Cell Cycle and Cell Division.pdf

EUKARYOTIC CELL CYCLE This presentation was created and is owned by Jill Hanah C. Palafox, RN, LPT  What is Cell Cycle? It refers to the series of events that take place in a cell leading to its maturity and subsequent division. These events include duplication...

EUKARYOTIC CELL CYCLE This presentation was created and is owned by Jill Hanah C. Palafox, RN, LPT  What is Cell Cycle? It refers to the series of events that take place in a cell leading to its maturity and subsequent division. These events include duplication of its genome and synthesis of the cell organelles followed by division of the cytoplasm. Interphase is the period of the cell cycle during which the cell may either be living and not dividing or in which it is preparing itself to divide. Most of the cells in a fully-developed multicellular organisms are typically found in interphase. Mitosis is the the point in the cell cycle associated with division or distribution of replicated genetic material to two daughter cells. During mitosis the cell nucleus breaks down and two new, fully functional, nuclei are formed. Cytokinesis is the process that divides the cytoplasm into two distinctive cells.  Phases in Interphase G1 Phase or first gap During the G1 stage: the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins the cell is accumulating enough energy reserves to complete the task of replicating each chromosome in the nucleus. involves cell growth and protein synthesis An important in the G1 phase is the G1/S checkpoint that determines if the cell is ready enough to proceed into the division phase. At this point, events like the detection of DNA damage and nutrient concentration are performed to ensure that the cell has enough machinery to undergo cell division. S Phase or synthesis phase During the S phase: DNA replication results in the formation of two identical copies of each chromosome—sister chromatids—that are firmly attached at the centromere region. centrosomes are duplicated. Centrosomes consists of a pair of rod-like centrioles composed of tubulin and other proteins that sit at right angles to one another other. The two resulting centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes later during mitosis DNA Replication in S phase When a cell divides, it is important that each daughter cell receives an identical copy of the DNA. This is accomplished by the process of DNA replication. Adenine nucleotides pair with Thymine nucleotides, and Cytosine with Guanine. This means that the two strands are complementary to each other. For example, a strand of DNA with a nucleotide sequence of CACGACTT will have a complementary strand with the sequence GTGCTGAA G2 Phase or second gap During the G2 phase: the cell replenishes its energy stores and synthesizes the proteins necessary for chromosome manipulation. Some cell organelles are duplicated the cytoskeleton is dismantled to provide resources for the mitotic spindle. the RNA, proteins, other macromolecules required for multiplication of cell organelles, spindle formation, and cell growth are produced as the cell prepares to go into the mitotic phase. G0 Phase or Resting phase Cells in G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G0 temporarily until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently. How long does the cell cycle take? Different cells take different lengths of time to complete the cell cycle. A typical human cell might take about 24 hours to divide, but fast- cycling mammalian cells, like the ones that line the intestine, can complete a cycle every 9-10 hours when they're grown in culture. Different types of cells also split their time between cell cycle phases in different ways. In early frog embryos, for example, cells spend almost no time in G1 and G2 and instead rapidly cycle between S and M phases—resulting in the division of one big cell, the zygote, into many smaller cells. Regulation at Internal Checkpoints To prevent a compromised cell from continuing to divide, there are internal control mechanisms that operate at three main cell cycle checkpoints at which the cell cycle can be stopped until conditions are favorable. These checkpoints occur near the end of G1, at the G2–M transition, and during metaphase.  Let’s have an interactive recap of all topics discussed. Instructions: 1. Write each phase of Interphase (G1, S, G2, and G0) on separate pieces of paper. 2. You will be given 20 seconds to read and analyze the question that will be displayed on the screen. 3. After analyzing the question, show your answer by raising the paper that corresponds to the correct phase.  During which phase of the cell cycle does the cell grow larger? correct answer: G1 phase  The phase of the cell cycle where important events like DNA replication and formation of histone proteins take place. correct answer: S phase  At this point, events like the detection of DNA damage and nutrient concentration are performed to ensure that the cell has enough machinery to undergo cell division. correct answer: G1 phase  At this phase, the cell replenishes its energy stores and synthesizes the proteins necessary for chromosome manipulation. correct answer: G2 phase  During this phase, the content of DNA doubles in the cell, but the number of chromosomes remain the same as the division of chromosome doesn’t take place just yet. correct answer: S phase  This phase is important as it checks for DNA damage (during replication) to ensure that the cell is in proper condition to undergo division. correct answer: G2 phase  In this phase, the cell cycle machinery of the cell is dismantled, and the cell continues to remain quiescent until there is a reason for the cell to divide. correct answer: G0 phase DNA repair is a crucial step in the this phase as it repairs breaks or damages that might be present in the DNA strand after replication. correct answer: G2 phase MITOSIS or M phase The term “mitosis” was coined by Walther Flemming in 1882 while documenting the process of chromosomal division in salamander larvae. The term comes from the Greek word ‘mitos’ meaning ‘thread,’ referring to the thread-like appearance of chromosomes. It is a type of cell division (cell cycle) through which the cell (parent cell) produces two identical daughter cells. It is the process of dividing the nucleus-Karyokinesis  Functions and Importance of Mitosis 1. Growth and Development: Multicellular organisms require mitosis for growth from a fertilized egg into a fully developed organism. Repeated rounds of mitosis give rise to the vast number of cells that make up the tissues and organs of a body.  Functions and Importance of Mitosis 2. Tissue Repair and Regeneration: Mitosis replaces the lost or damaged cells when tissues are damaged due to injury or wear and tear. This helps in healing wounds and regenerating tissues. For example, the human liver has a remarkable capacity to regenerate through mitotic cell division.  Functions and Importance of Mitosis 3. Asexual Reproduction: In some organisms, mitosis is a form of asexual reproduction called vegetative reproduction. Single-celled organisms, such as protozoa and yeasts, as well as some multicellular organisms like hydras and plants, reproduce asexually through mitosis. Here, mitosis creates clones of the original organism.  Developmental Plasticity and Cell Differentiation: Mitosis allows a single fertilized egg to become a complex organism with diverse cell types. As cells divide, they differentiate into various cell types with specialized functions. While the regulation of gene expression controls this process, mitotic cell division initiates it. Immune System Function: Mitosis is essential for the proliferation of lymphocytes, which are white blood cells that play a critical role in the immune response. When activated by antigens, lymphocytes rapidly divide by mitosis to build up a force capable of fighting infection.  Maintenance of Chromosome Number: Mitosis ensures that each daughter cell receives an exact copy of the parent cell’s genetic material. This is crucial for maintaining the species-specific chromosome number in all body cells, which is important for normal functioning. Genetic Consistency: By precisely duplicating the genetic material and segregating it equally into two daughter cells, mitosis ensures genetic consistency. This means that all body cells of an organism (except for the gametes, which form via meiosis) contain the same DNA.  Cancer Prevention: Normally, mitosis is a highly regulated process. However, when these regulatory mechanisms fail, it leads to uncontrolled cell division and cancer. Understanding mitosis is crucial for developing treatments and prevention strategies for cancer. Mitosis in plant cell: Onion Root tip  Mitotic phases: PPMAT Prophase (P) During prophase: the chromatin condenses into visible chromosomes. Since DNA replicated in interphase, each chromosome consists of two sister chromatids joined at the centromere. The nucleolus fades and the nuclear envelope begins to disintegrate. Outside the nucleus, the mitotic spindle, comprised of microtubules and other proteins, starts forming between the two centrosomes. The centrosomes begin moving toward opposite poles of the cell.  Mitotic phases: PPMAT Prometaphase (P) During prometaphase: the nuclear envelope completely breaks down and the spindle microtubules interact with the chromosomes. The kinetochores, protein structures on the chromatids at the centromeres, become attachment points for the spindle microtubules. This is crucial for chromosome movement. The microtubules begin moving the chromosomes toward the center of the cell, an area known as the metaphase plate.  Mitotic phases: PPMAT Metaphase (M) During metaphase: The hallmark is the alignment of chromosomes along the metaphase plate. Each sister chromatid attaches to the spindle fibers coming from opposite poles. The kinetochores are under tension, which is a signal of proper bipolar attachment. This alignment ensures that each new cell receives one copy of each chromosome.  Mitotic phases: PPMAT Anaphase (A) During anaphase: the proteins holding the sister chromatids together break apart, allowing them to separate. The microtubules attached to kinetochores shorten and the cell elongates due to the pushing forces exerted by overlapping non-kinetochore microtubules. The sister chromatids are now individual chromosomes that are pulled toward opposite poles of the cell.  Mitotic phases: PPMAT Telophase (T) During telophase: is the reversal of prophase and prometaphase events. The chromosomes arrive at the poles and begin decondensing back into chromatin. Nuclear envelopes re-form around each set of chromatids, resulting in two separate nuclei within the cell. The spindle apparatus disassembles and the nucleolus reappears within each nucleus.  Cytokinesis Cytokinesis follows telophase. the division of the cytoplasm It is often considered a separate process from mitosis. In cytokinesis, the cytoplasm divides and forms two daughter cells, each with one nucleus. For animal cells, this involves a contractile ring that pinches the cell in two forming a cleavage furrow. In plant cells, a cell plate forms along the line of the metaphase plate, eventually leading to the formation of two separate cell walls. Animal vs Plant Cell Mitosis Mitosis in plant and animal cells follows the same fundamental process, but with some differences that stem from their unique cellular structures. Here are the key distinctions: Centrosomes and Spindle Formation: In animal cells, centrosomes containing a pair of centrioles are the organizing centers for microtubules and thus spindle formation. The centrosomes migrate to opposite poles of the cell during prophase. Plant cells lack centrioles. Instead, spindle microtubules form around nucleating sites in the cytoplasm called microtubule organizing centers (MTOCs). Cytokinesis: Animal cells undergo cytokinesis through the formation of a cleavage furrow. Actin and myosin microfilaments constrict the middle of the cell, pinching it into two daughter cells. Plant cells are surrounded by a rigid cell wall, so they cannot be pinched. Instead, they form a cell plate during cytokinesis. Vesicles from the Golgi apparatus coalesce at the cell’s equator, forming a new cell wall that expands outward until it fuses with the existing cell wall. Presence of Cell Wall: The rigid cell wall in plant cells restricts the movement of the cell during mitosis. For example, plant cells do not form asters (star-shaped microtubule structures) as seen in animal cells. Animal cells change shape during mitosis, which aids in the division process. Structural Support: Animal cells utilize centrosomes and astral microtubules for spatial orientation during mitosis. Plant cells rely more on the spatial structure provided by the cell wall and vacuoles for the organization of their mitotic spindle. Formation of Mitotic Structures: In animal cells, the mitotic spindle forms from the centrosomes and extends across the cell to organize and separate the chromosomes. In plant cells, the spindle forms without centrosomes and establishes a bipolar structure without the aid of astral microtubules. Despite these differences, the end goal of mitosis in both plant and animal cells is the same: to produce two genetically identical daughter cells from a single parent cell. The variations in the process are adaptations to the structural and material constraints inherent in the different types of cells. Does Mitosis Occur in Prokaryotes? Mitosis does not occur in prokaryotes. Prokaryotic organisms, such as bacteria and archaea, have a simpler cell structure without a nucleus and lack the complex chromosome structures found in eukaryotes. Instead of mitosis, prokaryotes undergo a different process called binary fission to replicate and divide.  Please complete the remaining activities in your Assignment Mitosis worksheets. Answers will be checked during the next meeting.