Question

Which of the following statements about the initiation phase of translation is false? a. An initiation factor allows \(5^{\prime}\) mRNA to attach to the small ribosomal subunit. b. Initiation factors complex with GTP to help Met-tRNA and AUG pair. c. mRNA attaches first to the small ribosomal subunit. d. GTP is synthesized. e. \(3^{\prime}-\mathrm{UAC}-5^{\prime}\) on the tRNA binds \(5^{\prime}-\mathrm{AUG}-3^{\prime}\) on \(\mathrm{mRNA}\).

Short answer

The false statement is d. GTP is not synthesized during the initiation phase of translation, it is used.

Step-by-step solution

Identify the Process of Translation Initiation

During the initiation phase of translation, the mRNA must first attach to the small ribosomal subunit, and an initiation factor facilitates this process. This phase also involves the alignment and binding of the initiator tRNA with the start codon on the mRNA.

Evaluate the Statements

Each statement given must be examined against the known mechanisms of the initiation phase of translation to determine if it is accurate or false.

Determine the False Statement

Statements a, b, c, and e are all true aspects of the initiation phase of translation in eukaryotes. Statement a describes the role of initiation factors, statement b refers to the GTP-dependent formation of the initiation complex, statement c indicates that mRNA attaches first to the small ribosomal subunit which is true, and statement e describes the base pairing between the anticodon of the tRNA and the start codon on the mRNA. The false statement is d, because GTP is not synthesized during the initiation phase of translation; it is used as energy for the assembly of the initiation complex.

Key concepts

Ribosomal Subunit

The process of the translation initiation phase begins with the small ribosomal subunit playing a pivotal role. Ribosomes are the molecular machines responsible for protein synthesis, and they consist of two main components: the small and large ribosomal subunits. During initiation, the small subunit has the critical job of binding to the mRNA strand. This interaction is highly regulated and is essential for accurately decoding the mRNA's message into the correct sequence of amino acids. The precise attachment ensures that the ribosome reads the message from the correct start codon, which in eukaryotes is typically AUG, signaling the beginning of a protein-coding sequence.

Once the small subunit is properly positioned on the mRNA, the large subunit will subsequently join to complete the functional ribosome assembly. However, the large subunit only joins after another key player—the initiator tRNA—has paired with the start codon. It is this coordinated assembly that sets the stage for the elongation phase of protein synthesis, where amino acids are added to the growing polypeptide chain.

Initiation Factors

Initiation factors are proteins that are crucial in the initial stages of translation. They facilitate the correct association between the small ribosomal subunit, mRNA, and tRNA charged with the initiator amino acid methionine (Met-tRNA). These factors are often involved in the recruitment and stabilization of the initiation components, and they help ensure that initiation begins at the right place and time.

In the context of energy usage, initiation factors complex with guanosine triphosphate (GTP), a molecule that provides the necessary energy for translation initiation. This energy is utilized when the GTP is hydrolyzed to guanosine diphosphate (GDP), which releases energy to drive the formation of the initiation complex. It's important to note that GTP itself is not synthesized during the initiation phase—instead, it's utilized as a pre-existing energy source. Therefore, the role of initiation factors is not to create GTP but to use it to ensure that translation initiates efficiently and accurately.

mRNA Attachment

mRNA attachment to the small ribosomal subunit is one of the first and most important steps in the initiation phase of translation. The mRNA strand, which carries the genetic code transcribed from DNA, must be accurately recognized and bound by the small ribosomal subunit to begin the synthesis of a protein.

This process is initiated by an initiation factor that recognizes a specific sequence near the 5' end of the mRNA, often referred to as the mRNA cap in eukaryotic cells. Once bound, the small subunit migrates along the mRNA in a process called 'scanning' until it reaches the start codon, AUG. Proper mRNA attachment ensures that the ribosome interprets the correct reading frame of the mRNA, which is essential for producing a functional protein. Any errors in attachment or reading frame selection can lead to the synthesis of nonfunctional or harmful proteins.

tRNA and Codon Pairing

Transfer RNA (tRNA) and codon pairing is at the heart of translating the mRNA's nucleotide code into a chain of amino acids. Each tRNA molecule carries a specific amino acid and contains a three-nucleotide sequence called an anticodon. The anticodon is designed to pair with a complementary codon, a three-nucleotide sequence on the mRNA.

During the initiation phase, the initiator tRNA carries methionine and has an anticodon sequence that is complementary to the AUG start codon on the mRNA. The accurate matching of the initiator tRNA's anticodon (3'UAC5') with the start codon (5'AUG3') is crucial for setting the correct starting point for protein synthesis. This tRNA-codon pairing is not only the first step in building the protein but also establishes the reading frame for the ribosome to follow, thereby influencing the entire polypeptide sequence.