DNA+Replication-detailed23

Genetic Material of Life

Key Candidates for Genetic Material
- DNA and Proteins were considered.
- Chemical components of chromosomes played a role in this debate.
Proteins' Edge
- Proteins appeared to have an advantage due to:
  - Greater potential for structural variation.
  - Specificity of function.
- DNA seemed too uniform in structure to account for the diversity of inherited traits.
- Comparison: 20 amino acids (Proteins) vs 4 nucleotides (DNA).

Griffith's Experiment in 1928

Overview of Griffith's Procedure
- Utilized two types of bacteria:
  - Pathogenic (S cells)
  - Non-pathogenic (R cells)
- Experiment involved heat-killed pathogenic bacteria mixed with non-pathogenic bacteria.
Conclusion
- Griffith discovered that the R cells were transformed into S cells, becoming pathogenic.
- This process exemplified transformation, wherein external DNA uptake changes a cell's genotype and phenotype.

Avery, McCarty, and MacLeod's Contributions

Substance Responsible for Transformation
- Avery purified various chemicals from heat-killed pathogenic bacteria: proteins, RNA, and DNA.
- Systematic exposure of non-pathogenic bacteria to these chemicals showed DNA was the transforming agent by 1944.
- Skepticism persisted about DNA being the hereditary material:
  - Proteins were still considered more complex.
  - Observations noted that DNA in complex organisms might differ.

Hershey & Chase Experiment (1952)

Phages
- Investigated whether viral DNA or proteins serve as genetic material.
Experimental Procedure
- Used radioactive sulfur to label proteins and radioactive phosphorus to label DNA.
Conclusion
- Proved that DNA is the hereditary material of viruses.

Chargaff's Rules

Contribution to DNA Research
- Developed two key rules related to nucleotide composition:
  - Base composition varies between different species.
  - Within the same species, the ratios of adenine (A) to thymine (T) and guanine (G) to cytosine (C) are consistent.

Molecular Diversity in DNA (1950)

Chargaff's Analysis
- Conducted studies on base composition of DNA from various organisms, identifying significant diversity between species and consistency within the same species.
  - Example: A = 30.3%, T = 30.3%, G = 19.5%, C = 19.9%.
- Findings lent credibility to DNA as the genetic material.

Franklin's Contribution

X-ray Diffraction Photograph
- Rosalind Franklin's photograph helped determine the structure of DNA.
- Findings led Watson to establish its helical shape and dimensions, supporting a double helix model with base spacing.

DNA Structure Organization

Nitrogen Bases
- Bases located on the inside of DNA due to their hydrophobic nature.
- Matching bases didn't align due to uniform diameter; discovered that certain purines and pyrimidines can hydrogen bond with each other.

DNA Double Helix Characteristics

Base Pairing
- Bases in one strand are complementary to the bases in the opposite strand, enabling DNA replication.

Semi-conservative Model of DNA Replication

Replication Steps
- The parent molecule consists of two complementary DNA strands.
- Each strand serves as a template, resulting in two daughter molecules consisting of one parental strand and one new strand.

Meselson & Stahl Experiment (Late 1950s)

Models of DNA Replication
- Conservative Model: ancestral DNA remains intact; new copy is formed.
- Semiconservative Model: parental strands separate and each serves as a template for new strands.
- Dispersive Model: each daughter strand contains a mix of old and newly synthesized parts.

DNA Replication Initiation

Origins of Replication
- DNA replication begins at specific sites with multiple origins in eukaryotic DNA.
- Helicase unwinds and separates DNA strands, creating a replication bubble.
Role of DNA Polymerase
- Enzyme that adds nucleotides to the newly forming strand.

Directionality of DNA Strands

Anti-parallel Nature
- Sugar-phosphate backbones run in opposite directions (5' to 3').
- DNA polymerase can only add nucleotides to the 3' end, resulting in elongation in the 5' to 3' direction.

Elongation of the New Strand

DNA Polymerase III
- Adds complementary nucleotides at the replication fork, working at different rates.
- Nucleotides are added as nucleoside triphosphates that release energy during the reaction.

Issues During Replication

Tension in DNA Strands
- Corrected by specific enzymes such as topoisomerases.
Enzymes in Replication
- Primase lays down RNA primers for new DNA strand synthesis.
- DNA polymerase replaces primer with DNA nucleotides, and ligase seals gaps between fragments.

Problems with Completing Replication

DNA Polymerase Limitations
- Can only extend from pre-existing strands, resulting in shorter daughter strands over successive replications.

Telomeres and DNA Erosion

Function of Telomeres
- Repeated sequences protect genes at DNA ends.
- In each replication cycle, telomeres erode, not the genetic information.
- Telomerase enzyme present in germ line and some cancer cells extends these sequences.

Chromatin Structure

Composition
- Chromatin is made of DNA and histone proteins, forming nucleosomes.
Chromatin Forms
- Heterochromatin: tightly coiled, non-transcribable during mitosis.
- Euchromatin: loosely coiled, transcribable during interphase.

Summary of Contributions to DNA Understanding

Importance of contributions listed in chronological order to clarify the historical development of DNA knowledge:
- Avery: DNA as the transforming agent.
- Watson, Crick, Franklin: Structure and replication model.
- Griffith: Transformation discovery.
- Meselson & Stahl: Evidence for semi-conservative replication.
- Chargaff: Consistency of DNA content.
- Hershey & Chase: DNA as the genetic material of bacteriophages.

Key Questions

Where does DNA replication begin?
What is the name of the monomer before incorporation into a strand?

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