeptide chain is extended during protein synthesis and RNA strand is extended during transcription. It is a critical phase in both transcription and translation, where nucleotides or amino acids are sequentially added to form a longer RNA molecule or protein, respectively. This phase involves specific enzymatic activities that facilitate the incorporation of building blocks into the growing chain, ensuring accurate synthesis according to the genetic template.
Elongation factors: Elongation factors are proteins that play crucial roles during the elongation phase of protein synthesis, facilitating the addition of amino acids to the growing polypeptide chain. They assist in the accurate and efficient translation of mRNA into a functional protein by promoting the binding of aminoacyl-tRNA to the ribosome, ensuring that the correct amino acids are incorporated in the right sequence. These factors are vital for maintaining the fidelity and speed of translation, which is essential for proper cellular function.
Gtp hydrolysis: GTP hydrolysis is the chemical process in which guanosine triphosphate (GTP) is converted into guanosine diphosphate (GDP) and inorganic phosphate (Pi) through the addition of water. This reaction is crucial in various biological processes, particularly in protein synthesis, where it provides energy and signals for the initiation and elongation of translation.
Initiation: Initiation is the first step in the processes of DNA replication, transcription, and translation, where the molecular machinery assembles at the start site of a gene or a DNA strand. This process is critical because it ensures that the correct sequence of nucleotides or amino acids is synthesized, setting the stage for accurate replication, expression, or protein synthesis. Successful initiation is essential for the proper functioning of all cellular processes that involve genetic information.
Initiation factors: Initiation factors are proteins that play a crucial role in the initiation phase of translation, helping to assemble the ribosome and position the mRNA and the initiator tRNA correctly for protein synthesis. They facilitate the binding of the ribosomal subunits to mRNA, ensure proper codon-anticodon pairing, and help recruit the initiator tRNA, which carries the first amino acid in the polypeptide chain. These factors are essential for ensuring that translation begins accurately and efficiently.
Messenger RNA: Messenger RNA (mRNA) is a single-stranded molecule that carries genetic information from DNA to the ribosome, where proteins are synthesized. It serves as the template for translating the genetic code into a specific sequence of amino acids, thus playing a crucial role in gene expression and protein synthesis.
MRNA: mRNA, or messenger RNA, is a type of RNA that carries genetic information from DNA to the ribosome, where proteins are synthesized. It plays a crucial role in the process of gene expression, serving as a template for translating the encoded information into proteins, which perform essential functions in cells.
P site: The p site, or peptidyl site, is one of the three binding sites on the ribosome that plays a crucial role during protein synthesis. It is where the tRNA carrying the growing polypeptide chain is located during translation. This site facilitates the formation of peptide bonds between amino acids, which is essential for elongating the nascent protein chain.
Peptidyl transferase activity: Peptidyl transferase activity refers to the enzymatic function of forming peptide bonds between amino acids during protein synthesis, specifically during translation. This activity is primarily associated with the large subunit of the ribosome, where it catalyzes the reaction that links the carboxyl group of one amino acid to the amino group of another, thereby elongating the polypeptide chain. The precision and efficiency of this process are crucial for the accurate assembly of proteins based on mRNA templates.
Release Factors: Release factors are proteins that play a crucial role in the termination phase of translation by recognizing stop codons in the mRNA sequence and promoting the release of the newly synthesized polypeptide chain from the ribosome. These factors ensure that protein synthesis is completed accurately and efficiently, preventing errors that could lead to nonfunctional proteins. They work by facilitating the hydrolysis of the bond between the polypeptide and the tRNA, allowing the protein to be released into the cellular environment.
Ribosomal rna: Ribosomal RNA (rRNA) is a type of RNA that is a fundamental component of ribosomes, the molecular machines responsible for protein synthesis in all living cells. rRNA plays a crucial role in the structural and functional framework of ribosomes, enabling them to facilitate the translation of messenger RNA (mRNA) into amino acid sequences during protein synthesis. This process involves three main stages: initiation, elongation, and termination, where rRNA contributes to the accuracy and efficiency of translation.
Ribosome: A ribosome is a complex molecular machine found within all living cells that serves as the site of protein synthesis, where messenger RNA (mRNA) is translated into polypeptides. This essential cellular structure is composed of ribosomal RNA (rRNA) and proteins, functioning in three stages: initiation, elongation, and termination during the translation process, making it vital for gene expression and cellular function.
Start codon: A start codon is a specific sequence of nucleotides in mRNA that signals the beginning of translation, the process where ribosomes synthesize proteins. The most common start codon is AUG, which codes for the amino acid methionine. This codon plays a critical role in determining the reading frame for translation and ensures that the ribosome begins synthesizing the protein at the correct location.
Stop codon: A stop codon is a nucleotide triplet within mRNA that signals the termination of protein synthesis during translation. It plays a crucial role in ensuring that proteins are synthesized to their correct lengths by indicating to the ribosome when to stop adding amino acids to the growing polypeptide chain. There are three specific stop codons: UAA, UAG, and UGA, which do not correspond to any amino acid and instead trigger the release of the newly formed protein from the ribosome.
Termination: Termination is the final step in both transcription and translation processes, where the synthesis of RNA or protein is concluded. During termination, specific signals indicate to the cellular machinery that the process should stop, leading to the release of the newly formed molecule. This process ensures that genes are expressed accurately and that proteins are produced correctly, playing a crucial role in gene regulation and cellular function.
Transfer RNA: Transfer RNA (tRNA) is a type of RNA molecule that plays a crucial role in the process of translation by transporting amino acids to the ribosome, where proteins are synthesized. Each tRNA molecule has a specific anticodon that pairs with the corresponding codon on messenger RNA (mRNA), ensuring that the correct amino acid is added to the growing polypeptide chain. This pairing is essential for accurate protein synthesis, as it allows the genetic code to be translated into functional proteins.
Translocation: Translocation refers to the process in protein synthesis where the ribosome moves along the mRNA strand, allowing for the sequential addition of amino acids to the growing polypeptide chain. This movement is crucial during translation, enabling the correct reading of codons and the incorporation of the corresponding tRNAs, which brings the appropriate amino acids for protein formation.
TRNA: tRNA, or transfer RNA, is a type of RNA molecule that plays a crucial role in the process of translation by bringing amino acids to the ribosome, where proteins are synthesized. Each tRNA molecule has a specific anticodon that pairs with a complementary codon on mRNA, ensuring that the correct amino acid is added to the growing polypeptide chain. This connection is essential for translating genetic information into functional proteins.
Universal genetic code: The universal genetic code is a set of rules that defines how the sequence of nucleotides in DNA and RNA is translated into the sequence of amino acids in proteins. This code is nearly universal across all known organisms, indicating a common evolutionary origin and enabling the processes of translation—initiation, elongation, and termination—where ribosomes read mRNA to synthesize proteins.