Overview
This lecture covers the process of protein synthesis, including how genetic information in DNA leads to the creation of proteins through transcription and translation.
Genes and the Genetic Code
- A gene is a DNA segment that codes for a molecule of RNA involved in protein synthesis.
- Proteins are built from 20 amino acids, but DNA has only four nucleotides: A, T, C, G.
- The genetic code uses triplets of nucleotides (base triplets) in DNA to specify amino acids.
- Each group of three nucleotides forms a codon in messenger RNA (mRNA).
- There are 64 possible codons; 61 code for amino acids, and 3 are stop codons.
- The start codon is always AUG, which codes for methionine.
Transcription (DNA to mRNA)
- Transcription is the process of copying genetic instructions from DNA to mRNA.
- This occurs in the nucleus, as DNA cannot leave the nucleus.
- RNA polymerase enzyme binds DNA, unwinds it, and builds mRNA from one DNA strand.
- mRNA is synthesized with codons, each coding for a specific amino acid.
- Pre-mRNA contains introns (removed) and exons (joined), allowing one gene to code for multiple proteins.
Translation (mRNA to Protein)
- Translation occurs in the cytoplasm, assembling amino acids into proteins.
- Three main participants: mRNA (provides code), tRNA (brings specific amino acids), and ribosome (site of synthesis).
- tRNA has an anticodon that matches the mRNA codon and carries the corresponding amino acid.
- Binding an amino acid to tRNA requires one ATP molecule.
- Translation has three steps: initiation (start codon recognition), elongation (amino acid chain growth), and termination (stop codon, release factor frees finished protein).
- Multiple ribosomes can translate a single mRNA strand simultaneously.
- Proteins may be modified, stored, or transported after synthesis (e.g., by rough ER and Golgi apparatus).
Key Terms & Definitions
- Gene — DNA segment containing instructions for producing RNA and, ultimately, proteins.
- Codon — Three-nucleotide sequence in mRNA specifying an amino acid.
- Base triplet — Three-nucleotide unit in DNA.
- Transcription — Process of copying DNA code to mRNA in the nucleus.
- Translation — Building a protein from mRNA code in the cytoplasm.
- RNA polymerase — Enzyme that synthesizes mRNA from DNA template.
- mRNA (messenger RNA) — RNA copy of gene code sent to ribosomes.
- tRNA (transfer RNA) — RNA that brings amino acids to ribosome during protein synthesis.
- Anticodon — Three-base tRNA sequence complementary to an mRNA codon.
- Ribosome — Organelle where proteins are synthesized.
- Introns/Exons — Noncoding/coding sequences in pre-mRNA; introns are removed, exons joined.
Action Items / Next Steps
- Review the APR video on protein synthesis in Connect.
- Prepare for the next topic: cell division.