Molecular Biology: History and Key Discoveries

• Molecular biology studies the composition, structure, and interactions of cellular molecules like nucleic acids and proteins, which are essential for cell functions and maintenance.

Historical Perspective

• Gregor Mendel discovered heredity regularities almost a century before DNA's discovery.
• Scientists rediscovered Mendel's work and realized life was encoded in genes in the early 20th century.
• The discovery of DNA structure started a new era to cracking the genetic code, it showed that DNA directs protein synthesis
• Technical advances included DNA sequencing, genetic engineering, and gene cloning.
• The human genome was completely sequenced in 2001.

Key Discoveries in Molecular Biology

• 1866: Gregor Mendel published his work on inheritance of traits in peas, becoming known as the "Father of Modern Genetics".
• 1869: Friedrich Miescher isolated "nuclein," an acidic, phosphorus-rich substance, from the nuclei of white blood cells.
• 1928: Frederick Griffith demonstrated a heritable "transforming principle" that allows bacteria to cause pneumonia in mice.
• 1929: Phoebus Aaron Levene characterized ribonucleic acid and deoxyribonucleic acid, proposing a "tetra nucleotide" structure for DNA.
• 1952: Alfred Hershey and Martha Chase determined that DNA is genetic material.
• 1953: Watson and Crick deduced DNA's double helix conformation.
• 1970: Paul Berg developed Recombinant DNA Technology.
• 1977: Frederick Sanger invented DNA sequencing.
• 1983: Kary Mullis invented In Vitro Amplification of DNA (PCR).
• 2003: The Human Genome Project (HGP) was completed under the leadership of Francis Collins, with support from the U.S. Department of Energy and the National Institutes of Health (NIH).

Mendel's Pea Plants Experiment

• Mendel crossed true-breeding violet flower plants with true-breeding white flower plants (P generation).
• The F1 generation hybrids all had violet flowers.
• In the F2 generation, approximately three-quarters of the plants had violet flowers, and one-quarter had white flowers.
• Mendel's Law of Segregation states that individuals have two alleles for each trait, inheriting one allele from each parent and passing only one allele to offspring.
• Homozygous organisms have identical alleles, heterozygous organisms have different alleles.
• Genotype is the genetic makeup of an individual while phenotype is the physical appearance.

Griffith's Bacterial Transformation Experiment

• In 1928, Frederick Griffith experimented with Streptococcus pneumoniae and mice.
• Mice injected with live S strain bacteria died of pneumonia because the S-strain has a polysaccharide capsule protecting it from the host’s immune system.
• Mice injected with live R bacteria remained healthy because the R-strain lacks a protective capsule and is defeated by the host's immune system.
• Mice injected with heat-killed S-bacteria also remained healthy.
• Mice injected with heat-killed S-bacteria and live R-bacteria died because the "transforming principle" transferred from the S bacteria to the R bacteria, allowing it to make a polysaccharide coat and evade the immune response.

Hershey-Chase Experiments

• The availability of radioisotopes post-WWII was crucial.
• Alfred Hershey and Martha Chase showed that the genetical material of bacteriophage T2 is DNA in 1952.
• Experiments used the differences between protein and DNA chemical structures. DNA contains C, H, O, N, and P, while protein contains C, H, O, N, and S.
• DNA of phages was labeled with radioactive Phosphorus-32, which is present in DNA but not in any of the 20 amino acids.
• The phages were allowed to infect E. coli, the transfer of 32P-labeled phage DNA into the bacterium's cytoplasm was observed.
• Phages were labeled with radioactive Sulfur-35, which is present in the amino acids cysteine and methionine, but not in DNA.
• After infection of E. coli, viral protein shells were sheared off using a high-speed blender and separated via centrifuge.
• The radioactive 35S tracer was observed in the protein shells, not in the infected bacteria
• After synthesis of phage components, lysis of the bacteria occurred and Progeny phage particles only contained 32P
• All information required to make new phage was contained within the injected DNA.

Experimental Modeling in Molecular Biology

• Bacteria: Prokaryotic, unicellular, free-living cells with a single chromosome in the cytoplasm known as the nucleoid.
• Escherichia coli (E. coli) is a model organism that is east to culture; it has a short generation time (20 min), and it is easy to complete DNA replication, RNA transcription, and Protein synthesis within minutes
• Bacteriophages: Simplest form of life that infects bacteria like animal, plant, and human viruses. Bacteriophages are not free-living, they use the machines of a host cell to replicate, it is now used as a cloning vector
• Yeast: Eukaryotic experimental model that contains chromosomes within a true nucleus. Mutant strains of yeast are used to discover genes that control growth, division, and cell behavior
• Animal and plant cells: Can also be used as models in genetic experiments.

Human Genome Project (HGP)

• An international scientific research project successfully completed in 2003. Which sequenced the entire human genome of 3.3 billion base pairs
• The HGP led to growth of bioinformatics, the successful sequencing of the human genome could solve mysteries and help to cope with disorders
• Goals include optimization of data analysis, sequencing the entire genome, identifying the complete human genome, creating genome sequence databases, and addressing the project's legal, ethical, and social issues.
• Methods include expressed sequence tags to differentiate genes forming part of the genome and those expressing RNAs, in addition Sequence annotation that assigns functional tags.
• The human genome is made up of of 3164.7 million base pairs, with genes averaging 3000 nucleotides.
• Over 50% of the genes' functions are yet to be discovered.
• Proteins are coded by less than 2% of the genome.

Key Terms

• Genetics: The biology branch that studies genes, genetic variation, and heredity in organisms.
• Inheritance/Heredity: Transmission of characters from one generation to the next by gametes or asexual reproductive bodies.
• Genome: All the DNA present in a cell, including chromosomes and non-chromosomal sites.
• Gene: The basic unit of heredity, a DNA segment with information for a protein or RNA.
• Alleles: A pair of genes occupying a specific location on a chromosome that control the same trait.
• Genotype: The genetic makeup of an individual that contributes to their phenotype.
• Phenotype: An individual's observable traits, determined by both genotype and environmental factors.