Overview
This lecture covers the structure and function of DNA, the process of DNA replication, gene expression including transcription and translation, and the impact of mutations on protein synthesis and cancer development.
Structure of DNA
- DNA discovered in 1953 as a double helix composed of nucleotides.
- Each nucleotide consists of a phosphate, a sugar, and a nitrogen-containing base.
- Four DNA bases: adenine (A), guanine (G) (purines), thymine (T), cytosine (C) (pyrimidines).
- DNA strands have a sugar-phosphate backbone and complementary base pairing (A-T, G-C).
- Strands are anti-parallel (one runs 5' to 3', the other 3' to 5') and held by hydrogen bonds.
DNA Replication
- DNA replication is semi-conservative: each new DNA has one old and one new strand.
- Major enzymes: DNA helicase (unwinds DNA), DNA polymerase (adds nucleotides), DNA ligase (seals breaks).
- Replication results in two identical DNA molecules.
Gene Expression: Transcription and Translation
- Genes are DNA regions that code for proteins.
- Gene expression uses three types of RNA: messenger (mRNA), transfer (tRNA), and ribosomal (rRNA).
- DNA has deoxyribose sugar; RNA has ribose; RNA uses uracil (U) in place of thymine.
- Transcription (in nucleus): makes mRNA copy from DNA using RNA polymerase, with A-U, C-G pairing.
- mRNA processing: introns (non-coding) are removed, exons (coding) joined, with a cap and poly-A tail added before entering cytoplasm.
- Translation (in cytoplasm): mRNA codons (three-base units) code for amino acids, using tRNA with anticodons and rRNA in ribosomes to build the protein.
Translation: Steps and Genetic Code
- Genetic code: 64 codons; 61 code for amino acids, 3 are stop codons, 1 start codon (AUG).
- Translation steps: initiation (assembly and start), elongation (amino acid chain builds), termination (release at stop codon).
- Ribosomes have binding sites for mRNA and tRNAs; multiple ribosomes can translate one mRNA.
Mutations and Their Effects
- Point mutations affect single nucleotides and may have no effect, change amino acids, or produce abnormal/incomplete proteins.
- Sickle cell anemia is caused by a point mutation.
- Frameshift mutations (insertions/deletions) shift codon reading, often resulting in nonfunctional proteins.
- Most cancers are caused by mutations that disrupt normal cell growth, leading from benign to malignant tumors.
Key Terms & Definitions
- Nucleotide — DNA/RNA subunit: phosphate, sugar, and base.
- Purine — Double-ring base (A, G).
- Pyrimidine — Single-ring base (T, C in DNA; U, C in RNA).
- Replication — Copying DNA to make two identical molecules.
- Transcription — Making mRNA from DNA template.
- Translation — Making protein from mRNA code.
- Codon — Three-base sequence on mRNA coding for an amino acid.
- Mutation — Change in DNA sequence; includes point mutations and frameshifts.
- Benign — Non-cancerous tumor.
- Malignant — Cancerous tumor, can spread (metastasize).
Action Items / Next Steps
- Review genetic code table for codon-amino acid pairing.
- Study the steps of transcription and translation.
- Prepare for next chapter on tissue types, especially epithelial cells.