Key Concepts in Molecular Biology

DNA structure

Double helix with antiparallel strands; made of nucleotides (A, T, C, G).

Base pairing rules

A pairs with T, C pairs with G (RNA: A pairs with U).

DNA replication

Semi-conservative process in which DNA is copied during S phase.

Enzymes in DNA replication

Helicase (unzips), DNA polymerase (adds bases), Ligase (seals gaps), Primase (adds RNA primer).

Transcription

DNA → RNA; occurs in the nucleus; uses RNA polymerase.

Translation

mRNA → protein; occurs in ribosomes using tRNA to match codons with amino acids.

mRNA

Carries genetic code from DNA to ribosome.

tRNA

Transfers amino acids to the ribosome; has anticodon to match mRNA.

rRNA

Makes up ribosomes; helps form peptide bonds between amino acids.

Codon

Three-base mRNA sequence that codes for one amino acid.

Start codon

AUG - codes for methionine; signals beginning of translation.

Stop codons

UAA, UAG, UGA - signal end of translation.

RNA processing in eukaryotes

Add 5' cap, 3' poly-A tail, and splice out introns.

Introns vs Exons

Introns = removed; Exons = expressed and translated.

Mutation

A change in DNA sequence; can be silent, missense, or nonsense.

Frameshift mutation

Insertion or deletion shifts reading frame; often more damaging than point mutations.

Gene regulation in prokaryotes

Uses operons (e.g., lac operon, trp operon) to turn genes on/off.

Lac operon

Inducible; ON when lactose is present (breaks down lactose).

Trp operon

Repressible; OFF when tryptophan is present (saves energy).

Gene regulation in eukaryotes

Involves transcription factors, enhancers, silencers, and epigenetics (e.g., methylation).

Epigenetics

Reversible modifications (like methylation) that affect gene expression without changing DNA sequence.

Virus structure

Protein coat (capsid) and genetic material (DNA or RNA); some have envelopes.

Lytic cycle

Virus replicates and bursts host cell; fast and destructive.

Lysogenic cycle

Viral DNA integrates into host genome and lies dormant before entering lytic cycle.

Retrovirus (e.g., HIV)

RNA virus that uses reverse transcriptase to make DNA from RNA.

Horizontal gene transfer

Prokaryotes can exchange genes via transformation, transduction, or conjugation.

Gel electrophoresis

Separates DNA fragments by size; smaller pieces move farther.

PCR (polymerase chain reaction)

Technique to amplify DNA sequences rapidly in the lab.