Biology

Genetics is the study of heredity and the variation of inherited characteristics. Gregor Mendel's work with pea plants in the 1860s laid the foundation for genetics, establishing the laws of inheritance.

The discovery of DNA's structure in 1953 by Watson and Crick revolutionized our understanding of heredity. Genes, made of DNA, carry the instructions for building and maintaining organisms. These instructions are passed from parents to offspring through reproduction.

Mendel's laws of inheritance:

• Law of Segregation: Each individual has two alleles for each gene, and these separate during gamete formation
• Law of Independent Assortment: Genes for different traits assort independently during gamete formation
• Law of Dominance: In a heterozygote, one allele may mask the expression of another

Modern genetics encompasses molecular genetics (study of genes at the molecular level), population genetics (study of genetic variation in populations), and quantitative genetics (study of continuously varying traits).

Genetic variation arises from mutations, genetic recombination during meiosis, and gene flow between populations. This variation is essential for evolution and provides the raw material for natural selection.

Evolution by natural selection, independently proposed by Charles Darwin and Alfred Russel Wallace in the 19th century, explains the diversity of life and how species adapt to their environments.

Natural selection requires three conditions: variation in traits, heritability of those traits, and differential survival and reproduction. When these conditions are met, populations evolve as advantageous traits become more common over generations.

Evidence for evolution comes from multiple sources:

• Fossil record: Shows progression of life forms over geological time
• Comparative anatomy: Homologous structures indicate common ancestry
• Molecular biology: DNA and protein similarities reveal evolutionary relationships
• Biogeography: Distribution of species reflects evolutionary history
• Direct observation: Evolution observed in rapidly reproducing organisms (e.g., antibiotic resistance)

Evolution occurs through several mechanisms: natural selection, genetic drift, gene flow, and mutation. Natural selection is the primary mechanism driving adaptation, where traits that enhance survival and reproduction become more common.

The modern synthesis combines Darwin's theory of natural selection with Mendelian genetics and population genetics, providing a comprehensive framework for understanding how evolution works at both the individual and population levels.

Cell theory is one of the fundamental principles of biology, stating that:

• All living organisms are composed of one or more cells
• The cell is the basic unit of structure and function in living organisms
• All cells arise from pre-existing cells through cell division

The structure of cells determines their function. Prokaryotic cells lack a nucleus, while eukaryotic cells have membrane-bound organelles that compartmentalize cellular functions.

Key differences between cell types:

• Prokaryotic cells: Simpler structure, no membrane-bound organelles, DNA in nucleoid region (e.g., bacteria, archaea)
• Eukaryotic cells: Complex structure with nucleus and organelles, DNA enclosed in nuclear membrane (e.g., plants, animals, fungi)

The structure-function relationship is evident in specialized cells. For example, red blood cells lack nuclei to maximize space for hemoglobin, muscle cells contain many mitochondria for energy production, and neurons have long extensions (axons) for signal transmission.

Cell membranes are selectively permeable, controlling what enters and exits the cell. This regulation maintains homeostasis and allows cells to respond to their environment. The fluid mosaic model describes the membrane as a dynamic structure with embedded proteins floating in a lipid bilayer.

Homeostasis is the ability of organisms to maintain stable internal conditions despite external changes. This includes temperature regulation, pH balance, nutrient levels, and other critical physiological parameters.

Organisms use feedback mechanisms to detect changes and respond to maintain internal stability. Failure of homeostasis can lead to disease or death. Physiological systems work together to maintain this delicate balance.

Types of feedback mechanisms:

• Negative feedback: The most common type, where a change triggers a response that counteracts the change (e.g., body temperature regulation)
• Positive feedback: Less common, where a change triggers a response that amplifies the change (e.g., blood clotting, childbirth)

Key homeostatic systems include:

• Thermoregulation: Maintaining body temperature within narrow limits
• Osmoregulation: Maintaining water and salt balance
• Blood glucose regulation: Maintaining stable blood sugar levels
• pH regulation: Maintaining appropriate acid-base balance

Physiology studies how these systems function at the molecular, cellular, tissue, organ, and organismal levels. Understanding physiology is essential for medicine, as many diseases result from homeostatic imbalances.

The Central Dogma, proposed by Francis Crick in 1958, describes how genetic information flows in biological systems:

This unidirectional flow of information is fundamental to all life. DNA stores genetic information, RNA acts as an intermediary messenger, and proteins execute the functions encoded in genes.

Key components of the Central Dogma include:

• DNA replication: The process by which DNA makes a copy of itself before cell division
• Transcription: The synthesis of RNA from a DNA template, occurring in the nucleus
• Translation: The synthesis of proteins from mRNA templates, occurring at ribosomes
• Post-translational modification: Chemical modifications that alter protein function

The genetic code is universal, meaning the same codons code for the same amino acids across almost all organisms. This universality provides strong evidence for common ancestry and allows genetic engineering techniques to work across species.

Exceptions to the unidirectional flow include reverse transcription (RNA → DNA) in retroviruses and RNA replication in RNA viruses, but these are rare exceptions that do not contradict the fundamental principle.

Energy flows through biological systems, being transformed and conserved according to the laws of thermodynamics. Organisms obtain energy from food (heterotrophs) or sunlight (autotrophs) and use it for growth, reproduction, and maintenance.

Ecology studies the interactions between organisms and their environment. Energy flows through ecosystems from producers (plants) to consumers (animals), with energy being lost as heat at each trophic level. Understanding these relationships is crucial for conservation and managing environmental challenges.

Key metabolic processes:

• Photosynthesis: Converts light energy into chemical energy (glucose)
• Cellular respiration: Releases energy stored in glucose to produce ATP
• ATP (adenosine triphosphate): The universal energy currency of cells

Ecological levels of organization:

• Individual: A single organism
• Population: A group of individuals of the same species in an area
• Community: All populations of different species in an area
• Ecosystem: Community plus abiotic (non-living) factors
• Biome: Large-scale ecosystems with similar climate and organisms
• Biosphere: All life on Earth

Energy efficiency in ecosystems is typically only 10% between trophic levels, meaning 90% of energy is lost as heat. This limits the length of food chains and explains why top predators are rare.

Transcription is the first step, where RNA polymerase reads the DNA template and synthesizes messenger RNA (mRNA). This process occurs in the nucleus of eukaryotic cells.

Translation is the second step, where ribosomes read the mRNA sequence and assemble amino acids into a polypeptide chain. Transfer RNA (tRNA) molecules bring the correct amino acids to the ribosome based on the genetic code.

The genetic code is read in groups of three nucleotides called codons. Each codon specifies a particular amino acid or a stop signal. This code is nearly universal across all life forms.