Cell Division PDF

CELL DIVISION  Cell division is the process cells go through to divide. There are several types of cell division, depending upon what type of organism is dividing. Organisms have evolved over time to have different and more complex forms of cell division.  Most prokaryotes, or bact...

CELL DIVISION  Cell division is the process cells go through to divide. There are several types of cell division, depending upon what type of organism is dividing. Organisms have evolved over time to have different and more complex forms of cell division.  Most prokaryotes, or bacteria, use binary fission to divide the cell. Eukaryotes of all sizes use mitosis to divide. Sexually-reproducing eukaryotes use a special form of cell division called meiosis to reduce the genetic content in the cell.  This is necessary in sexual reproduction because each parent must give only half of the required genetic material, otherwise the offspring would have too much DNA, which can be a problem. The stages in cell division are known as Cell Cycle.  The cell cycle Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides.  - G1 phase. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and moves into the S phase. -S phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.  -G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis. -M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis). The period between mitotic divisions - that is, G1, S and G2 - is known as interphase.  Types of Cell Division Prokaryotic Cell Division: Binary Fission Prokaryotes replicate through a type of cell division known as binary fission. Prokaryotes are simple organism, with only one membrane and no division internally. Thus, when a prokaryote divides, it simply replicates the DNA and splits in half.  The process is a little more complicated than this, as DNA must first be unwound by special proteins. Although the DNA in prokaryotes usually exists in a ring, it can get quite tangled when it is being used by the cell. To copy the DNA efficiently, it must be stretched out.  This also allows the two new rings of DNA created to be separated after they are produced. The two strands of DNA separate into two different sides of the prokaryote cell. The cell then gets longer, and divides in the middle.  Eukaryotic Cell Division: Mitosis Eukaryotic organisms have membrane bound organelles and DNA that exists on chromosomes, which makes cell division harder. Eukaryotes must replicate their DNA, organelles, and cell mechanisms before dividing. Many of the organelles divide using a process that is essentially binary fission.  After the DNA and organelles are replicated during interphase of the cell cycle, the eukaryote can begin the process of mitosis. Prophase Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates.  A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organise the production of microtubules that form the spindle fibres that constitute the mitotic spindle.  The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere.  Prometaphase The chromosomes, led by their centromeres, migrate to the equatorial plane in the midline of cell - at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate.  The spindle fibres bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense.  Metaphase The chromosomes align themselves along the metaphase plate of the spindle apparatus. Anaphase The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions.  The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.)  Telophase The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibres disappear.  Cytokinesis The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions.  EUKARYOTIC CELL DIVISION: MEIOSIS In sexually reproducing animals, it is usually necessary to reduce the genetic information before fertilization. Some plants can exist with too many copies of the genetic code, but in most organisms it is highly detrimental to have too many copies. Humans with even one extra copy of one chromosome can experience detrimental changes to their body.  EUKARYOTIC CELL DIVISION: MEIOSIS In sexually reproducing animals, it is usually necessary to reduce the genetic information before fertilization. Some plants can exist with too many copies of the genetic code, but in most organisms it is highly detrimental to have too many copies. Humans with even one extra copy of one chromosome can experience detrimental changes to their body.  To counteract this, sexually reproducing organisms undergo a type of cell division known as meiosis. As before mitosis, the DNA and organelles are replicated. The process of meiosis contains two different cell divisions, which happen back-to-back. The first meiosis, meiosis I, separates homologous chromosomes.  The homologous chromosomes present in a cell represent the two alleles of each gene an organism has. These alleles are recombined and separated, so the resulting daughter cells have only one allele for each gene, and no homologous pairs of chromosomes.  The second division, meiosis II, separated the two copies of DNA, much like in mitosis. The end result of meiosis in one cell is 4 cells, each with only one copy of the genome, which is half the normal number. Organisms typically package these cells into gametes, which can travel into the environment to find other gametes.  When two gametes of the right type meet, one will fertilize the other and produce a zygote. The zygote is a single cell that will undergo mitosis to produce the millions of cells necessary for a large organism. Thus, most eukaryotes use both mitosis and meiosis, but at different stages of their lifecycle. Meiosis I  The first meiotic division consists of prolonged prophase in which the homologous chromosomes come in close contact with each other and exchange hereditary material between them. Similarly, in the first meiotic division, the reduction of chromosome number takes place and, thus, two haploid cells are resulted by this division.  The first meiotic division is also known as the heterotypic division. Meiosis I consists of the following steps: Interphase I Just like mitosis, meiosis also consists of a preparatory phase called interphase. The interphase is characterized by the following features :  The nuclear envelope remains intact, and the chromosomes occur in the form of diffused, long, coiled, and indistinctly visible chromatin fibers. The DNA amount becomes double. Due to the accumulation of ribosomal RNA (rRNA) and ribosomal proteins in the nucleolus, the size of the nucleolus is significantly increased.  In animal cells, a daughter pair of centrioles originates near the already existing centriole and, thus, an interphase cell has two pairs of centrioles. In the G phase of interphase, there is a decisive change that directs the cell toward meiosis, instead of mitosis.  At the beginning of the first meiotic division, the nucleus of the dividing cell starts to increase in size by absorbing the water from the cytoplasm, and the nuclear volume increases about three folds.  Prophase I The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases: 1. Leptotene: Chromosomes start to condense. 2. Zygotene: homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads).  3. Pachytene: crossing over between pairs of homologous chromosomes to form chiasmata (sing. chiasma). 4. Diplotene: homologous chromosomes start to separate but remain attached by chiasmata. 5. Diakinesis: homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.  Metaphase I Metaphase I consists of spindle fiber attachment to chromosomes and chromosomal alignment at the equator. During metaphase I, the spindle fibers are attached with the centromeres of the homologous chromosomes, which are directed towards the opposite poles.  Anaphase I At anaphase I homologous chromosomes are separated from each other, and due to the shortening of chromosomal fibers or microtubules, each homologous chromosome with its two chromatids and undivided centromere move towards the opposite poles of the cell. Because during the chiasma formation, one of the chromatids has changed its counterpart, therefore, the two chromatids of a chromosome are not genetically identical.  Telophase I The onset of telophase I is defined by the movement of a haploid set of chromosomes at each pole. The nuclear envelope is formed around the chromosomes, and the chromosomes become uncoiled. The nucleolus reappears and, thus, two daughter nuclei are formed.  Cytokinesis I In animals, cytokinesis occurs by the constriction of the cell membrane while in plants, it occurs through the formation of the cell plate, resulting in the creation of two daughter cells. MEIOSIS II  In the second phase of the meiotic division, the haploid cell divides mitotically and results in four haploid cells. This division is also known as the homotypic division. This division does not include the exchange of the genetic material and the reduction of the chromosome number as in the first meiotic division.  Meiosis II consists of the following steps: Prophase II: In prophase II, each centriole divides, resulting in two pairs of centrioles. The centrioles move towards the opposite poles and the nuclear membrane, and the nucleolus disappears.  Metaphase II During metaphase II, the chromosomes get arranged on the equator of the cell through the spindle fibers. The centromere divides and, thus, each chromosome produces two daughter chromosomes. The spindle apparatus is attached to the centromere of each chromosome.  Anaphase II The daughter chromosomes move towards the opposite poles due to the shortening of chromosomal microtubules and the stretching of interzonal microtubules of the spindle.  Telophase II The chromatids migrate to the opposite poles and now known as chromosomes. The endoplasmic reticulum forms the nuclear envelope around the chromosomes, and the nucleolus reappears due to the synthesis of ribosomal RNA.  Cytokinesis II The process of cytokinesis is identical to cytokinesis I resulting in the division of cytoplasm for each of the four daughter cells formed.  APPLICATIONS OF MEIOSIS Meiotic like mitosis is used for several lab-based technologies, some of which are given below: Tissue culture Like mitosis, meiosis is also used in biotechnology to acquire a gametic condition in cells. Meiosis often accompanies mitosis to generate variation which aids in studies regarding evolutionary processes.  In-vitro gamete formation In various gamete failure-derived infertility issues, the embryonic stem cells are differentiated into germ-like cells through the meiotic division. These gametes are formed in-vitro via meiosis and are inserted into the individuals with such disorders.  Importance of Mitosis and Meiosis Mitosis is important because it promotes genetic stability (Progeny are genetically identical to parent cells. Mitosis does not produce any variation in genetic information), it brings about growth (when number of cells increase by mitosis, multicellular organisms grow). Mitosis is responsible for cell replacement (existing body cells divide to replace dying body cells).  Mitosis brings about regeneration (some invertebrates such as crustacea replace a leg, starfish can replace an arm or the whole body parts by division of body cells) and asexual reproduction (binary fission in bacteria occurs by mitosis).  Meiosis is important because it enables sexual reproduction to take place (Meiosis forms gametes that are used in sexual reproduction). Meiosis also forms zygotes with a diploid number since it is formed from two gametes that are haploids. Meiosis is also important because it promotes genetic variation among living organisms.  This variation results from crossing over in prophase I (when homologous chromosomes pair up to form bivalets and then exchange parts) and during independent assortment of chromosomes in anaphase I (when homologous chromosomes are separated from each other randomly). MITOSIS: MEIOSIS: Occurs continuously in somatic cells Occurs in germ cells (testes/ovaries) during the process of gametogenesis Completes in one sequence or phase Completes in two successful divisions Prophase is short, no sub-divisions Prophase is long with six sub-stages: preleptotene, leptotene, zygotene, pachytene, diplotene and diakinesis Parental chromosomes duplicate into two Homologous chromosomes separate into two chromatids chromosomes (not chromatids) Chromosome number in parents is maintained in Chromosome number in parents is halved in daughter cells daughter cells No synapsis occurs Synapsis occurs between homologous chromosomes