List the steps in protein synthesis and briefly describe what occurs at each step.

Answer 1

1. Transcription: Protein synthesis begins with transcription. In this step, the DNA sequence of a specific gene is transcribed into a complementary RNA molecule called messenger RNA (mRNA). RNA polymerase enzyme binds to a specific region of the gene, unwinds the DNA helix, and synthesizes the mRNA molecule by adding complementary RNA nucleotides according to the base pairing rules. This results in the formation of pre-mRNA.

2. RNA processing: The pre-mRNA molecule undergoes a series of modifications known as RNA processing. It includes the removal of non-coding regions (introns) from the pre-mRNA and the joining of the remaining coding regions (exons) to form a mature mRNA. Additionally, a modified 5' cap and a poly-A tail are added to stabilize the mRNA molecule and assist in its export from the nucleus.

3. mRNA export: The mature mRNA molecule is transported from the nucleus to the cytoplasm through nuclear pores for further processing and translation. This step ensures that the mRNA reaches the ribosomes, where protein synthesis occurs.

4. Translation initiation: In translation initiation, the mRNA molecule binds to a ribosome, which consists of two subunits. The small ribosomal subunit recognizes and binds to a specific sequence on the mRNA called the start codon (typically AUG), while the large subunit attaches to the small subunit, forming a functional ribosome.

5. Elongation: During the elongation phase, the ribosome moves along the mRNA molecule, one codon at a time, and matches each codon with its complementary anticodon on a transfer RNA (tRNA) molecule. Each tRNA molecule carries a specific amino acid that corresponds to the codon it recognizes. The ribosome catalyzes the formation of peptide bonds between adjacent amino acids, assembling them into a growing polypeptide chain.

6. Termination: Termination signals the end of protein synthesis. When a stop codon (UAA, UAG, or UGA) is encountered on the mRNA, a release factor binds to the ribosome instead of a tRNA. This causes the ribosome to dissociate, and the newly synthesized polypeptide is released.

7. Post-translational modifications: After protein synthesis, the newly formed polypeptide may undergo post-translational modifications to become a functional protein. These modifications can include folding, cleavage of excess amino acids, attachment of chemical groups (such as phosphate or carbohydrate), or association with other protein subunits to form a complex protein structure.

Overall, the steps of protein synthesis involve transcribing the DNA sequence into mRNA, processing and exporting the mRNA, initiating translation, elongating the polypeptide chain, terminating translation, and potentially modifying the resulting protein to achieve its final form and function.