Overview
This lecture reviews the structure of DNA and details the entire process of DNA replication, covering key enzymes and terminology important for cell biology.
DNA Structure Recap
- DNA is a nucleic acid made of monomers called nucleotides.
- Each nucleotide contains deoxyribose sugar, a phosphate group, and a nitrogen base.
- The four nitrogen bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T).
- Purines (A, G) are double-ringed; pyrimidines (C, T, and uracil in RNA) are single-ringed.
- Complementary base pairing: A pairs with T (double hydrogen bond), G pairs with C (triple hydrogen bond).
- Nucleotides are joined by phosphodiester bonds between sugar and phosphate.
- Hydrogen bonds connect complementary nitrogen bases across the two DNA strands.
DNA Replication Process
- DNA replication occurs during the S phase of interphase.
- Three main enzymes are involved: DNA helicase, DNA polymerase, and DNA ligase.
- DNA helicase "unzips" DNA by breaking hydrogen bonds, forming a replication fork.
- DNA polymerase matches new nitrogen bases to original strands and proofreads for errors, correcting most mistakes.
- DNA ligase reforms hydrogen bonds between new base pairs, completing the new DNA strand.
- Replication is semi-conservative: each new DNA molecule consists of one old (parent) strand and one new strand.
DNA Organization Terms
- Chromatin: Unwound, ball-like DNA present during interphase.
- Chromatid: Condensed, duplicated DNA strands visible during prophase, anaphase, and telophase.
- Chromosome: Structure from joined sister chromatids, visible during metaphase.
- Kinetochore: Protein at the centromere where sister chromatids attach.
- Centromere: Region where kinetochores hold sister chromatids together.
Key Terms & Definitions
- Nucleotide — DNA building block made of deoxyribose, a phosphate, and a nitrogen base.
- Purine — Double-ringed nitrogen base (adenine, guanine).
- Pyrimidine — Single-ringed nitrogen base (cytosine, thymine, uracil).
- Phosphodiester bond — Covalent bond joining the sugar and phosphate of adjacent nucleotides.
- Hydrogen bond — Weak bond connecting complementary nitrogen bases.
- Replication fork — Y-shaped region where DNA is split for replication.
- Semi-conservative replication — Process where each new DNA molecule contains one parent strand and one new strand.
- Chromatin — Loose, uncondensed DNA present during interphase.
- Chromatid — One half of a duplicated, condensed chromosome.
- Chromosome — Structure of joined sister chromatids visible during certain cell cycle stages.
- Kinetochore — Protein complex at the centromere for chromatid attachment.
- Centromere — Region holding sister chromatids together via kinetochores.
Action Items / Next Steps
- Review the content from the first three videos to solidify understanding before moving on to videos four through seven.
- Follow a five-day study plan and avoid cramming.