15.5 Ribosomes and Protein Synthesis
4 min read•june 14, 2024
Ribosomes are the protein-making factories of cells. They read the from and build proteins by linking amino acids. This process, called , involves three main steps: , , and .
Protein synthesis is crucial for life, as proteins perform most cellular functions. Understanding how ribosomes work helps us grasp how genetic information becomes functional molecules and how cells adapt to their environment.
Ribosomes and Protein Synthesis
Steps of protein synthesis
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- Initiation
- Small ribosomal subunit binds to mRNA at the () which signals the beginning of the protein-coding sequence
- , carrying the , base pairs with the in the of the
- Large ribosomal subunit joins to form a complete ribosome ready to synthesize the chain
- Elongation
- with complementary to the next on mRNA enters the of the ribosome
- , an enzymatic component of the ribosome, catalyzes the formation of a between the amino acids attached to the tRNAs in the A and P sites
- Ribosome translocates to the next codon, moving the tRNA from the A site to the P site and leaving the A site vacant for the next tRNA to enter
- Process repeats, adding amino acids one by one to the growing polypeptide chain according to the mRNA sequence
- Termination
- (, , or ) is encountered on the mRNA, signaling the end of the protein-coding sequence
- Release factors bind to the stop codon in the A site, triggering the hydrolysis of the bond between the polypeptide chain and the tRNA in the P site
- Completed polypeptide is released from the ribosome
- Ribosome dissociates into its small and large subunits, which can be recycled for another round of protein synthesis
Interactions during translation
- Ribosomes
- Consist of a small and that work together to synthesize proteins
- Contain three binding sites for tRNA: A (aminoacyl) site where new tRNAs enter, P (peptidyl) site where the growing polypeptide chain is attached, and E (exit) site where uncharged tRNAs leave the ribosome
- Catalyze the formation of peptide bonds between amino acids using peptidyl transferase activity
- tRNAs
- Adapter molecules that carry specific amino acids to the ribosome based on the genetic code
- Contain an anticodon loop with a triplet of nucleotides complementary to the mRNA codon
- Deliver amino acids to the A site of the ribosome, allowing them to be added to the growing polypeptide chain
- Amino acids
- Building blocks of proteins that determine the structure and function of the final protein product
- Attached to the 3' end of tRNAs by aminoacyl-tRNA synthetases, enzymes that ensure the correct pairing of amino acids with their cognate tRNAs
- Joined together by peptide bonds formed in the ribosome during the elongation phase of protein synthesis
Genetic code and protein structure
- Genetic code: The set of rules by which information encoded in genetic material is translated into proteins
- Codons: Three- sequences in mRNA that specify particular amino acids or signal the start or stop of
- Amino acids: The building blocks of proteins, specified by codons in the genetic code
- Polypeptide: A chain of amino acids linked by peptide bonds, forming the of a protein
- Nucleotides: The basic structural units of nucleic acids (DNA and RNA), consisting of a sugar, a phosphate group, and a nitrogenous base
Prokaryotic vs eukaryotic synthesis
- Initiation
- Prokaryotes: , a purine-rich region upstream of the start codon on mRNA, binds to the complementary sequence on the of the small ribosomal subunit to initiate translation
- Eukaryotes: Require a and on mRNA for efficient translation initiation, along with multiple (eIFs) that help recruit the ribosome to the mRNA
- Ribosomes
- Prokaryotes: , consisting of a and a
- Eukaryotes: , consisting of a and a , which are larger and more complex than their prokaryotic counterparts
- mRNA processing
- Prokaryotes: mRNA can be translated directly as it is being transcribed due to the lack of a nuclear membrane (coupled and translation)
- Eukaryotes: mRNA must undergo modifications (5' capping, splicing to remove introns, and 3' polyadenylation) and be exported from the nucleus to the cytoplasm before translation can occur
- Antibiotic sensitivity
- Some antibiotics (, ) specifically target prokaryotic ribosomes by binding to sites that differ between prokaryotic and eukaryotic ribosomes, allowing selective inhibition of bacterial protein synthesis without harming eukaryotic cells
Key Terms to Review (62)
16S rRNA: 16S rRNA is a component of the small subunit of prokaryotic ribosomes and plays a crucial role in protein synthesis by facilitating the correct alignment of the ribosome with mRNA. This ribosomal RNA molecule is essential for the decoding process, allowing transfer RNA (tRNA) to properly match amino acids to the corresponding codons on the mRNA strand during translation. Its highly conserved nature across different species makes it a valuable tool in phylogenetic studies and microbial taxonomy.
30S small subunit: The 30S small subunit is one of the two components of the prokaryotic ribosome, playing a crucial role in the initiation and elongation phases of protein synthesis. It is responsible for binding mRNA and the initial tRNA, ensuring that the genetic code is accurately translated into a corresponding amino acid sequence. The 30S small subunit also contains important rRNA and proteins that contribute to its structural integrity and function.
40S small subunit: The 40S small subunit is one of the two components of the ribosome, specifically found in eukaryotic cells. It plays a crucial role in protein synthesis by facilitating the initiation of translation, where messenger RNA (mRNA) is decoded to produce proteins. This subunit, along with the larger 60S subunit, forms the functional ribosome that carries out the essential process of translating genetic information into polypeptide chains.
5' 7-methylguanosine cap: The 5' 7-methylguanosine cap is a modified guanine nucleotide that is added to the 5' end of eukaryotic mRNA transcripts during transcription. This cap plays a crucial role in protecting mRNA from degradation, facilitating mRNA transport out of the nucleus, and assisting in the initiation of translation by ribosomes.
50S large subunit: The 50S large subunit is one of the two components of prokaryotic ribosomes, playing a vital role in protein synthesis. It consists of rRNA and proteins that work together to facilitate the translation process by ensuring that amino acids are assembled into polypeptides in the correct order according to mRNA sequences. This subunit interacts with the small 30S subunit to form a functional ribosome that translates mRNA into proteins, making it essential for cellular function and growth.
60S large subunit: The 60S large subunit is one of the two main components of a eukaryotic ribosome, playing a crucial role in protein synthesis. This subunit contains ribosomal RNA (rRNA) and proteins, providing the structural framework for the ribosome and facilitating the enzymatic activity needed for peptide bond formation during translation. It works in conjunction with the smaller 40S subunit to decode mRNA and assemble amino acids into polypeptides.
70S ribosomes: 70S ribosomes are a type of ribosome found in prokaryotic cells, characterized by their sedimentation coefficient of 70S, which reflects their size and density. These ribosomes are essential for protein synthesis, translating messenger RNA (mRNA) into polypeptide chains by facilitating the binding of transfer RNA (tRNA) and the ribosomal subunits during translation. The 'S' stands for Svedberg units, which measure how fast particles sediment in a centrifugal field, indicating that 70S ribosomes are smaller than the 80S ribosomes found in eukaryotic cells.
80S ribosomes: 80S ribosomes are the type of ribosomes found in eukaryotic cells, composed of a large 60S subunit and a small 40S subunit. They play a crucial role in protein synthesis by facilitating the translation of messenger RNA (mRNA) into polypeptide chains, which eventually fold into functional proteins. The 'S' in 80S refers to the Svedberg unit, a measure of the sedimentation rate during centrifugation, indicating the size and density of the ribosomal particles.
A site: The A site, or aminoacyl site, is one of the three functional sites found on a ribosome during the process of protein synthesis. It plays a crucial role in the translation of mRNA into a polypeptide chain by providing a binding location for incoming aminoacyl-tRNA molecules that carry specific amino acids. The correct pairing of tRNA anticodons with mRNA codons occurs at this site, facilitating the addition of amino acids to the growing peptide chain.
Amino Acid: Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid contains a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain or R group, which determines the properties and identity of the amino acid. These compounds play crucial roles in cellular functions, including serving as precursors for neurotransmitters and hormones.
Aminoacyl tRNA synthetases: Aminoacyl tRNA synthetases are enzymes that attach the correct amino acid to its corresponding tRNA molecule during protein synthesis. They play a crucial role in translating genetic information into functional proteins.
Aminoacyl-tRNA synthetase: Aminoacyl-tRNA synthetase is an enzyme that plays a crucial role in protein synthesis by attaching the appropriate amino acid to its corresponding tRNA molecule, ensuring that the genetic code is accurately translated into proteins. This enzyme is essential for interpreting the genetic code, as it facilitates the correct pairing of amino acids with their codons during translation, thus linking the genetic information carried by mRNA to the polypeptide chain being formed on the ribosome.
Anticodon: An anticodon is a sequence of three nucleotides in transfer RNA (tRNA) that corresponds to a complementary codon in messenger RNA (mRNA). This pairing is crucial for the accurate translation of genetic information into proteins, as the anticodon ensures that the correct amino acid is added to the growing polypeptide chain during protein synthesis. The interaction between anticodons and codons underpins the genetic code and is essential for ribosomal function.
AUG: AUG is the nucleotide sequence that serves as the start codon in protein synthesis, signaling the beginning of translation. It codes for the amino acid methionine and plays a critical role in ensuring that ribosomes know where to begin assembling proteins. The presence of AUG allows the ribosome to correctly interpret the messenger RNA (mRNA) sequence, ensuring that proteins are synthesized accurately and efficiently.
Codon: A codon is a sequence of three nucleotides in DNA or RNA that corresponds to a specific amino acid or a stop signal during protein synthesis. Each codon plays a crucial role in translating genetic information into proteins, which are essential for various cellular functions. The arrangement of codons in mRNA determines the sequence of amino acids in a polypeptide chain, directly influencing the structure and function of proteins.
Conjugation: Conjugation is a process where genetic material is transferred from one bacterial cell to another through direct contact. It often involves the transfer of plasmids which can carry beneficial genes such as antibiotic resistance.
Deoxynucleotide: Deoxynucleotide is a molecule consisting of a nitrogenous base, a deoxyribose sugar, and one or more phosphate groups. It is the basic building block of DNA.
E site: The E site, or exit site, is one of the three binding sites for tRNA on the ribosome during protein synthesis. This site is crucial for the release of uncharged tRNA molecules after they have donated their amino acids to the growing polypeptide chain. Understanding the function of the E site helps clarify the process of translation and how ribosomes facilitate protein production.
Elongation: Elongation refers to the stage in transcription and translation where nucleotides or amino acids are sequentially added to a growing RNA or polypeptide chain, respectively. During this process, RNA polymerase or ribosomes catalyze the addition of these building blocks, allowing for the synthesis of RNA in transcription and proteins in translation. Elongation is crucial for gene expression and is characterized by specific mechanisms that ensure accuracy and efficiency.
Eukaryotic initiation factor-2 (eIF-2): Eukaryotic initiation factor-2 (eIF-2) is a protein complex essential for the initiation of mRNA translation in eukaryotic cells. It plays a critical role in the formation of the pre-initiation complex by binding GTP and the initiator tRNA, facilitating its attachment to the small ribosomal subunit.
Eukaryotic initiation factors: Eukaryotic initiation factors are a set of proteins that play essential roles in the initiation phase of protein synthesis in eukaryotic cells. These factors help recruit ribosomes to the mRNA and facilitate the assembly of the translation machinery, ensuring that protein synthesis begins correctly at the start codon. Their function is crucial for the accurate and efficient translation of genetic information into proteins.
Genetic Code: The genetic code is a set of rules that defines how sequences of nucleotides in DNA and RNA are translated into proteins. It is universal across nearly all living organisms, allowing the information stored in genes to be expressed as functional proteins, which play critical roles in biological processes and cellular functions.
Initiation: Initiation is the first step in the processes of transcription and translation, where the necessary components come together to begin the synthesis of RNA or proteins. In transcription, it involves the binding of RNA polymerase to the promoter region of DNA, while in translation, it marks the assembly of the ribosome and the first tRNA carrying the amino acid to start protein synthesis. This step is crucial as it sets the stage for accurate and efficient gene expression.
Initiation complex: The initiation complex is a multi-component assembly formed during the early stage of translation in protein synthesis. It consists of mRNA, the ribosomal subunits, initiator tRNA, and various initiation factors.
Initiator tRNA: Initiator tRNA (transfer RNA) is a specialized tRNA molecule that is responsible for recognizing the start codon (AUG) on mRNA and initiating protein synthesis. It carries the amino acid methionine in eukaryotes or a formylated methionine (fMet) in prokaryotes.
Kozak’s rules: Kozak's rules describe the optimal sequence of nucleotides surrounding the start codon (AUG) in eukaryotic mRNA for efficient translation initiation. The consensus sequence is gccRccAUGG, where R is a purine (adenine or guanine).
Large subunit: The large subunit is a crucial component of ribosomes, which are the cellular machinery responsible for protein synthesis. This subunit plays a significant role in forming peptide bonds between amino acids during translation, effectively building polypeptide chains that will fold into functional proteins. Its structure and interactions with the small subunit and tRNA are essential for the accurate translation of mRNA into proteins.
Leucine: Leucine is an essential branched-chain amino acid that plays a crucial role in protein synthesis and muscle repair. It is one of the building blocks of proteins and is necessary for the growth and recovery of muscle tissue, making it important for athletes and those engaging in resistance training. Additionally, leucine has regulatory functions in metabolic pathways, linking it to energy balance and the response to nutrient intake.
Messenger RNA (mRNA): Messenger RNA (mRNA) is a single-stranded molecule that carries genetic information from DNA to the ribosome, where proteins are synthesized. It serves as a template for translating genetic code into amino acids, forming proteins.
Methionine: Methionine is an essential amino acid that plays a critical role in protein synthesis and various metabolic processes in the body. It is one of the building blocks of proteins and is specified by the codon AUG in the genetic code, which also serves as the start codon for translation during protein synthesis.
MRNA: mRNA, or messenger RNA, is a single-stranded molecule that carries genetic information from DNA to the ribosome, where proteins are synthesized. This process is essential for translating the genetic code into functional proteins, connecting it to various cellular processes and regulation mechanisms.
Nucleotide: A nucleotide is the basic building block of nucleic acids, such as DNA and RNA, composed of three components: a phosphate group, a five-carbon sugar, and a nitrogenous base. These components work together to form the structure of DNA and RNA, enabling the storage and transmission of genetic information.
P site: The P site, or peptidyl site, is one of the three binding sites on a ribosome responsible for protein synthesis. This site holds the tRNA molecule that carries the growing polypeptide chain during translation. As ribosomes move along the mRNA, the P site plays a crucial role in ensuring that amino acids are added in the correct sequence to form a functional protein.
Peptide bond: A peptide bond is a covalent bond that forms between the amino group of one amino acid and the carboxyl group of another, releasing a molecule of water in a dehydration synthesis reaction. This bond is fundamental in linking amino acids together to form proteins, which are essential for various biological functions. Understanding peptide bonds is crucial for grasping how proteins are synthesized and how genetic information is translated into functional molecules.
Peptidyl transferase: Peptidyl transferase is an enzymatic activity associated with ribosomes that catalyzes the formation of peptide bonds between amino acids during protein synthesis. This process is crucial for linking amino acids together to form polypeptides, which are the building blocks of proteins. By facilitating the transfer of the growing polypeptide chain from the tRNA in the P-site to the amino acid in the A-site, peptidyl transferase plays a key role in the overall mechanism of translation.
Poly-A tail: A poly-A tail is a stretch of adenine nucleotides added to the 3' end of an mRNA molecule during post-transcriptional modification. This tail plays a crucial role in stabilizing the mRNA, aiding in its export from the nucleus, and enhancing the efficiency of translation by ribosomes. The presence of the poly-A tail ensures that mRNA can be successfully translated into proteins by providing a protective buffer against degradation and facilitating interactions with the ribosomal machinery.
Polypeptide: A polypeptide is a chain of amino acids linked together by peptide bonds, forming the basic structure of proteins. These chains can vary in length and sequence, determining the specific structure and function of the resulting protein. Polypeptides play a crucial role in various biological processes as they fold into unique three-dimensional shapes, which are essential for their activity.
Polysome: A polysome, also known as a polyribosome, is a complex formed by multiple ribosomes simultaneously translating a single mRNA strand. This structure allows for the efficient synthesis of many copies of a protein from one mRNA molecule.
Post-transcriptional: Post-transcriptional regulation refers to the control of gene expression at the RNA level, after transcription has occurred. This can include processes such as RNA splicing, editing, transport, and degradation.
Post-translational: Post-translational refers to modifications made to a protein after it has been synthesized in a cell. These changes can affect the protein’s function, localization, stability, or interactions with other molecules.
Primary structure: Primary structure is the unique sequence of amino acids in a polypeptide chain. This sequence determines the protein's final 3D shape and its specific function.
Primary Structure: Primary structure refers to the specific sequence of amino acids in a polypeptide chain, which determines the unique characteristics and functions of proteins. This sequence is crucial because it dictates how the protein will fold into its three-dimensional shape, ultimately affecting its biological activity. The primary structure is formed during protein synthesis when ribosomes translate mRNA into a chain of amino acids, highlighting the essential connection between genetic information and protein function.
Protein synthesis: Protein synthesis is the biological process through which cells generate new proteins, essential for various cellular functions and structures. This process is intricately linked to the flow of genetic information from DNA to RNA and ultimately to the formation of proteins, highlighting the connection between genes and the traits they encode.
Ribosomal RNA (rRNA): Ribosomal RNA (rRNA) is a type of RNA that combines with proteins to form ribosomes. These ribosomes are the cellular structures where protein synthesis occurs.
Ribosome: A ribosome is a complex molecular machine found within all living cells that serves as the site of protein synthesis. It reads the genetic code carried by messenger RNA (mRNA) and translates it into polypeptide chains, which then fold into functional proteins. Ribosomes can be found freely floating in the cytoplasm or attached to the endoplasmic reticulum, playing a crucial role in cellular function and gene expression.
RRNA: Ribosomal RNA (rRNA) is a type of non-coding RNA that is a fundamental component of ribosomes, which are the cellular structures responsible for protein synthesis. rRNA molecules provide structural support and catalyze the chemical reactions involved in translating messenger RNA (mRNA) into proteins. This makes rRNA crucial for cellular function and gene expression, linking it closely to the processes of transcription and translation.
Shine-Dalgarno sequence: The Shine-Dalgarno sequence is a ribosomal binding site located in bacterial mRNA that is essential for the initiation of translation. This short, conserved nucleotide sequence is recognized by the ribosomal RNA of the 30S ribosomal subunit, ensuring that the ribosome properly aligns with the start codon of the mRNA. Its proper functioning is crucial for the accurate synthesis of proteins in prokaryotic organisms.
Signal Recognition Particle: The signal recognition particle (SRP) is a ribonucleoprotein complex that plays a crucial role in directing the synthesis of proteins that are destined for secretion or for the cell membrane. It recognizes and binds to the signal peptide emerging from the ribosome during protein synthesis, temporarily pausing translation until the SRP can guide the ribosome to the endoplasmic reticulum (ER). This process ensures that proteins are correctly targeted to their appropriate cellular locations.
Signal sequence: A signal sequence is a short peptide chain that directs the transport of a newly synthesized protein to its specific location within or outside the cell. It is typically found at the beginning (N-terminus) of the protein.
Small subunit: The small subunit is a critical component of ribosomes, which are cellular machines responsible for synthesizing proteins. This subunit plays a key role in the initiation of protein synthesis by binding to messenger RNA (mRNA) and ensuring that the correct transfer RNA (tRNA) molecules are recruited to assemble the amino acids in the right order. Its structure and function are vital for accurate translation and overall protein production.
Start codon: A start codon is a specific sequence of nucleotides in mRNA that signals the beginning of translation. It typically codes for the amino acid methionine in eukaryotes and formyl-methionine in prokaryotes.
Start Codon: A start codon is a specific sequence of three nucleotides in mRNA that signals the beginning of translation, where the ribosome assembles to synthesize a protein. The most common start codon is AUG, which not only indicates the start of protein synthesis but also codes for the amino acid methionine, the first amino acid in newly formed polypeptides. Understanding the role of start codons is essential for grasping how the genetic code translates into functional proteins within cells.
Stop codon: A stop codon is a nucleotide triplet within mRNA that signals the termination of protein synthesis during translation. It does not code for any amino acid and serves as a crucial signal for the ribosome to release the newly formed polypeptide chain. The presence of stop codons ensures that proteins are synthesized to their correct lengths, preventing the addition of extra, unnecessary amino acids.
Streptomycin: Streptomycin is an antibiotic that belongs to the aminoglycoside class, derived from the bacterium Streptomyces griseus. It is primarily used to treat infections caused by Gram-negative bacteria and has a significant role in inhibiting protein synthesis by binding to the 30S ribosomal subunit, disrupting the function of ribosomes during translation.
Termination: Termination refers to the final step in the processes of transcription and translation, signaling the end of RNA synthesis in transcription and the completion of polypeptide synthesis in translation. It is a crucial event that ensures the accurate production of RNA and proteins, maintaining cellular functions. This process involves specific sequences and factors that recognize when to halt synthesis, ensuring that only the required genetic information is produced and that proteins are properly folded and functional.
Tetracycline: Tetracycline is a broad-spectrum antibiotic that inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit. This connection prevents the attachment of aminoacyl-tRNA to the ribosome, thereby halting the process of translation and limiting bacterial growth. Its effectiveness against a wide range of bacteria makes it an important tool in treating various infections.
Transcription: Transcription is the biological process where the DNA sequence of a gene is copied into RNA. This process is essential for gene expression, as it allows the genetic information stored in DNA to be transferred to messenger RNA (mRNA), which then guides protein synthesis. It serves as the first step in expressing genes, linking the genetic code found in DNA to the production of proteins necessary for cellular functions.
Translation: Translation is the biological process by which proteins are synthesized from messenger RNA (mRNA) templates. This process involves decoding the mRNA sequence into a specific sequence of amino acids, which are the building blocks of proteins, and occurs in the ribosomes, where transfer RNA (tRNA) brings amino acids to the growing polypeptide chain. Translation connects the genetic code carried by mRNA to functional proteins, playing a crucial role in gene expression and cellular function.
TRNA: tRNA, or transfer RNA, is a type of RNA molecule that plays a crucial role in protein synthesis by transporting specific amino acids to the ribosome during translation. It acts as an adapter, matching its anticodon with the corresponding codon on the mRNA strand, ensuring that the correct amino acid is added to the growing polypeptide chain. This process is essential for translating the genetic information encoded in DNA into functional proteins.
UAA: UAA is one of the three stop codons in the genetic code, signaling the termination of protein synthesis during translation. It plays a crucial role in ensuring that proteins are synthesized accurately by marking the end of a polypeptide chain. The presence of UAA in the messenger RNA (mRNA) indicates to the ribosome that it should halt translation, releasing the completed protein for folding and post-translational modifications.
UAG: UAG is one of the three stop codons in the genetic code, signaling the termination of protein synthesis during translation. It plays a critical role in ensuring that proteins are produced correctly, as it marks the end of an amino acid sequence. Understanding UAG is essential for grasping how genetic information is converted into functional proteins.
UGA: UGA is a codon in the genetic code that signifies a stop signal during protein synthesis. It is one of three stop codons (alongside UAA and UAG) that play a crucial role in terminating the translation process, signaling the ribosome to end protein production and release the newly formed polypeptide chain. Understanding UGA helps in grasping how genetic information is translated into functional proteins.