Unit 1: Basic Biology

UNIT 1: Basic Biology (i) Cell Cycle and Cell Division • Robert Hooke coined the term 'cell'. Cell Theory It was given by German Botanist M.J. Schleiden and German Zoologist T. Schwann in 1838 and 1839. It was later modified by Rudolf Virchow in 1855, who gave the modern cell theory, also known as the cell doctrine or cell principle. It states that: • • • • • All living organisms (plants and animals) are made up of one or more cells. Cell is the basic unit of life. New cells always arise from the pre-existing living cells by division (omnis cellula-ecellula). Each and every cell in an organism possesses three essential parts—cell membrane, cytoplasm and nucleus. Cell division is necessary for growth, development and survival of organisms. Types of Cell Division (i) Mitosis It occurs in somatic or vegetative cells and results in the formation of two daughter cells, which are qualitatively and quantitatively identical to the parent cell. It is an equational division. (ii) Meiosis It occurs only in the reproductive or germ cells and results in the formation of four daughter cells, where each daughter cell is genetically different from the parent cell. It is a reductional division where chromosome number in daughter cells is reduced to half. Cell Cycle It is the sequence of events that occurs between the cell formation and its division into the daughter cells. The cell cycle comprises two phases: • • Interphase M-phase (mitosis and meiosis) Interphase It is a series of changes that take place in a newly formed cell and its nucleus before it becomes capable of division again. Therefore, it is also called inter-mitosis. Interphase is also known as the biosynthetic phase in which the cell duplicates its cell organelles and replicates its DNA. Interphase has three stages: (i) G₁ Phase • Characterised by the synthesis of RNA and non-histone proteins. (ii) S-Phase • • Follows the G₁ phase. Characterised by DNA replication and complete duplication of chromosomes. (iii) G₂ Phase • • Centrioles, mitochondria, Golgi bodies and other cytoplasmic organelles are doubled. The cell contains double the amount of DNA as compared to the original diploid cell. Mitotic (M-phase) During mitosis, replicated DNA and cytoplasmic contents are separated and cell division occurs. It consists of: • • Karyokinesis (division of nucleus) Cytokinesis (division of cytoplasm) It is of shorter duration. • Mitosis was first observed by Strasburger in plant cells and by Flemming in animal cells. Karyokinesis Karyokinesis comprises four phases: (i) Prophase • • • Longest phase of division. Replicated chromosomes with sister chromatids condense and become visible. Nuclear membrane and nucleolus disappear. (ii) Metaphase • • Chromosomes arrange at the equator of the spindle to form the equatorial plate. Each chromosome attaches to spindle fibres by its centromere. (iii) Anaphase • • • • Centromere divides, separating sister chromatids into newly formed chromosomes. Sister chromatids move toward opposite poles. Shortest phase of cell division. Chromosomes are best studied in this phase. (iv) Telophase • • Nuclear membrane reforms around each group of chromatids (now chromosomes). Two nuclei are formed. Karyokinesis (division of nucleus) is followed by cytokinesis (division of cytoplasm). Significance of Mitosis • • • • Mitosis results in the formation of identical cells. Daughter cells have the same genetic constitution quantitatively and qualitatively as the parent cell. It brings about reproduction in unicellular organisms. It is necessary for growth, maintenance and repair in multicellular organisms. It is required for healing wounds and regeneration of lost parts in certain animals. Differences Between Animal and Plant Cell Mitosis S. No. Mitosis in Animal Cell Asters are formed. It is called astral mitosis. Cytokinesis occurs by furrowing of cytoplasm. Occurs in most tissues throughout the body. (a) (b) (c) Mitosis in Plant Cell Asters are not formed. It is called anastral mitosis. Cytokinesis occurs by cell plate formation. Occurs mainly at growing tips and lateral sides. Meiosis Meiosis is the cell division which occurs in sex cells and results in the formation of four daughter cells. The whole process consists of two successive coordinated divisions: • • Meiosis I (Heterotypic division) Meiosis II (Homotypic division) Heterotypic division is the actual reduction division where chromosome number is reduced to half, whereas homotypic division (Meiosis II) is fundamentally similar to mitosis. Synapsis Pairing of homologous chromosomes is called synapsis. Significance of Meiosis • • • • • During Meiosis I, the chromosome number is reduced to half, while Meiosis II is a simple mitotic type division. Meiosis maintains the same chromosome number through successive generations of species. The most significant feature of meiosis is that chromosome number is reduced to half in daughter cells. It helps to produce new combinations of characters as a result of crossing over (exchange of genetic material between two homologous chromosomes). The four daughter cells formed are qualitatively and quantitatively different from the parent cell. Differences Between Mitosis and Meiosis S. Mitosis No. (a) Takes place in somatic cells. Single division producing two daughter (b) cells. Chromosome number remains the same (c) after mitosis. (d) No crossing over takes place. Meiosis Takes place in reproductive or germ cells. Double division producing four daughter cells. Chromosome number is reduced to half after meiosis. Crossing over takes place during Prophase I.