Characteristics of Life

Defining Living Organisms

To be classified as a living organism, an entity must possess all seven fundamental characteristics. These criteria distinguish living things from non-living matter.

• Reproduction: Ability to produce offspring, either sexually or asexually.

• Growth and Development: Organisms undergo regulated growth and developmental changes.

• Energy Use: Living things acquire and utilize energy, converting it to useful forms and expelling waste.

• Cells: All living things consist of one or more cells, which are the basic units of life.

• Order: Organisms exhibit complex and organized structures, from molecules to biosphere.

• Response to Environment / Homeostasis: Organisms respond to stimuli and maintain internal equilibrium.

• Evolution: Populations evolve over time through genetic changes.

Example: Viruses cannot reproduce independently and are not considered living organisms.

Scientific Method

Process of Scientific Inquiry

The scientific method is a systematic approach to investigating natural phenomena. It ensures that scientific knowledge is reliable and reproducible.

• Observation: Gathering information about phenomena.

• Question: Formulating questions based on observations.

• Hypothesis: Proposing a testable explanation.

• Empirical Test: Conducting experiments or collecting data.

• Conclusion: Analyzing results to support or refute the hypothesis.

Example: Testing whether plants grow faster under red light compared to blue light.

Genetics and DNA

Mendelian Genetics

Gregor Mendel founded the field of genetics, discovering the principles of inheritance through experiments with pea plants.

• Genes: Units of heredity, located on chromosomes.

• Alleles: Different forms of a gene (dominant and recessive).

• Genotype: Genetic makeup of an organism.

• Phenotype: Observable traits resulting from genotype.

Example: Mendel's pea plants showed dominant and recessive traits for flower color.

Chromosomes and DNA Structure

Chromosomes are structures within cells that contain DNA and proteins. DNA is the molecule that carries genetic information.

• Somatic cells: Diploid (2n), containing two sets of chromosomes.

• Gametes: Haploid (n), containing one set of chromosomes.

• DNA: Double helix structure composed of nucleotides.

• Nucleotides: Consist of a nitrogenous base, deoxyribose sugar, and phosphate group.

Chargaff's Rules: DNA base composition varies among species, but the amount of adenine equals thymine, and cytosine equals guanine.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information from DNA to RNA to protein.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a polypeptide (protein) by ribosomes.

Equation:

Mutations and Genetic Variation

Types of Mutations

Mutations are changes in the genetic code that can affect phenotype and drive evolution.

• Substitution: Replacement of a single nucleotide and its partner.

• Silent mutation: Change does not affect phenotype.

• Missense mutation: Change results in a different amino acid.

• Nonsense mutation: Change introduces a stop codon.

• Frameshift mutation: Insertion or deletion alters the reading frame.

Example: Sickle cell disease is caused by a point mutation in the hemoglobin gene.

Evolutionary Theory

Descent with Modification

Evolution is the process by which populations change over generations through genetic variation and natural selection.

• Natural selection: Traits beneficial for survival and reproduction become more common in a population.

• Adaptation: Inherited characteristics that enhance survival and reproduction.

• Survival of the fittest: Individuals with advantageous traits are more likely to survive and reproduce.

Example: Darwin's finches in the Galápagos Islands evolved different beak shapes to exploit different food sources.

Mechanisms of Evolution

• Mutation: Source of genetic variation.

• Genetic drift: Random changes in allele frequencies, significant in small populations.

• Founder effect: Small group establishes a new population, leading to genetic differences.

• Bottleneck effect: Population size is drastically reduced, decreasing genetic variation.

• Gene flow: Movement of alleles between populations.

• Natural selection: Differential survival and reproduction based on phenotype.

Modes of Natural Selection

• Directional selection: Favors one extreme phenotype.

• Disruptive selection: Favors both extremes of phenotype.

• Stabilizing selection: Favors intermediate phenotypes.

Example: Stabilizing selection maintains average birth weight in human populations.

Population Genetics

Population and Demography

Population genetics studies the distribution and changes of allele frequencies within populations. Demography examines population size, structure, and dynamics over time.

• Population: Group of individuals of the same species in a given area.

• Dispersion: Spatial arrangement of individuals (clumped, uniform, random).

• Demographic studies: Analyze birth rates, death rates, and age structure.

Example: Age structure diagrams show the proportion of individuals in different age groups.

Speciation and Macroevolution

Formation of New Species

Speciation is the process by which new species arise. It can occur through geographic isolation (allopatric speciation) or within the same area (sympatric speciation).

• Allopatric speciation: Physical barriers separate populations, leading to divergence.

• Sympatric speciation: New species arise without geographic isolation, often through genetic changes.

• Polyploidy: Organisms have more than two sets of chromosomes, common in plants.

Equation:

Comparative Genomics and Evolutionary Biology

Genome Sequencing and Analysis

Comparative genomics compares the genomes of different organisms to understand evolutionary relationships and functional biology.

• Whole genome shotgun approach: DNA is fragmented, sequenced, and assembled using computational methods.

• Systems biology: Integrates data to model biological systems and interactions.

• Medical applications: Identifying genes responsible for diseases and cancer.

Example: Human genome contains about 3,000 MB of DNA.

Evolutionary Mechanisms and Adaptation

Homology, Analogy, and Vestigial Structures

Homologous structures indicate common ancestry, while analogous structures perform similar functions but do not share ancestry. Vestigial structures are remnants of ancestral features.

• Homology: Similarities due to shared ancestry.

• Analogy: Similar function, different evolutionary origin.

• Vestigial structures: Non-functional remnants of ancestral traits.

Example: The human appendix is a vestigial structure.

Life History Strategies and Population Growth

Population Growth Models

Population growth can be modeled using exponential or logistic equations, depending on resource availability and environmental constraints.

• Exponential growth: Rapid increase in population size under ideal conditions.

• Logistic growth: Population growth slows as it approaches carrying capacity.

Equation:

Carrying capacity (K): Maximum sustainable population size.

Life History Strategies

• r-selected species: Many offspring, low survival rate (e.g., insects).

• K-selected species: Few offspring, high survival rate (e.g., humans).

Example: Sea turtles are r-selected, while elephants are K-selected.

Summary Table: Key Terms and Definitions

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology curriculum.