DNA Notes

DNA Notes

Overview of Genetic Material

• Historical Perspective: Morgan's group identified that genes are associated with chromosomes, but the specific genetic material was unknown.

• Candidates for Genetic Material: DNA and protein were the two primary candidates.

• Widespread Skepticism: There was a prevailing belief that proteins were the genetic material due to a lack of understanding of nucleic acids.

Frederick Griffith's Experiment (1928)

• Objective: Determine if DNA could transform bacteria.

• Strains Used: Two strains of a bacterium;

  • S strain: pathogenic.

  • R strain: harmless.

Experiment Protocol

• Mixed heat-killed S cells with living R cells.

• Observed outcomes:

  • Living S cells: Mouse dies.

  • Living R cells: Mouse healthy.

  • Heat-killed S cells: Mouse healthy.

  • Mixture of heat-killed S and living R cells: Mouse dies.

Results and Conclusions

• Transformation Observed: Living R cells transformed into pathogenic cells after mixed with heat-killed S cells.

• Conclusion: Transfer of genetic material indicated that DNA carried the genetic information (phenomenon labeled as "transformation").

Avery, McCarty, and MacLeod (1944)

• Discovered that the transforming substance was DNA.

• Resistance to Acceptance: Skepticism persisted among biologists due to minimal knowledge about DNA.

Composition of DNA

• Form: DNA is a polymer made of nucleotides.

• Nucleotides:

  • Components: Nitrogenous base, deoxyribose sugar, phosphate group.

• Base Composition: Four nitrogenous bases: Adenine (A), Guanine (G), Thymine (T), Cytosine (C).

• Division of Bases:

  • Pyrimidines: Thymine (T), Cytosine (C).

  • Purines: Adenine (A), Guanine (G).

Chargaff's Rules (1950)

• Erwin Chargaff discovered:

  • Base composition varies between species.

  • In any given species, A=T and G=C, establishing significance in DNA structure and variability.

Viral DNA and Bacteriophages

• Bacteriophages (Phages): Viruses that infect bacteria; consist of DNA (or RNA) enclosed by a protein coat.

  • Provided additional evidence for DNA as the genetic material.

Hershey-Chase Experiment (1952)

• Demonstrated that DNA is the genetic material of phage T2.

• Experiment Setup: Used radioactive labeling of DNA and protein to track infection in E. coli.

  • Result: Only DNA entered bacterial cells, confirming its role as the genetic material.

Structure of DNA

• Investigators: Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study DNA.

• Major Findings:

  • DNA is helical with two strands.

  • Model of DNA structure was represented as a double helix.

• Watson and Crick determined base pair relationships and antiparallel structure of strands.

• Purine + Pyridine : with consistent with X-ray data

Base Pairing

• Key Pairs: A pairs with T, and G pairs with C.

• Adenine + thyme, Guanine + cytosine , pair with hydrogen bonds

• Consistency with Chargaff's rules validated by the model.

DNA Replication

Begins as origin replication

• Mechanisms:

  • Each strand serves as a template for a new complementary strand.

  • Direction of replication: 5' to 3' direction.

  • Eukaryotic are bigger and have a different way of copying

  • At the ends fat replication bubble is a replication fork

    • Helicase, primase, topoisomerase enzymes in the replication fork

Leading and Lagging Strands

Parent molecule strands split, formation of new strands connect its it own complimentary Starands

• Leading Strand: Synthesized continuously towards the replication fork.

• Lagging Strand: Synthesized in short segments (Okazaki fragments) away from the replication fork.

• DNA polymerase III synthesizes both strands with the help of primase and ligase.

Key Enzymes in Replication

• Helicases (1) Unwind the double helix.

• Single-strand binding proteins: Stabilize strands post unwinding.

• Topoisomerases: Normalize tension during replication.

• DNA polymerases: Catalyze nucleic acid elongation, begin synthesis at an RNA primer.

• Primase: Lays down the RNA primer needed for DNA synthesis.

• Ligase: Joins Okazaki fragments of the lagging strand.

DNA Repair and Proofreading

• DNA polymerases have proofreading functions, enhancing accuracy (error rates significantly decrease) - cannot start on its own

• Primer is there to put fiber 6 RNA nucleotides at the start to give primase a starting point

• Primer serves as starting point of DNA strand

• Damaged DNA can be repaired through mechanisms like nucleotide excision repair.

• Datp supplied adenine to DN A

Anti parallel Elongation:

This structure affects double helix replication

DNA strands can only replicate in the 5’ 3’ direction

There is a leading strand (following the helocase) and lagging strand (away from the helocase)

Leading strand makes DNA one big long strand

Lagging trend makes a series of Okazaki fragment which are joined together by DNA ligase to make the single strand

• Helicase strand binding (replications)\

1. Primase make RNA primser

2. 2. Polymers copes the DNA

3. 3 DNA pol falls off

4. 4 makes Okazaki fragments

5. DNA pol replaces RNA with DNA

6. DNA ligase joins the strands together

Telomeres and Their Importance

• Eukaryotic chromosomes end with telomeres to protect genes during replication.

• Telomerase at the enf) enzyme extends telomeres in germ cells, with implications in cell aging and cancer prevention.

Chromatin Structure

• Composition: DNA and protein form chromatin, tightly packed into chromosomes.

• Packing Levels: From double helix to nucleosome to higher order structures.

• Euchromatin vs Heterochromatin: Different packing density with implications for gene expression during interphase and mitosis.