Students can go through TS Inter 1st Year Botany Notes 11th Lesson Cell Cycle and Cell Division will help students in revising the entire concepts quickly.
TS Inter 1st Year Botany Notes 11th Lesson Cell Cycle and Cell Division
→ Cell division is a process by which a cell duplicates for growth and reproduction of an organism.
→ Virchow proposed that new cells arise from pre-existing cells by division. This is i called cell lineage theory.
→ Somatic cells divide by a process of mitosis while the germ cells divide by a process called meiosis (reduction division).
→ In mitosis daughter cells have exactly the same number of chromosomes of the parent cell.
→ Duplication of DNA molecules and doubling of chromosomal constituents are observed during S – phase of interphase.
→ Mitosis includes karyokinesis and cytokinesis. Karyokinesis occurs in four stages.
→ During prophase nucleolus and nuclear membrane disappear. Chromatin condenses into chromosomes, each having two chromatids.
→ During metaphase, spindle fibres are formed. Spindle fibres attach to the centromere of chromosomes. Chromosomes move to the centre of the spindle.
→ In Anaphase centromere divides. Daughter chromosomes move to opposite poles.
→ In Telophase nucleolus and nuclear membrane reappear. Chromosomes J decondense into chromatin. Two daughter nuclei are formed.
→ Cytokinesis, is by cell plate method. Two daughter cells are formed.
→ In meiosis, karyokinesis and cytokinesis occur two times.
→ Meiosis I is a reductional division.
→ Prophase t is the longest stage and is divided into 5 stages.
(a) In leptotene, chromosomes become distinct being quite long and uncoiled.
(b) In zygotene, pairing of homologous chromosomes – synapsis occurs.
(c) In pachytene, exchange of genetic material (crossing over) occurs between non sister chromatids of the bivalent.
(d) In diplotene, repulsion starts between homologous. Chiasmata show terminalisation process.
(e) In diakinesis, nucleolus and nuclear membrane disappear.
→ In Metaphase I, the bivalénts come to be at the equator.
→ In Anaphase I, homologous..chromosomes separate. Each chromosome of a pair moves to opposite poles of the spindle.
→ In Telophase I, reappearance of nuclide and nudear membrane results in the formation of two haploid nuclei.
→ The events of meiosis Il are similar to mitotic divisioñ. Thus it is an equational division. Four haploid cells are formed. Daughter cells produced are called gametes br spores.
→ Crossing over results in exchange of genetic information between individuals of spores and evolution of species.