DNA Structure and Replication

Key Topics for Midterm Exam 3

• Genetics and gene expression

• DNA structure review

• DNA packaging

• DNA replication (major focus)

DNA Structure

• DNA as a source of genetic information

• Comprised of polynucleotides made up of nucleotides

• Each nucleotide consists of a sugar, phosphate backbone, and base (A, T, C, G).

• Four essential properties of DNA:

  1. Stores all genetic information.

  2. Precise replication during cell division.

  3. Expressed as the organism's phenotype.

  4. Susceptible to mutations.

• Key Differences:

  • DNA has a deoxyribose sugar, while RNA has ribose.

  • DNA uses thymine (T) instead of uracil (U) found in RNA.

• DNA is synthesized primarily in the 5' to 3' direction, with the exception of few examples.

• Base pairing rules:

  • A pairs with T, C pairs with G.

• DNA structure: two antiparallel strands forming a double helix, stabilized by hydrogen bonds.

• Important historical note: Rosalind Franklin's contribution to understanding the structure through X-ray crystallography.

DNA Packaging

• Requires DNA to be highly compacted to fit in cells.

• Utilizes supercoiling and histones to form nucleosomes.

• Achieves higher order structures leading to chromosomal formation.

• In steady state, DNA is organized but not in the compact forms seen during replication.

Introduction to DNA Replication

• An essential process during the S phase of the cell cycle.

• Involves unwinding and separating strands at origins of replication.

• Prokaryotic DNA replication is quick, utilizing a single circular origin while eukaryotic cells have multiple origins.

• Bidirectional replication: proceeds in both directions from the origin.

Overview of Enzymes involved in DNA Replication

• Helicase: unwinds the DNA double helix and forms a replication fork.

• Single Stranded Binding Proteins (SSBs): stabilize unwound DNA strands.

• Topoisomerases: alleviate torsional strain ahead of the replication fork.

  • Type I: cuts one strand to relieve stress.

  • Type II: cuts both strands to manage supercoiling.

• Primase: synthesizes RNA primers necessary to start DNA replication.

• DNA Polymerase: synthesizes new DNA strands; requires a template and primer, adds nucleotides in a 5' to 3' direction.

  • DNA Polymerase I (replaces RNA primers with DNA).

  • DNA Polymerase III (is the primary enzyme for new DNA strand synthesis).

• DNA Ligase: seals nicks in DNA strands by forming phosphodiester bonds.

Challenges in DNA Replication

• Lagging strand synthesized discontinuously in Okazaki fragments, requiring multiple RNA primers.

• Considerations about directionality leading to leading and lagging strands and their different synthesis mechanisms.

The End Replication Problem

• Linear DNA cannot fully replicate the ends, leading to degradation over time in eukaryotic cells.

• Telomeres: sequences at the end of chromosomes that protect DNA from degradation.

  • Telomerase: enzyme that extends telomeres, has implications for aging and cellular replication.

• Telomeres limit the number of times a cell can divide.

Mutations and Proofreading

• Errors during DNA replication can lead to mutations if not corrected.

• High fidelity of DNA polymerases through proofreading mechanisms.

• Example of laboratory techniques: Polymerase Chain Reaction (PCR) for amplifying DNA segments.

Conclusion

• Summary of key enzymes and their roles, emphasizing the coordinated process of DNA replication.

• Upcoming lectures will cover mutations in detail, preparing for future evaluations.