Life's Organization & Diversity

Viruses Review

• Composition: Consist of genetic material (either DNA or RNA, but not both) enclosed within a protective protein coat called a capsid. Some viruses also have an outer lipid envelope derived from the host cell membrane.

• Nature: Acellular organisms, considered non-living outside a host cell because they cannot carry out metabolic processes or reproduce independently. Inside a host cell, they are considered living as they replicate and express their genetic material.

• Pathogenic: All viruses are obligate intracellular parasites, meaning they must infect a host cell to replicate, and in doing so, cause various diseases.

• Classification Factors: Viruses are classified based on several criteria including the type of genetic material (DNA or RNA, single or double-stranded), their capsid shape (e.g., helical, polyhedral, complex), the type of host they infect (e.g., bacteria, plants, animals), their mode of spread, and their replication mechanism.

• Replication Cycles:

  • Lytic Cycle: A bacteriophage (virus infecting bacteria) attaches to the host cell, injects its genetic material, takes over the host cell's machinery to synthesize viral components, assembles new viral particles, and then causes the host cell to lyse (break open), releasing the new virions.

  • Lysogenic Cycle: The virus attaches and injects its genetic material, which then integrates into the host cell's genetic material (forming a 'provirus' in eukaryotes or a 'prophage' in prokaryotes). The viral genetic material lies dormant, replicating along with the host chromosome. It can later detach and enter the lytic cycle due to environmental triggers like stress.

• Retroviruses (e.g., HIV): These are RNA viruses that use a unique enzyme called reverse transcriptase to convert their RNA genome into DNA. This viral DNA then integrates into the host cell's genome, where it can remain latent for years before becoming active and leading to disease.

• Prions: Unique infectious agents composed solely of protein, lacking any DNA or RNA. They cause diseases by inducing normal host proteins (PrPc) to refold into an abnormal, disease-causing shape (PrPSc), leading to neurodegenerative disorders like Creutzfeldt-Jakob disease (CJD) in humans and Bovine Spongiform Encephalopathy (BSE) in cattle.

• Biotechnology Use: Viruses are utilized as viral vectors in gene therapy and genetic engineering due to their ability to efficiently deliver genetic material into host cells.

Cellular Diversity & Organization Review

• Cell Division:

  • Mitosis: A process of nuclear division resulting in two genetically identical daughter cells, each with the same number of chromosomes as the parent cell (2N). Primarily functions for growth, repair, and asexual reproduction in eukaryotes.

  • Meiosis: A specialized type of cell division that reduces the chromosome number by half, producing four haploid (1N) reproductive cells (gametes in animals, spores in plants) from a single diploid cell. This process is crucial for sexual reproduction and genetic diversity.

  • Binary Fission: A form of asexual reproduction used by unicellular prokaryotic organisms (bacteria and archaea) where a single cell divides into two identical daughter cells after replicating its DNA.

• Fragmentation: A form of asexual reproduction in which an organism breaks into several pieces, each capable of growing into a new individual; common in certain species of starfish and worms.

Comparing Bacteria & Archaea Review

• Domains of Prokaryotes: The prokaryotic organisms are divided into two distinct domains: Bacteria (encompassing Kingdom Bacteria) and Archaea (encompassing Kingdom Archaea). Both are prokaryotes, meaning they lack a true nucleus and membrane-bound organelles.

• Morphology (Shapes): Both bacteria and archaea exhibit similar basic cell shapes:

  • Cocci: Spherical or ovoid cells.

  • Bacilli: Rod-shaped or cylindrical cells.

  • Spirilli: Spiral or helical-shaped cells.

  • Aggregations: Cells can also form characteristic arrangements, such as chains (e.g., Streptococcus for cocci chains, Streptobacillus for bacilli chains) or clusters (e.g., Staphylococcus).

• Nutrition/Feeding: They exhibit diverse nutritional strategies:

  • Photosynthesis: Some bacteria (e.g., cyanobacteria) perform photosynthesis, producing their own food using light energy.

  • Chemoheterotrophy: Consuming organic compounds from other organisms.

  • Chemoautotrophy: Utilizing inorganic compounds (e.g., hydrogen sulfide, ammonia) as an energy source to fix carbon dioxide.

  • Methanogenesis: A unique metabolic pathway found exclusively in some Archaea, where methane (CH_4) is produced as a byproduct of anaerobic respiration. These archaea are called methanogens.

  • Cyanobacteria: A crucial group of photosynthetic bacteria that were among the first organisms to produce oxygen, significantly contributing to the oxygenation of Earth's early atmosphere.

• Habitats:

  • Archaea: Renowned for being extremophiles, thriving in extreme environments. Examples include Thermophiles (high temperatures), Acidophiles (low pH/high acidity), and Halophiles (high salt concentrations).

  • Bacteria: Mostly mesophiles, meaning they prefer moderate environmental conditions (temperature, pH, salinity).

• Reproduction:

  • Binary Fission: The primary mode of reproduction for both bacteria and archaea, resulting in two genetically identical daughter cells.

  • Conjugation: A process in bacteria involving the direct transfer of genetic material (often plasmids containing beneficial genes like antibiotic resistance) from one bacterial cell to another through a pilus, increasing genetic diversity within a population.

  • Endospores: Formed by some Gram-positive bacteria (e.g., Bacillus, Clostridium) as a survival mechanism during unfavorable conditions. These dormant, metabolically inactive, and highly resistant structures can withstand extreme heat, radiation, and chemicals, allowing the bacterium to persist until conditions improve.

• Classification: A common method for classifying bacteria is the Gram Stain:

  • Gram-positive bacteria: Have a thick peptidoglycan cell wall outside the plasma membrane, which retains the crystal violet stain, appearing purple under a microscope.

  • Gram-negative bacteria: Have a thinner peptidoglycan layer located between two membranes (an inner plasma membrane and an outer membrane), which does not retain the crystal violet, instead appearing pink or red after counterstaining with safranin.

• Environmental Role: Both bacteria and archaea play indispensable roles in ecosystems as decomposers, breaking down dead organic matter and recycling nutrients. Cyanobacteria are major oxygen producers, and many bacteria are involved in vital processes like nitrogen fixation, converting atmospheric nitrogen into forms usable by plants.

• Biotechnology: Archaea, particularly extremophiles, produce enzymes that are remarkably stable under extreme conditions (e.g., high temperatures, high salinity). These thermostable enzymes, such as Taq polymerase from Thermus aquaticus, are invaluable in molecular biology techniques like Polymerase Chain Reaction (PCR).

Eukaryotic Evolution & Diversity Review

• Origin: Eukaryotic cells appeared approximately 2 billion years ago. They are characterized by their larger size, increased complexity, presence of membrane-bound organelles, and linear chromosomes housed within a nucleus (N refers to the number of chromosomes in a haploid set).

• Endosymbiosis Theory (Proposed by Lynn Margulis):

  • This theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. It posits that a large ancestral prokaryote engulfed smaller prokaryotic cells.

  • Instead of being digested, these smaller cells survived within the host. Over evolutionary time, they became obligate endosymbionts, evolving into mitochondria (believed to have originated from aerobic bacteria) and chloroplasts (believed to have originated from photosynthetic cyanobacteria).

  • Evidence Supporting Endosymbiosis: Both mitochondria and chloroplasts possess several features reminiscent of prokaryotic cells: they have double membranes (the inner membrane derived from the original prokaryote, the outer from the engulfing host vesicle), contain their own circular DNA (distinct from nuclear DNA), have ribosomes (70S type, similar to prokaryotic ribosomes), and reproduce independently within the cell via binary fission, much like bacteria.

• Multicellularity: The evolution of multicellularity, allowing for cellular specialization and increased complexity, is estimated to have appeared between 1.2-1.5 billion years ago. Early examples include certain red algae. This development allowed organisms to grow larger and perform more complex functions.

• Reproduction & Life Cycles:

  • Asexual: Eukaryotes can reproduce asexually through various means, such as binary fission in unicellular forms (like protists) or through budding, fragmentation, or vegetative reproduction in multicellular organisms, which also use similar processes for growth and tissue repair.

  • Sexual (Unique to Eukaryotes): Sexual reproduction is a defining characteristic of eukaryotes, involving:

    • Meiosis: A specialized cell division that reduces the chromosome number in half, producing haploid gametes (in animals) or spores (in plants/fungi).

    • Fertilization: The fusion of two haploid gametes (e.g., sperm and egg) to form a diploid zygote.

    • This combination of meiosis and fertilization is vital for generating genetic variation within a population, which is a key driver of evolution.

  • Life Cycle Patterns: Eukaryotes exhibit diverse life cycle patterns based on the ploidy of the dominant life stage:

    • Zygotic Life Cycle: The organism exists predominantly in a haploid (1N) state. The only diploid stage is the zygote, which immediately undergoes meiosis to produce haploid spores (e.g., most fungi and some algae).

    • Gametic Life Cycle: The organism is predominantly diploid (2N). Meiosis directly produces haploid gametes, which fuse to form a diploid zygote that develops into a new diploid organism (e.g., animals and some protists).

    • Sporic Life Cycle (Alternation of Generations): Characterized by having both a multicellular haploid stage (gametophyte) and a multicellular diploid stage (sporophyte) in their life cycle. The sporophyte produces haploid spores by meiosis, which grow into gametophytes. The gametophytes produce haploid gametes by mitosis, which fuse to form a diploid zygote that grows into a sporophyte (e.g., plants and many algae).

Protists: The Unicellular Eukaryotes Review

• Definition: Protists are a highly diverse group of eukaryotic organisms that are not classified as animals, plants, or fungi. Most protists are unicellular, though some are colonial or multicellular.

• Animal-Like Protists (Protozoans): These protists are heterotrophic (ingest food) and lack a rigid cell wall, giving them animal-like mobility and feeding behaviors.

  • Cercozoons (Amoebas): Move and engulf food particles using temporary extensions of their cytoplasm called pseudopods. They are often found in aquatic environments. A significant example is Entamoeba histolytica, which causes amoebic dysentery in humans.

  • Ciliates: Characterized by being covered in numerous short, hair-like appendages called cilia, which they use for both locomotion and feeding (to sweep food particles into their oral groove). They typically possess two nuclei: a large macronucleus (controls daily functions) and one or more small micronuclei (involved in genetic recombination during conjugation). Paramecium is a well-known example.

  • Flagellates (Zoomastigina): Move using one or more long, whip-like flagella. Many are parasitic or symbiotic. Examples include Trypanosoma (causes sleeping sickness, transmitted by tsetse flies) and Giardia lamblia (causes giardiasis, an intestinal infection).

  • Sporozoans (Sporozoa): These protists are non-motile and are obligate intracellular parasites. They typically have complex life cycles involving multiple hosts. Plasmodium, the causative agent of malaria (transmitted by Anopheles mosquitoes), is a notorious example.

• Fungus-Like Protists: These protists are decomposers or absorptive heterotrophs, obtaining nutrients by secreting enzymes and absorbing dissolved organic molecules. They often produce spores and some have cellulose in their cell walls.

  • Slime Moulds: Can exist in two main forms: Plasmodial slime moulds (which form a large, single-celled, multinucleate mass called a plasmodium that creeps and engulfs food) and Cellular slime moulds (which spend most of their lives as individual amoeboid cells but aggregate into a slug-like mass when food is scarce, eventually forming a fruiting body).

  • Water Moulds (Oomycota): Characterized by filamentous (hyphae-like) growth and cellulose in their cell walls, unlike true fungi which have chitin. Many are important plant parasites, such as Phytophthora infestans, which caused the Irish potato famine.

Plant-Like Protists (Unicellular Algae)

• These are autotrophic protists that contain chloroplasts and perform photosynthesis. They form the base of many aquatic food chains.

• Diatoms (Chrysophyta): Possess unique, intricate cell walls made of hydrated silica (SiO_2 ) that act as a protective shell, and are major components of phytoplankton, contributing significantly to global oxygen production.

• Dinoflagellates (Dinoflagellata): Characterized by two flagella (one that wraps around the cell and one that trails behind) that cause them to spin as they move. Some are responsible for "red tides," harmful algal blooms that produce toxins, while others are bioluminescent.

• Euglenoids (Euglenozoa): Mixotrophic protists, meaning they can perform photosynthesis in the presence of light (due to chloroplasts) and also consume organic matter when light is absent. They lack a rigid cell wall but have a flexible pellicle for shape