Chapter 4 Cell cycle _ cellular reproduction 2.pdf

Fundamentals of Biology Chapter 4: Cell Cycle & Cellular Reproduction Learning objectives By the end of this section, you will be able to: 1. Distinguish different stages of the cell cycle and their characteristics. 2. Describe the different types of cell divisions (i.e. mitosi...

Fundamentals of Biology Chapter 4: Cell Cycle & Cellular Reproduction Learning objectives By the end of this section, you will be able to: 1. Distinguish different stages of the cell cycle and their characteristics. 2. Describe the different types of cell divisions (i.e. mitosis, meiosis, binary fission, etc…) and their characteristics. 3. Recognize the functions of cell divisions. 4. Justify how the cell cycle control system functions in cell division. 5. Illustrate the role of meiosis in genetic variation. Covered topics 4.1. The Cell Cycle. 4.2. Mitosis and Cytokinesis. 4.3. Prokaryotic Cell Division. 4.4. The Cell Cycle Control System & Cancer Cells. 4.5. Meiosis & Genetic Variation. 4.1. The Cell Cycle What is the role of cell division? The continuity of life is based upon the reproduction of cells or cell division. Unicellular organisms reproduce by cell division. Multicellular organisms depend on cell division for: ○ Development from a fertilized cell. ○ Growth. ○ Repair.  4.1. The Cell Cycle What is the cell cycle? ○ It is a set of stages that take place between two successive cell divisions. What is happen just before each cell division? ○ The cell grows larger. ○ The number of organelles doubles. ○ The DNA is replicated. Cell cycle consists of two major stages: 1. Interphase, and; 2. Mitotic phase. Gametes 4.1. The Cell Cycle 1. Interphase: Consists of three different stages: ○ G1, ○ S, and; ○ G2. 1. Mitotic phase: Consists of two stages: ○ Mitosis, and; ○ Cytokinesis. 4.1. The Cell Cycle: Interphase Interphase (about 90% of the cell cycle) can be divided into subphases. Interphase consists of three stages (G1, S, and G2): 1. G1 (first gap): ○ The cell increases in size. ○ Doubles its organelles (such as mitochondria and ribosomes). ○ Accumulates materials that will be used for DNA synthesis. 4.1. The Cell Cycle: Interphase 2. S (synthesis): ○ DNA synthesis or replication occurs. ○ Proteins associated with DNA are synthesized. 3. G2 (Second gap): ○ G2 is the stage from the completion of DNA replication to the onset of mitosis. ○ Cell synthesizes proteins necessary for division. 4.1. The Cell Cycle: Mitotic phase The mitotic phase alternates with the interphase in the cell cycle. Eukaryotic cell division consists of two stages: 1. Mitosis, the division of the nucleus. 2. Cytokinesis, the division of the cytoplasm. Mitotic stage  4.2. Mitosis and Cytokinesis: Chromosomes Eukaryotic Chromosome: ○ The DNA in the chromosomes is associated with various proteins, including histones. ○ When the eukaryotic cell is not undergoing division, the DNA is located within thread- like chromatin. ○ Before mitosis begins, chromatin becomes highly coiled and condensed, and it is easy to see the individual chromosome which is composed of two sister chromatids held together at a centromere. 4.2. Mitosis and Cytokinesis: Chromosomes Diploid chromosome number Eukaryotic Chromosome: Type of Name of Number of ○ Each species has a characteristic organism organism Chromosome chromosome number. ○ This is the full or diploid (2n) number of chromosomes that are found in all cells Animal Human 46 of the individual. Fruit fly 8 ○ Somatic cells have two sets or diploid (2n) number of chromosomes. Plant Potato 48 ○ Gametes (sperm and eggs) have one set Garden Pea 14 or haploid (n) number of chromosomes. Fungi Yeast 32 4.2. Mitosis and Cytokinesis: Chromosomes 4.2. Mitosis and Cytokinesis: Phases of Mitosis Mitosis consists of five phases: ○ Prophase. ○ Prometaphase. ○ Metaphase. ○ Anaphase. ○ Telophase. The cell before mitosis 4.2. Mitosis and Cytokinesis: Phases of Mitosis Prophase: ○ Chromatin has condensed and the chromosomes are visible. ○ The nucleolus starts to disappear. ○ The nuclear envelope disintegrates. ○ The spindle begins to assemble as the two centrosomes migrate away from one another. ○ In animal cells, microtubules in star-like arrays known as asters, radiate from each centrosome. 4.2. Mitosis and Cytokinesis: Phases of Mitosis Prometaphase: ○ Centromeres of each chromosome develop pair of Kinetochores on each side. ○ Spindle fibres attach to the kinetochores and begin to move chromosomes. Metaphase: ○ At metaphase, chromosomes line up at the metaphase plate (the center of the cell). ○ Spindle fibers attached to the sister chromatids come from opposite spindle poles. 4.2. Mitosis and Cytokinesis: Phases of Mitosis Anaphase: ○ Sister chromatids separate, become daughter chromosomes, and move toward the spindle poles. ○ Each pole receives the same number and kinds of chromosomes as the parent cell. 4.2. Mitosis and Cytokinesis: Phases of Mitosis Telophase: ○ The spindle fibres disappear. ○ The nuclear envelopes form around the daughter chromosomes. ○ Each daughter nucleus contains the same number and kinds of chromosomes as the original parent cell. ○ The chromosomes become more diffuse chromatin once again. ○ A nucleolus appears in each daughter nucleus. 4.2.Mitosis and Cytokinesis: Cytokinesis Cytokinesis is the division of the cytoplasm. When mitosis occurs, cytokinesis doesn’t occur, the result is a multinucleated cell. Cytokinesis differs for plant and animal cells. In animal cells: ○ Cytokinesis occurs by a process known as cleavage, forming a cleavage furrow. In plant cells: ○ a cell plate forms during cytokinesis between the daughter cells. 4.2.Mitosis and Cytokinesis: Functions of Mitosis Mitosis permits growth and repair. Activity No 4 Using drag and drop, rearrange the mitotic phases according to the normal sequence. Metaphase 1. ………………………….. Anaphase 2..………………………….. Prophase 3. ………………………….. Telophase 4. ………………………….. Prometaphase 5. ………………………….. 4.3. Prokaryotic Cell Division Cell division in unicellular organisms, such as prokaryotes, produces two new individuals. This is asexual reproduction. The type of this asexual reproduction is called Binary fission.  4.4. The Cell Cycle Control System & Cancer Cells The Cell Cycle Control System: ○ The cell cycle appears to be controlled by specific chemical signals present in the cytoplasm. ○ The different stages and activities of the cell cycle are directed by the cell cycle control system: ○ Is similar to a clock. ○ Is regulated by internal and external signals. A signal is a molecule that may stimulate or inhibits a metabolic event.  4.4. The Cell Cycle Control System & Cancer Cells Cell Cycle Checkpoints (FL-Cancer/02) This video lecture explains... Cell cycle control System Cell cycle checkpoints G1 checkpoint G2 checkpoint M checkpoint Video Lecture Links: The Cell Cycle - https://www.youtube.com/watch?v=AYr1ZHsmlH8  4.4. The Cell Cycle Control System & Cancer Cells Apoptosis: ○ Is programmed cell death, and it involves: Fragmenting of the nucleus, Blistering of the plasma membrane engulfing cell fragments. ○ Mitosis and apoptosis are opposing forces; Mitosis increases cell number, and Apoptosis decreases cell number. ○ Apoptosis is caused by enzymes called caspases.  4.4. The Cell Cycle Control System & Cancer Cells Loss of Cell Cycle Control in Cancer Cells: ○ Cancer cells do not respond normally to the body’s control mechanisms. As a result, they divide uncontrollably and form a mass of cells called a tumor. ○ Two types: Benign tumor (not cancerous). Malignant tumors (Cancerous). 4.5. Meiosis & Genetic Variation: Meiosis & Sexual Reproduction Meiosis: ○ Is the type of nuclear division that reduces the chromosome number from the diploid (2n) number to the haploid (n) number. ○ It takes place in the gametes of the organisms that reproduce by sexual reproduction. The diploid (2n) number refers to the total number of chromosomes. The haploid (n) number of chromosomes is half the diploid number. In humans, the diploid number of 46 (i.e. 2n = 46) is reduced to the haploid number of 23 (n = 23). 4.5. Meiosis & Genetic Variation: Meiosis & Sexual Reproduction At sexual maturity, the ovaries and testes produce haploid gametes by meiosis. Gametes (the reproductive cells); the sperm and egg each has only 23 chromosomes. During fertilization, the egg and sperm fuse, forming a zygote. A zygote always has the full or diploid (2n) number of chromosomes. The zygote then develops (by mitosis) into an adult organism. 4.5. Meiosis & Genetic Variation: Homologous Chromosomes Human somatic cells (all cells except a gamete) have 23 pairs of chromosomes The two chromosomes in each pair are called homologous chromosomes, or homologues. The chromosome homologous pair have the same length and carry genes controlling the same inherited characters. Each pair of homologous chromosomes includes one chromosome from each parent. The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father. 4.5. Meiosis & Genetic Variation: Sex Chromosomes The sex chromosomes are called X and Y. Human females have a homologous pair of X chromosomes (XX). Human males have one X and one Y chromosome. The 22 pairs of chromosomes that do not determine sex are called autosomes. 4.5. Meiosis & Genetic Variation: Stages of Meiosis Meiosis is also preceded by the replication of chromosomes. First, DNA synthesis occurs. All the chromosomes are duplicated, and each consists of two identical sister chromatids. 4.5. Meiosis & Genetic Variation: Stages of Meiosis Meiosis requires two nuclear divisions and produces four haploid daughter cells, each having half the total number of chromosomes. 4.5. Meiosis & Genetic Variation: Stages of Meiosis Meiosis takes place in two sets of cell divisions, Known as meiosis I and meiosis II. Meiosis I occurs in four phases: ○ Prophase I. ○ Metaphase I. ○ Anaphase I. ○ Telophase I and cytokinesis. Meiosis II also has four phases: ○ Prophase I. ○ Metaphase I. ○ Anaphase I. ○ Telophase I and cytokinesis. 4.5. Meiosis & Genetic Variation: Stages of Meiosis 4.5. Meiosis & Genetic Variation: Daughter Cells In the animal life cycle, the daughter cells become the gametes, either sperm or eggs. In the plant life cycle, the daughter cells become haploid spores that germinate to become a haploid generation. This generation produces the gametes by mitosis. 4.5. Meiosis & Genetic Variation: Genetic Variation In all eukaryotes, meiosis ensures the maintain both: ○ Genetic diversity, and; ○ Genetic integrity. Three mechanisms contribute to genetic variation: ○ Crossing over. ○ Independent assortment of chromosomes. ○ Random fertilization. 4.5. Meiosis & Genetic Variation: Crossing Over Crossing over: ○ Exchange of genetic material (i.e. homologous portions) between non-sister chromatids of a bivalent during meiosis. ○ Crossing over produces recombinant chromosomes, which combine genes inherited from each parent. ○ It starts very early in prophase I, as homologous chromosomes pair up gene by gene. ○ Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome. 4.5. Meiosis & Genetic Variation: Crossing Over Crossing Over Crossing Over 4.5. Meiosis & Genetic Variation: Independent assortment Independent assortment of chromosomes: ○ Homologous pairs of chromosomes orient randomly at metaphase I of meiosis 1. ○ Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs. ○ The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number. ○ For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes. 4.5. Meiosis & Genetic Variation: Independent assortment 4.5. Meiosis & Genetic Variation: Random Fertilization Random fertilization: ○ Random fertilization adds to genetic variation because the fusion of the gametes will produce a zygote with any of about 70 trillion diploid combinations. ○ Crossing over adds even more variation. ○ Each zygote has a unique genetic identity.  Mitosis Vs Meiosis Mitosis Meiosis 1 Requires only one nuclear division Requires two nuclear divisions 2 Produces two diploid daughter cells Produces four haploid daughter cells 3 The daughter cells are genetically Identical to The daughter cells are neither genetically identical to each each other and to the parent cell other or to the parent cell 4 Occurs in all tissues for growth and repair Occurs only in the reproductive organs and produces the gametes 5 Do not occur During prophase I, bivalents form and crossing-over occurs 6 During metaphase, individual chromosomes During metaphase I, bivalents independently align at the align at the metaphase plate metaphase plate 7 During anaphase, sister chromatids separate, During anaphase I, homologues of each bivalent separate becoming daughter chromosomes that move and duplicated chromosomes move to opposite poles to opposite poles Mitosis Vs Meiosis Assignment No 4 Compare between mitosis and meiosis.