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CELL DIVISION - AN INTRODUCTION Dear Reader In the previous chapter you have read about the diversity in the living world. One of the fundamental feature of all living organisms is reproduction. Reproduction as you are aware is the process by which parents can produce young ones of the similar kind. You are also familiar with asexual and sexual reproduction. Try to recall the differences between the two. Asexual Reproduction- only single parent is involved and genetically identical copies are produced. Sexual Reproduction- requires fertilization of male and female gametes; results in variation. Let us try to recall your early childhood (or of some other child you know). What is the approximate height of a child who is: just born one-month old one-year old five-year old ten-year old? Compare it with your present height. I am sure you agree that there is an increase in height at each stage as compared to the previous one. You might have noticed increase in height of a plant as it grows from a seedling into an adult plant. Growth is another fundamental feature of all living organisms. Did you ever try to find out the mechanism of growth in living organisms? Is growth the result of an increase in cell size or an increase in cell number? 201 One of the changes that is associated with the growth of an organism is increase in the number of cells. How do cells increase in number? Cells divide to increase their numbers. Try to recall the process of binary fission in Amoeba and budding in yeast. You are also aware about the fact that higher plants and animals cannot divide by binary fission and budding. Cell division is the key phenomenon for both growth and reproduction. Cell division in higher organisms is mainly of two types: mitosis and meiosis. Mitosis is an equational division and meiosis is a reductional division. In this chapter we will study some basic concepts related with these two types of cell division. Chromosomes In eukaryotes, the nucleus of all cells contains thread - like structures called chromosomes. Each chromosome is further made up of helically coiled DNA. DNA is the hereditary material in all organisms except some viruses. During cell division, DNA is transmitted from parent cells to the daughter cells, thus DNA is the unit of inheritance. You will learn more about the structure of chromosome in Chapter 8- Cell: The Unit of Life in class XI. Figure 1: Relationship between Nuclear, Chromosomes, DNA and Genes 202 Haploid and Diploid Cells Try to recall the number of chromosomes in human sperm and ovum. The number of sets of chromosomes in a cell is called its ploidy. A cell with one set of chromosomes is called haploid (n), whereas a cell with two sets of chromosomes is called diploid (2n). Gametes such as sperm and ovum have only one set of chromosomes, so they are haploid (n). Somatic cells have two sets of chromosomes, so they are diploid (2n). sperm (male gamete) Haploid(n) ovum (female gamete) Ploidy Diploid(2n) somatic cells Figure 2: Ploidy of cells Video link: http://www.britannica.com/science/diploid-phase/images-videos/The-terms- haploid-and-diploid-refer-to-the-number-of/68422 Let us know more about somatic cells and gametes present in higher animals. Somatic Cells and Reproductive Cells Somatic cells or vegetal cells make up all tissues and organs of the body such as skin, stomach, intestine, lungs, bones and blood. In a multicellular organism, any cell other than gametes, germ cells or stem cells is a somatic cell. Somatic cells are diploid and divide by mitosis or equational division to form more somatic cells which are also diploid. What is the number of chromosomes in human somatic cells? 203 Stem cells (2n) are undifferentiated cells normally present in the bone marrow of an adult. These cells divide by mitosis to form more cells of the same type from which some cells can differentiate into specialised cells. In animals, the egg cell and sperms are the gametes. They are also called reproductive cells, as they participate in the process of sexual reproduction. Each mature reproductive cell is haploid. Haploid gametes are formed from diploid germ cells by a type of cell division called meiosis or reductional division. Gametes: Haploid cells (n) Human egg cell(n=23) Human sperm cell(n=23) Somatic cells: Diploid (2n) Chromosome number in somatic cells of human being is 2n = 2 x 23 = 46 Here we have taken the example of animals particularly humans to understand the basics of cell division, the process is similar in plants also. In higher plants, meristematic cells divide by mitosis to form simple and complex permanent tissues. Microspore mother cells in the anther and megaspore mother cell present in the ovule of a flower 204 divide by meiosis to form haploid microspore and megaspore, respectively. Further development inside the microspores and megaspore leads to formation of male & female gametes respectively. Homologous Chromosomes Figure 3: Fertilisation of a haploid male and a female gamete helps to restore diploidy in the zygote When a haploid sperm (n) fertilises with a haploid ovum(n), it results in the formation of a diploid zygote(2n). The zygote thus receives one set of paternal(n) and one set of maternal chromosomes(n). These chromosomes are similar but not truly identical. Thus, for each paternal chromosome there is a homolog or similar maternal chromosome. Each of such a pair of similar chromosomes is called homologous chromosome. Each chromosome of a homologous pair carries same genes in the same order although their expression may vary. You will study in Chapter 10- Cell Cycle and Cell Division in class XI that in germ cells, homologous chromosomes pair up at the time of meiotic cell division. Zygote undergoes a special type of mitotic cell division resulting in the formation of diploid daughter cells. Try to recall the term used for this type of cell division from chapter 1 - How life begins? 205 Mitosis and Meiosis Each human somatic cell contains 23 pairs of chromosomes (2n) whereas each gamete in humans contains only 23 chromosomes (n). Try to analyse the fact that although “all the cells in an organism are formed from the same zygote but still the somatic cells and gametes vary in the number of the sets of chromosomes possessed by them”. Actually, it happens because of the two different types of cell division taking place in them. Somatic cells(2n) divide by the process of mitosis. Mitosis generally takes place when plants and animals need to make more cells for growth and repair. Mitosis is an equational division, resulting in the formation of two daughter cells(2n) which are genetically identical to the parent cell. The chromosome number in each daughter cell is equal to the number of chromosomes in the parent cell (2n).The daughter cells grow and behave as parent cell and enter another round of equational cell division and the process continues as the organism continues to grow. Thus, a human somatic cell (2n = 2 x 23) will divide to produce two daughter cells each having 23 pairs of chromosomes. Video link for mitosis - https://www.youtube.com/watch?v=DwAFZb8juMQ Diploid (2n) germ cells (also known as meiocytes)present in the testes of a male and ovary of a female animal divide by the process of meiosis to form haploid male and female gamete respectively (n).For example, in humans, germ cells in the testes of a male and germ cells in ovary of a female, each having 23 pairs of chromosomes will divide by meiosis to form sperm and ovum respectively, each having only 23 206 chromosomes respectively (n). Thus, meiosis is a reductional division. It has two main phases - Meiosis I and II. Meiosis also introduces variations in the next generation. Reductional division during gametogenesis is essential to restore diploid chromosome number in the next generation. What will happen if sperm and ovum are also diploid? Video link – meiosis- https://www.youtube.com/watch?v=nMEyeKQClqI, https://www.youtube.com/watch?v=D1_-mQS_FZ0 Video link - mitosis vs. meiosis - https://www.youtube.com/watch?v=Ba9LXKH2ztU Do you know that chromosome number is constant for a species? The number of chromosomes does not correlate with the apparent complexity of an organism. Chromosome number (2n) in some common organismsHuman (Homo sapiens) = 46, Gorilla gorilla= 48, Pea (Pisum sativum) = 14, Onion (Allium cepa) = 16 Types of cells Somatic cell (2n) Somatic cell(2n) Germ cell(2n) Mitosis Somatic cell (2n) Gamete (n) Gamete (n) Meiosis Gamete (n) Gamete (n) You will study the details of mitosis and meiosis in Chapter 10 - Cell cycle and Cell Division in Class XI. 207 DO IT YOURSELF 1. 2. 3. Identify the ploidy of the following cells: a. Zygote: _________ b. Intestinal cells: ________ c. Pollen mother cells: _______ d. Ovum: ____________ Which type of cell division will take place in the following cells: a. Pollen mother cells: ____________ b. Skin cells: ____________________ c. Megaspore mother cells: ____________ d. Meristematic cells: ____________ Write the chromosome number in the following cells: S. No. Name of organism Chromosome number in meiocyte Chromosome number in gamete A. Onion (Allium cepa) 16 ……….. B. Human being (Homo sapiens) 46 ……….. C. Fruitfly (Drosophila melanogaster) ……….. 4 D. Rice (Oryza sativa) ……….. 12 REFERENCES http://evolution.about.com/od/Overview/a/Asexual-Vs-SexualReproduction.htm 208