Viruses and Bacteriophages

Types and Life Cycles of Viruses

Viruses are infectious agents that require a host cell to reproduce. They can infect a wide range of organisms, including bacteria (bacteriophages), animals, and plants.

• Bacteriophage: A virus that infects bacteria. It can undergo two main life cycles:

  • Lytic cycle: The virus injects its genome into the host, replicates, and causes the host cell to burst, releasing new viruses.

  • Lysogenic cycle: The viral genome integrates into the host genome and replicates along with it, remaining dormant until triggered to enter the lytic cycle.

• Types of Viruses:

  • Herpesviruses: DNA viruses causing diseases like chickenpox, cold sores, and shingles.

  • Rhinoviruses: RNA viruses causing the common cold.

  • HIV: An RNA virus that attacks the immune system, leading to AIDS.

  • Tobacco mosaic virus: Infects plants, causing mosaic-like symptoms on leaves.

Example: The HIV virus uses reverse transcriptase to convert its RNA genome into DNA, which integrates into the host genome.

7.1 – Darwin’s Theory of Evolution

Introduction to Evolution by Natural Selection

Charles Darwin’s theory of evolution by natural selection explains how species change over time due to inherited traits that enhance survival and reproduction.

• Species are not permanent and can change over time.

• Natural selection: Individuals with traits better suited to their environment are more likely to survive and reproduce.

• Evidence for evolution includes fossils, comparative anatomy, and biogeography.

Example: Darwin’s observations of finches on the Galápagos Islands showed variations in beak shape related to food sources.

7.3 – Evolution Affects Our Daily Lives

Applications and Impacts of Evolution

Evolutionary principles are observed in many aspects of daily life, including medicine and agriculture.

• Antibiotic resistance: Bacteria evolve resistance to antibiotics through natural selection.

• Pesticide resistance: Insects evolve resistance to chemical pesticides.

• Evolution explains the emergence of new diseases and the adaptation of organisms to changing environments.

Example: MRSA is a strain of bacteria resistant to several antibiotics due to evolutionary adaptation.

7.5 – Populations Are the Units of Evolution

Genetic Variation and the Gene Pool

Evolution acts on populations, not individuals. The gene pool is the total collection of genes in a population at any one time.

• Genetic variation arises through mutation and sexual reproduction.

• Evolution is measured as a change in allele frequencies in the gene pool over generations.

• Microevolution: Small-scale changes in allele frequencies within a population.

Example: The frequency of a gene for antibiotic resistance increases in a bacterial population exposed to antibiotics.

7.7 – Evolution Proceeds Through Several Mechanisms

Mechanisms of Evolution

Several mechanisms contribute to changes in the genetic makeup of populations over time.

• Mutation: Random changes in DNA that create new alleles.

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

• Gene flow: Movement of alleles between populations through migration.

• Natural selection: Differential survival and reproduction of individuals with advantageous traits.

Equation:

This is the Hardy-Weinberg equation, used to calculate allele and genotype frequencies in a population.

7.9 – Macroevolution Encompasses Large-Scale Changes

Speciation and Extinction

Macroevolution refers to large-scale evolutionary changes, such as the formation of new species (speciation), mass extinctions, and the emergence of major new groups.

• Speciation: The process by which one species splits into two or more distinct species.

• Branching evolution: A population splits into two or more populations that evolve independently.

• Non-branching evolution: A population changes over time without splitting.

• Mass extinction: Periods when large numbers of species go extinct in a relatively short time.

Example: The extinction of the dinosaurs at the end of the Cretaceous period.

7.13 – Phylogenetic Trees Represent Hypotheses About Evolutionary History

Phylogenetic Trees and Systematics

Phylogenetic trees are diagrams that represent hypotheses about the evolutionary relationships among species.

• Based on evidence such as fossil records, anatomical similarities, and DNA analysis.

• Systematics is the scientific study of the diversity and relationships of organisms.

Example: A phylogenetic tree showing the evolutionary relationships among primates.

Species and Reproductive Barriers

What is a Species?

A species is a group of organisms capable of interbreeding and producing fertile offspring. Reproductive barriers prevent different species from interbreeding.

• Prezygotic barriers: Prevent mating or fertilization between species (e.g., habitat isolation, temporal isolation, behavioral isolation).

• Postzygotic barriers: Prevent the development of viable or fertile offspring (e.g., hybrid sterility).

Example: Horses and donkeys can mate to produce a mule, but mules are sterile.

Speciation Can Occur Through Various Mechanisms

Mechanisms of Speciation

Speciation can occur through different mechanisms, including:

• Allopatric speciation: Physical separation of populations leads to the formation of new species.

• Sympatric speciation: New species arise within the same geographic area, often through genetic changes.

Example: A population of insects separated by a mountain range evolves into two distinct species.

8.6 – Eukaryotic Cells Evolved from Prokaryotic Cells

Origin of Eukaryotes

Eukaryotic cells are thought to have evolved from prokaryotic cells through a process called endosymbiosis.

• Prokaryotes: Simple cells without a nucleus or membrane-bound organelles.

• Eukaryotes: Complex cells with a nucleus and organelles such as mitochondria and chloroplasts.

• Endosymbiotic theory: Mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

Example: Mitochondria have their own DNA, similar to bacterial DNA, supporting the endosymbiotic theory.

Major Groups of Plants and Their Evolution

Evolution of Plant Diversity

Plants evolved from green algae and diversified into several major groups, each with unique adaptations for life on land.

Example: Angiosperms are the most diverse group of plants, including grasses, trees, and flowers.

Human Evolution

Origins and Characteristics of Humans

Humans evolved from ancestral primates and share many characteristics with other mammals.

• Primates: Mammals with flexible hands and feet, forward-facing eyes, and large brains.

• Hominins: The group that includes modern humans and their extinct relatives.

• DNA evidence suggests humans and chimpanzees share a common ancestor about 6-7 million years ago.

Example: Fossil evidence and genetic data support the evolutionary relationship between humans and other primates.

The Immune System

Components and Functions

The immune system protects the body from pathogens using a variety of defenses.

• External defenses: Skin, mucous membranes, stomach acid.

• Internal defenses: White blood cells, lymphatic system, inflammatory response.

• Specific immunity: Lymphocytes (B cells and T cells) recognize and respond to specific antigens.

• Antibodies: Proteins produced by B cells that bind to specific antigens and help neutralize pathogens.

Example: Vaccines stimulate the immune system to produce memory cells that provide long-term protection against diseases.

The Endocrine System

Hormones and Regulation

The endocrine system consists of glands that secrete hormones to regulate body functions.

• Hormones are chemical messengers that travel through the bloodstream to target organs.

• Major endocrine glands include the pituitary, thyroid, and adrenal glands.

• Hormones control processes such as growth, metabolism, and reproduction.

Example: Insulin is a hormone produced by the pancreas that regulates blood sugar levels.