We are continually learning more about how life works. As we progress in our understanding, we are able to reject old hypotheses and introduce new ones. Many people have contributed to our current concept of life and though we may have revised their thought, we have not completely disregarded their efforts, but continue to build upon their findings.

Molecules and Cells

• Hooke, Schwann, Virchow: Cell Discovery and Theory

Robert Hooke observed cork under a microscope, and coined the term "cell" as a name for the compartments he saw. (1665)

Theodor Schwann proposed the first two tenets of the cell theory: (1839)

• All organisms are composed of one or more cells.
• The cell is the basic unit of life.

Rudolf Virchow added the third piece of the cell theory: (1858)

• Cells come from preexisting cells.

**Don't worry about the dates and names too much: know the theory!

• Plasma Membrane

Investigations in the 1920's concluded that the plasma membrane is indeed a phopholipid bilayer. In the 1970's the fluid-mosaic model of the membrane was developed. This model depicts the membrane as a fluid structure in which are embedded proteins and cholesterol.

• Membrane Invagination Hypothesis and Endosymbiont Hypothesis: from Prokaryote to Eukaryote

Each of these hypotheses is a proposed explanation of how cells went from the primitive prokaryotic cell to the more complex eukaryotic cell: a major step in the evolution of organisms.

• Biotechnology: Harnessing our Knowledge and Putting it to Use

We are now putting our collective knowledge of various processes and functions of animal and plant life to good use. We use bacteria to produce human insulin. We have developed gene therapy techniques thanks to our understanding of genes and viral infection. We can make plants resistant to insects because we have come to understand and use transformation.

Heredity and Evolution

• Chemical Evolution

Urey and Miller, Oparin, and Haldane did various experiments that support the idea of chemical evolution in a primitive, anaerobic environment. It is, however, still a controversial concept.

• Gregor Mendel: Father of Genetics

Mendel did extensive work with pea plants, and set down the framework for our current understanding of patterns of inheritance. He recognized dominant and recessive forms, or phenotypes, controlled by what he called factors. We now call these differing forms of genes alleles. Mendel's law of segregation states that a pair of alleles becomes separated during the formation of gametes, and that each gamete only contains one allele for each trait. (We now know that meiosis is the process by which alleles separate, and that polygenic inheritance means that a trait can be controlled by more than one gene.) His law of independent assortment states that alleles are separated into gametes randomly - the way in which one pair of alleles separates does not affect the separation of another pair. It was not until later that the concept of linkage was realized. From his studies, it has been established that there is always a common ratio in the two following crosses:

• Monohybrid Cross:

Aa x Aa

1 AA: 2 Aa: 1: aa

3 dominant phenotypes: 1 recessive phenotype

• Dihybrid Cross:

AaBb x AaBb

9 with dominant phenotypes for trait A, B: 3 dominant phenotypes for trait A, recessive for trait B: 3 dominant phenotypes for trait B, recessive for trait A: 1 recessive phenotype for trait A, B

**We know that Mendel's laws really relate to whole chromosomes, not single alleles.

• DNA: Structure

James Watson and Francis Crick developed the current model of DNA as a double-stranded helix. They proposed that the two strands run in opposite directions. The phosphate and sugar backbone of each chain winds around the outside, and the bases project into the center. The bases, and strands, are connected by hydrogen bonds that can easily be broken for replication and transcription.

James Watson and Francis Crick developed 3 models for DNA replication and confirmed, through radioactive marking, that the semi-conservative model is the correct one. It states that every strand of DNA is composed of one original piece of DNA, and one newly assembled one.

• DNA as the Hereditary Material: The Use of Radioactive Markers and the Lytic Cycle

DNA was confirmed as the genetic material during experiments that involved radioactive sulfur and phosphorus. This research proved that the DNA of a bacteriophage, and not the protein coat, was directing the assembly and production of progeny viruses. These results provide strong evidence that DNA is the hereditary information in bacteriophages, as well as all other organisms.

• Charles Darwin: Gradual Evolution

Darwin made his famous trip to the Galapagos islands in the 19th century. There, he came to some revolutionary conclusions. He proposed that populations evolve in response to environmental changes. He began to question creationist beliefs, and suggested that all organisms are the result of evolution and adaptation. He insisted that change occurs gradually and steadily. His theory is the basis for the gradualism model of evolution.

• Stephen Jay Gould and Niles Eldredge: Another Approach to Evolution

These two scientists have proposed a hypothesis known as punctuated equilibrium. They suggested that speciation occurs rapidly and that species remain unchanged between cataclysmic events.

• Barbara McClintock: Jumping Genes

Doing extensive work on maize, Barbara McClintock concluded that pieces of genetic material were "jumping" from chromosome to chromosome. She called this process transposition, and the "jumping" pieces she named transposones. They provide for more variation within a genome.

Organisms and Populations

• Jan Baptiste Van Helmont: Steps to Understanding Photosynthesis

Jan Baptiste Van Helmont was interested in plant growth, and performed two experiments that were important in initially understanding photosynthesis. He determined that a potted plant gains mass, and was curious as to the cause. We now know that the the glucose produced by the process of photosynthesis accounts for the observed mass gain. He performed another experiment involving a mouse and a bell jar. He put a mouse under the bell jar, and observed that the mouse died after an extended period of time. He then put a mouse and a plant under the bell jar. This time, the mouse and plant both survived. Van Helmont concluded that the plant was producing something which the mouse required. We have determined that this product of photosynthesis is oxygen.

• Louis Pasteur: Spontaneous Generation Disproved

Before Pasteur's experiments it was largely believed that animals spontaneously appeared: if one left meat outside, within a few days maggots and then flies would "appear." Pasteur's experiment involved a sterilized broth, exposed indirectly to air. Any microbes present in the air would become trapped on their way into the container, never reaching the broth. When the broth remained sterile after many days, Pasteur concluded that organisms come from other organisms, and are not spontaneously generated.