Protein Synthesis Steps to Know for AP Biology

Protein synthesis is a vital process where DNA is converted into functional proteins. It involves several key steps: transcription, mRNA processing, translation, and post-translational modifications, all crucial for cellular function and regulation in living organisms.

1. DNA Transcription

   • The process begins in the nucleus where DNA serves as a template for synthesizing RNA.
   • RNA polymerase binds to the promoter region of the gene, unwinding the DNA strands.
   • Complementary RNA nucleotides are added to form a single-stranded mRNA molecule.
   • Transcription continues until a termination signal is reached, signaling the end of the gene.

2. mRNA Processing

   • The primary mRNA transcript undergoes capping, where a 5' cap is added for stability and recognition.
   • Introns (non-coding regions) are removed, and exons (coding regions) are spliced together.
   • A poly-A tail is added to the 3' end to protect the mRNA from degradation and assist in export.

3. mRNA Export from Nucleus

   • Processed mRNA is transported from the nucleus to the cytoplasm through nuclear pores.
   • The export is facilitated by proteins that recognize the 5' cap and poly-A tail.
   • Once in the cytoplasm, the mRNA is ready for translation into a protein.

4. Translation Initiation

   • The small ribosomal subunit binds to the mRNA at the start codon (AUG).
   • The initiator tRNA carrying methionine pairs with the start codon.
   • The large ribosomal subunit then assembles with the small subunit, forming a complete ribosome ready for elongation.

5. Elongation

   • tRNA molecules bring amino acids to the ribosome, matching their anticodons with mRNA codons.
   • Peptide bonds form between adjacent amino acids, elongating the polypeptide chain.
   • The ribosome moves along the mRNA, facilitating the addition of amino acids until a stop codon is reached.

6. Termination

   • The process concludes when a stop codon (UAA, UAG, UGA) is encountered on the mRNA.
   • Release factors bind to the ribosome, prompting the release of the newly synthesized polypeptide.
   • The ribosomal subunits disassemble, and the mRNA is released for potential reuse.

7. Post-Translational Modifications

   • Newly synthesized proteins may undergo modifications such as phosphorylation, glycosylation, or cleavage.
   • These modifications can affect protein activity, localization, and stability.
   • They are crucial for the protein to achieve its final functional form and perform its biological role.

8. Protein Folding

   • Proteins fold into specific three-dimensional shapes, which are essential for their function.
   • Chaperone proteins assist in the proper folding process, preventing misfolding and aggregation.
   • Properly folded proteins are then transported to their functional locations within the cell or secreted outside.