Overview
This lecture explains how DNA and its cousin RNA work together to produce proteins in cells, including the steps of transcription, translation, and protein folding.
DNA as the Instruction Manual
- DNA contains genes, which are instructions for building proteins.
- Each gene is transcribed and translated to make specific proteins, like titin, the largest known human protein.
- Animals, plants, and foods are mainly made of water, carbohydrates, fats, and proteins arranged by DNA instructions.
Transcription: Copying DNA into RNA
- Transcription occurs in the cell nucleus, copying a DNA segment (transcription unit) into messenger RNA (mRNA).
- The promoter sequence (often the TATA box) signals where transcription should begin.
- RNA polymerase enzyme binds to the promoter, unzips DNA, and builds mRNA by matching nucleotide bases (A-U, C-G; RNA uses uracil (U) instead of thymine (T)).
- Transcription ends at a termination signal, releasing the mRNA.
mRNA Processing and Splicing
- A 5' cap (guanine) is added to the start of the mRNA and a poly-A tail (many adenines) to the end for protection and transport.
- Introns (non-coding regions) are removed from mRNA during RNA splicing, leaving only exons (coding regions).
- snRNPs (snurps) and the spliceosome perform the splicing.
Translation: Building Proteins from mRNA
- mRNA exits the nucleus and enters a ribosome for translation.
- Ribosomes are made of ribosomal RNA (rRNA) and proteins, serving as the site for protein synthesis.
- Transfer RNA (tRNA) brings amino acids to the ribosome, matching mRNA codons with tRNA anticodons.
- Each triplet codon on mRNA specifies an amino acid; translation starts at the AUG codon (methionine).
- The amino acid sequence forms a polypeptide chain, the primary protein structure.
- Multiple codons may code for the same amino acid, allowing for some error tolerance.
Protein Folding and Structure
- Polypeptide chains fold into secondary structures (alpha helices and beta sheets) via hydrogen bonds.
- Tertiary structure forms through interactions among amino acid R groups (hydrophobic/hydrophilic).
- Some proteins have quaternary structure, with multiple chains combining, like hemoglobin.
- Protein structure (primary, secondary, tertiary, quaternary) determines its function as structural protein or enzyme.
Key Terms & Definitions
- Gene — a segment of DNA with instructions to make a protein.
- Transcription — process of copying DNA into mRNA.
- Promoter (TATA box) — DNA sequence signaling where transcription starts.
- RNA polymerase — enzyme that creates mRNA from DNA.
- mRNA (messenger RNA) — RNA copy that carries instructions from DNA to ribosome.
- Intron — non-coding segment, removed during splicing.
- Exon — coding segment, kept in mRNA for translation.
- Spliceosome — complex that removes introns from mRNA.
- tRNA (transfer RNA) — brings amino acids to the ribosome during translation.
- Codon — three-base mRNA sequence coding for an amino acid.
- Anticodon — three-base tRNA sequence that pairs with codon.
- Polypeptide — chain of amino acids; primary structure of protein.
- Primary structure — linear amino acid sequence.
- Secondary structure — local folding into helices or sheets.
- Tertiary structure — overall 3D folding due to R group interactions.
- Quaternary structure — assembly of multiple polypeptide chains.
Action Items / Next Steps
- Review the steps of transcription and translation.
- Memorize the key definitions and terminology.
- Prepare for the next lecture on genetic information exchange during reproduction.