DNA Structure and Function

In 1869, Friedrich Miescher isolated phosphate-rich acidic compounds from the nuclei of leukocytes found in pus from hospital bandages.
Miescher termed this material "nuclein" due to its isolation from prepared cell nuclei.
This material was later identified as DNA (Deoxyribonucleic Acid).

Frederick Griffith conducted experiments using two strains of Streptococcus pneumoniae.
- R strain: Rough colonies, non-pathogenic.
- S strain: Smooth colonies, pathogenic, known to cause death.
Griffith injected a mouse with a mixture of heat-killed S strain and live R strain, resulting in the mouse's death.
He recovered live S strain bacteria from the dead mouse.
Griffith concluded that a factor passed from the heat-killed S strain to the R strain, transforming the R strain into the S strain.

Demonstrated that the transforming principle was likely nucleic acids.
They recovered cell extracts of the S strain from dead mice and systematically degraded macromolecules using specific enzymes.
They mixed these extracts with the R strain and injected the mice, concluding that DNA serves as the informational component transferred during transformation.

Hershey and Chase hypothesized that bacteriophages inject their genes into bacterial hosts for reproduction.
They aimed to isolate the injected material using two different radiolabeled phages:
- 35S: labels proteins.
- 32P: labels DNA.
The method involved infecting bacteria with the phages, then using a blender to knock off the phage coats, followed by centrifugation to analyze what entered the bacterial cells.
Only 32P (DNA) was found inside the bacterial cells, indicating that DNA is the genetic material.

Each nucleotide is composed of:
- A sugar: Deoxyribose in DNA; ribose in RNA.
- A nitrogenous base: Adenine, thymine, cytosine, guanine (DNA), or uracil (RNA).
- A phosphate group.

Erwin Chargaff analyzed nucleic acids and found:
- DNA consists of 4 nucleotide building blocks: adenine, thymine, cytosine, guanine.
- Discovered that the amounts of adenine and thymine were roughly equal, and the same for cytosine and guanine.
- This observation led to Chargaff's rules: % adenine = % thymine and % cytosine = % guanine.

Chargaff's rules and Rosalind Franklin's X-ray diffraction findings informed Watson and Crick’s determination of DNA structure in 1953.
Watson and Crick, along with Maclyn McCarty's contributions, shaped the modern understanding of DNA.
Rosalind Franklin provided crucial data through X-ray diffraction, supporting the conclusion that DNA has a double-helix structure.

DNA is described as a:
- Double helix polymer comprising repeating nucleotides.
- Strands run in opposite directions (antiparallel) and are directional.
- Two strands are held together by specific complementary hydrogen bonds between nitrogenous bases.
- The double helix has major and minor grooves allowing access for DNA-binding proteins during transcription and replication.

Frederick Sanger's dideoxy chain termination method is used in DNA sequencing, employing dye-labeled dideoxynucleotides to create terminating DNA fragments.
DNA fragments are then separated by capillary electrophoresis based on their size, enabling the reading of the DNA sequence from an electropherogram produced by a laser scanner.

Three proposed models for DNA replication:
- Conservative replication: Parent strands remain intact while new double strands form.
- Semi-conservative replication: Strands separate; each serves as a template for new complementary strands.
- Dispersive replication: Original strands are broken and interspersed within new strands.

Meselson and Stahl used E. coli grown in heavy nitrogen (15N) and switched to 14N to demonstrate that DNA replication is semi-conservative.
Results showed after one cell division, DNA from 15N and 14N sediments indicated that it was half 14N.
Subsequent divisions produced DNA with varying levels of 14N, supporting that half of the new DNA came from the original, thus confirming semi-conservative replication.

Replication Fork Formation: Initiated when helicase separates DNA strands at the origin.
Topoisomerase: Relieves tension ahead of the fork to prevent over-winding.
Single-strand binding proteins: Stabilize single strands.
Primase: Synthesizes RNA primer for DNA polymerase III, which synthesizes new strands.
DNA polymerase I: Replaces RNA primers with DNA nucleotides.
DNA ligase: Connects Okazaki fragments into a continuous strand.

DNA polymerases have proofreading activities to correct errors during replication.
DNA repair mechanisms include:
1. Proofreading: Corrects errors immediately during replication using DNA polymerase.
2. Mismatch repair: Repairs incorrect base pairings by utilizing the methylation status of strands.
3. Nucleotide excision repair: Targets large structural changes like thymine dimers caused by UV exposure.

Point mutations: Changes in a single base pair, leading to several types of mutations:
- Silent mutations: Do not change amino acid sequences.
- Missense mutations: Result in a different amino acid.
- Nonsense mutations: Create a premature stop codon.
- Frameshift mutations: Include insertions or deletions that alter reading frames.
Chromosomal mutations: Larger structural changes such as insertions, deletions, translocations, inversions, fusions, and duplications.

Eukaryotic DNA is compacted through multiple levels:
- DNA helix: Wrapped around histone proteins to form nucleosomes.
- Nucleosomes: Coiled into a solenoid shape, which is looped and attached to scaffolding proteins to form a fully compacted chromosome.

Method for separating DNA fragments based on size:
- DNA is negatively charged, so it migrates toward a positive pole in an electric field.
- Fragments are loaded into an electrified gel, acting as a sieve that sorts by size; smaller fragments move faster than larger ones.

Eukaryotic chromosome ends require telomerase for maintenance:
- Telomerase, a reverse transcriptase, extends the 3′ ends of chromosomes by synthesizing DNA using an RNA template.
- This counteracts the shortening of chromosomes that occurs during replication.
- Elizabeth Blackburn was awarded the Nobel Prize in 2009 for her work on telomerase function.

Understanding DNA structure, replication, repair mechanisms, and mutation types is paramount for genetics, molecular biology, and biotechnological applications. These concepts form the foundation for studies in cell biology, heredity, and evolution, highlighting the intimate relationship between structure and function in biological systems.