Campbell Biology, 12th Edition by Lisa A. Urry, Micheal L. Cain, Steven A. Wasserman, Peter V. Minorsky, Rebecca B. Orr, Neil A. Campbel-363-404

The double-helical structure of DNA was proposed by James Watson and Francis Crick in April 1953.
DNA contains all genes and genetic information inherited from parents.

DNA replication allows genetic information to be passed from parent cells to daughter cells during mitosis and generationally during meiosis.
Each chromosome consists of a single DNA molecule and proteins. When duplicated, chromosomes contain two DNA molecules.
Replication begins at multiple origins, forming replication bubbles that contain forks at each end, allowing for efficient DNA copying.

British medical officer Frederick Griffith conducted experiments in 1928 with Streptococcus pneumoniae, demonstrating transformation.
Griffith found that heat-killed pathogenic bacteria could transform nonpathogenic strains into pathogenic ones when mixed.
The term transformation refers to a change in genotype and phenotype due to the assimilation of external DNA by a cell.

Griffith’s Findings:
- Pathogenic strain (S) with a protective capsule causes disease.
- Nonpathogenic strain (R) without a capsule does not.
- Mixing heat-killed S with live R led to the transformation of R into S, indicating that some substance from S caused this change.
Avery-MacLeod-McCarty Experiment:
- Identified DNA as the transforming substance responsible for the transformation observed by Griffith.
- Scientists remained skeptical, particularly regarding the role of proteins as genetic material.

Conducted by Alfred Hershey and Martha Chase in 1952, it demonstrated that DNA, not protein, is the genetic material of bacteriophage T2.
Utilized radioactive isotopes to label DNA and protein, revealing that only DNA entered bacterial cells, leading to new phage production.

Erwin Chargaff discovered in 1950 that DNA varies among species and that the amount of adenine equals thymine (A=T) and the amount of guanine equals cytosine (G≡C). These findings contributed to the understanding of DNA structure.

DNA is a polymer of nucleotides, each containing a phosphate group, deoxyribose sugar, and a nitrogenous base (A, T, G, C).
The sugar-phosphate backbone runs in opposite directions, with nitrogenous bases extending inwards.
The double helix structure has antiparallel strands, with hydrogen bonds stabilizing between complementary bases.

Initiation:
- Begins at the origin of replication, helicases unwind the DNA, and primase synthesizes RNA primers.
Elongation:
- DNA polymerases synthesize new DNA strands by adding nucleotides to the primer in the 5' to 3' direction.
Termination:
- Replication forks meet, and enzymes complete the synthesis of DNA.

The pre-mRNA receives a 5' cap and a poly-A tail which are critical for its stability and export from the nucleus.
Introns (non-coding regions) are spliced out while exons (coding regions) are joined to form the mature mRNA.

mRNA is translated into a polypeptide chain in ribosomes. The process involves:
1. Initiation: Ribosome assembles at the start codon (AUG).
2. Elongation: tRNA delivers specific amino acids to the growing chain. Ribosome facilitates peptide bond formation.
3. Termination: Occurs when a stop codon is reached, releasing the completed polypeptide.

Ribosomes, composed of rRNA and proteins, have binding sites for mRNA and tRNA, crucial for the translation of the genetic code into proteins.

Understanding the structure and function of DNA and the processes of replication, transcription, and translation is fundamental to molecular biology and genetics, highlighting the intricate flow of information from genes to proteins.