While going through the chapter pay special attention to the following –
Terms and Definition –
(1) Synaptonemal complex
(2) Chiasmata
(3) Terminalization
(4) Brachymeiosis
(5) Bouquet stage
(6) Tetrad
(7) G0 stage
(8) Asters
(9) Karyochorosis
(10) Recombinase enzyme
(11) Disjunction
(12) Bivalents
(13) Interkinesis
(14) Homotypic division
(15) Heterotypic division
Differences –
(1) Cytokinesis and Karyokinesis
(2) Cytokinesis in plants and animals
• New cells arise from the pre-existing cells by process known as cell division.
• Cell division is an important phenomenon that occurs in all living organisms.
• In unicellular organisms, cell division directly produces two individual thus, it is a method of reproduction in these organisms.
• In multicellular organisms, life begins from a single cell zygote or fertilized egg, which divides & redivides to form a complete organisms.
• Each parent cell give rise to two daughter cells each time they divide. These newly formed daughter cells themselves grow and divide, giving rise to a new cell population that is formed by the growth and division of a single parental cell and its progeny.
CELL CYCLE
• Cell cycle is a genetically controlled series of changes that occur in a newly formed cell by which it duplicates its genome, synthesises other constituents, undergoes growth and divides to form two daughter cells.
• Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specifi c stage in the cell cycle.
• The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control.
• Cell cycle consists of two basic phases, states or periods. There is a long non-dividing growing I-phase (Inter-phase) and a short-dividing M-phase. Both have substages.
M-phase is the period or phase of actual cell division.
Phases of Cell Cycle
• Interphase : The resting phase or stage between the two mitotic divisions is called inter-mitotic phase or interphase. During interphase, cell prepares for division by undergoing both cell growth and DNA replication in an orderly manner.
• Most of DNA, RNA and protein are made during interphase, so it is the period when metabolic activity is greatest.
• The interphase is the longest phase of the mitotic cycle, last more than 95% of the duration of cell cycle. It has three sub-phases: G1, S and G2 which occurs in this order followed by M phase.
• Different eukaryotic cells vary in the length of time taken to complete an entire cell cycle; they also differ in the relative proportions of time allotted to each of the four stages. Thus, in continuously dividing cells, an individual cell passes through the following four phases of cell cycle.
(i) G1 phase: (G stands for gap) : In this phase synthesis of substrate and enzyme necessary for DNA synthesis take place. Therefore, G1 is marked by the transcription of rRNA, tRNA, mRNA and synthesis of different types of proteins. It corresponds to the interval between mitosis and initiation of DNA replication. During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
• Time taken for completion of this phase is about 30–40% of the total cell cycle (mitotic cycle).
• Decision of cell division occurs during this phase.
