Overview
This lecture explains the structure of DNA, how it stores genetic information, and the process and significance of DNA replication.
DNA as Genetic Information
- DNA is the instruction manual for building and maintaining all living organisms.
- Each cell in your body contains a complete copy of your DNA.
- The human genome consists of about three billion base pairs.
- DNA's code determines what makes us human and impacts traits and health.
Structure of DNA
- DNA is made of nucleotides, each with a sugar, phosphate group, and one of four bases.
- The four DNA bases are adenine (A), guanine (G), thymine (T), and cytosine (C).
- DNA is a double helix with sugar-phosphate backbones and paired bases forming the rungs.
- Strands are anti-parallel: one runs 5’ to 3’, the other 3’ to 5’.
Base Pairing Rules
- Adenine always pairs with thymine (A-T); guanine always pairs with cytosine (G-C).
- Pairing occurs via weak hydrogen bonds.
- The sequence of A, T, G, and C encodes genetic information.
Chromosomes and Packing
- DNA coils tightly around proteins to form chromosomes.
- Chromosomes allow DNA to fit inside the nucleus of each cell.
DNA Replication Process
- DNA replication is needed for cell division and is semi-conservative (one old and one new strand).
- Enzyme helicase unwinds DNA, creating replication bubbles.
- DNA polymerase adds new complementary bases to each template strand.
- On the leading strand, replication is continuous; on the lagging strand, it's in fragments (Okazaki fragments).
- Enzymes check and repair errors during replication.
Mutations and Their Effects
- Errors or environmental damage can cause base pairing mistakes, leading to mutations.
- Most mutations are neutral; some can cause disease or provide beneficial traits.
- Proofreading reduces mutation rates to about 1 in 10 billion base pairs.
Discovery of Okazaki Fragments
- Drs. Reiji and Tsuneko Okazaki discovered short DNA fragments on the lagging strand during replication.
- Okazaki fragments are later joined into a continuous strand.
Key Terms & Definitions
- Nucleotide — DNA building block, made of a sugar, phosphate, and base.
- Double Helix — Twisted ladder shape of DNA.
- Base Pair — Two nitrogenous bases paired by hydrogen bonds (A-T and G-C).
- Chromosome — Tightly coiled DNA around proteins inside the cell nucleus.
- Helicase — Enzyme that unwinds DNA for replication.
- DNA Polymerase — Enzyme that assembles new strands of DNA.
- Okazaki Fragment — Short DNA piece on the lagging strand during replication.
- Mutation — Change in DNA sequence.
Action Items / Next Steps
- Review how DNA structure supports replication.
- Prepare to learn how cells use DNA to make proteins in the next lecture.