DNA Replication and Structure

DNA Replication

• DNA is a double helix and double-stranded, while RNA is single-stranded.
• Chargaff's Law states that the relative amount of Adenine (A) equals Thymine (T), and Guanine (G) equals Cytosine (C).
• Watson and Crick proposed the first correct model of DNA as a double helix.
• The 2 strands of DNA are antiparallel.
• Sugars and phosphates face outward in DNA's structure.
• Nitrogen bases face inward.
• Nitrogenous base pairing: A=T and G=C.

DNA Replication Models

• Conservative: the parental DNA molecule is used to make a new copy.
• Semiconservative: the parental strand is used as a template to make a new strand.
• Semiconservative is the correct model as it follows the DNA double helix structure.
• Dispersive: portions of the parental strand are used as templates for new regions.

Purines and Pyrimidines

• Purines are nitrogenous bases with a double-ring structure (Adenine and Guanine).
• Pyrimidines are 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 existing strand as a template.
• The leading strand is synthesized continuously, while the lagging strand is synthesized in fragments.
• DNA Ligase joins the fragments of the lagging strand together to create two DNA molecules.
• DNA polymerase synthesizes DNA by adding complementary nucleotides to the new DNA strand.
• DNA polymerase requires a DNA template and a primer with a 3' -OH group.
• DNA polymerase can only add nucleotides in a 5' → 3' direction.
• Helicase unwinds the DNA double helix.
• Nucleotides provide the energy needed for DNA replication.
• Telomeres protect the ends of chromosomes.
• In normal cells, telomeres shorten with each division.
• In 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 enzymes scan the genome for mistakes and damage.
• Excision repair involves cutting out a damaged region of DNA, filling it in with DNA polymerase, and sealing it with DNA ligase.
• Nuclease cuts out damaged sections of DNA.
• Primase synthesizes short RNA primers.

Transcription & Translation

• Codon: 3 nucleotides of mRNA that code for an amino acid.
• More than one codon can code for the same amino acid.
• The DNA template strand is the strand that RNA polymerase reads to make mRNA, and is complementary to the mRNA (A pairs with U, T pairs with A).
• The DNA coding strand looks like the mRNA and is identical to the mRNA (A stays the same, T pairs with U).
• The "universal genetic code" means that in all living things, the same codons code for the same amino acid.
• All life shares a common ancestor.
• Translation converts the genetic code of nucleotide sequence into 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.
• Not true for all genes because some genes code for RNA that is never translated.
• Transcription: the synthesis of RNA from DNA (DNA → RNA).
• Translation: the synthesis of polypeptides from RNA (RNA → proteins).

DNA vs. RNA

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

Mutation

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