DNA as the Genetic Material

The Search for the Genetic Material

Early 20th-century biologists sought to identify the molecules responsible for inheritance. Chromosomes, composed of DNA and protein, were the primary candidates. The role of DNA in heredity was first discovered through studies of bacteria and viruses.

• Genes are located on chromosomes.

• DNA and protein were considered possible genetic materials.

• Experiments with bacteria and viruses provided key evidence.

Evidence That DNA Can Transform Bacteria

Frederick Griffith's 1928 experiment demonstrated that DNA could transfer genetic traits between bacterial strains, a process called transformation.

• Pathogenic and harmless strains of bacteria were used.

• Mixing heat-killed pathogenic bacteria with living harmless bacteria resulted in some harmless bacteria becoming pathogenic.

• Transformation is defined as a change in genotype and phenotype due to uptake of foreign DNA.

Evidence That Viral DNA Can Program Cells

Viruses called bacteriophages (phages) provided further evidence. The Hershey-Chase experiment showed that DNA, not protein, is the genetic material of phage T2.

• Phages consist of DNA (or RNA) enclosed in a protein coat.

• Only DNA enters the bacterial cell during infection, providing genetic information.

Chargaff's Rules and DNA Structure

Erwin Chargaff discovered that DNA composition varies between species and that the number of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C). These findings supported DNA as the genetic material.

• DNA is a polymer of nucleotides: each nucleotide contains a nitrogenous base, a sugar, and a phosphate group.

• Chargaff's rules: A = T, G = C in any species.

Building a Structural Model of DNA

X-ray Crystallography and the Double Helix

Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study DNA's molecular structure. Franklin's images enabled Watson and Crick to deduce the double-helical structure of DNA.

• DNA is composed of two strands forming a double helix.

• The strands are antiparallel, running in opposite directions.

Base Pairing and the Double Helix

Watson and Crick determined that base pairing is specific: adenine pairs with thymine, and guanine pairs with cytosine. Pairing a purine with a pyrimidine results in a uniform width consistent with X-ray data.

• Purine + pyrimidine = correct width for double helix.

• Hydrogen bonds hold base pairs together.

DNA Replication and Repair

Base Pairing to a Template Strand

DNA replication relies on the complementary nature of the two strands. Each strand serves as a template for building a new strand, resulting in two identical DNA molecules.

• Replication is essential for inheritance and cell division.

• Each new DNA molecule contains one old strand and one new strand (semiconservative model).

Initiation of DNA Replication

Replication begins at origins of replication, where the DNA strands are separated, forming replication bubbles. Replication proceeds in both directions until the entire molecule is copied.

• Eukaryotic chromosomes have multiple origins of replication.

• Replication forks are Y-shaped regions where DNA is unwound.

Synthesizing a New DNA Strand

DNA polymerases require a primer to begin synthesis. Primase synthesizes a short RNA primer, and DNA polymerases add nucleotides to the 3' end of the primer.

• Nucleotides are added as nucleoside triphosphates.

• Dehydration reactions join monomers, releasing pyrophosphate.

Antiparallel Elongation: Leading and Lagging Strands

DNA polymerases can only add nucleotides to the 3' end, so replication occurs in the 5' → 3' direction. The leading strand is synthesized continuously, while the lagging strand is synthesized in segments called Okazaki fragments.

• Okazaki fragments are joined by DNA ligase.

Bacterial DNA Replication Proteins and Their Functions

Several proteins are involved in DNA replication, each with a specific function.

The DNA Replication Complex

Proteins involved in DNA replication form a large complex, sometimes called the "DNA replication machine." DNA polymerase may be stationary, reeling in parental DNA and extruding daughter DNA.

Proofreading and Repairing DNA

DNA polymerases proofread newly synthesized DNA, replacing incorrect nucleotides. Repair enzymes correct mismatches and excise damaged DNA.

• Nucleotide excision repair involves a nuclease, DNA polymerase, and DNA ligase.

Replicating the Ends of DNA Molecules

Linear DNA molecules face challenges in replicating their ends. Eukaryotic chromosomes have telomeres at their ends, which postpone gene erosion. Telomerase catalyzes the lengthening of telomeres in germ cells.

• Telomere shortening is associated with aging.

• Telomerase activity is observed in cancer cells.

Chromosome Structure and DNA Packing

Chromatin and Chromosome Organization

Bacterial chromosomes are circular DNA molecules with little protein. Eukaryotic chromosomes are linear and associated with large amounts of protein, forming chromatin.

• DNA is packed into nucleosomes, each composed of DNA wound around histone proteins.

• Histone tails extend outward and regulate gene expression.

Chromatin Packing and Gene Expression

Chromatin can be loosely packed (euchromatin) or highly condensed (heterochromatin). Dense packing makes gene expression difficult.

• Chromatin undergoes changes during the cell cycle, condensing into metaphase chromosomes.

• Histone modifications affect chromatin condensation and gene expression.

Summary

DNA is the hereditary material, with a double-helical structure that allows for accurate replication and transmission of genetic information. Replication involves a complex interplay of enzymes and proteins, and chromatin structure regulates gene expression and chromosome organization.