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BIOLOGICAL SCIENCE #856 - PSC 201 1ST SEM | 2024-2025 | PROF. RHEYNIEL A. ESCOBEL Reproduction:...

BIOLOGICAL SCIENCE #856 - PSC 201 1ST SEM | 2024-2025 | PROF. RHEYNIEL A. ESCOBEL Reproduction: MODULE 3 CELLULAR REPRODUCTION ○ Asexual Reproduction - Organisms create exact copies of themselves to produce offspring. Lesson 1: Cell Division Cells are replicated by all living things to grow, repair, and reproduce. Mitosis, an asexual method of reproduction, produces ○ Sexual Reproduction - Gametes (eggs and cells that are genetically identical to their parent cells. On the other sperm), which contain half the genetic hand, meiosis, a unique form of cell division used in sexual information of their parent cells, combine to reproduction, results in cells that have half the genetic makeup of form new individuals. their parent cells. Meiosis and sexual reproduction increase population genetic diversity in progeny. Cell Division Cells replicate by copying all of their components, including their DNA, before dividing to create new cells. Although meiosis and mitosis share many characteristics, they also This process, called asexual reproduction, produces differ greatly. In the sections that follow, we will go over both identical replicas of the original cell. processes (as well as the interphase). Single-celled prokaryotes, like bacteria, reproduce asexually via binary fission, allowing them to divide quickly and multiply in as little as 10 to 20 minutes. A sea urchin begins life as a single cell that splits into two cells as shown above (a), which may be seen with a scanning electron Lesson 2: The Role of Cell Cycle microscope (SEM). After four rounds of cell division, there are 16 cells, as shown in an SEM image (b). The sea urchin eventually The Cell Cycle evolves into a complex, multicellular organism, as seen in (c) mature sea urchin. The division and development of a single cell into two new daughter cells is the ordered sequence of events known as Cells divide for several important reasons: the cell cycle. To create two identical clone cells, cells that are ready to Growth - All of the trillions of cells in your body originate divide go through carefully planned and regulated phases from a single cell formed after fertilization. Mitosis of growth, DNA replication, and division. The mitotic produces all these cells, enabling growth. phase and interphase are the two primary phases of the Repair - Cells wear out and need replacement. For cell cycle. instance, skin cells are continually shed and must be The nuclear DNA of the cell duplicates and expands during replaced to maintain healthy skin. interphase. The mitotic phase, which separates and distributes the duplicated chromosomes into daughter nuclei, comes next. Usually, when a cell splits, its cytoplasm does too, producing two daughter cells. M.D.ALORIA MODULE 3 1  BIOLOGICAL SCIENCE #856 - PSC 201 1ST SEM | 2024-2025 | PROF. RHEYNIEL A. ESCOBEL In animal cells, two centrioles that aid in organizing cell division are connected to the centrosomes. Plants and the majority of fungi lack centrioles in their centrosomes. Interphase Structure of Chromosomes There are two copies of each chromosome in every cell During interphase, the cell undergoes normal growth following DNA replication but before cell division. At a processes while also preparing for division. structure known as the centromere, each copy, often In order for a cell to move from interphase into the mitotic referred to as a sister chromatid, is physically attached to phase, many internal and external conditions must be its counterpart. met. The three stages of interphase are G₁, S, and G₂. While there are a few cells in the body that do not undergo cell division (such as red blood cells, most neurons, and some muscle cells), most somatic cells divide regularly. G1 Phase (First Gap) A somatic cell is a general term for a body cell, and all human cells, except for the cells that produce eggs and It is characterized by minimal apparent change under the sperm (which are referred to as germ cells), are somatic microscope. During the G₁ phase, the cell exhibits high cells. biochemical activity. (Cell growth is primary) S Phase (Synthesis of DNA) Sister chromatids or identical pairs of DNA molecules joined at the centromeric region, are the product of DNA replication in the S phase. Somatic cells contain two copies of each of their chromosomes (one copy received from each parent) for a total of 46 (23 pairs). Cells in the body replace themselves over the lifetime of a person. M.D.ALORIA MODULE 3 2  BIOLOGICAL SCIENCE #856 - PSC 201 1ST SEM | 2024-2025 | PROF. RHEYNIEL A. ESCOBEL DNA condenses and divides the chromosomal material a homologous pair of chromosomes, in which each equally between the two halves during karyokinesis, chromosome was inherited from a single parent, is ensuring that each daughter cell has an identical set of represented by the colors red and blue. chromosomes. A sister chromatid that is identical to each chromosome in the homologous pair is formed during DNA replication. This produces the well-known "X" shape, where a sister chromatid is represented by each arm of the "X." G2 Phase (Second Gap) In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. These series of phases are: Mitotic Phase Prophase: Chromatin condenses into visible chromosomes, and the mitotic spindle begins to form. The The mitotic phase is a multistep process during which the nuclear envelope starts to break down. duplicated chromosomes are aligned, separated, and Prometaphase: The nuclear envelope completely move into two new, identical daughter cells. disassembles, allowing spindle fibers to attach to the The first portion of the mitotic phase is called karyokinesis kinetochores of chromosomes. or nuclear division. Metaphase: Chromosomes align at the metaphase plate The second portion of the mitotic phase, called (the cell's equator), ensuring that each daughter cell will cytokinesis, is the physical separation of the cytoplasmic receive one copy of each chromosome. components intro the two daughter cells. Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell as the spindle fibers shorten. Karyokinesis Telophase: Chromosomes reach the opposite poles and begin to decondense back into chromatin. The nuclear The phase of cell division known as karyokinesis is when envelope re-forms around each set of chromosomes, the nucleus divides into two daughter nuclei. Usually, this resulting in two separate nuclei within the same cell. procedure comes before cytokinesis. Cytokinesis The term "cytoplasmic division" is cytokinesis. The cytoplasm divides after the nucleus divides in half, a process known as karyokinesis in cells. M.D.ALORIA MODULE 3 3 BIOLOGICAL SCIENCE #856 - PSC 201 1ST SEM | 2024-2025 | PROF. RHEYNIEL A. ESCOBEL All of the mother cell's cytoplasm and cell organelles are Lesson 3: Control of the Cell Cycle transferred to each daughter cell during cytokinesis. The effective completion of karyokinesis is a prerequisite Regulation of the Cell Cycle by External Events for this process. Cells move through a cyclical pattern in contrast to organisms, which have a linear life cycle from conception to death. Parent cells give rise to offspring cells, which can proliferate to form more cells. Although, in theory, cells might never die as long as they divide endlessly, in practice, cells only survive as long as the organism does. Before entering mitosis to divide, most cells spend time in interphase, a non-dividing stage. The division of cells is controlled by outside forces. Events like the death of a neighboring cell or the production of growth hormones, including human growth hormone (HGH), can cause initiation. While too much HGH can result in gigantism, too little While most eukaryotes go through comparable stages of HGH can produce dwarfism. Division can also be impeded mitosis, eukaryotes with cell walls, like plant cells, go by cell congestion. Furthermore, cells divide to address through a very different cytokinesis process. the problem of growing larger and less efficient due to A cleavage furrow is formed when the membrane is pulled their surface-to-volume ratio inward by this ring. Until the cell divides into two distinct daughter cells, the furrow gets deeper. Regulation at Internal Checkpoints Between daughter cells in plant cells, a new cell wall It is essential that daughter cells replicate exactly like the develops. A cell plate is formed when the Golgi apparatus parent cell. Chromosome duplication or distribution errors divides into vesicles that congregate in the middle of the can result in mutations that are transferred to offspring cell. cells. Enzymes use glucose to construct the new wall as this internal control systems function at three major plate develops until it fuses with the cell walls. On either checkpoints in the cell cycle—near the end of G₁, at the side of the new wall, the Golgi membranes merge into the G₂/M transition, and during metaphase—to prevent plasma membrane. damaged cells from proliferating. These checkpoints guarantee favorable conditions, like DNA repair before development. M.D.ALORIA MODULE 3 4  BIOLOGICAL SCIENCE #856 - PSC 201 1ST SEM | 2024-2025 | PROF. RHEYNIEL A. ESCOBEL Inhibitors generally inhibit cell division under unfavorable The G1 Checkpoint situations, but their action can be reduced, potentially encouraging malignancy. The G₁ checkpoint ensures all conditions are favorable for Conversely, excessive activity of positive regulators may cell division. It is a critical point where the cell commits to lead to cancer. Mutations in genes that encode cell cycle dividing. External factors like growth factors help the cell regulator proteins are frequently the cause of these pass this checkpoint. alterations. The cell must be of the right size, have enough energy reserves, and check for DNA damage before proceeding. Failure to achieve the conditions prevents a cell from entering the S phase. It can either go into the G₀ (inactive) phase and wait for better conditions, or it can interrupt the cycle to address the problem. The G2 Checkpoint If specific requirements are not fulfilled, the G2 checkpoint blocks entry into the mitotic phase. It evaluates protein stores, and cell size, and makes sure all chromosomes are properly and damage-free duplicated. without the addition of growth factors or signals from The cell cycle stops if issues are found so that DNA can be growth-stimulating proteins, cancer cells can proliferate in a dish. repaired or replication can be finished. Mitosis begins In contrast, normal cells need growth factors to proliferate in a when cyclin-dependent kinases (CDKs) initiate replication culture. correctly. Gene mutations that regulate cell division eventually cause cells to become malignant. The majority of cancer cells have at least 60 mutations, according to research from the Cancer Genome Project. Since many of the mutations in these cells do not contribute to the progression of cancer, it is difficult for researchers to determine which mutations cause certain tumors. Tumor suppressor genes, which are genes that inhibit cell division or trigger apoptosis, are rendered inactive by other mutations linked to cancer These genes function as brakes on cell division; uncontrolled division cannot happen unless a mutation occurs in both copies. The M Checkpoint In the final stages of metaphase in mitosis, the M checkpoint, also known as the spindle checkpoint, takes place. It ensures that all sister chromatids are appropriately anchored to spindle microtubules. Because the separation of sibling chromatids during anaphase is irreversible, the cell cycle will not begin until each pair's kinetochores are securely anchored to spindle fibers at opposing ends. Lesson 4: Cancer and the Cell Cycle Cell cycle regulators that do not operate properly can result in uncontrolled cell division, which is a characteristic of cancer. M.D.ALORIA MODULE 3 5  BIOLOGICAL SCIENCE #856 - PSC 201 1ST SEM | 2024-2025 | PROF. RHEYNIEL A. ESCOBEL For example, the p53 gene, which senses damage to DNA and Lesson 5: Prokaryotic Cell Division functions as a transcription factor for checkpoint control genes, is found in two mutant copies in many cancer cells. Prokaryotic Cell Division Atypical hyperplasia results from a cell dividing more than its neighbors due to a succession of mutations. This may The nucleoid area of prokaryotic cells houses a single eventually progress to cancer (carcinoma). circular DNA chromosome. Compared to eukaryotic cell division, their method of cell division, known as binary fission, is less complicated. The single chromosome allows for speedier DNA replication and eliminates the need for mitosis because If the malignant cells' ability to adhere to surrounding cells there isn't a nucleus or numerous chromosomes to declines and they can travel to other areas, they may coordinate. undergo metastasis, or the spread of cancer to other tissues and organs. Binary Fission In biology, binary fission is a type of asexual reproduction where a parent cell divides, resulting in two identical cells, each having the potential to grow to the size of the original cell. In the early stages, tumors are usually benign and stay within their tissue boundaries. As they progress to malignancy, they gain the ability to invade nearby tissues. The word asexual describes a reproduction that occurs without involving sex cells (gametes). Instead, the somatic cells undergo an asexual process that will produce a clone of the parent. The offspring is a clone because its genome will be identical to that of the parent. Binary fission is common among prokaryotes, e.g. archaea, eubacteria, cyanobacteria, and certain protozoans (e.g.amoeba). Furthermore, invasive cancer cells often release proteases to break down the extracellular matrix and create new pathways, allowing them to move into and invade neighboring tissues. M.D.ALORIA MODULE 3 6 BIOLOGICAL SCIENCE #856 - PSC 201 1ST SEM | 2024-2025 | PROF. RHEYNIEL A. ESCOBEL Some bacteria like E.coli can divide in as little as twenty minutes. Salmonella typhimurium, one of the species of bacteria that causes food poisoning, can divide in 40 minutes allowing for their population to grow rapidly. Most people infected with Salmonella show signs of the illness 12-72 hours after being exposed to the bacteria. M.D.ALORIA MODULE 3 7