General Biology Reviewer PDF - G11

University of Saint Louis 4 GENETICS- the study of how genes and how traits are passed down from one generation to GENERAL the next....

University of Saint Louis 4 GENETICS- the study of how genes and how traits are passed down from one generation to GENERAL the next. SOME CAREERS INVOLVING BIOLOGY BIOLOGY FORENSIC SCIENCE- is the application of science to answer questions related to the law. REVIEWER NEUROBIOLOGY - is the study of the nervous system. 1st Semester - Midterm _________________________ PALEONTOLOGY - which uses fossils to study life’s history. Week 1: ZOOLOGY - which studies animals. What is Science? BOTANY - which studies plants. Science is the pursuit and application of knowledge THE PROPERTIES OF LIFE and ➔ All living organisms share several key understanding of the natural and social characteristics or functions: order, world following sensitivity or response to the environment, reproduction, growth and development, a regulation, homeostasis, and energy systematic processing. When viewed together, these methodology based on evidence. eight characteristics serve to define life. What are the branches of Science? ORDER 1. Basic Science or Pure Science Organisms are highly organized, - It seeks to expand knowledge regardless of the coordinated structures that consist of one short-term application of that knowledge. or more cells. 2. Applied Science SENSITIVITY OR RESPONSE TO STIMULI - Aims to use science to solve real-world Organisms can respond to diverse stimuli. problems such as improving crop yields, finding a Example: The leaves of the sensitive plant cure for a particular disease, or saving animals (Mimosa pudica) will instantly droop and fold threatened by a natural disaster. when touched. After a few minutes, the plant returns to normal. BRANCHES OF BIOLOGICAL STUDY REPRODUCTION When reproduction occurs, genes 1. MICROBIOLOGY - study of microorganisms. containing DNA are passed along to an 2. PHYSIOLOGY- deals with the normal organism's offspring. These genes ensure functions of living organisms and their parts. that the offspring will belong to the same species and will have similar 3. ECOLOGY- study of the relationships between characteristics, such as size and shape. living organisms, including humans, and their physical environment. Example: Kittens have inherited genes from both physiological; as such, adaptations parents and share many of the same frequently involve other properties of characteristics. organisms such as homeostasis, reproduction, and growth and GROWTH AND DEVELOPMENT development. All organisms grow and develop following specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and Week 2: CELL THEORY development, ensuring that. species' young will grow up to exhibit many of the INTRODUCTION: same characteristics as its parents. - Aristotle proposed the idea of Spontaneous Generation Theory which states that life may REGULATION spontaneously arise from any inanimate matter. Even the smallest organisms are complex - This theory persisted until 17 th century. Some and require multiple regulatory scientists proved that life cannot come from mechanisms to coordinate internal inanimate objects. functions, respond to stimuli, and - They believed that life has a source. All living Stresses. organisms are composed of at least one cell. Today, we studied that the cell is the smallest HOMEOSTASIS structure that can perform all activities required to To function properly, cells need to have sustain life. appropriate conditions such as proper temperature, pH, and appropriate SIZE OF THE CELL concentration of diverse chemicals. ➔ Cells are generally small. Most cells are Homeostasis is 6 the ability of an smaller than 1mm, and some are even as organism to maintain constant internal small as 1um. conditions. ➔ The cell can only be viewed under the Example: Polar bears (Ursus maritimus) and microscope to magnify its size in the field other mammals living in ice-covered regions of view. maintain their body temperature by generating heat and reducing heat loss through thick fur and SURFACE AREA TO VOLUME RATION IN a dense layer of fat under their skin. CELLS ➔ The cell itself is a system. ENERGY PROCESSING ➔ A cell needs a surface area large enough All organisms use a source of energy for relative to its volume to allow adequate their metabolic activities. Some organisms nutrients to enter and sufficient waste to capture energy from the sun and convert it be eliminated. into chemical energy in food; others use ➔ Surface area to volume ratio is important chemical energy in molecules they take in that it favors a smaller cell size in terms of as food. the efficiency of the movement of Example: The California condor (Gymnogyps molecules. californianus) uses chemical energy derived from food to power flight. SURFACE AREA TO VOLUME RATION IN CELLS EVOLUTION (ADAPTATION) ➔ The small size of the cell provides them Adaptations help organisms survive in with a relatively higher ratio of surface their ecological niches, and adaptive traits area to volume. may be structural, behavioral, or  ➔ A living entity may maintain the same total HISTORY OF THE DEVELOPMENT OF THE volume, but having small cells will allow it CELL THEORY to have a greater surface area available for the movement of molecules. 1. Zacharias Janssen (1585 - 1632) - The discovery of the cell as the basic unit of life GENERAL FUNCTIONS OF THE CELL involves different scientists. Hans Janssen and 1. Regulation of Internal Environment his son, Zacharias Janssen, invented the first 2. Acquisition and Utilization of Energy primitive microscope which was used to view 3. Responsiveness to its Environment microscopic cells. 4. Protection and support 2. Robert Hooke (1635 - 1703) REGULATION OF THE INTERNAL - Robert Hooke was the first to observe the cells ENVIRONMENT by using cork from the bark of an oak tree under ➔ The small size of the cell provides them the microscope. with a relatively higher ratio of surface area to volume. 3. Francesco Redi (1626 - 1697) ➔ A living entity may maintain the same total - The experiment of Francesco Redi during his volume, but having small cells will allow it time disproved the long-believed theory of to have a greater surface area available spontaneous generation, which states that life for the movement of molecules. comes from inanimate objects. Example: Shivering generates metabolic heat while perspiration serve as a cooling system to 4. Anton Van Leeuwenhoek (1632 - 1723) remove excess heat in the body. - Anton Van Leeuwenhoek was the first to see and describe microorganisms from a drop of ACQUISTITION AND UTILIZATION OF water by using his self-made practical ENERGY microscope. ➔ Energy is needed by cells to drive most of the chemical reactions and other functions 5. Matthias Schleiden (1804 - 1881) in the organism’s body. - Matthias Schleiden is one of the proponents of ➔ Cells acquire energy from the nutrients in cell theory who stated that plants are made up of food that organisms consume. cells and that cell is the basic unit of life. RESPONSIVENESS TO THEIR ENVIRONMENT 6. Theodore Schwann (1810 - 1882) ➔ The cell’s environment changes - Theodore Schwann is also a proponent of cell constantly and rapidly. To survive, cells theory who stated that all organisms are made up respond to various signals that indicate of cells. any form of change in their environment. Example: A tanned skin means more pigment is 7. Rudolf Virchow (1821 - 1902) released in that area to impact protection to the - Rudolf Virchow another proponent of the cell underlying cells, especially UV radiation that can theory who stated that cells come from damage DNA. preexisting cells. Through successive cell division, many cells can be produced from a PROTECTION AND SUPPORT single cell. ➔ Cells protect and support their internal environment through their cellular membranes. Immune cells impart protection against pathogens and other foreign bodies that may enter the general circulation. PRINCIPLES OF THE CELL THEORY the fusion of egg cell and sperm cell to form a fertilized cell call zygote. NOTE: Reproduction, being one of the major PRINCIPLES PROPONENT attributes of life, is exhibited at the cellular level 1. Every living Theodore Schwann through cell division. organisms consists of one or more cells THE MODERN VERSION OF CELL THEORY Energy flow occurs within the cell. 2. The cell is the Matthias Schleiden The DNA of the cell is passed from cell to fundamental unit of cell. life. It is the smallest All cells have similar basic cellular structural and chemical composition. functional unit of all organisms. WEEK 3: MAJOR PARTS OF THE CELL 3. Cells come from Rudolf Virchow INTRODUCTION preexisting cells. Cells - Similar to the buildings we know, our cells also contain hereditary rely on biomolecules that provide shape and material, which they rigidity. In addition, buildings also have pass to their offspring structures and other security measures that when they divide. regulate the entry and exit of people just as the cell manages to become selective to the molecules that will move through it. THE CELLS AS A COMMON FEATURE AMONG ALL ORGANISMS - One of the principles of cell theory states that all Unicellular organisms consist of only one living organisms are composed of at least one cell but are already considered living cell. Cells are the fundamental units of structure organisms. and function in all living organisms. Organisms, from unicellular forms, such However, microscopic in size, cells are as in paramecium (left), to multicellular composed of structures that are complex in forms, such as in plants (right), are made function. of cells. 1. PLASMA MEMBRANE THE CELL AS THE FUNDAMENTAL UNIT OF ★ Plasma membranes mostly consist of LIFE phospholipids, a form of lipid molecule. The cell is the basic level that exhibits all Each phospholipid of the cell membrane is the important attributes of life. It is at the composed of two distinct regions. level of the cell that important biochemical ★ The first part is the “head” with a reactions take place to keep living negatively charged phosphate group with organisms alive. polar covalent bonds. NOTE: Plants cells perform various functions that ★ The second part is made up of the two contribute to the survival of the entire plant. nonpolar fatty acids “tails”. These phospholipids group to form a two-layered THE CELL AS A PRODUCT OF PREEXISTING barrier called a phospholipid bilayer. CELLS ★ The hydrophilic (water-loving) head region Cells can only come from preexisting cells of the phospholipid bilayer faces outward. similar to how bacterial and yeast cells This orientation exposes it to the aqueous produce produce their daughter cells solutions on the outside and inside of the through binary fission. Another example is cell.  ★ By contrast, the hydrophobic CARRIER PROTEINS (waterfearing) tails are oriented inward - These are involved in the transport of and are together shielded from water. The substances across the membrane. They middle portion of the phospholipid bilayer receive certain molecules but change their makes the plasma membrane selectively shape to be able to move these permeable. substances across the membrane. These ★ The CELL MEMBRANE are very specific with the molecules that - is also composed of proteins and other they transport across the plasma molecules that are free to drift laterally membrane. within the bilayer. The phospholipid bilayer is often described as a fluid mosaic model. According to this model, proteins, carbohydrates, and cholesterol molecules are embedded in the membrane so that they look like a mosaic. ROLE OF THE LIPID BILAYER - It serves as a primary wall or barrier of the cell from its external environment. - It is semipermeable in nature. It means it only allows certain molecules to pass through. The molecules that are free to move across the CELL RECOGNITION PROTEINS membrane generally require no energy to pass - These proteins help recognize the through, i.e., they can move passively. presence of pathogens so that certain immune responses can be elicited. ROLE OF MEMBRANE PROTEINS RECEPTOR PROTEINS - These proteins have a shape that allows only specific molecules to bind to them. When a molecule binds to a receptor protein, the protein changes its shape, bringing about particular cellular responses. Most molecules that cannot readily pass through the lipid bilayer do so with the aid of the proteins embedded in the membrane. CHANNEL PROTEINS - Channel proteins allow transport of certain molecules through the membrane. It forms a channel or passageway that simply allows certain substances to move from one side to the other. CYTOSKELETON - Cells are constantly exposed to various physical forces. All eukaryotic cells have a  system of interconnected protein filaments by special proteins. It can also facilitate from their nucleus to the plasma cell movement. They can move the cell membrane. This, collectively, is known as and its organelles by interacting with the cytoskeleton which extends motor molecules such as that in muscle throughout the cytoplasm (and outside the proteins. cytoplasm in some cells). INTERMEDIATE FILAMENTS - Intermediate filaments, as the name implies, have characteristics that lie MICROTUBULE between those of microfilaments and - Microtubules are straight, long, hollow microtubules. Intermediate filaments are cylinders with globular proteins called made of a variety of proteins. tubulin. There are two types of tubulin, - They form a ropelike assembly of fibrous namely, the alpha-tubulin and beta-tubulin polypeptides. Some intermediate - Alpha and beta-tubulin proteins come filaments support the nuclear envelope by together by forming dimers. These dimers anchoring the nucleus, while others are arranged in rows. Microtubules have support the plasma membrane. 13 rows of tubulin dimers, surrounding an - They also impart great strength to skin empty central core. Generally, these cells because one of their components cytoskeletal elements grow out and are includes the structural protein keratin. organized by microtubule organizing centers or MTOCs. The MTOCs in animal cells are in the form of centrosomes. THE GENERAL ROLE OF THE CYTOSKELETON If compared to a factory building, the cytoskeleton serves as the foundation and beams that support the whole structure of the factory. The cytoskeleton also Microtubules are also helpful during cell division. supports the structure of the cell itself and They assemble into structures called spindle provides anchorage and reinforcement for fibers that separate the duplicated chromosomes many organelles. then disassemble afterward. - It also serves as both the skeleton and the “muscles” of the cell, which provides support and MICROFILAMENTS movement. Furthermore, it maintains cell shape - Microfilaments are long and flexible fibers and it allows the cell and its organelles to move. composed of the protein actin which is the thinnest component of the cytoskeleton. THE CYTOPLASM - It provides structural support as a dense, The entire region of the cell between the complex web, to which they are anchored nucleus and the plasma membrane is the  cytoplasm. In eukaryotic cells, it is in the A cell wall is permeable to water which is cytoplasm that all the various subcellular why it is easier for dissolved substances structures are suspended. to cross it to the plasma membrane. The plasma membrane encloses it together with Although permeable to water, the cell wall all other cellular contents. The cytoplasm is maintains cell shape, and it keeps cells composed of the cytosol, a semifluid solution from absorbing too much water so that that consists of water and inorganic and organic they will not burst especially in plant cells. molecules. MAJOR PARTS OF THE CELL THE ROLE OF CYTOPLASM 1. PLASMA MEMBRANE If the factory building has a floor where all 2. CYTOPLASM raw materials, machines, other 3. CYTOSKELETON equipment, and employees work and 4. CELL WALL manufacture products, the cell has its cytoplasm to serve as a vessel for various COMPONENTS OF THE CELL: functions to take place. The role of the SUBCELLULAR ORGANELLES cytoplasm is simply to put all organelles and other cellular contents in place. It is where all other molecules inside the cell are suspended, most of which are substrates for enzymatic reactions to take place. THE STRUCTURE OF THE CELL WALL Some scientists believed that the nucleus evolved as a result of invagination. Much scientific evidence also shows that mitochondria and chloroplasts arose when a large eukaryotic cell engulfed independent prokaryotes. This relationship is later on referred to as the endosymbiotic theory. This theory is the explanation for the double membrane of mitochondria and chloroplasts and also the reason for having their own genetic material aside from the one found in the The cell wall is a rigid structure that surrounds nucleus. most prokaryotic cells, including archaea and bacteria. GENETIC CONTROL OF THE CELL A cell must maintain its structures and Peptidoglycan – the cell wall of bacteria also process energy to support its cellular Cellulose - the cell wall of plants activities. But which of the organelles Chitin – the cell wall of fungi. controls the orders, masters the plan, commands the responses to its environment? Just like a factory building, ROLE OF CELL WALL the cell needs a “command center” that It is a rigid structure that provides would direct and give instructions on what protection and structural support to cells. processes or activities will take place. NUCLEUS The three types of RNA produced in the nucleus: ribosomal RNA (rRNA), messenger RNA (mRNA) and transfer RNA (tRNA). ★ THE COMMAND CENTER OF THE CELL ★ The importance of the nucleus in the cell is that it is the one that specifies the code for protein synthesis. NUCLEOLUS CHROMATIN ★ The nucleolus, which is a dark region of ★ The nucleus contains chromatin and a chromatin, produces the ribosomal RNA. semifluid matrix called the nucleoplasm. This is where the rRNA joins with proteins Chromatin is a network of strands that to form the subunits of ribosomes. condenses and undergoes coiling into rod-like structures called chromosomes. NUCLEAR ENVELOPE CHROMOSOMES ★ The nuclear envelope is a double membrane that is similar to the plasma ★ The chromosomes are carriers of genetic membrane of the cell. It physically information and that the nucleus is the separates the nucleus from the cytoplasm command center of the cell. The number but is still able to communicate with the of chromosomes in a cell depends on the cytoplasm through the nuclear pores species; for example, each cell of the human body contains 46 chromosomes, NUCLEAR PORES rice cells have 24 and dog cells have 78. ★ The nuclear pores permit the passage of ribosomal subunits and mRNA out of the The chromatin and the chromosomes contain nucleus into the cytoplasm. It also DNA and some RNA. Genes, are composed of facilitates the passage of proteins from the DNA which are units of heredity located on the cytoplasm into the nucleus. chromosome. ★ The nuclear pore complexes act like gatekeepers to regulate what comes in and out of the nucleus. RIBOSOMES ROUGH ENDOPLASMIC RETICULUM - There are attached ribosomes in the RER. These ribosomes produce proteins that will be inserted into the growing endoplasmic reticulum membrane, transported to other organelles and eventually exported. - Some products manufactured by rough endoplasmic reticulum are dispatched to other ★ Ribosomes are the organelles that use locations in the cell by means of transport instructions from the nucleus, written in vesicles which are sacs made of the membrane mRNA, to build proteins. Ribosomes are that bud off from the rough endoplasmic composed of small and large subunits. reticulum. ★ These can be found in the cytosol, which are called free ribosomes. They can also SMOOTH ENDOPLASMIC RETICULUM be attached to the outside of the - Just like the rough endoplasmic reticulum, it endoplasmic reticulum or nuclear also forms vesicles that transport molecules to envelope, as bound ribosomes. other parts of the cell. - Enzymes of the smooth endoplasmic reticulum The DNA programs protein production in the are important in the synthesis of lipids, including cytoplasm by transferring its coded information to oils, phospholipids, and steroids. a molecule called messenger RNA. The mRNA - Another function of smooth endoplasmic molecule then carries the order to build a certain reticulum is the storage of ions. It also houses type of protein from the nucleus to the cytoplasm. enzymes that detoxify drugs and poisons. The mRNA exits through pores in the nuclear COLGI BODIES envelope and travels to the cytoplasm, where it ★ An organelle named for its discoverer, an then binds to ribosomes. The ribosome moves Italian scientist named Camillo Golgi, the along the mRNA, translating the genetic Golgi apparatus or Golgi bodies work in message into a protein with a specific amino acid close partnership with the endoplasmic sequence. reticulum. It serves as a warehouse and processing station for products ENDOMEMBRANE SYSTEM manufactured by the endoplasmic - The endomembrane system consists of the reticulum. endoplasmic reticulum, the Golgi bodies, and many vesicles. The vesicles transport molecules LYSOSOMES from one part of the system to another. It is a series of interacting organelles between the nucleus and the plasma membrane. Many of these organelles interact in the synthesis, distribution, storage, and export of molecules. ENDOPLASMIC RETICULUM - The endoplasmic reticulum is one of the main manufacturing facilities within the cell. It consists of a system of membranous channels and saccules that is physically continuous with the nuclear envelope.  It takes up to 90% of the cell’s volume which is filled with fluid called cell sap that gives added support to the cell. ENERGY HOUSES ★ The cell needs energy houses that can manage the production of ATP. The cell requires a continuous supply of energy to perform the work of life. Energy is very important in every cellular process ★ A lysosome is a membrane-enclosed sac because it powers each process to make of digestive enzymes that is absent in it possible for the cell to survive. plant cells. It is developed from vesicles MITOCHONDRIA that bud off from the Golgi bodies. It has a ★ Mitochondria are often called the very low pH and stores powerful powerhouse of the cell because it hydrolytic-enzymes in an inactive state. converts the chemical energy of foods that ★ Lysosomes destroy nonfunctional produce most of the cell’s ATP in the organelles and portions of cytoplasm, process of cellular respiration, wherein the breakdown wastes, ingests cells, and oxygen is used and the carbon dioxide is cellular debris delivered by other vesicles. given off. ★ It provides an acidic environment for its enzymes without affecting other CHLOROPLAST organelles of the cell. This organelle also ★ Chloroplasts are unique to plant and algal serves as recycling centers. cells which perform photosynthesis. PEROXISOMES ★ All peroxisomes contain enzymes that MITOCHONDRIA VS. CHLOROPLAST result in the creation of hydrogen peroxide (H2O2 ) which is a toxic molecule that is immediately broken down to water (H2O) and oxygen (O2 ) by another peroxisomal enzyme called catalase. ★ Peroxisomes perform metabolic assistance to organelles. They have varied functions but they specialize in synthesizing and breaking down lipids. They also break down fatty acids, amino Mitochondria are present in all eukaryotic cells acids, and toxins such as alcohol. including plant and algal cells. Even though mitochondria are smaller than chloroplasts, they VACUOLES can usually be seen using a light microscope. ★ The large sacs of membrane or vesicles The number of mitochondria can vary depending that bud from the endoplasmic reticulum, on the metabolic activities and energy needed Golgi bodies, or plasma membrane are within a cell. called vacuoles. ★ The central vacuole of a plant cell is a On the other hand, chloroplasts are only versatile compartment that stores organic present in plant cells and some algal cells have nutrients, such as proteins and poisons, one chloroplast only. It can be quite large, twice that protect against plant-eating animals. as wide and as much as five times the length of the mitochondria. Chloroplasts have a three-membrane system. The grana are the solar This is comparable to a mansion that has power packs of the chloroplasts which trap light several rooms or compartments. energy and convert it into chemical energy. Domain Eukarya which includes protists, fungi, plants, and animals are examples of eukaryotes. Eukaryotes are organisms that consist of eukaryotic cells. PRESENCE OF A NUCLEUS In a prokaryotic cell, the genetic materials (DNA) are concentrated in a region of the cytoplasm called the nucleoid. The term prokaryotic comes from the Greek terms prowhich means before and karyon or kernel that refers to the nucleus.On the other hand, the term eukaryotic comes from the Greek terms eu meaning true and karyon or kernel. CELL WALL AND CELL MEMBRANE The cell wall is present in almost all prokaryotic cells, but not in most eukaryotic cells (these are not found in WEEK 4: animals and most protists]. Structurally, the cell wall of prokaryotes is made up of PROKARYOTIC CELLS peptidoglycan - a complex sugar A prokaryotic cell is a type of cell that (polysaccharide] and a few amino acids. does not have a nucleus and eukaryotes, it is either made up of membrane-bound organelles within its cellulose as in plants and chitin in fungi. cytoplasm. Prokaryotes do not have sterols in the cell Prokaryotes are small, single-celled membrane but have a sterol-like lipid organisms that have prokaryotic cells. component called hopanoid. These organisms are metabolically In eukaryotic cells, the sterols that are diverse because they can utilize different present in the cell membrane are nutrients and energy sources and they cholesterol (animals], phytosterol (plants], can inhabit all types of environments on and ergosterol (fungi) Earth. All bacteria that include the organisms of domains Archaea and ENDOMEMBRANE SYSTEM AND OTHER Bacteria are considered prokaryotes. ORGANELLES The endomembrane system is bresent EUKARYOTIC CELLS in eukaryotic cells, but not in prokaryoti The other type of cell that is characterized cells. Other organelles like mitochondria by the presence of a nucleus and and chloroplast are also present in membrane-bound organelles within its eukaryotic cells, but not in prokaryotic cytoplasm is called a eukaryotic cell. cells. Membrane-bound organelles of eukaryotic The presence of organelles in eukaryotic cells provide compartmentalization in the cells provides compartmentalization. It cell. increases the surface area-volume ratio of  the cells, allows the occurrence of simultaneous cell activities without MODE OF REPRODUCTION interference from each other, and allows Most prokaryotic cells reproduce through the separation of DNA the nucleus, binary fission and some reproduce mitochondria, and chloroplast. through spores. Binary fission is a process of reproduction RIBOSOMES wherein one cell is divided into two new Ribosomes are present in both prokaryotic cells. and eukaryotic cells. These have the In eukaryotic cells, individual cells same function in both, which is protein reproduce through mitosis and meiosis. synthesis. Mitosis (somatic or body cell division] and. In a prokaryotic cell, all ribosomes are meiosis (sex cell division! are types of cell found in the cytoplasm. In eukaryotic cells, division, in eukaryotic cells. they can be found in the cytoplasm, outer nuclear membrane, rough endoplasmic DNA REPLICATION reticulum, mitochondrion, and chloroplast. DNA replication in eukaryotes takes a longer time than in prokaryotes because SHAPE OF DNA AND NUMBER OF eukaryotes have more DNA than CHROMOSOMES prokaryotes. In prokaryotic cells, DNA DNA is the genetic material in replication occurs in two opposing chromosome) prokaryotic and eukaryotic directions at the same time in the cells. It codes for protein and RNA. In cytoplasm. prokaryotic cells, DNA is found in the On the other hand, eukaryotic cells have nucleoid region of the cytoplasm. multiple points of origin and use Prokaryotes, as well as the mitochondrion unidirectional replication within the and the chloroplast, have covalent, nucleus. closed, circular DNA that is not coiled in protein "spools" called histones. Most TRANSCRIPTION AND TRANSLATION prokaryotes have only one chromosome In prokaryotic cells, transcription and and an extrachromosomal DNA called a translation can be done at the same time plasmid. in the cytoplasm. In eukaryotic cells, the linear DNA is Aside from that, there is no coiled in histones and is found inside the post-transcriptional processing because nucleus. the DNA of prokaryotes does not have a Eukaryotes have more than one non-coding part called introns. chromosome so histones are essential in In eukaryotic cells, the transcription occurs packaging DNA into nucleosomes and in the nucleus and the translation occurs in helping it to the cytoplasm. Condense into chromatin. Through this, Eukaryotic DNA contains exons (coding several chromatins can fit into the nucleus part] and introns (non-coding part). without the high risk of mechanical Post-transcriptional processing is done in damage. eukaryotes to remove introns and come up with the final RNA CELL SIZE Prokaryotic cells are smaller compared to STRUCTURES AND FUNCTIONS OF ANIMAL eukaryotic cells. Typical, prokaryotic cells CELLS can range from 0.1 to 5um in. diameter while eukaryotie cells are typically 10 to - Tissues are groups of cells that are similar in 100um in diameter. structure and function. In animals, there are four main types of tissue—epithelial, connective, and multinucleated (with more than one muscle, and nervous tissues. nucleus). -They pull the bone and skin when they contract to cause movement. EPITHELIAL TISSUE 2. SMOOTH MUSCLE ➔ Epithelial tissue or epithelium is a type of - Smooth muscle or visceral muscle is a animal tissue that forms the inner and type of muscle tissue commonly found in outerlining of organs, the covering in the walls of hollow organs such as surfaces, and the primary glandular tissue intestines, stomach, bladder, blood of the body. In terms of structure, vessels, and uterus. epithelial cells are closely packed to form continuous sheets. This kind of structure - It involuntarily contracts slower than the allows epithelium to form linings and other two types of muscle tissue. Smooth impart protection to bodily structures muscles are nonstriated, uninucleated, and spindle-shaped (have pointed ends) Cell junctions like desmosomes and tight cells. junctions permit the cells of epithelial tissue to absorb and filter different substances CARDIAC MUSCLE CONNECTIVE TISSUE - Cardiac muscle is a muscle tissue found ➔ Connective tissue is the most abundant in the heart. Unlike a skeletal muscle, it is tissue in the body that connects body uninucleated (one nucleus) and it moves parts. Unlike the avascularized epithelial involuntarily (cannot be controlled tissues, most connective tissues are consciously). However, it has striations vascularized (with constant lood supply like skeletal muscle. from blood vessels) except tendons and ligaments. - Cardiac muscle cells are branching together and fit tightly together at If epithelial tissues have a basement membrane, junctions called intercalated disks. These connective tissues have an extracellular matrix. disks contain gap junctions that facilitate the rapid conduction of electrical impulses MUSCULAR TISSUE across the heart ➔ Muscular tissue, or simply muscle, is made up of specialized cells that can NERVOUS TISSUES shorten or contract to produce ➔ Nervous tissue makes up the central movements. Muscle tissues consist of nervous system and peripheral nervous long and extensive muscle fibers. Three system. It is composed of neurons and types of muscle tissues differ in structure neuroglia or supporting cells. and function. ➔ A neuron is made up of the cell body or soma, dendrite, and axon. TYPES OF MUSCLE TISSUES The dendrite is the receiver of electrochemical signals from external stimuli or adjacent neurons. 1. SKELETAL MUSCLE It transmits incoming signals towards the cell - Skeletal muscle is a muscle tissue body. attached to the skeleton or bones. These muscles can be controlled consciously or The cell body or soma contains the nucleus and voluntarily. Skeletal muscle cells are long, specialized organelles that produce molecules cylindrical, striated (with visible stripes), needed by the neuron. Electrochemical signals will be transmitted away from the cell body by Rudolph Virchow. Cells, in a particular part of through the axon a body, have specific functions. - When a group of cells is performing the same ➔ The cell body or soma contains the function, they are considered as tissues. There nucleus and specialized organelles that are different types of cells found in an organism’s produce molecules needed by the neuron. body. ➔ Electrochemical signals will be - In plants, tissues can be classified as transmitted away from the cell body meristematic and permanent tissues. These through the axon. tissues are generally responsible for growth, support, and transport. The axon is surrounded by an insulating layer called the myelin sheath that allows impulses to MERISTEMATIC TISSUES transmit quickly and efficiently along the neuron. ★ Meristematic tissues are composed of The periodic gaps between myelin sheaths on an cells that give rise to another set of cells. axon are called Nodes of Ranvier. ★ The cells produced from meristematic tissues can either stay as meristematic Between two neurons, or a neuron and a muscle cells to continually produce more cells or or gland, there is a neural junction or synapse transform into specialized cells which will where the transmission of electrochemical become parts of some tissues and organs signals occurs. of a growing plant. ★ Meristematic tissues are mainly THREE TYPES OF NEURONS responsible for the growth of a plant. They 1. SENSORY NEURON also give rise to essential parts of a - Sensory neurons are usually unipolar or growing plant. Apical and lateral pseudounipolar with an axon that meristems are the major meristematic branches into two extensions. The first tissues found in plants. one is connected to the dendrite that receives sensory input, and the other one APICAL TISSUES transmits the information to the central ★ The height of plants is attributed to the nervous system. work of apical meristems. These meristems are located on the shoots and 2. INTERNEURON roots of the plant. As apical meristems - Interneurons are bipolar or multipolar divide to produce new cells, they are also neurons with one axon and multiple elongating the root and shoot systems by dendrites. It connects the sensory neuron producing the primary plants’ body which to the motor neuron. includes the dermal, vascular, and ground 3. MOTOR NEURON tissues. - Motor neurons are multipolar neurons ★ Elongation of the root and shoot systems that carry electrochemical signals from the is referred to as primary growth in plants. CNS to the muscles or glands. Apical meristems, located at the root tips and terminal buds in a plant’s shoots, are continuously dividing to produce primary meristems. STRUCTURES AND FUNCTIONS OF PLANT ★ Primary meristems are derivatives of CELLS apical meristems consisting of protoderm, procambium, and ground meristem which PLANT TISSUES will, later on, give rise to the plants’ three - All organisms are made of cells,” is one of the tissue systems-- epidermis, stele (xylem statements in cell theory which was formulated  and phloem), and ground tissues, ★ In leaves and in stems, the epidermis has respectively. tiny outgrowths called trichomes. These hairlike outgrowths prevent water loss and LATERAL MERISTEMS reflect excess light. When young woody ★ A plant does not only increase in height plants mature, the epidermis is replaced but also increases in diameter. In most by periderm which is produced by cork plants, growth in diameter and girth is an cambium. essential factor for survival because it The lower and upper parts of leaves are increases the plants’ rigidity and protected by the epidermis. The lower epidermis sturdiness. of a leaf has guard cells. ★ A growth in diameter and girth is called secondary growth which results from the continuous division of lateral meristems. Guard cells are specialized structures that ★ Vascular cambium and cork cambium are regulate the opening and closing of stomata. the lateral meristems found along the Stomata are slit-like structures on the lower length of the plant which causes the epidermis of leaves which aids in the exchange increase in diameter and girth of plants. of gases between plants and the environment. VASCULAR CAMBIUM ★ Vascular cambium gives rise to secondary GROUND TISSUES xylem and phloem. Inward growth of ★ Cells that are neither dermal nor vascular vascular cambium produces xylem, while are called ground tissues. They are an outward growth produces phloem considered as fillers of plants and form the ★ As vascular cambium grows, layers of bulk of plants. secondary xylem are added which ★ Ground tissues are usually found between becomes wood. When wood develops, dermal and vascular tissues. The ground plants become sturdy and rigid. tissues found on the exterior of vascular tissues are referred to as cortex. SIMPLE PERMANENT TISSUES ★.Simple permanent tissues are composed Meanwhile, pith is found at the center of the of one kind of cell, while complex stem. These simple permanent tissues are permanent tissues are composed of two generally responsible for photosynthesis, or more kinds of cells. support, and storage. Parenchyma, collenchyma, ★ Ground and epidermal tissues are and sclerenchyma are the kinds of ground considered as simple permanent tissues. tissues found in plants that differ in function and On the other hand, xylem and phloem are in location. considered as complex permanent tissues. PARENCHYMA CELLS ★ Parenchyma cells have thin and flexible primary walls but lack secondary walls. EPIDERMAL TISSUES These cells are mostly responsible for the ★ Epidermis covers the whole body of synthesis and storage of plant food. nonwoody and young woody plants and is ★ Parenchyma cells in leaves contain protected by a waxy cuticle. The cuticle chloroplasts and perform photosynthesis. prevents loss of water and invasion of In stems and in roots, parenchyma cells disease-causing microorganisms. contain plastids that act as storage of ★ The epidermis in roots has tiny projections starch, which is a complex sugar found in called root hairs which help in increasing most plants. the absorption capacity of roots. COLLENCHYMA CELLS ★ Sugar and other organic nutrients are ★ Unlike parenchyma, the primary walls of distributed throughout the plant’s body by collenchyma are relatively thick although phloem. Phloem is composed of sieve its thickness is uneven. tubes consisting of sievetube elements. ★ Collenchyma lacks secondary walls and ★ Sieve-tube elements are cells where their primary walls do not contain lignin, nutrients, sucrose, and organic which is a polymer providing rigidity. This compounds are transported. thus provides a furnishing flexible msupport to immature parts of plants. SPECIALIZES CELL AND CELL STRUCTURES SCLERENCHYMA ★ Sclerenchyma provides support and - Cells with these unique structures are called rigidity to plants. It has two kinds, namely modified or specialized cells. All specialized cells sclereids and fibers. Sclereids strengthen have undergone cell modification, a process in seed coats and are responsible for which cells develop special structures to carry gritty-textured flesh of some fruits. out their specific functions. Modification in cells ★ On the other hand, fibers are used includes an increase in the number of organelles commercially as components of making like mitochondria in the muscle cells or the loss rope and flax fibers. of organelles like the nucleus in the red blood cells. COMPLEX PERMANENT TISSUES ★ Plants need a constant supply of food, SPECIALIZED CELLS AND CELLS water, and minerals for their survival. The STRUCTURE IN ANIMALS distribution of food, water, and minerals are acted upon by xylem and phloem. MICROVILLI Microvilli (singular, microvillus) are specialized structures on the surface of epithelial cells. These cellular extensions XYLEM are made of actin microfilaments that ★ Xylem is composed of two types of serve as their structural core. conducting cells— tracheids and vessel A microvillus’ average diameter and length are elements. Tracheids are thin and 0.1 and 2 micrometers, respectively. They are elongated cells where water passes found in the small intestines, kidneys, egg cells, through. and white blood cells. ★ Tracheids, as well as vessel elements, have thin primary walls but thick CILIA AND STEREOCILIA secondary walls. Their secondary walls Cilia (singular, cilium) are tiny hair-like have pits that allow the transport of water projections on the surface of epithelial from one cell to another. cells. Cilia in the bronchioles which are ★ The thick secondary walls of tracheids responsible for ensuring that the serve as a rigid and strong wall against respiratory tract is free of dust, dirt, and the tension of water transport. Moreover, pathogens. vessel elements are thin-walled cells of Cilia are found on the respiratory tract. xylem. Unlike tracheids, vessel elements These hairlike structures above the have perforated plates that allow the pseudostratified epithelia of the trachea transport of water through the vessels. prevent mucus (from goblet cells), bacteria, and dirt from entering the lungs. PHLOEM Stereocilia (singular, stereocilium) are surface extensions of the epidermis which  can be found in the epididymis of the junctions are cell junctions found in testis and in the inner ear of humans and epithelial tissues. animals. Tight junctions are found surrounding each cell Stereocilia in the epididymis (which is a and serve as impermeable structures that duct or passageway in the male prevent leakage of substances when transmitted reproductive system through which sperm from one cell to another. cells pass) absorb the fluid that propels Adherens junctions and desmosomes are sperm cells from the seminiferous tubules specialized protein structures responsible stereocilia in the inner ear serve as for connecting adjacent cells. sensors of vibrations. Aside from sensing Adherens junctions are found below tight vibrations, stereocilia also transform these junctions, while desmosomes are found vibrations into neural signals that will be below adherens junctions. interpreted by the brain. Gap junctions serve as channels of ions, water, and other essential substances FLAGELLA needed by cells. The flagella (singular, flagellum) are tail-like structures that provide motility to RED AND WHITE BLOOD CELLS cells. Red blood cells or erythrocytes are blood Flagella are found in sperm cells which cells that lack nuclei and mitochondria. help them travel from the male The absence of a nucleus and reproductive tract to the egg cell in the mitochondria in each red blood cell allows fallopian tube. it to perform its work properly. Red blood cells are responsible for transporting oxygen throughout the body. The lack of BASAL AND HEMIDESMOSOMES nucleus in red blood cells gives more Basal infoldings and hemidesmosomes space for hemoglobin—a protein that are both found at the basement carries oxygen; thus, more oxygen membrane of epithelial cells. molecules can be transported. Epithelial cells sit on a basement White blood cells, also known as membrane that serves as their foundation. leukocytes, are specialized mcells that are Hemidesmosomes are the ones that classified into granulocytes and anchor epithelial cells to their basement agranulocytes. Granulocytes are white membrane. blood cells that contain granules in their Epithelial cells found in the salivary glands cytoplasms which are observable under a and excretory duct contain basal light microscope. These cytoplasmic infoldings in their basement membrane. granules are enzymes that digest These basal infoldings increase the pathogens that can cause diseases in the surface area of these cells. Moreover, body. basal infoldings are responsible for ion and fluid transport. CELL JUNCTIONS SPERM AND EGG CELLS Cell junctions are specialized structures Gametes or sex cells are considered as found on the lateral membrane of the structures that specialize cells because of cells. They are responsible for adhering their distinct structures that help them cells to other cells or to the extracellular carry out their specific functions. matrix. Tight junctions, adherens Egg cells or ova (singular, ovum), with a junctions, desmosomes, and gap diameter of 0.12 micrometers, are the largest cells in humans. Egg cells have  two outer membrane layers namely zona Root hairs are tiny hair-like structures that pellucida and corona radiata. originated from the epidermis of plants. These Zona pellucida is the inner layer of the egg’s specialized structures die off after 2 to 3 weeks outer membrane. It assists the sperm cell upon but are constantly replaced by new ones to fusion with the egg cell. maintain the efficiency of water absorption. The outermost layer is the corona radiata which consists of three layers of cells derived from follicles. The two outer membrane layers of the egg cells prevent MESOPHYLL CELLS polyspermy—an event when an egg cell is Photosynthesis happens in the mesophyll fertilized by more than one sperm cell. layer of the leaves located between the Sperm cells, by contrast, are relatively lower and upper epidermis. small compared with the egg cells. Sperm This layer is the specific site of cells are specialized cells that have a photosynthesis as it contains a large flagellum. The flagella in sperm cells help number of chloroplasts them move from the reproductive tract of ★ The palisade cells are elongated and males to that of females. The sperm cells cylindrical and form a palisade layer have three distinct parts— head, beneath the upper epidermis of the midpiece, and flagellum. leaves. Because of their shape, palisade The sperm’s head contains acrosome that cells accommodate 70% of the plant’s helps them penetrate the egg cell’s outer chloroplasts. The arrangement and membrane, while the midpiece contains location of palisade cells allow them to numerous mitochondria that provide obtain enough sunlight needed for energy to the flagellum. The tail-like photosynthesis. region in the figure is the flagellum itself ★ Spongy mesophyll cells are which provides motility to the sperm cell. irregularly-shaped cells found beneath the palisade layer and above the lower epidermis of leaves, Cells in this layer are SPECIALIZED CELLS AND CELL loosely packed and have many spaces in STRUCTURE IN PLANTS between them. The spaces between the ❖ Trichomes are hair-like structures that cells in the spongy layer allow more develop from the plant’s epidermis and efficient gas exchange during are mostly found on the leaves. They may photosynthesis. be unicellular or multicellular. Their size and functions vary from one type of plant CELLS OF XYLEM AND PHLOEM to another. ★ Xylem and phloem are not themselves ❖ Trichomes with glands on their tip release cells but are instead composed of chemicals that prevent some insects from specialized cells that play a vital role in a feeding on plants. But trichomes may also plant’s growth and development. These produce chemicals that some useful are tube-like cells that transport water, products are derived from, like mint nutrients, and food throughout the plant’s fragrance. body. ★ Xylem comprises cells that are specifically responsible for transporting water and ROOT HAIRS nutrients obtained from the soil. ❖ Root hairs are specialized cell structures ★ Phloem comprises cells that are that help plants to efficiently absorb water responsible for transporting food, usually from the soil by increasing the surface the by-products of photosynthesis. area for absorption.  - Much like entire organisms, individual cells also WEEK 5 : CELL CYCLE AND CELL DIVISION have their own life cycle. The life cycle of a cell is known as the cell cycle, and this describes how - Any sexually reproducing multicellular cells grow develop, and reproduce. organisms begins as a single-celled entity. - The single-celled entity then relies on The cell cycle involves different phases that cellular events to produce more cells, the constitute a series of preparations for a cell to early stage of which is shown in the blastula divide to produce new cells. GENETIC MATERIAL OF THE CELL DNA MOLECULE ➔ Almost, every cell in our bodies has genetic material in the form of deoxyribonucleic acid (DNA]. ➔ Most biological traits such as eye color, hair color, and height are coded for by the DNA. STAGES OF THE CELL CYCLE ➔ DNA, the primary genetic material of living organisms, stores genetic information in the sequences of its nucleotides. ➔ Positively charged Histone proteins aid in DNA packaging. CHROMATIN ➔ Interphase DNA is found in the form of chromatin, a complex of DNA and proteins. - Chromatin in dispersed inside the intact cell nucleus during interphases - Chromatin will sometimes need to further condense into a highly coiled and compact structure. - This highly condensed form of DNA is known as a chromosome. NOTE: Humans have 46 individual (or 23 pairs) of chromosomes. Chromatin condenses into chromosomes. Each half of the chromosome is a chromatid. 1. INTERPHASE - GAP 1 Growth of cytoplasm and doubling of INTRODUCTION TO CELL CYCLE - All species of organisms have their own life cycle organelles that shows how they are born, develop, and Produce proteins, enzymes, nutrients, and mature. energy Highest rate of protein synthesis  Movement of centrioles of the centrosomes away to await and assist the THE CELL CYCLE CHECKPOINTS events of mitosis. 2. INTERPHASE - S This stage is named so because it is when the cell synthesizes a copy of its DNA in a very notable process called DNA replication. - This refers to the event when an existing DNA produces another copy of itself. G1 TO S CHECKPOINT - Is there any damage in the cell’s DNA? - Is the cell large enough with all the necessary energy reserves and doubled organelles? G2 TO M CHECKPOINT - Are the environmental conditions favorable for cell division to take place? Is there a place? - is the DNA properly replicated? - is there a presence of DNA damage? M CHECKPOINT - Are there spindles properly attached? 3. INTERPHASE - GAP 2 Continued growth and the production of materials that are necessary for cell division to occur. Protein synthesis but not at the rate similar to that of the G1 phase. 4. M- PHASE MITOSIS AND MEIOSIS The M phase in eukaryotic organisms involves either one of two processes, namely, mitosis and meiosis. STAGES OF MITOSIS Mitosis refers to how a cell divides to - Sex cells or gametes undergo produce new cells. meiosis, whereas somatic cells or Only somatic cells (also known as body non-sex cells) undergo mitosis. - cells undergo mitosis. 5. CYTOKINESIS NOTE: A cell will only begin mitosis if the conditions Cytokinesis is marked by the formation of are correct and favorable for cell division. a cleavage furrow in animal cells. THE ROLE OF MITOSIS IN THE CELL CYCLE M Phase - The cell undergoes mitosis if it is a somatic cell or meiosis if it is a sex cell. - It allows the cell to divide and produce new cells. G0 Resting State G1 CHECKPOINT Checks for: Nutrients, Growth factors, DNA damage METAPHASE CHECKPOINT Checks for: Chromosome spindle THE PHASES OF MITOSIS attachment. G2 CHECKPOINT 1. PROPHASE Checks for: Cell Size, DNA replication NOTE The synthesis phase involves the duplication of the cell’s DNA. The Chromosome DNA in interphase is found in the form of chromatin which condenses during mitosis. ➔ The chromatin condense into Each chromosome is made up of two chromosomes. sister chromatids. ➔ The nucleolus disintegrates. ➔ The mitotic spindle starts to form. Haploid and Diploid Cells ➔ The human genome has 23 pairs of chromosomes for a total of 46 chromosomes in each cell. 1.1 PROMETAPHASE ➔ 23 is the haploid number whereas 46 is ➔ It begins after the nuclear envelope the diploid number. ➔ breaks down. ➔ Haploid is often represented as n while ➔ The centrosomes move to opposite poles. diploid is 2n. ➔ The kinetochores found at the ➔ The number of sets of chromosomes that centromeres become apparent. a cell or an organism has is known as ploidy 2. METAPHASE  ➔ The microtubules of the mitotic spindle In Telophase, the parent cell starts to form attach and interact with the kinetochores Two new cells. of the chromosomes. ➔ The chromosomes align at the center of CYTOKINESIS the cell, in the metaphase plate. ➔ After telophase, cytokinesis will divide the cell into two. ➔ Cytokinesis is not a phase of mitosis but 3. ANAPHASE is closely related to it. ➔ Recall from the previous lesson that each of the two daughter cells will inherit a nucleus, similar genetic material, organelles, and more. ➔ The mitotic spindle pulls and separates the sister chromatids apart. ➔ The chromatids, now called daughter chromatids, are then pulled toward the opposite poles. WEEK 6: 4. TELOPHASE STAGES OF MEIOSIS ➔ Like mitosis, meiosis also refers to how cells divide to produce new cells. ➔ Sex cells or gametes are produced from meiotic division. Meiosis is the M phase that leads to the ➔ Daughter chromatids decondense. production of either sperm cells or egg cells. This ➔ In animals, the boundary of the type of division is very closely tied with the new cells is known as the cleavage process of sexual reproduction. furrow. ➔ In plants, it is known as the cell MEIOSIS AND ITS FUNCTION plate. Production of functional sex cells or gametes NOTE!: Two rounds: meiosis I and meiosis II In Prophase, the cell’s genetic material Reduction in the chromosome number resembles Polka dots or strings. Allows restoration of original chromosome In Metaphase, the chromosomes Migrate number during fertilization to the Middle of the cell. In Anaphase, the chromosomes move away from each other.  ➔ Maternal and paternal chromosomes are randomly arranged. ➔ Facilitates reduction in chromosome number. 3. ANAPHASE 1 ➔ Separation of the homologous pairs ➔ Maternal and paternal members move to either pole NOTE!: 4. TELOPHASE 1 AND CYTOKINESIS Meiosis involves two rounds of cytokinesis ➔ Chromosomes reach opposite poles. which will ultimately produce four ➔ The cytoplasm divides. daughter cells with a reduced chromosome number. NOTE!: Meiosis II is an EQUATIONAL division. Meiosis I comes after the G2 phase of the cell cycle. 1. PROPHASE II Meiosis I is a REDUCTIONAL division. ★ A new spidle forms around the chromosomes 1. PROPHASE 1 ➔ Chromosome condensation 2. METAPHASE II ➔ Sypnasis between homologous ★ Chromosomes line up at the equator. pairs ➔ Crossing over between 3. ANAPHASE II homologous pairs ★ Centrosomes divide. ➔ Chromosomes after meiosis ★ Chromatids move to the opposite poles of the cells. 4. TELOPHASE II ★ A nuclear envelope forms around each set of chromosomes. ★ The cytoplasm divides. DIFFERENCES BETWEEN MITOSIS AND MEIOSIS 1.1 PROMETAPHASE 1 ➔ Further chromosomal condensation ➔ Further disintegration of nuclear envelope ➔ Meiotic spindle formation 2. METAPHASE 1 ➔ Alignment of the homologous chromosomes at metaphase plate  ❖ Sister chromatids line up and separate into individual chromosomes in meiosis II. IMPORTANCE OF CROSSING OVER Facilitates genetic recombination Enhances genetic variation Increases chances of survival CHANGES IN THE NUMBER OF CHROMOSOMES BEFORE S PHASE - Chromosomes are assumed to be single at G1 phase (2n = 6) AFTER S PHASE - Chromosomes are duplicated by the end of S phase (2n = 6) DURING MEIOSIS I - Duplicated chromosomes enter meiosis I (2n = 6) To help you remember: MiSO: Mitosis for Somatic cells AFTER MEIOSIS I - Chromosome number is MeGa: Meiosis for Gametes reduced after meiosis I (n = 3) MEIOSIS II - Sister chromatids are separated after meiosis II ( n= 3) THANK YOU!! GOODLUCK SA EXAM MGA BEBELUVS! PADAYON BESt STUDENTS!!! Mitosis maintains an identical set of genetic material in the daughter cells whereas meiosis reduces this amount and introduces new combinations. These have different consequences and applications as a result. MEIOSIS I VS. MEIOSIS II ❖ Sypnasis occurs only in Meiosis I ❖ Tetrads line up and separate into individual homologous chromosomes in meiosis I ❖ Crossing over occurs only in meiosis I