Biology: The Study of Life

Defining Life

Biology is the scientific study of living organisms and their interactions with the environment. To determine if something is alive, biologists use several key criteria that distinguish living things from non-living matter.

• Cells: All organisms are composed of one or more membrane-bound cells, which are the basic units of life.

• Replication: All living things have the capacity to reproduce, either sexually or asexually.

• Information: Organisms process hereditary information encoded in genes (DNA), as well as information from their environment.

• Energy: All organisms acquire and use energy to maintain order and support life processes.

• Evolution: Populations of organisms change over time through evolutionary processes.

Core Theories in Modern Biology

Three major scientific theories provide the framework for understanding biological science:

1. Cell Theory

2. Chromosome Theory of Inheritance

3. Theory of Evolution by Natural Selection

Cell Theory

The cell theory is a fundamental concept in biology, stating that:

• All organisms are made up of cells.

• All cells come from pre-existing cells.

This theory was developed through the work of scientists such as Robert Hooke and Anton van Leeuwenhoek, who used microscopes to observe cells in various organisms.

Historical Observations

• Robert Hooke (1665): Observed cork tissue (dead plant cells) with a 30X microscope, coining the term "cell."

• Anton van Leeuwenhoek: Used a 300X microscope to observe single-celled organisms ("animalcules").

These discoveries led to the realization that cells are the fundamental units of life.

Testing the Origin of Cells

In the 1800s, the dominant belief was spontaneous generation—the idea that living organisms could arise from non-living matter under certain conditions. This was challenged by the hypothesis that all cells come from pre-existing cells ("all-cells-from-cells").

• Louis Pasteur's Experiment: Pasteur used swan-necked flasks to show that cells do not spontaneously appear in sterile broth, supporting the idea that all cells arise from existing cells.

Conclusion: All cells come from pre-existing cells, not from spontaneous generation.

Chromosome Theory of Inheritance

The chromosome theory of inheritance explains how genetic information is transmitted from one generation to the next.

• Genes: Units of hereditary information, located on chromosomes.

• Chromosomes: Structures within cells that contain DNA, the molecule that encodes genetic instructions.

In the early 1900s, Sutton and Boveri observed chromosomes during cell division and proposed that genes are located on chromosomes. Later, it was discovered that chromosomes are composed of DNA, and genes are specific segments of DNA that determine traits.

Structure of DNA

• James Watson and Francis Crick (1953): Proposed the double-helix structure of DNA, with base pairs forming the rungs of the helix.

Flow of Genetic Information

• DNA is transcribed into RNA, which is then translated into proteins.

• Changes (mutations) in DNA can lead to different proteins and, consequently, different traits.

This flow of information is often summarized as the "central dogma" of molecular biology:

Theory of Evolution by Natural Selection

The theory of evolution by natural selection, first articulated by Charles Darwin and Alfred Wallace, explains how populations of organisms change over time.

• Common Ancestry: All species are related by descent from a common ancestor.

• Descent with Modification: Species change over generations, acquiring new traits.

Natural Selection

Natural selection is the process by which certain traits become more common in a population because they confer a survival or reproductive advantage.

• Individuals in a population vary in their traits.

• Some of these traits are heritable (can be passed to offspring).

• In a given environment, individuals with advantageous traits are more likely to survive and reproduce ("survival of the fittest").

Over time, this leads to changes in the frequency of traits within a population, driving evolution.

The Tree of Life and Phylogeny

The "Tree of Life" is a metaphor for the evolutionary relationships among all organisms. By comparing DNA sequences, scientists can infer how closely related different species are and construct phylogenetic trees that depict evolutionary history.

• Phylogeny: The evolutionary history and relationships among species.

• DNA analysis is a key tool for determining phylogenetic relationships.