CELL DIVISION, GAMETOGENESIS & THEORY OF OVULATION Cell Cycle • The cell cycle is the series of events that take place in the cell leading to its division and duplication that produces two daughter cells. • Periods of cell cycle: • Interphase • Cell division • Cytokinesis Interphase • It is also called the Interkinesis / Resting phase. • During this stage, the nucleus is not dividing but the DNA in the nucleus is duplicated in preparation for the next division. • Phases of interphase: • Gap 0 (G0) • Gap 1 (G1) • Synthesis (S) • Gap 2 (G2) • Gap 1(G1) • The cell grows and functions normally. • A high amount of protein synthesis occurs and the cell grows to double its original size. • More organelles are produced. • The volume of cytoplasm increases and mitochondria and chloroplasts divide. • If the cell is not to divide again, it will enter G0. • Synthesis (S) • The cell duplicates its DNA. • This is also known as The Swanson Phase. • Gap 2 (G2) • The cell resumes its growth in preparation for division. • Gap Zero (G0) • Some cells do not further divided means their cell cycle is arrested so it is termed as Gap Zero (G0). Cell Division • Cell division is the process by which a parent cell divides into two or more daughter cells. • Eukaryotes undergo two type of cell division : • Mitotis • Meiosis • Prokaryotes undergo cell division : • Binary fission Mitosis (Karyokinesis) • It is the process in which the parental cell divides into two daughter cells which are genetically identical to the parent cell. • All somatic (body) cells multiply by the mitosis cell division. • Stages of Mitosis: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase Prophase • This process is also called as Chromatin condensation. • The chromatin condenses into double rod-shaped structures (chromosomes). • Centrioles move to the opposite sides of the nucleus. • Nucleolus disappears. • Nuclear membrane disintegrates. Prometaphase • It is the phase of mitosis following prophase and preceding metaphase. • Kinetochore (the protein structure) on chromatids where the spindle fibers attach, during cell division to pull sister chromatids apart. Metaphase • Chromosomes meet in the middle. • Chromosomes arrange at equator of cell. • Become attached to spindle fibers by centromeres. • Homologous chromosomes do not associate. Anaphase • Chromosomes get pulled apart. • Spindle fibers contract pulling chromatids to the opposite poles of the cell. Telophase • Chromosomes uncoil. • Spindle fibres disintegrate. • Centrioles replicate. • Nuclear membrane forms. • Cell divides. Cytokinesis • Cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis. • In this stage of cell division, there is invagination of cell wall at the equator toward the center of the parental cell which results into equal division of the cytoplasm and two daughter cells are formed. • After cytokinesis, each daughter cell is in the interphase portion of the cell cycle. Meiosis • During this cell division, a nucleus divides into four daughter nuclei having half of the chromosome number to the parent nucleus. • Gametes (sex cells) divided by this type of cell division. • Stages of meiosis: 1) Meiosis I 2) Meiosis II Meiosis I • Meiosis I segregates homologous chromosomes, producing two haploid cells from a diploid cell, so it is referred to as a Reduction division. • Phases of Meiosis I : 1. Prophase I a) Leptotene b) Zygotene c) Pachytene d) Diplotene e) Diakinesis 2. Metaphase I 3. Anaphase I 4. Telophase I Prophase I • Prophase I is the longest phase of meiosis. • During prophase I, DNA is exchanged between homologous chromosomes in a process called Homologous Recombination. • It can be subdivided into; • Leptotene • Zygotene • Pachytene • Diplotene • Diakinesis Leptotene(Leptonema) • Greek words meaning "Thin threads". • Leptotene is of very short duration. • During this phase the chromosomes become visible. • Progressive condensation and coiling of chromosome fibers takes place. Zygotene(Zygonema) • Greek words meaning "Paired threads“. • chromosomes line up with each other into homologous chromosome pairs. • Pairing is brought about in a zipper-like fashion and may start at the centromere (Procentric) or at the chromosome ends (Proterminal) or at any other portion (Intermediate). • The paired chromosomes are called Bivalent Or Tetrad Chromosomes. Pachytene(Pachynema) • Greek words meaning "Thick threads“. • At this point a tetrad of the chromosomes has formed known as a bivalent. • Here chromosomal crossover occurs called as Crossing Over. • Nonsister chromatids of homologous chromosomes may exchange segments over regions of homology. • At this site they become attached, this point is called as Chiasmata. Diplotene(Diplonema) • Greek words meaning "Two threads“. • In mammalian and human fetal oogenesis all developing oocytes develop to this stage and are arrested before birth. • This suspended state is referred to as the Dictyotene Stage or Dictyate. • • It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later. Diakinesis • Greek words meaning "Moving through". • This is the first point in meiosis where the four parts of the tetrads are actually visible. • Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. • Other than this observation, the rest of the stage closely resembles prophase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. Metaphase I • The nuclear membrane disappears. • Spindle is formed as in mitosis. • Chromosomes are attached by centromeres. • Bivalents gather on metaphase plate. Anaphase I • Differs from that in mitosis. There is no splitting of the centromeres. • One entire chromosome of each pair moves to each pole of the spindle. • The resulting daughter cells therefore have half chromosomes, each made up of 2 chromatids. Telophase I • Karyokinesis takes place or without it cell enters in 2nd division of meiosis, which is similar to mitosis. • Cells may enter a period of rest known as Interkinesis or Interphase II. Meiosis II • Also known as Equational division. • Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. • The end result is production of four haploid cells from the two haploid cells, produced in Meiosis I. Phases of Meiosis II 1. Prophase II 2. Metaphase II 3. Anaphase II 4. Telophase II Prophase II • The disappearance of the nucleoli and the nuclear envelope. • The shortening and thickening of the chromatids. • Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division. Metaphase II • The centromeres contain kinetochores that attach to spindle fibers from the centrosomes at opposite poles. Anaphase II • Chromosomes get pulled apart. • Spindle fibers contract pulling chromatids to the opposite poles of the cell. Telophase II • It is marked by decondensation and lengthening of the chromosomes. • The disassembly of the spindle occurs. • Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells. • Meiosis is now complete and ends up with four new daughter cells. Gametogenesis • Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. • Animals produce gametes directly through meiosis in organs called gonads (testicle in males and ovary in females). • Males and females of a species that reproduces sexually have different forms of gametogenesis. Spermatogenesis (In male) oogenesis (In female) Spermatogenesis • Spermatogenesis is the process in which spermatozoa are produced from male primordial germ cells by way of mitosis and meiosis. • It comprises: i. Spermatocytogenesis- Process of formation of Spermatocyte from Spermatogonium. ii. Spermiogenesis(Metamorphosis)- Process of transformation of Spermatids into Spermatozoa. • A Spermatogonium is an undifferentiated male germ cell, originating in a seminiferous tubule and dividing into two primary spermatocytes in the production of spermatozoa. • There are three subtypes: Type A(d) cells (with dark nuclei): These cells replicate to ensure a constant supply of spermatogonia to fuel spermatogenesis. Type A(p) cells (with pale nuclei): These cells divide by mitosis to produce Type B cells. Type B cells: which divide to give rise to Primary spermatocytes. • Each Primary Spermatocyte duplicates its DNA and subsequently undergoes meiosis I to produce two haploid Secondary Spermatocytes. • Each of the two Secondary Spermatocytes further undergo meiosis II to produce two Spermatids (haploid). • So 1 primary spermatocyte => 4 spermatids. • The process of spermatogenesis is divided into the following phases • Multiplication phase • Growth phase • Maturation phase • Metamorphosis of spermatid Multiplication phase • Also known as Spermatocytogenesis. • The sperm mother cells present in the germinal epithelium of the seminiferous tubules divide repeatedly by mitosis to form large number of diploid rounded sperm mother cells which are called as spermatogonia. • Some of these sex cells move towards the lumen of seminiferous tubules and enter the growth phase. These cells are called primary spermatocytes. • The primary spermatocytes are diploid and contain chromosomes. • Some of the sex cells produced by the division of spermatogonia remain in their original condition