Bio_Cell_Cycle_1-3.pdf
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Cell Cycle The time it takes to complete one cell cycle is the generation time. It is the series of events that take place in a cell leading to its division and duplication. It is the...
Cell Cycle The time it takes to complete one cell cycle is the generation time. It is the series of events that take place in a cell leading to its division and duplication. It is the Further Information process by which a cell grows, replicates its DNA, and divides into 2 daughter cells. It is the Cells divide when they reach a certain sequence of cell growth and division. size. o Nerve, skeletal, muscle, and red Why animals grow to be big. blood cells do not divide. Why animals shed their skin. Cell division involves mitosis or (Molting/Ecdysis) meiosis, and cytokinesis. Mitosis or Meiosis involves the division Stages of the Cell Cycle of chromosomes. Cytokinesis refers to the division of the cytoplasm. Karyokinesis refers tot the division of the nucleus. Mitosis without cytokinesis results in multinucleate cells. Cell Division All cells are derived from pre-existing cells. New cells are produced for growth and to replace damaged or old cells. Cell Division is different for prokaryotes (bacteria) who undergo binary fission, and eukaryotes (Protist, Fungi, Plants, Animals) who undergo the meiosis and/or mitosis. It is necessary for reproduction of unicellular or multicellular organisms. Prokaryotes Lack a nucleus. A single circular chromosome. 1. Interphase (G1, S, G2) Divide via Binary Fission. 2. Mitosis (PMAT – Prophase, Metaphase, Anaphase, Telophase) Eukaryotes 3. Cytokinesis Must divide their nucleus and other Why do Cells Divide? organelles in preparation for cell division. They divide for growth, repair, and Before nuclear division, genetic reproduction. material replicates. Other Definitions of Cell Cycle Mitosis The period from the beginning of one Mitotic division results in genetically identical division, to the beginning of the next. eukaryotic cells (a clone). Mitosis is the basis of asexual reproduction. 2. Centromere – joins the chromatids together. The point where sister It is simply the division of somatic (body) cells. chromatids are joined together. Meiosis 3. P arm – otherwise known as the short arm; upwards. It results in the halving of the chromosome 4. Q arm – otherwise known as the long number in preparation for fertilization. arm; downward. 5. Telomere – tips of the chromosome. Meiosis shuffles genes in new combinations. Karyotype Meiosis results in genetically different cells. Picture of an individual’s Meiosis and fertilization are the basis of sexual chromosomes. reproduction. o Identify sex and It is simply the division of gametes (sex cells). chromosome defects. Final pair identifies sex. Terminologies o Same Size: XX = Female o Different Size: XY = Male Chromatin – thin fibrous form of DNA and proteins A normal human will have 46 chromosomes Sister Chromatids – identical structures that result from Chromosome Segregation in Prokaryotes chromosome replication, formed during the S phase. Reproduce by a type of cell division Chromosomes – tightly coiled called binary fission. strands of DNA. The bacterial chromosome replicate. Diploid (somatic) cells contain the The two daughter chromosomes entire set of chromosomes. actively move apart. Haploid (gametes) cells contain only half of the total number of chromosomes. Cell Cycle – sequence of phases in the life cycle of the cell. Anatomy of a Chromosome Two Parts of the Cell Cycle 1. Growth and Preparation (Interphase) Occurs between divisions. Longest part of the cycle 3 Stages (G1, S, G2) o G1 (Gap 1) Phase – cell grows in size, and synthesizes proteins and organelles. The cell is recovering from mitosis. o S (Synthesis) Phase – 1. Chromatids – two identical parts of a DNA Replication occurs. chromosome. o G2 (Gap 2) Phase – the cell continues to grow and prepare for mitosis. The Cell Cycle Control System (Internal) Organelles are replicated and more The sequential events of the cell cycle are growth occurs. directed by a distinct cycle control system o Interphase before similar to a clock. Mitosis – DNA is The clock has specific checkpoints where the replicated along with cell cycle stops until a go-ahead signal is other organelles and received. cellular components. The cell prepares for Two types of regulatory proteins involved: division. Cyclins 2. Cell Division o a family of proteins that fluctuate in concentration throughout the a. Mitosis (Nuclear Division) i. Prophase – chromatin cell cycle. There are different types of cyclins, each associated condenses into with a specific phase of the cell chromosomes, nuclear cycle. The levels of cyclins rise envelope breaks down, and fall at specific times, helping and mitotic spindle begins to control the progression of the to form. cell cycle. ii. Metaphase - chromosomes align at the Cyclin-dependent kinases (CDK) cell’s equator. (Equatorial o a group of enzymes that are Plate) activated only when they are iii. Anaphase – sister bound to a specific cyclin. The chromatids separate and activation of CDKs is essential for move to opposite poles of the phosphorylation of target the cell. proteins, which triggers various iv. Telophase – nuclear cellular processes involved in the envelope forms around the cell cycle. separated chromosomes The activity of cyclins and CDKs fluctuate and the chromosomes during the cell cycle. begin to decondense. b. Cytokinesis (Cytoplasm External Control: In density-dependent Division) – the final stage where inhibition the cytoplasm divides, resulting Crowded cells stop dividing. in 2 separate daughter cells. Most animal cells exhibit anchorage Mitotic Phase dependence, in which they must be attached This includes the division of the cell nucleus to a substratum to divide. (karyokinesis) and the division of the Loss of Cell Cycle Controls in Cancer Cells cytoplasm (cytokinesis). It can be observed through the light microscope. Cancer Cells – do not respond normally to the body’s control mechanisms. They form 1. Prophase tumors, and exhibit neither density- 2. Metaphase dependent inhibition nor anchorage 3. Anaphase dependence. 4. Telophase Malignant tumors – invade surrounding tissues and can metastasize, or they export cancer cells to other parts of the body where 2. Metaphase they may form secondary tumors. a. Chromosomes lineup along the equatorial plate Mitosis (Growth and Repair) (metaphase plate) It is a type of cell division in eukaryotic cells b. Movement of chromsomes in where a parent cell divides to produce two, the equatorial plate genetically identical daughter cells. (metaphase plate or metaphase plane) This process is crucial for growth, c. Spindle fibers attach to the development, and repair in multicellular centromeres of each organisms. chromosome. d. Shortest phase in Mitosis. Examples: Imagine you have a skin cut. The cells around the wound need to divide rapidly to heal the injury. This utilizes mitosis. 4 Stages of Mitosis These stages are often preceded by interphase (part of the cell cycle, but not mitosis itself), and followed by cytokinesis. 1. Prophase and Prometaphase a. Chromosomes condense into visible chromosomes. b. Microtubules form c. Nuclear envelope begins to break down. d. Centrosomes move to opposite poles of the cell. e. Spindle fibers begin to form. 3. Anaphase a. Centromeres divide. b. Spindle fibers pull one set of chromosomes to each pole c. Precise alignment is critical to division. d. Formation of Cleavage – sister chromatids separate and move towards opposite poles. The cell elongates as the spindle fibers pull the chromatids apart. bacterial chromosomes replicate and the 2 daughter chromosomes actively move apart. 4. Telophase a. Nuclear envelope forms around the chromosomes. (Reformation of nuclear envelop around the chromosome at each pole) b. Chromosomes uncoil, spindle fibers breakdown and dissolve. c. Cytokinesis i. Animals – cytokinesis occurs by a process Cytokinesis known as cleavage, The actual splitting of the daughter cells into 2 forming of cleavage separate cells. furrow. (With the help of contractile ring, or Centriole actin ring.) A barrel-shaped structure in most animal cells ii. Plants – during involved in the organization of the mitotic cytokinesis, a cell plate (source) spindle, and in the completion of forms. (Golgi derived cytokinesis. vesicles) A disc-like Centromere structure in the plane of the equator of the It is the region where 2 sister chromatids spindle that separates meet. two sets of chromosomes. Kinetocore The protein structure on chromosomes where the spindle fibers attach during cell division. iii. Bacteria – reproduce via binary fission: Illustrations of Mitosis Human Somatic Cells (46n) Human Gametes (23 N) Stages of Meiosis 1. Interphase – chromosome replication 2. Meiosis 1 – reductional phase a. Prophase I, Metaphase I, Anaphase I, Telophase I 3. Meiosis 2 – equational phase a. Prophase II, Metaphase II, Anaphase II, Telophase II Why do we need Meiosis? It is the fundamental basis for sexual reproduction. Tracing the Process 2 Haploid (1n) gametes are brought gametes are brought together through fertilization to A cell starts with one nucleus containing DNA form a diploid (2n) zygote. → The DNA condenses and becomes visible as chromosomes → These chromosomes line up Meiosis 1: Reductional Phase in the middle → They're pulled apart to opposite sides → Two new nuclei form → The cell splits into two daughter cells. Meiosis It reduces the number of chromosomes by half. Prophase 1 Fertilization restores the diploid state of a cell. Meiosis is a specialized type of cell division that produces gametes (sex cells) in sexually reproducing organisms. It results in 4 genetically diverse haploid daughter cells, each containing half the number of chromosomes as the parent cell. This process is crucial for maintaining the correct chromosome number of offspring and generating genetic diversity through Chromosomes condense and recombination. become visible, and migrate towards the nuclear envelope. It is called reduction-division. The original cel o Each chromosome is made up is diploid (2n), and four daughter cells of 2 identical chromatids, produced are monoploid (1n). known as sister chromatids. The chromosomes undergo It produces gametes (eggs and sperm), and thickening and move away from each occurs in the testes in males, in the ovaries for other. females. Homologous chromosomes pair up o synapsis – pairing of homologous chromosomes, cross-over between non-sister chromatids o Chiasma – point of location where cross over occurs. o Tetrads form in Prophase I Anaphase 1 Homologous chromosomes separate and move to opposite poles. Centromeres remain intact. Crossing over occurs, exchanging genetic material. The chromosomes migrate to opposite poles. The homologous chromosomes are separated. The sister chromatids are not separated. o Homologous chromosomes Telophase 1 during crossing over – multiplies the already huge Chromosomes reach the poles. number of different gamete Nuclear envelopes may reform. types produced by Cytokinesis occurs, forming 2 independent assortment. haploid cells. Nuclear envelope and nucleoli break down. Spindle fibers begin to form. Metaphase 1 Homologous (pairs of chromosomes that are similar in size, shape, and genetic content. They are inherited, one from each parent.) chromosome pairs align at the Chromosomes continue to migrate equator. towards the pole. Spindle Fibers attach to the centromeres. Both poles have haploid number Homologous chromosomes lie on the (reductional phase) but still double in equatorial plate. Human eggs are arrested the amount of DNA. during this phase, the egg will proceed to Condensation of chromosomes and the next stage if it is fertilized. cytokinesis takes place. Nuclear envelope begins forming. Metaphase II Outcome: 2 daughter cells (n) Meiosis 2: Reducing Chromosome Number In human males, meiosis occurs continuously in the testes to produce sperm. A diploid spermatogonium (46 chromosomes) undergoes meiosis to produce 4 haploid spermatids (23 chromosomes each), which matures into sperm cells. Chromosomes align at the equator In human females, meiosis begins in fetal Spindle fibers attach to individual ovaries but pauses until puberty. At ovulation, chromatids. meiosis I completes in one oocyte, producing o Human eggs are arrested a secondary oocyte and a polar body. during this phase and will Meiosis II only completes if fertilization proceed when fertilized. occurs, resulting in the ovum and another If fertilization does not occur, the egg polar body. remains static, but eventually degrades during a certain period of The process ensures genetic diversity in time. offspring, and is a fundamental aspect of sexual reproduction. Anaphase II Sister chromatids separate and The same process in mitosis (equational move to opposite poles. phase) but results in 4 haploid cells. Telophase II Prophase II Chromatids reach the poles, nuclear envelopes reform, cytokinesis occurs. Chromosomes condense again Spindle fibers form Results of Meiosis Comparison of Cell Divisions Cell Cycle Regulation For all living eukaryotic organisms, it is essential that the different phases of the cell cycle are precisely coordinated. Errors in the coordination may lead to chromosomal alterations. Chromosomes or parts of chromosomes may be lost, rearranged, or distributed unequally between the 2 daughter cells. Checkpoints Much of the control of the progression through the phases of a cell cycle are exerted at checkpoints. There are many such checkpoints but the three most critical are those that occur near the end of G1, prior to S-phase entry. As well as near the end of G2, before mitosis, and at metaphase.
