Cell Division: Mitosis and Meiosis (Biology)

 CELL DIVISION It is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis...

 CELL DIVISION It is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis is an equational division which gives rise to genetically identical cells in which the total number of chromosomes is maintained.  Mitosis was first discovered in plant cells by Strasburger (1875) W. Flemming, (1879) discovered it in animal cells. The term mitosis was coined by Flemmingin 1882.  The cells undergoing mitosis are called mitocytes. In plants, the mitocytes are mostly meristematic cells. In animals, the mitocytes are stem cells, germinal epithelium and embryonic cells. It also occurs during regeneration. Root tip is the best material to study mitosis. There are three main reasons: 1.Growth 2.Repair/healing 3.Asexual reproduction  INTERPHA The cell grows and developsSEand manufactures proteins The cells’ organelles perform their functions and metabolic activities for survival The chromosomes duplicate for cell division to ensure that the daughter cells would have appropriate numbers of genetic materials.  INTERPHAS E During G1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps.  INTERPHAS E In S phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase.  INTERPHAS E During the second gap phase, or G2 phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G2 phase ends when mitosis begins. CHECKPOINTS The G1 checkpoint, at the G1/S transition. The G2 checkpoint, at the G2/M transition. The spindle checkpoint, at the transition from metaphase to anaphase.  G CHECKPOINTS The G1 checkpoint is the main decision point for a cell 1– that is, the primary point at which it must choose whether or not to divide. Once the cell passes the G1 checkpoint and enters S phase, it becomes irreversibly committed to division. Size. Is the cell large enough to divide? Nutrients. Does the cell have enough energy reserves or available nutrients to divide? Molecular signals. Is the cell receiving positive cues (such as growth factors) from neighbors? DNA integrity. Is any of the DNA damaged?  G0 CHECKPOINTS If a cell doesn’t get the go-ahead cues it needs at the G1 checkpoint, it may leave the cell cycle and enter a resting state called G1 phase. Some cells stay permanently in G1, while others resume dividing if conditions improve. G0 CHECKPOINTS  G2 CHECKPOINTS To make sure that cell division goes smoothly (produces healthy daughter cells with complete, undamaged DNA), the cell has an additional checkpoint before M phase, called the G2 checkpoint. DNA integrity. Is any of the DNA damaged? DNA replication. Was the DNA completely copied during S phase?  SPINDLE CHECKPOINTS The cell examines whether all the sister chromatids are correctly attached to the spindle microtubules What will happen if there is an uncontrolled cell cycle? Apoptosis: the process of programmed cell death Metastasis: the movement or spreading of cancer cells from one organ or tissue to another What will happen if there is an uncontrolled cell cycle?  The meiotic cell division first time described by Van Beneden in 1883 Meiotic cell division occurs in germ cells of all living organism. During meiosis, the genetic material of a diploid germ cell undergoes two nuclear divisions and resulting in to four haploid daughter cells. Each daughter cells has one half of the number of chromosomes as the parent cell. There are two successive nuclear divisions in  One germ cell makes 4 gametes with ½ the # of chromosome Occurs only in germ cells in gonads testes /ovaries Occurs in flowers ovary and anther Pair of chromosomes are similar in shape , size, and types of genes. Each locus is in same position on chromosome. eye color eye color locus locus hair color hair color locus locus Paternal Maternal (from Dad) (from Mom)  The two stages of meiosis are: Meiosis I : also called as Reductional Division Meiosis II : also called as Equational Division Before a dividing cell enters meiosis, it undergoes a period of growth called Interphase. INTERPHAS EI The interphase just prior to the entry of cell in to meiosis is known as pre meiotic interphase. During the S phase of pre meiotic interphase, chromosome replication takes place. PROPHASE I The Prophase I divided into 5 stages: 1. Leptotene 2. Zygotene 3. Pachytene 4. Diplotene 5. Diakinesis PROPHASE I: LEPTOTENE The first stage of Prophase I is called Leptotene or leptonema. All the chromosomes begin to condense, so, they become visible as fine thread. There is marked increase in the nuclear volume. A process of 'homology search' which is essential to the initial pairing of homologs, begins during PROPHASE I: ZYGOTENE The zygotene stage also known as zygonema. This stage begins with the initiation of pairing between homologus chromosomes and it ends with complete pairing. The process of pairing between homologus chromosomes is known as Synapsis. PROPHASE I Prophase I - Synapsis Nonsister chromatids sister chromatids Tetrad sister chromatids PROPHASE I: PACHYTENE The pachytene stage also known as pachynema. The process of synapsis is complete. The two homologus of each bivalent appears to be attached with each other at one or more points, these attachments are known as chiasmata. crossing over It is a precise breakage, swapping and reunion between two non- sister chromatids. Crossovers make new gene combinations and which are an important source of genetic variations in populations. nonsister chromatids Tetrad Chiasma: site of variation crossing over PROPHASE I: diplotene The diplotene stage also known as diplonema. DNA recombination is complete. The chromatids continue to shorten and thicken and the four sister chromatids in a group is called a tetrad. PROPHASE I: diplotene The paired chromatids begin to pull apart, causing the strands to separate longitudinally. The chiasmata tend to become terminalised as the meiotic prophase continues. PROPHASE I: diakinesis The chromomes become shorter and thicker due to condensation. Nucleolus and nuclear envelope disappear towards the end of diakinesis. The spindle apparatus becomes organized. The centrioles migrate away from one another. metaphase i  Shortest phase; paired homologues align.  INDEPENDENT ASSORTMENT occurs  pairs of homologues line up independently of other pairs’ orientation toward the poles -- random.  Adds variation. anaphase i  Homologous chromosomes separate towards the poles.  Sister chromatids remain attached telophase i  Each pole now has haploid set of chromosomes (however – still doubled).  Cytokinesis occurs: two haploid daughter cells formed. meiosis ii  No interphase II ( no more DNA replication)  Meiosis II is similar to mitosis Prophase II / Metaphase II Anaphase II meiosis ii: telophase ii  Same as telophase in mitosis.  Nuclei form.  Cytokinesis occurs (2nd time).  Four haploid daughter cells produced gametes ~ sperm or egg; ovule or pollen grain Spermatogen esis: The Oogenesis: process of The process of sperm egg cell production in production in the testes of the ovaries of males females Spermatogenesis n=23 human germ cell in testes n=23 sperm Still doubled chromosomes n=23 2n = 46 haploid (n) n=23 n=23 diploid (2n) Still doubled chromosomes n=23 meiosis I meiosis II Oogenesis 23 human germ cell in ovary Polar n=23 Still doubled 23 Bodies 2n = 46 23 n=23 Still doubled diploid (2n) Ovum n=23 meiosis I meiosis II ASSIGNMENT: Mitosis and Meiosis Foldable *Bring WHITE cartolina and coloring materials on our next meeting