Central dogma: DNA → RNA → Protein (replication, transcription, translation)
Central Dogma
Genetic information flows in one direction — from DNA to protein
Replication: DNA → DNA. Transcription: DNA → mRNA (nucleus). Translation: mRNA → protein (ribosomes). Reverse transcriptase (HIV) breaks the rule: RNA → DNA.
DNA Base Pairing
DNA base pairs: A-T (2 H-bonds), G-C (3 H-bonds). RNA uses U instead of T.
DNA Base Pairing
Complementary base pairing — which bases pair together
A pairs with T (or U in RNA). G pairs with C. G-C has 3 hydrogen bonds (stronger). A-T has 2. Higher G-C content → more thermally stable DNA.
The Genetic Code
Codon = 3 bases = 1 amino acid. AUG = start. UAA UAG UGA = stop.
The Genetic Code
Three-letter codons specify which amino acid to add
64 possible codons (4³). 61 encode amino acids. 3 are stop codons. AUG = start (methionine). Code is degenerate — most AAs have multiple codons.
Point mutations and frameshift mutations — and their consequences
Substitution: one base replaced. Missense: changes amino acid. Nonsense: premature stop. Silent: same amino acid. Insertion/deletion: shifts reading frame → garbled protein from that point onward.
Missense
Wrong amino acid
Nonsense
Premature stop codon
Silent
Same amino acid — synonymous
Frameshift
Insertion/deletion shifts reading frame
Polymerase Chain Reaction
PCR: denature → anneal primers → extend with Taq polymerase. Amplifies DNA exponentially.
Polymerase Chain Reaction
PCR amplifies tiny DNA samples into millions of copies
Three steps: (1) Denaturation: heat to 95°C, separate DNA strands. (2) Annealing: cool to ~55°C, primers bind. (3) Extension: 72°C, Taq polymerase builds new strand. Each cycle doubles the DNA. 30 cycles = ~1 billion copies.
Semiconservative Replication
DNA replication: semiconservative — each new DNA has one old strand and one new strand
Semiconservative Replication
How DNA copies itself — proven by Meselson-Stitch experiment
Each daughter DNA molecule retains one parental strand and one newly synthesized strand. Meselson-Stitch (1958): used nitrogen isotope labeling to prove this. DNA polymerase: synthesizes new strand 5'→3'. Needs a primer (RNA) to start. Leading strand: continuous synthesis. Lagging strand: Okazaki fragments.
RNA Types
RNA types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal). Each has a distinct function in protein synthesis.
RNA Types
Three major types of RNA — each with a specific role
mRNA: carries genetic message from DNA to ribosome — the instruction. tRNA: brings correct amino acid to ribosome, anticodon pairs with codon — the delivery system. rRNA: structural and catalytic component of ribosome — the factory. Also: snRNA (splicing), miRNA (gene regulation), siRNA (gene silencing).
Promoter: where RNA polymerase binds to start. Template strand: read 3'→5'. mRNA synthesized 5'→3' (complementary to template, same sequence as coding strand but with U instead of T). Terminator: signals end of transcription. In eukaryotes: pre-mRNA processed → 5' cap, poly-A tail, introns spliced out.
Translation Mechanism
Translation: ribosome reads mRNA codon by codon. tRNA anticodon pairs with codon. Peptide bond forms.
Translation Mechanism
How the ribosome reads mRNA to build a protein
Initiation: ribosome assembles at start codon (AUG). Elongation: A site (incoming aminoacyl-tRNA), P site (peptidyl-tRNA, growing chain), E site (exiting tRNA). Peptide bond forms between A and P site. Ribosome advances (translocation). Termination: stop codon → release factors → protein released.
Prokaryotes: operons — lac operon (repressor blocks transcription when lactose absent). Eukaryotes: more complex. Transcription factors bind specific DNA sequences near genes. Enhancers: regulatory sequences far from gene, loop to promoter. Epigenetics: heritable changes in gene expression without DNA sequence change. Methylation silences genes, histone acetylation activates.
CRISPR Gene Editing
CRISPR-Cas9: guide RNA directs Cas9 protein to cut specific DNA sequence — gene editing tool
CRISPR Gene Editing
The revolutionary technology that allows precise DNA editing
CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats): naturally occurring bacterial immune system. Cas9: a DNA-cutting protein. Guide RNA: designed to match the target DNA sequence. Cas9 cuts both strands at target. Cell repairs cut: can introduce mutations (knockout) or insert new sequence. Applications: disease research, potential genetic therapies.
Restriction Enzymes
Restriction enzymes: cut DNA at specific recognition sequences — molecular scissors for biotechnology
Restriction Enzymes
The tools that made recombinant DNA technology possible
Restriction endonucleases: bacterial defense against foreign DNA. Cut at specific palindromic sequences (EcoRI cuts GAATTC). Create 'sticky ends' (overhanging single strands) or blunt ends. Sticky ends facilitate ligation (joining) with complementary pieces. Foundation of recombinant DNA, cloning, genetic engineering, PCR.