DNA Replication and Repair: Key Experiments and Mechanisms

DNA as the Hereditary Material

Historical Experiments Demonstrating DNA as Genetic Material

For much of the early 20th century, scientists debated whether DNA or protein was the hereditary material. Several classic experiments established DNA as the molecule responsible for inheritance.

• Griffith's Experiment (1928): Frederick Griffith discovered the phenomenon of transformation in Streptococcus pneumoniae. He worked with two strains: the nonvirulent R (rough) strain and the virulent S (smooth) strain. When heat-killed S strain was mixed with live R strain, the R strain was transformed into a virulent form, indicating the transfer of hereditary material.

• Avery, MacLeod, and McCarty (1940s): These researchers identified DNA as the 'transforming factor' by treating extracts with enzymes that destroyed proteins, RNA, or DNA. Only destruction of DNA prevented transformation, showing that DNA was necessary for heredity.

• Hershey-Chase Experiment (1952): Martha Chase and Alfred Hershey used bacteriophage T2 to infect Escherichia coli. By labeling viral DNA with 32P and protein with 35S, they showed that only DNA entered the bacterial cells and directed viral replication, confirming DNA as the genetic material.

Example: In the Hershey-Chase experiment, after blending and centrifugation, radioactive DNA was found in the bacterial pellet, while radioactive protein remained in the supernatant.

DNA Replication

Structure of DNA

DNA is a linear polymer composed of monomers called nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, guanine, or cytosine).

• Sugar-Phosphate Backbone: Nucleotides are joined by phosphodiester bonds between the 5' phosphate and 3' hydroxyl groups, giving DNA directionality (5' to 3').

• Antiparallel Strands: DNA consists of two strands running in opposite directions, stabilized by hydrogen bonds between complementary bases (A-T, G-C).

• Double Helix: The antiparallel, complementary strands twist to form a double helix.

Models of DNA Replication

Three models were proposed for DNA replication:

• Semiconservative Replication: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

• Conservative Replication: The parental molecule remains intact, and an entirely new molecule is synthesized.

• Dispersive Replication: Each new DNA molecule contains interspersed segments of old and new DNA.

Meselson-Stahl Experiment

Matthew Meselson and Franklin Stahl used isotopic labeling with 15N and 14N to distinguish old and new DNA strands in E. coli. After one generation in 14N, DNA had intermediate density, supporting the semiconservative model of replication.

Mechanism of DNA Replication

DNA replication is a highly regulated, accurate process involving several key steps and enzymes:

• Initiation: Replication begins at origins of replication, forming replication bubbles with two replication forks.

• Helicase: Unwinds the double helix by breaking hydrogen bonds between bases.

• Single-Stranded Binding Proteins (SSBPs): Stabilize unwound DNA, preventing re-annealing.

• Topoisomerase: Relieves supercoiling ahead of the replication fork.

• Primase: Synthesizes short RNA primers to provide a free 3' OH group for DNA polymerase.

• DNA Polymerase: Adds nucleotides to the 3' end of the primer, synthesizing new DNA in the 5' to 3' direction.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork in short fragments called Okazaki fragments.

• DNA Polymerase I: Removes RNA primers and fills in gaps with DNA.

• DNA Ligase: Seals nicks between Okazaki fragments, forming a continuous strand.

Example: In eukaryotes, multiple origins of replication allow rapid duplication of large chromosomes.

Replication Complex (Replisome)

The replisome is a multi-enzyme complex that coordinates the activities of all proteins involved in DNA replication, ensuring efficiency and accuracy.

Telomeres and Telomerase

Linear chromosomes have ends called telomeres. The lagging strand cannot be fully replicated at the ends, leading to progressive shortening. Telomerase extends telomeres in certain cell types (e.g., germ cells, stem cells), preventing loss of genetic information.

DNA Repair Mechanisms

Proofreading

DNA polymerase has proofreading activity: it detects and removes incorrectly paired bases during synthesis using its 3' to 5' exonuclease activity, reducing the error rate to less than 1 in 109 bases.

Mismatch Repair

After replication, mismatch repair enzymes recognize and correct mismatched bases missed by DNA polymerase. They remove a segment of the newly synthesized strand and fill in the correct sequence.

Nucleotide Excision Repair

This system repairs bulky DNA lesions, such as thymine dimers caused by UV light. Enzymes remove the damaged DNA segment, and DNA polymerase fills in the gap using the undamaged strand as a template. DNA ligase seals the final nick.

• Example: Individuals with Xeroderma pigmentosum have defective nucleotide excision repair, making them highly sensitive to UV-induced DNA damage.

Summary Table: Key DNA Replication and Repair Enzymes

Enzyme/Protein	Function
Helicase	Unwinds DNA double helix
SSBPs	Stabilize single-stranded DNA
Topoisomerase	Relieves supercoiling
Primase	Synthesizes RNA primers
DNA Polymerase	Synthesizes new DNA strand
DNA Polymerase I	Removes RNA primers, fills gaps
DNA Ligase	Seals nicks in DNA backbone
Telomerase	Extends telomeres

Key Equations

• Phosphodiester Bond Formation:

$\text{(DNA strand)}_{n} + \text{dNTP} \rightarrow \text{(DNA strand)}_{n+1} + PP_i$

• Base Pairing: A pairs with T, G pairs with C (Chargaff's rules)

Additional info: These notes cover content relevant to General Biology topics, specifically DNA structure, replication, and repair, as outlined in chapters 15 and 16 of standard biology textbooks.