prokaryotes

Practice flashcards covering key concepts from the lecture notes: prokaryotes vs. eukaryotes, DNA/RNA features, ribosomes, flagella, taxis, cell shapes, reproduction, plasmids, genetic recombination, and autotrophy/heterotrophy.

What does the term 'prokaryote' mean in relation to cellular organization?

The term 'prokaryote' literally means 'before the nucleus'. This refers to organisms whose cells lack a membrane-bound nucleus to house their genetic material and typically lack most other membrane-bound organelles, meaning their internal structure is much simpler compared to eukaryotic cells.

What is a Prokaryote?

A prokaryote is a single-celled organism (such as bacteria and archaea) that lacks a membrane-bound nucleus and other specialized membrane-bound organelles. Its genetic material (DNA) is usually a single circular chromosome located in a region called the nucleoid.

What is a Eukaryote?

A eukaryote is an organism whose cells contain a membrane-bound nucleus that encloses their genetic material (DNA), as well as other specialized membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus). Examples include animals, plants, fungi, and protists.

What is the Nucleus?

In eukaryotic cells, the nucleus is a prominent membrane-bound organelle that houses the cell's main genetic material, organized into linear chromosomes. It serves as the control center, regulating gene expression and mediating the replication of DNA.

Do prokaryotic cells have membrane-bound organelles?

No; prokaryotic cells are defined by their fundamental lack of membrane-bound organelles. While they possess structures like ribosomes (for protein synthesis), these are not enclosed by a membrane. Cellular functions are often carried out in the cytoplasm or on the cell membrane.

What are Membrane-bound organelles?

Membrane-bound organelles are specialized compartments within a cell that are enclosed by lipid bilayers, separating their internal environment from the rest of the cytoplasm. These structures, such as the mitochondria, endoplasmic reticulum, and Golgi apparatus, are characteristic features of eukaryotic cells and perform distinct functions.

What is the Nucleoid?

The nucleoid is an irregularly shaped region within the cytoplasm of a prokaryotic cell that contains the genetic material (the main circular chromosome) but is not enclosed by a membrane.

How do prokaryotic and eukaryotic genomes differ in DNA organization?

Prokaryotic DNA is typically a single, circular chromosome located in the nucleoid and is not associated with histones (though it can be supercoiled). Eukaryotic DNA is linear, organized into multiple chromosomes (composed of DNA tightly wound around histone proteins), and housed within a membrane-bound nucleus.

What is Prokaryotic DNA?

Prokaryotic DNA primarily consists of a single, large, circular molecule of double-stranded DNA located in the nucleoid region. In addition, many prokaryotes also carry smaller, independently replicating circular DNA molecules called plasmids.

What is Eukaryotic DNA?

Eukaryotic DNA is typically organized into multiple linear chromosomes (varying in number depending on the species) that are found within the nucleus. These chromosomes consist of DNA tightly complexed with histone proteins to form chromatin.

What are Chromosomes?

Chromosomes are thread-like structures made of DNA and associated proteins that carry genetic information in the form of genes. In eukaryotes, they are linear and located in the nucleus; in prokaryotes, the main chromosome is typically circular and found in the nucleoid.

What are the typical ribosome sizes in prokaryotes and eukaryotes?

Prokaryotic cytoplasmic ribosomes are 70S (Svedberg unit), composed of 30S and 50S subunits. Eukaryotic cytoplasmic ribosomes are 80S, composed of 40S and 60S subunits. Interestingly, mitochondria and chloroplasts (organelles within eukaryotic cells) contain their own 70S ribosomes, providing evidence for their endosymbiotic origin.

What are Ribosomes?

Ribosomes are complex molecular machines found in all cells (prokaryotic and eukaryotic) that are responsible for protein synthesis (translation). They read the genetic code on messenger RNA (mRNA) and facilitate the linking of amino acids into polypeptide chains. They are not membrane-bound organelles.

What is the Svedberg unit (S) in relation to ribosomes?

The Svedberg unit (S) is a measure of a particle's sedimentation rate in a centrifuge, which is influenced by both its mass and shape. It is used to characterize the size and density of ribosomes and their subunits; higher S values generally indicate larger, denser particles.

Do all cells have mitochondria?

No; mitochondria are sophisticated membrane-bound organelles that are exclusively found in eukaryotic cells, where they play a crucial role in cellular respiration and ATP production. Prokaryotic cells lack mitochondria and perform similar functions using their cell membrane.

What are Mitochondria?

Mitochondria are double-membrane-bound organelles, often called the 'powerhouses' of the cell, found in most eukaryotic cells. They are primarily responsible for generating large quantities of adenosine triphosphate (ATP) through cellular respiration, which is used as a source of chemical energy.

What are Chloroplasts?

Chloroplasts are specialized double-membrane-bound organelles found in plant cells and other eukaryotic photosynthetic organisms. They are the sites of photosynthesis, converting light energy into chemical energy in the form of glucose. Like mitochondria, they also contain their own 70S ribosomes and circular DNA.

What is the nine-plus-two arrangement in eukaryotic flagella and cilia?

The nine-plus-two arrangement (or 9+2 axoneme) refers to the characteristic internal structure of eukaryotic flagella and cilia. It consists of nine outer pairs of microtubules (doublets) arranged in a circle, surrounding two central singular microtubules.

What are Eukaryotic flagella?

Eukaryotic flagella are longer, whip-like appendages on the cell surface that facilitate cell motility (e.g., in sperm cells and some protists). They are complex structures, powered by ATP hydrolysis, and possess the characteristic internal 9+2 microtubule arrangement (axoneme).

What are Cilia?

Cilia are short, hair-like appendages on the surface of some eukaryotic cells. They can be numerous and function in cell motility (propelling the cell), moving fluids or particles across the cell surface, or as sensory organelles. Like flagella, they also exhibit the 9+2 microtubule arrangement.

What is the 9+2 arrangement (or Axoneme)?

The 9+2 arrangement is the defining structural pattern of the core of eukaryotic flagella and cilia, known as the axoneme. It describes the presence of nine peripheral doublet microtubules surrounding two central single microtubules.

What are Microtubules?

Microtubules are hollow cylinders made of tubulin protein found in eukaryotic cells. They are a crucial component of the cytoskeleton, providing structural support, facilitating intracellular transport, and forming structures like cilia, flagella, and the spindle fibers during cell division.

What drives the rotation of prokaryotic flagella?

The rotation of prokaryotic flagella is driven by a proton gradient (specifically, the proton motive force) across the bacterial cell membrane. Hydrogen ions (H^+) flow back into the cell through a motor complex at the base of the flagellum, causing it to spin like a propeller.

What are Prokaryotic flagella?

Prokaryotic flagella are simpler, helical protein filaments that extend from the cell surface and rotate like a propeller, enabling bacterial motility. They are composed of a protein called flagellin and are powered by the proton motive force, not ATP hydrolysis.

What is the Proton Motive Force (PMF)?

The Proton Motive Force (PMF) is an electrochemical gradient of protons (hydrogen ions, H^+) across a membrane, typically generated by electron transport chains. This force represents stored energy that can be used to power various cellular processes, such as ATP synthesis (via ATP synthase) and the rotation of prokaryotic flagella.

How do eukaryotic flagella differ from prokaryotic flagella in structure and energy usage?

Eukaryotic flagella are larger, whip-like structures composed of microtubules with a 9+2 arrangement (axoneme), and their movement is powered by the hydrolysis of ATP. In contrast, prokaryotic flagella are smaller, simpler filaments made of flagellin, lack the 9+2 structure, and rotate via a motor driven by a proton gradient (proton motive force) across the cell membrane.

What is ATP hydrolysis?

ATP hydrolysis is the chemical reaction in which the terminal phosphate bond of adenosine triphosphate (ATP) is broken by water, releasing inorganic phosphate (P_i) and a significant amount of energy, forming adenosine diphosphate (ADP). This energy is used to power various cellular processes, including muscle contraction and eukaryotic flagella movement.

Do prokaryotes have microtubules?

No; microtubules are complex protein structures and key components of the cytoskeleton found exclusively in eukaryotic cells. They are not present in prokaryotes.

What is taxis?

Taxis refers to the directed movement or orientation of an organism (especially a motile one) in response to a specific environmental stimulus. This movement can be either towards (positive taxis) or away from (negative taxis) the stimulus.

What is Phototaxis?

Phototaxis is the directed movement of an organism in response to light. For example, some photosynthetic bacteria will move towards light (positive phototaxis) to optimize their light harvesting, while others might move away from excessive light (negative phototaxis).

What is Chemotaxis?

Chemotaxis is the directed movement of an organism in response to chemical gradients. Bacteria, for instance, can move towards attractive chemicals like nutrients (positive chemotaxis) or away from repellent chemicals like toxins (negative chemotaxis).

What is Magnetotaxis?

Magnetotaxis is the directed movement of an organism in response to a magnetic field. Some aquatic bacteria, known as magnetotactic bacteria, synthesize magnetic crystals (magnetosomes) that allow them to align with the Earth's geomagnetic field and navigate to optimal oxygen concentrations in sediments.

Name the three basic shapes of prokaryotic cells.

The three basic shapes of prokaryotic cells are: 1. Cocci (spherical), 2. Bacilli (rod-shaped), and 3. Spirochetes (spiral-shaped). Other shapes like vibrios (comma-shaped) and spirilla (rigid spirals) also exist.

What are Cocci (singular: coccus)?

Cocci are spherical or roughly spherical-shaped bacteria. They can appear individually (monococci), in pairs (diplococci), in chains (streptococci), or in grape-like clusters (staphylococci).

What are Bacilli (singular: bacillus)?

Bacilli are rod-shaped or cylindrical bacteria. They can appear individually, in pairs (diplobacilli), or in chains (streptobacilli).

What are Spirochetes?

Spirochetes are a specific type of spiral-shaped bacteria that are typically long, thin, and flexible, with an internal flagellar arrangement (axial filaments) that allows for a characteristic corkscrew-like motility. Examples include bacteria causing syphilis and Lyme disease.

What is binary fission?

Binary fission is the primary method of asexual reproduction in prokaryotes (and some organelles like mitochondria). It involves the replication of the cell's single circular DNA chromosome, followed by the division of the parent cell into two genetically identical daughter cells.

What are plasmids?

Plasmids are small, extrachromosomal, circular DNA molecules found in many prokaryotic cells (and some eukaryotes). They replicate independently of the main bacterial chromosome and often carry accessory genes that provide advantageous traits, such as antibiotic resistance genes or virulence factors. They can be readily transferred between bacteria.

What are Accessory genes?

Accessory genes are genes carried on plasmids or other mobile genetic elements that are not essential for the basic survival or primary metabolism of the organism but can provide selective advantages under certain conditions. Common examples include genes for antibiotic resistance, heavy metal resistance, or virulence factors.

What is transformation in prokaryotes?

Transformation is a mechanism of horizontal gene transfer in prokaryotes where a bacterial cell directly takes up and incorporates foreign genetic material (extracellular DNA) from its surrounding environment into its own genome. Cells capable of this are called 'competent'.

What is Transduction?

Transduction is a process of horizontal gene transfer in prokaryotes where bacterial DNA is transferred from one bacterium to another via a bacteriophage (a virus that infects bacteria). During viral replication, bacterial DNA can be accidentally packaged into new phage particles and then delivered to another cell upon infection.

What are Bacteriophages (Phages)?

Bacteriophages (often shortened to phages) are viruses that specifically infect and replicate within bacteria. They play a crucial role in bacterial genetics by mediating transduction, transferring genetic material between bacteria.

What is Conjugation?

Conjugation is a major mechanism of horizontal gene transfer in prokaryotes involving the direct, unidirectional transfer of genetic material (typically a plasmid like the F factor, or a portion of the chromosome) from a donor bacterium to a recipient bacterium through a direct cell-to-cell contact, often facilitated by a specialized pilus (sex pilus). It is not considered true sexual reproduction as it does not involve gamete fusion.

What is a Pilus (plural: pili)?

A pilus is a hair-like appendage found on the surface of many bacteria. Sex pili are specialized pili that enable conjugation by forming a bridge between donor and recipient cells, facilitating the transfer of genetic material (e.g., the F factor plasmid).

What is the F factor (Fertility Factor)?

The F factor (or Fertility Factor) is a specific type of plasmid or a region of the bacterial chromosome that carries genes required for conjugation, including those that encode the production of a sex pilus. Bacteria possessing the F factor (F+ cells) can act as DNA donors, initiating genetic transfer to recipient F- cells.

What is Autotrophy?

Autotrophy is a mode of nutrition where an organism produces its own food (organic compounds) from inorganic sources. Autotrophs are often called 'producers' and typically use either light energy (photoautotrophs) or chemical energy (chemoautotrophs) to fix inorganic carbon (like CO_2).

What is Heterotrophy?

Heterotrophy is a mode of nutrition where an organism obtains its nutrition by consuming organic carbon compounds produced by other organisms. Heterotrophs are often called 'consumers' and cannot synthesize their own food from inorganic sources.

What are Photoautotrophs and Chemoautotrophs?

Photoautotrophs are autotrophs that use light energy as their energy source to fix inorganic carbon dioxide (CO_2) into organic compounds (e.g., plants, algae, cyanobacteria). Chemoautotrophs are autotrophs that obtain energy from the oxidation of inorganic chemical compounds (e.g., hydrogen sulfide, ammonia, ferrous iron) to fix inorganic carbon; many are prokaryotes found in extreme environments.

What is Carbon Fixation?

Carbon fixation is the process by which inorganic carbon (primarily carbon dioxide, CO_2) is converted into organic compounds by living organisms. This process is central to life on Earth, forming the basis of most food webs, and is carried out by autotrophs (photoautotrophs and chemoautotrophs).

How does bicarbonate relate to carbon fixation in aquatic systems?

In aquatic environments, carbon dioxide (CO2) reacts with water to form carbonic acid (H2CO3), which then dissociates into bicarbonate (HCO3^-) and carbonate (CO3^{2-}). Many aquatic photoautotrophs (like algae and some cyanobacteria) are capable of utilizing bicarbonate as an alternative carbon source for carbon fixation when CO2 concentrations are low, using specific transport mechanisms.

What is Bicarbonate (HCO_3^-)?

Bicarbonate (HCO3^-) is an ion formed from the dissociation of carbonic acid (H2CO3) in water, which itself is formed from the dissolution of carbon dioxide (CO2) in water. It serves as a significant carbon source for primary producers in aquatic ecosystems.

Why are plasmids important for antibiotic resistance?

Plasmids are critically important for the spread of antibiotic resistance because they often carry genes that confer resistance to various antibiotics. Being mobile genetic elements, plasmids can be efficiently transferred between different bacteria (even of different species) through conjugation, transformation, or transduction, rapidly disseminating resistance genes throughout bacterial populations and contributing to the global challenge of drug-resistant infections.

What is Antibiotic Resistance?

Antibiotic resistance is the ability of bacteria or other microorganisms to survive and grow in the presence of an antibiotic drug that would normally kill or inhibit them. This resistance often arises from mutations in bacterial chromosomal genes or, more commonly, through the acquisition of antibiotic resistance genes carried on plasmids via horizontal gene transfer.