Molecular Biology of DNA and RNA

• DNA: double helix, double stranded
• RNA: single stranded
• Chargaff's Law: the relative amount of Adenine equals Thymine and Guanine equals Cytosine (A=T and G=C)
• Watson and Crick proposed the first correct model of DNA as a double helix

DNA Structure

• Two DNA strands are antiparallel
• Sugars and phosphates face outward
• Nitrogen bases face inward
• Nitrogenous base pairings: A=T and G=C

DNA Replication Models

• Conservative: parent DNA molecule is used to make a new copy
• Semiconservative: parent strand is used as a template to make a new strand
• Semiconservative model is the correct model
• Dispersive: portions of the parent strand are used as templates for new regions

Nitrogenous Bases

• Purines: nitrogenous bases with a double-ring structure (Adenine and Guanine)
• Pyrimidines: nitrogenous bases with a single-ring structure (Cytosine and Thymine)

DNA Replication in Prokaryotic Cells

• One chromosome has one origin of replication
• Helicase unwinds and separates the two strands of DNA, creating a replication fork
• DNA polymerase builds new DNA, using the old strand as a template
• Leading strand synthesizes continuously
• Lagging strand synthesizes fragments
• DNA Ligase joins fragments together, creating two DNA molecules

DNA Polymerase

• DNA polymerase synthesizes DNA by adding complementary nucleotides to the new DNA strand
• Requires a DNA template
• Requires a primer with a 3' - OH group
• DNA plymerase can only synthesize in a 5' to 3' direction

Leading and Lagging Strands

• Leading strand is synthesized continuously
• Lagging strand is synthesized in short discontinuous segments joined by DNA ligase
• Helicase unwinds the DNA double helix
• Nucleotides provide the energy needed for DNA replication

Telomeres and Telomerase

• Telomeres protect the ends of chromosomes
• Normal cells telomeres shorten with each division
• Cancer cells telomeres are maintained to allow continuous division
• Telomerase is the enzyme that makes telomeres
• Normal cells have low or absent telomerase activity
• Cancer cells often have reactivated telomerase, enabling unlimited cell division

DNA Repair

• DNA repair enzyme scan the genome for mistakes and damage
• Excision repair involves cutting out a damaged region of DNA, filling it back in with DNA polymerase, and sealing it with DNA ligase
• Nuclease cuts out damaged sections of DNA
• Primase synthesizes short RNA primers

Transcription and Translation

• Codon: 3 nucleotides of mRNA that code for an amino acid
• More than one codon can code for the same amino acid
• DNA template strand is the strand that RNA polymerase reads to make mRNA
• DNA template strand is complementary to the mRNA (A pairs with U, T pairs with A)
• DNA coding strand is the strand that looks like the mRNA
• DNA coding strand is identical to the mRNA (A stays the same, T pairs with U)
• Universal genetic code: all living things use the same codons to code for the same amino acid
• All life share a common ancestor
• Translation converts the genetic code of a nucleotide sequence into an amino acid sequence
• mRNA reads the codons and brings the amino acid to the ribosome

Central Dogma Theory

• DNA is copied into RNA, and RNA makes proteins
• The central dogma theory is not true for all genes because some genes code for RNA that is never translated
• Transcription synthesizes of RNA (DNA to RNA)
• Translation synthesizes polypeptides (RNA to proteins)

DNA vs RNA

• DNA nucleotides: A, C, G, T
• In eukaryotes, DNA stays in the nucleus wrapped around proteins
• DNA is double stranded and more stable with deoxyribose sugar
• RNA nucleotides: A, C, G, U
• In eukaryotes, RNA is mobile in the nucleus
• In eukaryotes, RNA is mostly naked
• RNA is single stranded and more "labile" with ribose sugar

Mutation

• Mutation is a permanent change in DNA sequence
• Point mutation: change in one base pair
• Missense mutation: new codon codes for a different amino acid
• Nonsense mutation: mutation creates a stop codon
• Frameshift mutation: inserts or deletes DNA sequence, most damaging because it changes everything
• Silent mutation: base changes, amino acid stays the same, least damaging because there is no change in the protein