Chapter 16

Gregor Mendel: Proposed the idea of "heritable factors" that control traits in offspring.
Thomas Hunt Morgan: Suggested that genes are located on chromosomes, leading to the understanding that chromosomes, composed of DNA and proteins, become prime candidates for the genetic material.

Transforming bacteria: Research by Fredrick Griffith in 1928 demonstrated that DNA can transform non-pathogenic bacteria into pathogenic strains through a process known as transformation.
Griffith utilized two strains of a bacterium: one that was pathogenic (disease-causing) and one that was harmless.
Oswald Avery's Contribution (1944): Alongside Maclyn McCarty and Colin MacLeod, Avery isolated the transforming substance and concluded it was DNA.
- Many biologists initially remained skeptical about the role of DNA due to limited prior knowledge regarding its structure and function.
Viruses as Genetic Material: Further evidence supporting the role of DNA as the genetic material came from bacteriophage studies; these are viruses that infect bacteria.
- A phage is understood to be a virus consisting of DNA (or RNA) encased in a protein coat.

Alfred Hershey and Martha Chase: In 1952, they demonstrated through experiments with the T2 phage that DNA is the genetic material.
- They designed an experiment to prove that only one of the two components, either DNA or protein, enters an E. coli cell during infection and concluded that the injected DNA carries the genetic information.

DNA is recognized as a polymer comprising nucleotides, each composed of:
  - A nitrogenous base
  - A sugar
  - A phosphate group
Erwin Chargaff's Findings (1950): He reported that the composition of DNA varies across different species, leading to the formulation of Chargaff's Rules:
- The amount of adenine (A) equals the amount of thymine (T), and the amount of cytosine (C) equals the amount of guanine (G) in any given species.

Maurice Wilkins and Rosalind Franklin: Employed X-ray crystallography to study the molecular structure of DNA.
- Franklin's X-ray crystallographic images allowed James Watson to deduce that DNA is helical in nature.
- The images provided crucial insights into the dimensions of the helix and the spacing of the nitrogenous bases.
Watson and Crick's Model: They proposed a structural model of DNA based on these insights and concluded that:
  - DNA is a double helix made up of two strands.
  - The pairing of nitrogenous bases is specific: adenine pairs with thymine (A-T), and guanine pairs with cytosine (G-C).
  - This pairing is consistent with Chargaff's rules, confirming A = T and G = C in any organism.

The two strands of DNA serve as complementary templates during replication. Each strand allows for the synthesis of a corresponding new strand based on specific base-pairing rules.
During DNA replication, the parental DNA molecule unwinds, and two new daughter strands are formed as templates.
Meselson and Stahl's Experiment: Supported the semi-conservative model of DNA replication by labeling nucleotides with different isotopes of nitrogen, showing the replication process.
Key Terms and Components:
  - Replication Fork: A "Y" shaped region where new DNA strands elongate.
  - Helicases: Enzymes responsible for unwinding the DNA double helix at the replication fork.
  - Single-strand binding proteins: Stabilize and bind to single-stranded DNA.
  - Topoisomerases: Correct overwinding ahead of the replication fork by breaking, swiveling, and rejoining DNA strands.

Overview of replication involves leading and lagging strands:
  - Leading strand synthesized continuously, while the lagging strand is synthesized in short segments known as Okazaki fragments.
  - Primase: Synthesizes RNA primers at the 5' ends of the leading and Okazaki fragments.
  - DNA Polymerases: Include DNA pol III, which synthesizes new DNA strands, and DNA pol I, which replaces RNA primers with DNA.
  - DNA Ligase: Joins Okazaki fragments on the lagging strand and connects the 3' ends of the newly synthesized segments to form a continuous strand.

DNA Polymerases: These enzymes proofread the newly synthesized DNA and replace any incorrect nucleotides.
Mismatch Repair: Repair enzymes fix errors in base pairing during DNA replication.
DNA can sustain damage from harmful agents like cigarette smoke and X-rays or can undergo spontaneous changes.
Nucleotide Excision Repair: A process in which a nuclease cuts out damaged stretches of DNA and replaces them with correctly synthesized segments.

While the error rate post-repair is low, it remains non-zero, and some sequence changes can become permanent.
Such mutations are the foundation of genetic variation, serving as the substrate for natural selection and evolution.

Eukaryotic DNA molecules, especially linear chromosomes, face limitations during replication processes, particularly at the 5' ends.
Telomeres: Special nucleotide sequences like TTAGGG exist at the ends of eukaryotic chromosomes to delay the erosion of genes.
Though they do not fully prevent DNA shortening, they help safeguard genetic information from degradation, which has implications for aging and cellular senescence.

A chromosome consists of DNA packed with proteins.
Chromatin: The form of DNA packaging varies: during interphase, chromatin is loosely packed (euchromatin) allowing gene expression, while regions like centromeres and telomeres condense into heterochromatin, making genetic information difficult to access for transcription processes.