The Structural Basis of Cellular Information: DNA, Chromosomes, and the Nucleus

Chapter 16: The Structural Basis of Cellular Information

Introduction

This chapter explores the molecular foundation of genetic information in cells, focusing on the structure and function of DNA, chromosomes, and the nucleus. It covers the experiments that led to the identification of DNA as the genetic material, the flow of genetic information, and key terms and processes essential for understanding cellular heredity.

Genetic Material and Its Discovery

Key Experiments and Scientists

• Frederick Griffith: Discovered the "transforming principle" through experiments with Pneumococcus bacteria, showing that non-pathogenic bacteria could be transformed into pathogenic forms.

• Avery, MacLeod, and McCarty: Demonstrated that DNA is the substance responsible for transformation in bacteria.

• Hershey and Chase: Used bacteriophages to show that DNA, not protein, is the genetic material in viruses.

Key Terms: Pathogenic, Transforming Principle, Transcription, Phage coats (ghosts), Bacteriophage, Retrovirus, Reverse transcriptase.

Genes and DNA

Genes Consist of DNA

• Definition: Genes are discrete units of hereditary information, consisting of DNA sequences that code for functional products, usually proteins.

• Replication: The process by which DNA is copied to produce two identical molecules for distribution to daughter cells during cell division.

• Hereditary Transmission: Genetic instructions are faithfully passed from one generation to the next via DNA.

Flow of Genetic Information

Transcription and Translation

The flow of genetic information in cells occurs through two main processes:

• Transcription: The synthesis of RNA from a DNA template, catalyzed by RNA polymerase.

• Translation: The process by which the sequence of bases in messenger RNA (mRNA) directs the synthesis of a polypeptide (protein).

Central Dogma of Molecular Biology:

• DNA → RNA → Protein

Equations:

• Transcription: $\text{DNA} \xrightarrow{\text{RNA polymerase}} \text{RNA}$

• Translation: $\text{mRNA} \xrightarrow{\text{ribosome}} \text{Protein}$

Diagrams: Flow of Information in Cells

• Between Generations: DNA replication during cell division ensures genetic continuity.

• Within a Cell: DNA is transcribed to mRNA, which is then translated into protein.

Chemical Nature of Genetic Material

Discovery of DNA

• Johann Friedrich Miescher (1869): Discovered "nuclein" (now known as DNA) in white blood cells.

• Walther Flemming: First observed chromosomes in dividing cells.

• Eduard Zacharias: Removal of DNA from cells abolished chromosome staining, suggesting DNA is the genetic material.

Experimental Evidence for DNA as Genetic Material

Griffith's Transformation Experiment

• Pathogenic S-strain: Causes fatal infection in mice.

• Non-pathogenic R-strain: Does not cause infection.

• Mixing dead S-strain with live R-strain: Mice died, and live S-strain bacteria were recovered, indicating transformation.

• Transforming Principle: Substance from S-strain converted R-strain into pathogenic form.

Avery, MacLeod, and McCarty's Experiment

• Fractionated S-strain extracts to identify the transforming substance.

• Only the nucleic acid fraction (DNA) could transform R-strain bacteria.

• Destruction of DNA prevented transformation, confirming DNA as the genetic material.

Hershey-Chase Experiment

• Bacteriophage T2: Used to infect bacteria.

• Radioactive labeling: Proteins labeled with $^{35}S$, DNA labeled with $^{32}P$.

• After infection, only $^{32}P$ (DNA) entered the bacterial cells, not $^{35}S$ (protein).

• Conclusion: DNA is the genetic material of phages.

RNA as Genetic Material in Some Viruses

RNA Viruses

• Some viruses, such as Tobacco Mosaic Virus (TMV), use RNA as their genetic material.

• Experiments showed that the type of RNA present determines the type of virus produced.

Retroviruses

• Definition: Retroviruses are RNA viruses that replicate through a DNA intermediate.

• Example: Human Immunodeficiency Virus (HIV).

• Reverse Transcriptase: Enzyme that synthesizes complementary DNA (cDNA) from an RNA template.

• Viral RNA is reverse transcribed into DNA, which integrates into the host genome as a provirus.

• Proviral DNA is replicated with the host cell's DNA and transcribed to produce new viral RNA and proteins.

Key Terms and Definitions

Term	Definition
Pathogenic	Causing disease
Transforming Principle	Substance responsible for genetic transformation (later identified as DNA)
Transcription	Synthesis of RNA from a DNA template
Phage coats (ghosts)	Protein shells of bacteriophages left outside the host cell after DNA injection
Bacteriophage	Virus that infects bacteria
Retrovirus	RNA virus that replicates via a DNA intermediate
Reverse transcriptase	Enzyme that synthesizes DNA from an RNA template

Summary Table: Experiments Identifying Genetic Material

I Experiment	Organism	Key Finding
Griffith	Pneumococcus (bacteria)	Transformation of non-pathogenic to pathogenic bacteria
Avery, MacLeod, McCarty	Pneumococcus (bacteria)	DNA is the transforming principle
Hershey-Chase	Bacteriophage T2	DNA, not protein, is injected into bacteria
TMV Experiment	Tobacco Mosaic Virus	RNA is the genetic material in some viruses
Retrovirus Cycle	HIV (human)	RNA genome reverse transcribed to DNA, integrates into host

Conclusion

The identification of DNA as the genetic material was a pivotal moment in cell biology, supported by key experiments and discoveries. The flow of genetic information from DNA to RNA to protein underlies cellular function and heredity, with exceptions in certain viruses where RNA serves as the genetic material. Understanding these processes is fundamental to the study of molecular and cellular biology.