Chapter 16: The Structural Basis of Cellular Information—DNA, Chromosomes, and the Nucleus

Genetic Material and Cellular Information

Introduction to Genetic Material

Cells contain a set of instructions that determine their structure, function, and regulation of activities. These instructions are faithfully transmitted to daughter cells during cell division. The hereditary information is organized into distinct units called genes.

• Genes: Segments of DNA that code for functional products, typically proteins.

• Replication: The process by which DNA is copied to ensure each daughter cell receives genetic information.

Genes Consist of DNA

Genes are composed of DNA, which encodes the information necessary for the synthesis of proteins and other functional molecules. During cell division, DNA undergoes replication to produce two identical copies for distribution to daughter cells.

• DNA: Deoxyribonucleic acid, the molecule that stores genetic information.

• Replication: Ensures genetic continuity between generations of cells.

Transcription and Translation

The flow of genetic information in cells occurs through a two-stage process: transcription and translation.

• Transcription: The synthesis of RNA from a DNA template by RNA polymerase. This process copies genetic information from DNA into messenger RNA (mRNA).

• Translation: The process by which the sequence of bases in mRNA is used to direct the synthesis of a polypeptide (protein) at the ribosome.

Central Dogma of Molecular Biology

The central dogma describes the directional flow of genetic information:

• DNA → RNA → Protein

Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Chemical Nature and Discovery of Genetic Material

Historical Discoveries

• Johann Friedrich Miescher (1869): Discovered DNA, originally called "nuclein," in white blood cells.

• Walther Flemming: First observed chromosomes in dividing cells under the microscope.

• Eduard Zacharias: Demonstrated that removing DNA from cells abolished chromosome staining, suggesting DNA's role in heredity.

Debate Over Genetic Material

From 1910 to the 1940s, most scientists believed that genes were made of protein, as proteins were considered more complex than DNA. This view changed after key experiments demonstrated that DNA is the genetic material.

Key Experiments Demonstrating DNA as Genetic Material

Griffith's Transformation Experiment

Frederick Griffith studied Streptococcus pneumoniae and discovered two strains:

• S-strain: Pathogenic, causes fatal infection in mice.

• R-strain: Non-pathogenic, does not cause infection.

• Mixing heat-killed S-strain with live R-strain caused fatal infection, indicating transformation of R-strain into S-strain.

• Griffith called the responsible factor the transforming principle.

Avery, MacLeod, and McCarty's Experiment

Oswald Avery and colleagues identified the transforming principle as DNA by fractionating S-strain extracts and showing only the nucleic acid fraction could transform R-strain. Digestion of DNA prevented transformation.

Table: Macromolecules in Heat-Killed Cells

Option	Macromolecules Present
a	Carbohydrates, lipids, proteins, nucleic acids
b	Carbohydrates, lipids, proteins (not nucleic acids)
c	Proteins, nucleic acids (not carbohydrates or lipids)
d	Nucleic acids (not proteins, carbohydrates, or lipids)

Hershey-Chase Experiment

Alfred Hershey and Martha Chase used bacteriophage T2 to distinguish DNA from protein as genetic material:

• Phage proteins labeled with radioactive sulfur ($^{35}$S), DNA labeled with radioactive phosphorus ($^{32}$P).

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

• Conclusion: DNA is the genetic material of phage T2.

Genetic Material in Viruses

RNA as Genetic Material

Some viruses use RNA as their genetic material. For example, the tobacco mosaic virus (TMV) and certain bacteriophages.

• Experiments with TMV showed that the type of RNA present determines the type of lesion on plants.

Retroviruses

Retroviruses are RNA viruses important to human health, such as HIV. They use reverse transcriptase to synthesize complementary DNA from their RNA genome.

• Two copies of RNA genome enclosed in a protein capsid.

• Reverse transcriptase synthesizes a DNA strand complementary to viral RNA.

• Double-stranded DNA integrates into the host genome as a provirus.

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

• New viruses bud from the host cell membrane.

Table: Key Terms and Definitions

Term	Definition
Gene	Segment of DNA coding for a functional product
DNA	Deoxyribonucleic acid, hereditary material
Transcription	Synthesis of RNA from DNA template
Translation	Synthesis of protein from mRNA template
Nuclein	Original name for DNA discovered by Miescher
Bacteriophage	Virus that infects bacteria
Reverse transcriptase	Enzyme synthesizing DNA from RNA template
Provirus	Integrated viral DNA in host genome

Summary of Key Experiments

• Griffith: Discovered transformation in bacteria.

• Avery, MacLeod, McCarty: Identified DNA as the transforming principle.

• Hershey-Chase: Demonstrated DNA is the genetic material in viruses.

• TMV Experiments: Showed RNA can be genetic material in some viruses.

• Retrovirus Studies: Revealed reverse transcription and integration into host genome.

Additional info:

• Diagrams referenced in the notes illustrate the flow of genetic information, experimental setups, and viral life cycles.

• Key historical figures include Miescher, Flemming, Zacharias, Griffith, Avery, MacLeod, McCarty, Hershey, and Chase.