Genetic Material, DNA Structure, Chromosomes, and Experiments Demonstrating DNA as Genetic Material

Genetic Material: DNA and RNA

Introduction to Nucleic Acids

Nucleic acids are essential biomolecules that store and transmit genetic information in all living organisms. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These molecules are polymers made up of repeating units called nucleotides.

• DNA is the genetic blueprint for most organisms, encoding instructions for protein synthesis.

• RNA plays various roles in gene expression and protein synthesis.

Structure of Nucleotides

Each nucleotide consists of three components:

• A pentose (five-carbon) sugar

• A nitrogenous base attached to the 1' carbon

• A phosphate group attached to the 5' carbon

For DNA, the sugar is deoxyribose (lacking an oxygen atom at the 2' carbon), while for RNA, it is ribose (with an OH group at the 2' carbon).

• There are four nitrogenous bases in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T).

• RNA contains uracil (U) instead of thymine.

Chargaff's Rules

Chargaff's rules describe the base pairing regularities in DNA:

• The number of purines (A + G) equals the number of pyrimidines (C + T): $A + G = C + T$

• The amount of adenine equals thymine, and the amount of guanine equals cytosine: $A = T$ and $G = C$

Example: If a DNA segment has 60 A and 50 C, the total number of nucleotides is 220 (A = T = 60, G = C = 50; total = 60 + 60 + 50 + 50 = 220).

Genomic DNA and Chromosomes

Genome and Chromosome Structure

The genome is the complete set of genetic material in a cell. In eukaryotes, the genome is organized into multiple linear chromosomes composed of DNA and associated proteins (chromatin complexes). Prokaryotes typically have a single circular chromosome.

• Human somatic cells have 46 chromosomes (23 pairs).

• Gametes (egg and sperm) have 23 chromosomes and are haploid (n).

• Somatic cells are diploid (2n), containing two matched sets of chromosomes.

Homologous Chromosomes

In diploid organisms, chromosomes exist in pairs called homologous chromosomes. Each pair contains the same genes in the same order, but may have different versions (alleles) of those genes.

• Homologous chromosomes are similar in length and gene position.

• Sex chromosomes (X and Y) determine biological sex and are not always homologous.

Levels of DNA Compaction

DNA is highly compacted to fit within the cell nucleus. The levels of compaction include:

• Double helix: The basic structure of DNA.

• Nucleosome: DNA wrapped around a core of 8 histone proteins.

• Chromatin: DNA plus nucleosomes.

• Higher-order chromatin: Further folding and coiling of chromatin.

• Condensed chromosome: The most compact form, visible during cell division.

In prokaryotes, DNA is supercoiled but not organized into chromatin. In eukaryotes, chromatin is found during interphase (G1, S, G2, G0), and chromosomes condense during mitosis and meiosis.

Experimental Evidence for DNA as Genetic Material

Griffith's Transformation Experiment (1920s)

Frederick Griffith discovered the phenomenon of transformation using two strains of Streptococcus pneumoniae:

• R strain: Rough colonies, non-virulent (did not cause disease in mice).

• S strain: Smooth colonies, virulent (caused disease in mice due to a polysaccharide capsule).

Key findings:

• Heat-killed S strain did not cause disease.

• Mixing heat-killed S strain with live R strain caused disease, indicating that some "transforming principle" from the S strain converted R strain into virulent S strain.

Avery, MacLeod, and McCarty Experiment (1940s)

These scientists identified DNA as the "transforming principle" by selectively destroying proteins, RNA, or DNA in the heat-killed S strain:

Enzyme	Target Molecule	Result
Proteases	Proteins	Transformation occurred
Nucleases	DNA	No transformation
Amylases	Polysaccharides	Transformation occurred
Control	None	Transformation occurred

Conclusion: Only destruction of DNA prevented transformation, indicating DNA is the genetic material.

Hershey-Chase Experiment

Alfred Hershey and Martha Chase used bacteriophages (viruses that infect bacteria) to determine whether DNA or protein is the genetic material. They labeled phage DNA with radioactive phosphorus ($^{32}P$) and protein with radioactive sulfur ($^{35}S$), then tracked which component entered bacterial cells during infection.

• Only DNA entered the bacteria and directed the production of new phages.

• Conclusion: DNA, not protein, is the hereditary material in viruses.

Process of Bacteriophage Infection

• Bacteriophage attaches to the bacterial cell surface.

• Phage injects its DNA into the host cell.

• Viral DNA directs the host cell to produce new viral components.

• New phages are assembled and released from the host cell.

Summary Table: Key Experiments Demonstrating DNA as Genetic Material

Experiment	Organism/System	Key Finding
Griffith	Bacteria (R and S strains)	Transformation of non-virulent to virulent by a "transforming principle"
Avery, MacLeod, McCarty	Bacteria	DNA is the transforming principle
Hershey-Chase	Bacteriophage and bacteria	DNA, not protein, is the genetic material

Additional info:

• These notes cover material relevant to General Biology topics, including nucleic acid structure, chromosome organization, and classic experiments in molecular genetics.

• For further study, review the processes of DNA replication, transcription, and translation, as well as the cell cycle and its regulation.