Cells (1) PDF - Biology Past Paper 2024-2025

General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 1. Anton van Leeuwenhoek Lesson 8: Cell A Dutch invent...

General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 1. Anton van Leeuwenhoek Lesson 8: Cell A Dutch inventor Basic unit of life First to device his own light 100 trillion cells microscope Only 200 types of cells (include red Revealed a wide variety of blood cells) organisms not visible to the naked All share the same basic structures eyes. Thin pliable plasma (cell) membrane His contribution is the beginning to surrounds the jelly like cytoplasm understand the chemical reactions Cytoplasm - contains different involved in the living material. organelles Little organelles - mitochondria and 2. Robert Hooke ribosomes A British scientist Spherical Nucleus - control center of 300 years ago the cell Observed tiny slices of cork through Red Blood cell - standard a microscope comparison reference. Average Described it as a mass of tiny diameter = 7.5 microns cavities similar to honeycomb Many are spherical (Skeletal muscle Compared it to small rooms in the cell - makes move movement monastery and coined the term possible) CELLS. Neuron - number of processes Discovered the nucleus as the Axon = 1.2 to 1.5 meters long central organelle Dendrites are shorter than axon but branching 3. Robert Brown Cell - jelly like, granular, grayish to colorless, transparent, translucent, Scottish botanist microscopic, sticky, slippery, viscous, He discovered the nucleus of a cell heavier and denser than water He is perhaps best known for the Cytology - study of cells discovery of the random movement of microscopic particles in a surrounding solution called Brownian Major Contributors to the Discovery of Cells Movement The following scientist contributed to the He developed alternative plant discovery and our knowledge about cells: classifications. 1. Anton van Leeuwenhoek 2. Robert Hooke 3. Felix Dujardin 3. Robert Brown 1835, He viewed living cells with a 4. Felix Dujardin microscope. French biologist and cytologist born 5. Matthias Jacob Schleidan in Tours 6. Theodore Schwann Noted for his studies in the 7. Rudolf Virchow classification of protozoans and invertebrates.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 4. Matthias Jacob Schleidan Other Contributors to the Discovery of Cells A German botanist and a 1595 – Zacharias Janssen credited with the 1st co-founder of cell theory compound microscope. He proposed that all plants are 1840 – Albrecht von Rolliker realized that made up of cells. sperm cells and egg cells are also cells. Recognized the importance of cell 1857 – Kolliker described mitochondria. nucleus and sensed its connection with cell division 1879 – Flemming described chromosome First German botanist to accept behavior during mitosis. Charles Darwin’s theory of evolution. 1898 – Golgi described the golgi apparatus 5. Theodore Schwann Lesson 9: Common Structures of a Cell A German physiologist and zoologist The following are the typical parts of a cell: All animals are composed of animal 1. Cell membrane/Plasma cells and that within an individual Membrane/Plasmalemma organisms all the cells are identical. 2. Nucleus Founded modern histology be 3. Cytoplasm defining the cell as the basic unit of animal structure. 1. Cell Membrane 6. Rudolf Virchow For support and protection the outermost covering of animal cell, Viennese pathologist semi-permeable, thin and flexible Published his observation that new It is composed of protein and cells arise only from pre-existing cells phospholipids (Omnis cellula e cellula) Works with the other scientist to The Function of a Cell Membrane established the Cell Theory Used the theory to lay the Maintains the shape of the cell groundwork for cellular pathology or Contains the cell contents the disease at the cellular level. Prevents the contents of one cell from mixing with those of other cells Cell Theory Controls the entrance and exit of materials in the cell Theodor Schwann and Matthias Jakob Schleiden in 1838, 2. Nucleus All living things are made up of cells The cell is the basic structure and Rounded, darkly stained structure function of all living things. That is, separated from the cytoplasm by a the cell carries all the processes that double-walled nuclear membrane. are characteristics of all living things. Cells arise from one another cell Functions of a Nucleus through the process of cell division. Control center of the cell which directs cell division since it contains the hereditary information in the form of gene Control protein synthesis and other metabolic activities of the cell  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Parts of a Nucleus 7. Excretion – The ability of the cell to The following are parts of a nucleus eliminate waste materials A. Nuclear Membrane 8. Respiration – This is taking in of B. Nucleoplasm/Nuclear Sap/Karyoplasms oxygen and using this for oxidation of food substances with resulting A. Nuclear Membrane liberation of energy There are two subsections in a nuclear membrane a. Outer Nuclear Membrane Nuclear membrane that is continuous with a system of the endoplasmic reticulum. It is perforated with pores, which facilitates the passage of large organic molecules between the nucleus and the rest of the cell. b. Inner Nuclear Membrane Nuclear membrane that is continuous and composed of a membrane system with DNA as the principal nucleic acid, some RNA, and protein. It is impermeable to the exit of DNA but permeable to m-RNA. Physiology of a Cell Membrane The following are the physiology of the cell membrane: 1. Cell division – This is the ability of the cell to grow to a limited extent and produce new cells 2. Contractility – The ability of the cell to be stimulated so as to shorten and return to its original length when stimulus is removed 3. Conductivity – The ability to transmit a wave of excitation throughout the substance of the cell 4. Irritability – This is the property that enables the cell to respond to stimulus 5. Secretion – Cells that can synthesize useful substances from those that they absorb and can give of these substances as secretory products. 6. Absorption and Assimilation – Living cells can take food and other substances and utilize it  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Cell Membrane  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 A. Nucleoplasm/Nuclear Sap/Karyoplasm Gel-like, no organelles, rich in nucleic acid. Inside the nucleoplasm are the following: Nucleoplasm is a darkly stained spherical body which produced mRNA. Chromatin is clumped of a dense granular, threadlike network which is transformed into chromosomes during mitosis. It also contains the genes which carry the genetic information necessary for replication and protein synthesis. 3. Cytoplasm The living substance of the cell The protoplasm that surrounds the nucleus of the cell which contains the organelles and inclusion bodies. Physiological properties of the cell Cytoplasmic Organelles The following are the Membranous Organelles: I. Mitochondria/Chondrisomes II. Golgi Apparatus/Dictyosomes III. Endoplasmic Reticulum IV. Rough ER V. Smooth ER VI. Lysosomes VII. Peroxisome VIII. Vacuoles  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Illustrations Descriptions MEMBRANOUS ORGANELLES Mitochondria/Chondrisomes Spherical, rod-shaped, cigar or sausage-shaped hollow structure Composed of double membrane, where the outer membrane is smoothly and tightly stretched while the inner membrane is invaginated with folds forming shelves called ‘cristae’ (this folds contain enzymes which are used in the conservation of food energy by the cell to do cellular work Functions: Serve as the power of the cell (production of energy in the form of ATP; support the mechanical and chemical work performed by the cell. Golgi apparatus/dictyosomes Series of smooth membrane that is continuous with the endoplasmic reticulum. Enzymes are concentrated along the surface of the membranes. Consists of several flatten tubular membranes known as “cisternae” which are stacked upon each other. Vacuoles are found at the dilated terminal at either end of the cisternae. Function: for packaging of food materials Endoplasmic reticulum Series of parallel arrays of membrane creating Rough ER canals like membranous tubules and vesicles that run throughout the cytoplasm of the cell. The canal carries substances from the cell membrane or throughout the cytoplasm. Function: Transport substances throughout the cell. Types of ER: (1) Rough or granular ER – canals are studded with ribosomes. The proteins produced in the ribosome Smooth ER are secreted from the cell through the canals. Function RER is active in the secretion of protein such as pancreatic exocrine cells and liver cells. (2) Smooth or granular ER – does not contain ribosomes. Function: site of the synthesis of steroid hormones such as that of the adrenal glands; involved with lipid or fat synthesis as in striated muscles and concerned with rapid transport of metabolites needed for muscular contraction  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Lysosome Large spherical body which is membrane bound and dense appearing structures that contain enzymes which are collectively referred to as acid hydrolases, that are capable of breaking down intracellular molecules and digesting like bacteria that enter the cells. Function: phagocytosis and cellular digestion or breaking down complex particles and cellular products Peroxisome Similar to lysosomes in the way that they are membrane bound sacs containing enzymes. The enzymes in this organelle are involved in either the production of hydrogen peroxide or destruction of hydrogen peroxide to water. Function: involved in purine catabolism, the breaking down of nucleic acid and conversion of fats to glucose. Vacuoles Spherical, empty sacs for storage of food (food vacuole in animals), water vacuole (as in for plants), and contractile vacuole (nitrogenous waste, in amoeba). Vacuoles have a simple structure: they are surrounded by a thin membrane and filled with fluid and any molecules they take in. They look similar to vesicles, another organelle, because both are membrane-bound sacs, but vacuoles are significantly larger than vesicles and are formed when multiple vesicles fuse together. NON-MEMBRANOUS ORGANELLES Ribosomes Tiny spherical structure scattered throughout the cytoplasm. Types of ribosomes: (1) attached ribosomes – found attached to the endoplasmic reticulum; (2) free-living ribosome - found scattered throughout the cell’s cytoplasm Function: site of protein synthesis within the cell. It is in the ribosomes that amino acids are linked together to form protein. Centrosome Organelle is only present in animal cells, which means ‘cell center’.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 The region of centrosome is near the nucleus. within it are pair of small rod-like structures called centrioles. Around the centrioles (distinct minute cylindrical structure) are single microtubules called aster. Functions: Centrioles are active in the process of animal cell division since they serve as poles which are sources of spindle fibers and determine the plane of cell reproduction; Asters is the one that initiates the formation of spindle fibers during cell division. Microtubules Hollow microscopic tubules which are made up of protein molecules. Function: form the cytoskeleton of the cell which serves as supportive structure and maintains the shape of the cell; form spindle fiber (chain of microtubules) which control the movement of the chromosome; form the centrosome of the animal cell. Microfilaments Solid microscopic tubules with a property of contractility. These are the structural units of cilia and flagella, the locomotory structures of the cell. Cilia – microscopic hair-like projection as in paramecium Flagellum – a long whip-like structure of the cell as in euglena and bacteria Function: control of the movement of the cell.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Photo of a Cell  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Photo of an Animal Cell An animal cell contains membrane-bound organs, or organelles.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Lesson 10: Prokaryotic & Eukaryotic Prokaryotic Eukaryotic Means “before nucleus” Means “true nucleus” No membrane-bounded nucleus Nucleus is bounded by a membrane Circular strands of DNA DNA in several linear chromosomes Few cell organelles Many specialized organelles Unicellular Multicellular, except for unicellular protist like Small ribosome Euglena Microtubules are usually absent Large ribosomes Cell wall contains murein Microtubules are present May contain chlorophyll but not within the chloroplast Cell wall, when present as in plants does not Cell reproduction: undergo direct cell division contain murein (amitosis) Chlorophyll, when present as in plant cells, is Intercellular links: pili for DNA exchange within the chloroplast Flagella: lack 9 + 2 tubular structure Cell reproduction: undergo indirect cell division (mitosis) Does not contain pili Flagella, when present, have 9 + 2 tubular structure Anatomy of a Bacteria Cell  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 LESSON 11 – PHYSIOLOGY OF THE CELLS Physiology of Cells Cells are regarded as highly organized units engaged in ceaseless chemical activities. These activities are dependent on the continuous reception of the substances from the so-called internal environment and the elimination of substances to the tissue fluid. Walter C. Canon, an American physiologist, coined the term homeostasis, which is defined as the overall process of maintaining optimum internal environmental conditions. Intercellular materials are the materials located or occurring between cells and also known as interstitial fluid. Described as a viscous solution containing inorganic chemicals, proteins, carbohydrates, and lipids. The primary difference from intercellular material is the kind of protein and amounts of various chemicals present. Physical Processes that Govern the Movement of Materials Across Cell Membrane 1. Passive Transport – those that do not require a source of energy. a. Diffusion Diffusion is the movement of the solute to spread through the solution until the composition is homogenous. The movement of the region of higher concentration to that of the lower concentration. The net movement of molecules from the area where they are higher concentration to an area where they are more scarce. Factors affecting the rate of diffusion o Size of molecules and ions – the smaller the size, the faster the rate of diffusion o The concentration of gradient – the greater the difference in a concentration gradient, the faster the rate of diffusion o Temperature – the higher the temperature, the faster the movement of the molecules. b. Osmosis The passage of solvent through a semipermeable membrane. Osmosis is the given name to the diffusion of water through a semipermeable membrane, such membrane is permeable only to the solvent, not to the solute. The pressure applied to the solution to prevent solvent flow through the membrane is called osmotic pressure. Osmotic pressure is vital to living cells because of the enclosing semi-permeable membrane of the cells through which they communicate with their environment. Osmotic Characteristics of Solutions  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 o Isotonic – have the same osmotic characteristics to the blood serum. This will not affect the size and shape of RBC. o Hypertonic – have a higher osmotic characteristic than that of the blood serum. In this type of solution, the RBC will shrink. This process is called crenation in animals and plasmolysis in plants. Shrinking of plant cell result to wilting o Hypotonic – have lower osmotic characteristics than that of the blood serum. In this solution, the RBC will swell and may burst. The process is called hemolysis in animals and plasmoptysis in plants. In plants, swelling of the cell does not result in bursting since the plant cell is covered with a cell wall. Instead, the plant becomes turgid and crisp. c. Dialysis Dialysis is a common biological chemical method of separating and purifying by selective passage of ions and minute molecules through a semipermeable membrane that will not allow proteins to pass through. Rate of dialysis depends of the following factors: o The area of dialyzer o The size of the pores o Temperature o Electric charges o The relative concentration of solution on the two sides of the membrane d. Filtration The passage of solution across a semipermeable membrane as a result of mechanical force (gravity). 2. Active Transport – those that require a source of energy, involves the movement of substances regardless of concentration gradient. (ex. Sodium and potassium pump) 1. Endocytosis An active process wherein the cell encloses the substance in membrane-bounded vesicles pinched off from the cell membrane. Term for a phenomena involving the surrounding and ingestion of various substances by the plasma membrane. Types of Endocytosis o Phagocytosis ▪ A process wherein the material is in the form of large particles or chunks of matter. ▪ It is also known as the “cell eating” process. ▪ The vacuoles formed within the cell are called “phagosomes” and are attached to lysosomes. The  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 hydrolytic enzymes of lysosomes digest the particular matter. ▪ For instance, in amoeba, arm-like processes called “pseudopodia” flow around the material, enclosing it within a vesicle which then becomes detached from the plasma membrane and migrates into the interior of the cell. o Pinocytosis ▪ Is a process of engulfing liquid materials or small particles, it is also known as the “cell drinking” process. ▪ The step in endocytosis is that the material first becomes attached to the cell membrane, on a selective binding site. Then the loaded membrane either flows inward to form deep narrow channels. At the end of which vesicles are simply detached directly from the membrane at the cell surface. 2. Exocytosis An energy requiring process by which secretory granules discharge their contents by fusing with the cell membrane. It is a reverse of endocytosis, materials contained in a membranous vesicles are carried to the side of the cell where the vesicular membrane fuses with the cell membrane and the bursts, releasing the materials to the exterior. This is the way by which hormones and waste materials are released from the cell. LESSON 12 – CELL DIVISIONS Introduction All living forms on earth have their own life cycles: a series of progressive stages of an individual that goes through from the time it is born until the time it reproduces. The cell cycle can be thought of as the life cycle of a cell. A series of growth and development steps a cell undergoes between its “birth”—formation by the division of a mother cell—and reproduction—division to make two new daughter cells. Stages of the Cell Cycle A cell must complete several important tasks: it must grow, copy its genetic material (DNA), and physically split into two daughter cells. The cells do the tasks in a systematic, predictable series of stages that make up the cycle. It is a cell cycle because, at the end of each go-round, the daughter cells will start the same process over again from the beginning.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 In eukaryotic cells or cells with a nucleus, cell cycle stages are divided into two major phases: interphase and the mitotic (M) phase. The interphase, the cell grows and copies the DNA; during the mitotic (M) phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells. Interphase Let’s enter the cell cycle just as a cell form by the division of its mother cell. What must this newborn cell do next if it wants to go on and divide itself? Preparation for division happens in three steps: o G1 starts subscript, 1, end subscript phase. During G1 ​start subscript, 1, end subscript phase, known the primary gap phase, the cell grows larger, organelles becomes two in number and makes the molecular building blocks it will need in later steps. o S phase. In the S phase, the cell produces a complete and the same copy of deoxyribonucleic acid in its nucleus. It also copies a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during the M phase. o G2 ​starts subscript, 2, end subscript phase. During the second gap phase or G2 start subscript, 2, end subscript phase, the cells develops more, produce proteins and organelles, and begin to reorganize its contents in preparation for mitosis. G2 starts subscript, 2; end subscript phase ends when mitosis begins. The G11 start subscript, 1, end subscript, S, and G2 ​start subscript, 2, end subscript phases together are known as interphase. The prefix inter- means between, reflecting that interphase occurs between one mitotic (M) phase and the next. Image of the cell cycle. Interphase is composed of G1 phase (cell growth), followed by S phase (DNA synthesis), followed by G2 phase (cell growth). At the end of interphase  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 comes the mitotic phase, which is made up of mitosis and cytokinesis and leads to the formation of two daughter cells. Mitosis precedes cytokinesis, though the two processes typically overlap somewhat. Image credit: "The cell cycle: Figure 1" by OpenStax College, Biology (CC BY 3.0). M phase During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells. M phase involves two distinct division-related processes: mitosis and cytokinesis. In mitotic division, the nuclear DNA of the cell condenses into visible chromosomes. It is pulled apart by the mitotic spindle, a specialized structure made out of microtubules. Mitosis occurs in four stages: prophase (sometimes divided into early prophase and prometaphase), metaphase, anaphase, and telophase. In the process of cytokinesis, the cytoplasm of the cell is split into two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. Importantly, cytokinesis takes place differently in animal and plant cells.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Cytokinesis in animal and plant cells In an animal cell, a contractile ring of cytoskeletal fibers forms in the middle of the cell and contracts inward, producing an indentation called the cleavage furrow. Eventually, the contractile ring pinches the mother cell in two, creating two daughter cells. In a plant cell, vesicles derived from the Golgi apparatus move to the middle of the cell, where they fuse to form a cell plate structure. The cell plate expands outwards and connects with the cell’s sidewalls, creating a new cell wall that partitions the mother cell to make two daughter cells. What is mitosis? Mitosis is a cell division wherein one cell (the mother) undergoes division to have genetically identical daughters. In a cell cycle, mitosis is the division process where the nucleus’s DNA is dividing into two equal sets of chromosomes. The majority of the cell divisions that are happening in the body involves mitosis. During development and growth, mitosis increases in the body of an organism within cells, and throughout an organism’s life, it changes old cells with new ones. For eukaryotes (a single-celled organism) such as the yeast, mitotic cell divisions are a form of reproduction, that adds individuals to the population of that certain organisms. In these cases, the “aim” of mitotic division is to make sure that the daughter cell gets a complete sets of chromosomes with too few or too many chromosomes that is typically doesn't function well: this may not endure, or cause tumor or cancer. When these cells undergo mitosis, they don’t just divide their DNA at random and piles it into the two daughter cells. But, they divides their duplicated chromosomes in an organized series of stages. Phases of mitosis Mitotic cell divisions have four basic phases: prophase, metaphase, anaphase, and telophase. In some books it has five stages, the breaking of prophase into an early prophase and a late prophase. These phases occur in strict consecutive order, and the division of cytoplasm - the process of dividing the cell contents to make two new cells - starts in anaphase or telophase.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Stages of mitosis: prophase, metaphase, anaphase, telophase. Cytokinesis typically overlaps with anaphase and/or telophase. You can remember the order of the phases with the famous mnemonic: [Please] Pee on the MAT. But don’t get too hung up on names – what’s most important to understand is what’s happening at each stage, and why it’s important for the division of the chromosomes. Late G2 phase The cell has two centrosomes, each with two centrioles, and the DNA has been copied. At this stage, the DNA is surrounded by an intact nuclear membrane, and the nucleolus is present in the nucleus. This cell is in interphase (late G22​start subscript, 2, end subscript phase) and has already copied its DNA, so the chromosomes in the nucleus each consist of two connected copies, called sister chromatids. You can’t see the chromosomes very clearly at this point, because they are still in their long, stringy, decondensed form. This animal cell has also made a copy of its centrosome, an organelle that will play a key role in orchestrating mitosis, so there are two centrosomes. (Plant cells generally don’t have centrosomes with centrioles, but have a different type of microtubule organizing center that plays a similar role.)  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Early prophase. The mitotic spindle starts to form, the chromosomes start to condense, and the nucleolus disappears. In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes. The chromosomes start to condense (making them easier to pull apart later on). The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the centrosomes as they move apart. The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made, disappears. This is a sign that the nucleus is getting ready to break down.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Late prophase (prometaphase). The nuclear envelope breaks down and the chromosomes are fully condensed. In late prophase (sometimes also called prometaphase), the mitotic spindle begins to capture and organize the chromosomes. The chromosomes become even more condensed, so they are very compact. The nuclear envelope breaks down, releasing the chromosomes. The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Metaphase Chromosomes line up at the metaphase plate, under tension from the mitotic spindle. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide. All the chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up). At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles. Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed. Anaphase The sister chromatids separate from one another and are pulled towards opposite poles of the cell. The microtubules that are not attached to chromosomes push the two poles of the spindle apart, while the kinetochore microtubules pull the chromosomes towards the poles.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell. The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell. Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer. All of these processes are driven by motor proteins, molecular machines that can “walk” along microtubule tracks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they walk. Telophase The spindle disappears, a nuclear membrane re-forms around each set of chromosomes, and a nucleolus reappears in each new nucleus. The chromosomes also start to decondense. In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place. The mitotic spindle is broken down into its building blocks.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear. The chromosomes begin to decondense and return to their “stringy” form. Cytokinesis in animal and plant cells. Cytokinesis in an animal cell: an actin ring around the middle of the cell pinches inward, creating an indentation called the cleavage furrow. Cytokinesis in a plant cell: the cell plate forms down the middle of the cell, creating a new wall that partitions it in two. Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase. In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The “drawstring” is a band of filaments made of a protein called actin, and the pinch crease is known as the cleavage furrow. Plant cells can’t be divided like this because they have a cell wall and are too stiff. Instead, a structure called the cell plate forms down the middle of the cell, splitting it into two daughter cells separated by a new wall.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 When division is complete, it produces two daughter cells. Each daughter cell has a complete set of chromosomes, identical to that of its sister (and that of the mother cell). The daughter cells enter the cell cycle in G1. When cytokinesis finishes, we end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell. The daughter cells can now begin their own cellular works. https://www.khanacademy.org/science/biology/cellular-molecular-biology/mitosis/a/ph ases-of-mitosis  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Meiosis Meiosis, on the other hand, is used for just one purpose in the human body: the production of gametes—sex cells, or sperm and eggs. Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell. To put that another way, meiosis in humans is a division process that takes us from a diploid cell—one with two sets of chromosomes—to haploid cells—ones with a single set of chromosomes. In humans, the haploid cells made in meiosis are sperm and eggs. When a sperm and an egg join in fertilization, the two haploid sets of chromosomes form a complete diploid set: a new genome. Phases of meiosis In many ways, meiosis is a lot like mitosis. The cell goes through similar stages and uses similar strategies to organize and separate chromosomes. In meiosis, however, the cell has a more complex task. It still needs to separate sister chromatids (the two halves of a duplicated chromosome), as in mitosis. But it must also separate homologous chromosomes, the similar but nonidentical chromosome pairs an organism receives from its two parents. These goals are accomplished in meiosis using a two-step division process. Homologue pairs separate during a first round of cell division, called meiosis I. Sister chromatids separate during a second round, called meiosis II. Since cell division occurs twice during meiosis, one starting cell can produce four gametes (eggs or sperm). In each round of division, cells go through four stages: prophase, metaphase, anaphase, and telophase. Meiosis I Before entering meiosis I, a cell must first go through interphase. As in mitosis, the cell grows during G11​start subscript, 1, end subscript phase, copies all of its chromosomes during S phase, and prepares for division during G22​start subscript, 2, end subscript phase. During prophase I, differences from mitosis begin to appear. As in mitosis, the chromosomes begin to condense, but in meiosis I, they also pair up. Each chromosome carefully aligns with its homologue partner so that the two match up at corresponding positions along their full length. For instance, in the image below, the letters A, B, and C represent genes found at particular spots on the chromosome, with capital and lowercase letters for different forms, or alleles, of each gene. The DNA is broken at the same spot on each homologue—here, between genes B and C—and reconnected in a criss-cross pattern so that the homologues exchange part of their DNA.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Image credit: based on "The process of meiosis: Figure 2" by OpenStax College, Biology, CC BY 3.0 Image of crossing over Two homologous chromosomes carry different versions of three genes. One has the A, B, and C versions, while the other has the a, b, and c versions. A crossover event in which two chromatids—one from each homologue—exchange fragments swaps the C and c genes. Now, each homologue has two dissimilar chromatids. One has A, B, C on one chromatid and A, B, c on the other chromatid. The other homologue has a, b, c on one chromatid and a, b, C on the other chromatid. This process, in which homologous chromosomes trade parts, is called crossing over. It's helped along by a protein structure called the synaptonemal complex that holds the homologues together. The chromosomes would actually be positioned one on top of the other—as in the image below—throughout crossing over; they're only shown side-by-side in the image above so that it's easier to see the exchange of genetic material.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Image of two homologous chromosomes, positioned one on top of the other and held together by the synaptonemal complex. Image credit: based on "The process of meiosis: Figure 1" by OpenStax College, Biology, CC BY 3.0  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 The phases of meiosis I. Prophase I: The starting cell is diploid, 2n = 4. Homologous chromosomes pair up and exchange fragments in the process of crossing over. Metaphase I: Homologue pairs line up at the metaphase plate. Anaphase I: Homologues separate to opposite ends of the cell. Sister chromatids stay together. Telophase I: Newly forming cells are haploid, n = 2. Each chromosome still has two sister chromatids, but the chromatids of each chromosome are no longer identical to each other. When the homologous pairs line up at the metaphase plate, the orientation of each pair is random. For instance, in the diagram above, the pink version of the big chromosome and the purple version of the little chromosome happen to be positioned towards the same pole and go into the same cell. But the orientation could have equally well been flipped, so that both purple chromosomes went into the cell together. This allows for the formation of gametes with different sets of homologues. In anaphase I, the homologues are pulled apart and move apart to opposite ends of the cell. The sister chromatids of each chromosome, however, remain attached to one another and don't come apart. Finally, in telophase I, the chromosomes arrive at opposite poles of the cell. In some organisms, the nuclear membrane re-forms and the chromosomes decondense, although in others, this step is skipped—since cells will soon go through another round of division, meiosis II2,32,3start superscript, 2, comma, 3, end superscript. Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells.  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Meiosis II Cells move from meiosis I to meiosis II without copying their DNA. Meiosis II is a shorter and simpler process than meiosis I, and you may find it helpful to think of meiosis II as “mitosis for haploid cells." The cells that enter meiosis II are the ones made in meiosis I. These cells are haploid—have just one chromosome from each homologue pair—but their chromosomes still consist of two sister chromatids. In meiosis II, the sister chromatids separate, making haploid cells with non-duplicated chromosomes. Phases of meiosis II  General Lesson 8 to 12: Cells Biology 1st Semester |2024 - 2025 Prophase II: Starting cells are the haploid cells made in meiosis I. Chromosomes condense. Metaphase II: Chromosomes line up at the metaphase plate. Anaphase II: Sister chromatids separate to opposite ends of the cell. Telophase II: Newly forming gametes are haploid, and each chromosome now has just one chromatid. During prophase II, chromosomes condense and the nuclear envelope breaks down, if needed. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase II, the chromosomes line up individually along the metaphase plate. In anaphase II, the sister chromatids separate and are pulled towards opposite poles of the cell. In telophase II, nuclear membranes form around each set of chromosomes, and the chromosomes decondense. Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells. How meiosis "mixes and matches" genes The gametes produced in meiosis are all haploid, but they're not genetically identical. For example, take a look the meiosis II diagram above, which shows the products of meiosis for a cell with 2n=42n=42, n, equals, 4 chromosomes. Each gamete has a unique "sample" of the genetic material present in the starting cell. As it turns out, there are many more potential gamete types than just the four shown in the diagram, even for a cell with only four chromosomes. The two main reasons we can get many genetically different gametes are: Crossing over. The points where homologues cross over and exchange genetic material are chosen more or less at random, and they will be different in each cell that goes through meiosis. If meiosis happens many times, as in humans, crossovers will happen at many different points. Random orientation of homologue pairs. The random orientation of homologue pairs in metaphase I allows for the production of gametes with many different assortments of homologous chromosomes.