BIOLOGY TRANS CHAPTER 2: “Cellular Reproduction” Lesson 1: THE CELL CYCLE Cell Cycle – cycle of growing and dividing of a cell MAIN STAGES OF CELL CYCLE: • INTERPHASE Longest phase in the cell cycle A 24-hr cycle • MITOSIS Spends about one hour only Division of the nucleus, after the completion of the interphase • CYTOKINESIS Results of the mitosis Cytoplasm from the original cell divides and forms two new cells THE DISCOVERY OF CYCLINS Early 1880’s biologists studied on sea urchin the protein in cells that are responsible for regulating the cell cycle. The protein was named CYCLIN. REGULATORY PROTEINS • Internal Regulators Proteins found inside the cell Regulate the processes that happen inside the cell Include the protein that make sure that mitosis does not occur unless the chromosomes have replicated Protein that prevents the cells from entering anaphase unless spindle fibers have already attached themselves to the chromosomes • External Regulators Proteins that respond to events outside the cell Example is growth factors that stimulate the growth and division of cells (GROWTH FACTORS – plays an important role in healing wounds and embryonic development) DISEASES ASSOCIATED WITH THE CELL CYCLE MECHANISM - - CARDIOVASCULAR DISEASES A diminishing loss of control in the cell cycle leads to the proliferation of cancer. Cancer cells are basically cells that just keep on dividing. Cancer cells infect other healthy living cells. METASTASIS, process by which cancer cells spread from where they originated. - another group of diseases associated with the cell cycle mechanism Leading causes of death in the Philippines DOH identifies 170 000 deaths Leading cause of morbidity and mortality in industrialized countries ASTHEROSCLEROSIS A chronic and progressive inflammatory condition in which an artery wall thickens as a result of the accumulation of white blood cells (WBC’s/Leukocytes) and the proliferation of vascular smooth muscle cells, leading to vascular diseases. VASCULAR DISEASES include MYOCARDIAL AND STROKE MYOCARDIAL INFARCTION (MI) also known as a heart attack, occurs when blood flow decreases or stops to a part of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may travel into the shoulder, arm, back, neck or jaw. STROKE a sudden interruption in the blood supply of the brain. Most strokes are caused by an abrupt blockage of arteries leading to the brain (ischemic stroke). Other strokes are caused by bleeding into brain tissue when a blood vessel bursts (hemorrhagic stroke). APOPTOSIS The programmed death of cells in the body Cells undergo a series of programmed steps: shrinkage of the chromatin and the breakage of parts of the cell membrane, after which the cells are eventually destroyed Plays an important role in shaping plants and animal structure Lesson 2: THE STAGES OF THE CELL CYCLE CELL CYCLE – period of growth, maturation, and reproduction of the cell 1 THREE MAJOR STAGES: • INTERPHASE 1. 2. 3. - Longest phase in the cell cycle Cell prepares for cell division Cell performs its regular functions, taking in nutrients and growing GAP 1 (G1) STAGE – primary growth phase First stage of interphase that comes before synthesis of DNA The cell increases in mass and/or size and organelles increase in number in preparation for cell division The longest stage of interphase Length of the G1 stage is responsible for the difference between fast dividing cells and slowly dividing cells Can be slowed by reducing the nutrients available in the cell, thus, the cell will take longer time to build up the resources needed for cell division Great depletion of nutrients, the cells can virtually stop growing SYNTHESIS (S) STAGE – replication of DNA A very crucial part of interphase Part when DNA is synthesized Begins with the replication of the DNA After replication the cell has twice as many chromosomes as before GAP 2 (G2) STAGE – secondary growth phase After DNA synthetization Cell synthesizes proteins and histones and continues to increase in size Proteins necessary for cell division such as kinase and histones, are synthesized including the microtubules that will comprise to spindle fibers Chromosomes starts to condense and become visible CELL DIVISION Two Phases: • NUCLEAR DIVISION Genetic material in the nucleus is divided 2 kinds of nuclear division = MITOSIS – the nucleus is divided to two new genetically identical daughter cells = MEIOSIS – a reduction division, producing daughter cells that contain half the genetic information of the parent cells • CYTOKINESIS Cytoplasm is divided • MITOSIS Initiated after the completion of interphase Division of nucleus or Karyokinesis Important in growth and repair Produces diploids, two new daughter cells Period of continuous activity and necessary for all eukaryotic organisms Means asexual reproduction in some organisms like protozoans and algae Takes a short period of time FOUR MAIN PHASES: 1. PROPHASE Chromatin condenses into more discrete chromosomes Nuclear envelope starts to disappear/disintegrate and spindles forms at opposite poles of the cells Nucleolus disappears Chromatin fibers become coiled with each chromosome having two chromatids joined at a centromere Centrosome divides into two, each attached with a pair of centrioles Centrioles then move to opposite ends to form mitotic spindle In animal cells, the mitotic spindle initially appears as ASTERS 2. METAPHASE Spindle fibers are fully developed and chromosomes align at equatorial plate 2 - • Chromosome are attached through the centromeres of every two chromatids or one chromosome 3. ANAPHASE Separation of the chromosomes Centromeres in each distinct chromosome are pulled and moved apart Paired sister chromatids are separated and formed a new full chromosome Spindle apparatus move the daughter chromosomes toward the poles or centrioles 4. TELOPHASE Complete sets of chromosomes during this stage are formed New distinct nuclei of the daughter cells have developed nuclear envelope reappears nucleolus reappears chromatin fibers of chromosomes uncoil CYTOKINESIS follows right after the process of mitosis and completes the full stage of the cell cycle occurs when the cytoplasm from the original cell divides and forms two new cells separated the organelles and other cytoplasmic inclusions such as ribosomes, ER, Golgi bodies and Mitochondria ANIMAL CELLS, cell membrane forms a CLEAVAGE FURROW, pulled inward by a tiny filament, where gradually, the membrane pinches inward and forms separate cells around each nucleus PLANT CELLS, cannot pinch due to the presence of cell wall, instead CELL PLATE forms between two nuclei, cell plate is produced by the Golgi Body which supplies the new plasma membrane • o MEIOSIS Process of cell division that takes place in sexually mature organisms Process that results in the reduction of the chromosome number from diploid to haploid in germ cells (sperm and egg) ▪ SOMATIC CELLS – body cells and contain the normal number of chromosomes called DIPLOID (2n); examples are: SKIN CELLS, MUSCLE CELLS, BONE CELLS, BRAIN CELLS ▪ GAMETES – sex cells and contain only ½ the normal number of chromosomes called HAPLOID (n); examples are: SPERM CELLS, EGG CELLS/OVUM MEIOSIS 1 PROPHASE 1 ▪ Early Prophase 1, Homologs pair and crossing over occurs ▪ Late Prophase 1, chromosomes condense, spindle fiber forms, and nuclear envelope fragments ▪ LEPTOTENE / LEPTONEMA – condensation stage, where initial condensation of chromatin material to form chromosome ▪ ZYGOTENE / ZYGONEMA – tetrad formation, homologous chromosomes are joint to form tetrad called SYNAPSIS ▪ PACHYTENE / PACHYNEMA – crossing over stage, homologous chromosomes in a tetrad cross over each other and pieces of chromosomes or genes are exchanged and produces genetic recombination in the offspring; creates variation or diversity in the offspring’s traits ▪ DIPLOTENE – terminalization of chiasmata and the late stage of prophase in meiosis in which the paired homologous chromosmes begin to repel each other and move apart, but are usually held together by CHIASMATA ▪ DIAKINESIS – final stage of the prophase in meiosis and characterized by shortening and thickening of the paired chromosomes and the formation of spindle fibers and the disappearance of the nucleolus and degeneration of the nuclear membrane - METAPHASE 1 3 - - ▪ Homologous pairs of chromosomes align along the equator of the cell ▪ Kinetochore of the chromosome will attach to facilitate movement ANAPHASE 1 ▪ Homologous pairs of chromosomes separate, migrate and move to opposite poles ▪ Sister chromatids are still attached at their centromere TELOPHASE 1 ▪ Two daughter cells are completely divided ▪ Nuclear envelope reassembles ▪ Spindle fibers disappears ▪ Cytokinesis divides cell into two (CLEAVAGE FURROW / CELL PLATE) INTERKINESIS 1 ▪ A short pause between meiosis 1 and meiosis II ▪ Prepares the cell for the 2nd meiotic division ▪ Similar to interphase of mitosis except there is no replication of the chromosomes ▪ NO MORE DNA REPLICATION ▪ ▪ ▪ ▪ Nuclear envelope assembles Chromosomes decondense Spindle fibers disappears Cytokinesis divides the cell into two (CLEAVAGE FURROW / CELL PLATE) COMPARISONS BETWEEN MITOSIS AND MEIOSIS MITOSIS o MEIOSIS II PROPHASE II ▪ Nuclear envelope and nucleoli disappear and chromatids start to shorten and thicken ▪ Centrioles move to opposite poles and spindle fibers occurs METAPHASE II ▪ Chromosomes align along the equator of the cell ▪ Each chromosome composed of two sister chromatids that are joined by a centromere and attached to the kinetochore from the spindle fiber ANAPHASE II ▪ Sister chromatids separate and move to opposite poles ▪ Centromeres joining the chromatids divide, allowing the microtubules attached to the kinetochores to pull the sister chromatids apart ▪ Sister chromatids and now called SISTER CHROMOSOMES as they are moved toward the opposite poles TELOPHASE II MEIOSIS • Produces genetically identical cell • Produces genetically unique cells • Results in diploid chromosome number • Results in haploid chromosome number • Results in diploid cells • Results in haploid cells • Produces two new cells • Produces four new cells • Takes place throughout the organisms life time • Takes place only during the reproductive years of the organisms • Involved in asexual reproduction • Involved in sexual reproduction COMPARISONS BETWEEN MITOSIS AND MEIOSIS MITOSIS MEIOSIS Number of Division 1 2 Number of Daughter Cells 2 4 YES NO Genetically Identical? 4 Chromosome Number Same as PARENT Half of PARENT Where SOMATIC CELLS GERM CELLS When Throughout life Sexual maturity GROWTH & REPAIR SEXUAL REPRODUCTION Role • • SPERMATOGENESIS Process of sperm formation Begins forming at the age of puberty SPERMATOGONIA, numerous thin and tightly coiled seminiferous tubules with immature cells found in the testes SERTOLI CELLS, abound along the walls of the tubules help nourish the immature germ cells by providing them nutrients and blood products Immature cells develop into primary spermatocyte, secondary spermatocyte, spermatids, and sperm cells and are transported closer to the lumen, then to the seminiferous tubules, and into the epididymis OOGENESIS Process of egg formation oogenesis takes place at pubertal age OOGONIA, primordial cells inside the ovaries These matures and develop into primary oocyte, secondary oocyte, ootids, and ovum or egg cell NOTE: In both MALE and FEMALE, 22 AUTOSOMAL CHROMOSOMES and 1 pair of SEX CHROMOSOMES, the X and Y chromosomes. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome. CELL CYCLE CHECKPOINTS: o G1 CHECKPOINT Checks for the size of the cell and integrity of their DNA o G2 CHECKPOINT Checkpoint prior to mitosis; it checks for the integrity of DNA o M CHECKPOINT Occurs during mitosis found during the metaphase stage; checks proper alignment of the chromosomes SCITRIVIA NOTE: GAMETOGENESIS Process where diploid cells undergo cell division and differentiation through meiosis to form mature haploid gametes or sex cells Usually take place in the reproductive organs in the body Primordial or undifferentiated germ cells found in the reproductive organs undergo growth and development to eventually become sex cells. COLCHICINE A medicine used for treating gout. It is originally extracted from plants of the genus COLCHICUM, one of which is the autumn crocus. Colchicine is a mitosis-inhibiting drug. Used to view chromosomes only on metaphase stage. Colchicine is used to arrest mitosis during the metaphase stage. 5 6
