Question

Which component is the last to join the initiation complex during the initiation of translation? a. the mRNA molecule b. the small ribosomal subunit c. the large ribosomal subunit d. the initiator tRNA

Short answer

The large ribosomal subunit is the last to join the initiation complex during the initiation of translation.

Step-by-step solution

Understand Initiation of Translation

The initiation of translation in protein synthesis involves the assembly of the components needed to begin translating mRNA into a protein. This process occurs in a specific order.

Identify the Order of Assembly

The correct order for the assembly of the initiation complex is as follows: first, the small ribosomal subunit binds to the mRNA molecule. Then, the initiator tRNA binds to the start codon on the mRNA. Lastly, the large ribosomal subunit joins the complex to complete the initiation phase.

Determine the Last to Join

Based on the order of assembly, the last component to join the initiation complex is the large ribosomal subunit.

Key concepts

Initiation Complex Assembly

The initiation complex assembly is a critical step in translation, the process by which cells build proteins. During this phase, various components come together to form a complete unit capable of synthesizing a protein from the mRNA template.

First, the small ribosomal subunit locates the mRNA and binds to it, recognizing the start codon, which is typically AUG. This is followed by the pairing of the initiator tRNA, with an anticodon that is complementary to the start codon, carrying the amino acid methionine. Methionine marks the beginning of the amino acid sequence of the protein.

Lastly, the large ribosomal subunit arrives, creating a functional ribosome. This is the final step in forming the initiation complex, marking the end of the initiation phase and readying the machinery for the elongation phase, where the protein chain is synthesized.

Ribosomal Subunits

Ribosomes are the cellular machines responsible for protein synthesis. They are comprised of two distinct subunits, aptly named the small and large ribosomal subunits. The small subunit is involved in the decoding process, where it reads the mRNA. The large subunit, on the other hand, is responsible for the formation of peptide bonds, which link amino acids together to form proteins.

Both subunits are required for translation and each plays a unique role. They exist separately when not actively synthesizing proteins but come together during initiation to kickstart the process. In eukaryotic cells, the small subunit is designated as the 40S subunit, and the large as the 60S subunit, combining to form an 80S ribosome.

Initiator tRNA

Initiator tRNA is the tRNA molecule that begins the synthesis of proteins by being the first to bind to the start codon on the mRNA. This specialized tRNA carries the amino acid methionine in eukaryotes or a modified form of methionine in prokaryotes. It is unique because it can directly bind to the P site of the ribosome, unlike other tRNAs that enter at the A site during the elongation stages of translation.

The initiator tRNA sets the reading frame for the mRNA and is essential for accurate translation of the genetic code into a corresponding string of amino acids, eventually forming a functional protein.

Protein Synthesis

Protein synthesis is the cellular process of building proteins, which are crucial to countless cell functions. It occurs in two primary stages: transcription, where the DNA code is converted into mRNA, and translation, where the mRNA is read by the ribosome to assemble proteins.

Translation further divides into three phases: initiation, elongation, and termination. Initiation, involving the assembly of an initiation complex, sets the stage. Elongation is the step-by-step addition of amino acids guided by mRNA codons, and termination occurs when a stop codon is reached, signaling the completion of the protein. The newly synthesized protein then undergoes various modifications and folding to become a functional molecule that can perform tasks vital for the survival and functioning of the cell.