Protein Synthesis Process

Overview

This lecture covers the process of protein synthesis in cells, breaking it down into the stages of transcription and translation, and explaining key molecules and steps involved.

Overview of Protein Synthesis

• Protein synthesis is the process by which proteins (polypeptide chains) are made in cells.
• It occurs in two stages: transcription (in the nucleus) and translation (in the cytoplasm on ribosomes).

Transcription (Stage 1)

• Transcription takes place inside the nucleus where a gene from DNA is copied into messenger RNA (mRNA).
• DNA is too large and valuable to leave the nucleus, so only the gene of interest is copied.
• The DNA double helix unwinds and the enzyme DNA helicase breaks hydrogen bonds to expose a template strand.
• Free RNA nucleotides pair up with complementary bases on the exposed DNA strand.
• RNA polymerase joins RNA nucleotides together by forming phosphodiester bonds, creating pre-mRNA.
• Pre-mRNA is modified by splicing; introns (non-coding regions) are removed and exons (coding regions) are joined together.
• The modified mRNA, now only exons, leaves the nucleus via nuclear pores to enter the cytoplasm.

Translation (Stage 2)

• Translation occurs in the cytoplasm when mRNA attaches to a ribosome at the start codon (first three bases).
• Transfer RNA (tRNA) molecules, each with a specific anticodon and amino acid binding site, pair with complementary mRNA codons.
• The ribosome holds mRNA and tRNA in place, facilitating peptide bond formation between amino acids.
• Peptide bond formation requires an enzyme and ATP (energy).
• The ribosome moves along mRNA, matching codons with tRNA until it reaches a stop codon, which signals translation to end.
• The polypeptide chain (primary structure) is released and later modified and folded in the Golgi body.

Key Terms & Definitions

• Transcription — Process of copying a gene from DNA to mRNA in the nucleus.
• Translation — Process of assembling amino acids into a polypeptide using mRNA on ribosomes.
• mRNA (Messenger RNA) — RNA molecule that carries genetic code from DNA to ribosome.
• tRNA (Transfer RNA) — RNA molecule that brings specific amino acids to the ribosome during translation.
• Codon — Three-base sequence on mRNA that codes for an amino acid.
• Anticodon — Three-base sequence on tRNA complementary to mRNA codon.
• Introns — Non-coding sequences in DNA; removed during mRNA splicing.
• Exons — Coding sequences in DNA; remain in mRNA after splicing.
• Start Codon — First codon (AUG) on mRNA signaling start of translation.
• Stop Codon — Codon signaling end of translation; does not code for an amino acid.

Action Items / Next Steps

• Review the processes of transcription and translation.
• Learn the role of key enzymes (DNA helicase, RNA polymerase, spliceosome).
• Understand the significance of introns and exons in pre-mRNA processing.