Overview
This lecture covers genetics, focusing on the structure and function of genetic material, DNA replication, gene expression (transcription and translation), gene regulation, horizontal gene transfer, and mutations in both prokaryotes and eukaryotes.
Genetics Basics
- Genetics is the study of inheritance and how traits are passed from parents to offspring.
- The genome is all the genetic material in an organism; includes nuclear DNA and extra-chromosomal DNA.
- Chromosomes are DNA structures; prokaryotes have one circular chromosome, eukaryotes have multiple linear ones.
- A gene is a DNA segment that codes for a protein.
- Genotype is an organism’s genetic makeup; phenotype is the observable traits.
DNA Structure and Replication
- DNA is made of nucleotides (sugar, phosphate, nitrogen base).
- Bases pair as A-T and G-C in DNA; in RNA, A pairs with U.
- DNA strands are antiparallel (run in opposite 5' to 3'/3' to 5' directions).
- DNA replication is semi-conservative: each new DNA has one old, one new strand.
- Key enzymes: helicase (unwinds DNA), DNA polymerase III (adds nucleotides, proofreads), DNA polymerase I (removes primers), ligase (joins DNA fragments).
- Replication in bacteria starts at the origin, proceeds bi-directionally, and produces two daughter DNA molecules.
Gene Expression: Transcription and Translation
- Central dogma: DNA → RNA → Protein.
- Transcription: RNA polymerase binds promoter, copies DNA into mRNA.
- mRNA uses uracil (U) instead of thymine (T); three main types: mRNA (messenger), rRNA (ribosomal), tRNA (transfer).
- Translation: mRNA codons (triplets) determine amino acid sequence; tRNA matches codons with attached amino acids using anticodons.
- The start codon is always AUG (methionine); three stop codons exist.
- In prokaryotes, transcription and translation can occur simultaneously; in eukaryotes, they happen in separate cellular locations.
Gene Regulation
- Constitutive genes are always active; others are regulated by repression (off) or induction (on).
- Operons (e.g., lac operon) include regulator, promoter, operator, and structural genes for control.
- The repressor protein blocks transcription unless inactivated by an inducer.
Horizontal Gene Transfer
- Three main methods: conjugation (direct cell-to-cell), transformation (uptake of free DNA), and transduction (bacteriophage-mediated).
- Transposons (jumping genes) can move within or between DNA molecules, introducing variability.
Mutations
- Mutations are changes in genetic information, sometimes beneficial, usually neutral or harmful.
- Mutagens can be chemical or radiation (e.g., UV light).
- Types: missense (single amino acid change), frameshift (insertion/deletion shifts reading frame), nonsense (premature stop codon).
- Most mutations are rare and often neutral; beneficial mutations drive evolution.
Key Terms & Definitions
- Genome — all genetic material in an organism.
- Gene — segment of DNA coding for a protein.
- Genotype — genetic makeup.
- Phenotype — observable trait.
- Operon — cluster of genes under single regulation.
- Codon — three-nucleotide sequence in mRNA coding for amino acids.
- Mutation — permanent change in DNA sequence.
- Transposon — movable genetic element (jumping gene).
Action Items / Next Steps
- Review key terms and focus on DNA/RNA structure, replication, transcription, translation, and gene regulation.
- Complete any assigned homework, including questions on genetic engineering (section not covered in lecture, but may appear in homework).
- Prepare for Lecture Exam 2, covering chapters 6, 7, and 8.