D1.2.17 Initiation of translation (HL) Notes

Initiation of Translation: How Protein Synthesis Begins

1. Imagine you’re assembling a complex machine.
2. Before you start, you need the right tools, instructions, and a clear starting point.
3. In cells, this process begins with translation initiation, the first step in building proteins from mRNA.

The Key Players in Translation

1. Before diving into initiation, let’s review the essential components:
   1. mRNA: Carries the genetic instructions from DNA.
   2. Ribosome: The molecular machine that reads mRNA and assembles amino acids.
   3. tRNA: Transfers the correct amino acids to the ribosome.

Step-by-Step Process of Translation Initiation

1. Attachment of the Small Ribosomal Subunit

1. The process begins with the small subunit of the ribosome binding to the 5′ end of the mRNA.
2. This end is marked by a 5′ cap in eukaryotes, which helps the ribosome recognize and attach to the mRNA.

2. Scanning for the Start Codon

1. Once attached, the small subunit moves along the mRNA in the 5′ to 3′ direction, scanning for the start codon (AUG).
2. This codon signals where translation should begin.

3. Binding of the Initiator tRNA

1. The start codon AUG codes for the amino acid methionine.
2. A special tRNA, called the initiator tRNA, carries methionine and has an anticodon (UAC) that pairs with AUG.
3. This tRNA binds to the P site of the small ribosomal subunit.

4. Attachment of the Large Ribosomal Subunit

1. Once the initiator tRNA is in place, the large ribosomal subunit joins the small subunit.
2. This forms a complete ribosome, ready to begin translation.
3. The initiator tRNA remains in the P site, while the A site is empty and ready to accept the next tRNA.

The Role of Ribosomal Binding Sites: A, P, and E

1. The ribosome has three critical binding sites for tRNA:
   1. A Site (Aminoacyl Site): Holds the tRNA carrying the next amino acid to be added.
   2. P Site (Peptidyl Site): Holds the tRNA with the growing polypeptide chain.
   3. E Site (Exit Site): Where tRNA, now empty, exits the ribosome.

Initiation Sets the Stage for Accurate Protein Synthesis

1. Without it, the ribosome might start translating at the wrong codon, leading to a completely different and nonfunctional protein.