Overview
This lecture explains protein synthesis, covering both transcription and translation processes, focusing on key steps and exam-marking points required by AQA.
Overview of Protein Synthesis
- Protein synthesis occurs in two stages: transcription and translation.
- Transcription happens in the nucleus, creating an mRNA copy of a gene.
- Translation occurs at ribosomes in the cytoplasm, using the mRNA to assemble amino acids into a protein.
Transcription
- Hydrogen bonds between complementary DNA bases break, causing the DNA double helix to unwind.
- Only one DNA strand acts as a template for mRNA formation.
- Free RNA nucleotides align with complementary bases on the DNA template strand.
- RNA uses uracil (U) instead of thymine (T) to pair with adenine.
- RNA polymerase catalyzes the joining of RNA nucleotides, forming phosphodiester bonds to synthesize an mRNA strand.
- The initial RNA transcript in eukaryotes is called pre-mRNA and requires modification.
- Introns (non-coding sequences) are removed from pre-mRNA through splicing, leaving only exons (coding sequences).
- In prokaryotes, mRNA is produced directly as their DNA lacks introns, so no splicing is needed.
Translation
- Modified mRNA leaves the nucleus and attaches to a ribosome.
- The ribosome reads the mRNA codons one at a time.
- tRNA molecules, each with an anticodon complementary to the mRNA codon, bring specific amino acids.
- Each tRNA carries a specific amino acid matching its anticodon sequence.
- Amino acids are joined by peptide bonds, a process requiring ATP.
- tRNA is released after amino acid delivery; the ribosome moves along to the next codon.
- Process continues until a stop codon is reached, completing the polypeptide chain.
- The polypeptide chain enters the Golgi apparatus for folding and modification.
Key Terms & Definitions
- Transcription — process of making a complementary mRNA copy from a DNA gene.
- Translation — process where ribosomes use mRNA to assemble amino acids into a polypeptide.
- mRNA — messenger RNA; carries genetic code from DNA to ribosome.
- tRNA — transfer RNA; brings specific amino acids to ribosome during translation.
- Codon — sequence of three mRNA bases coding for an amino acid.
- Anticodon — sequence of three bases on tRNA, complementary to mRNA codon.
- Introns — non-coding DNA or RNA sequences removed during splicing.
- Exons — coding sequences remaining after intron removal.
- RNA Polymerase — enzyme that synthesizes mRNA from the DNA template.
- Peptide Bond — bond linking amino acids in a protein.
Action Items / Next Steps
- Use workbook to reinforce understanding of transcription and translation steps.
- Review flashcards covering key marking points for exam preparation.