Structure of chromosomes, cell cycle, cell division

Class 10 ICSE Biology
Lesson 2 Structure of Chromosomes< Cell cycle, Cell division

1. What do you wonder about today’s topic structure of Chromosomes?

2. Chromosomes During cell division, chromatin fibers condensed to form chromosomes. Chromosomes are the highly coiled and condensed form of chromatin fibers.

3. Chromatin fibres Are made up of Nucleosome subunits. Each nucleosome comprises 40% of DNA molecule and 60% of Histone Octamers.

4. DNA Molecule Each DNA is very large and is composed of two complementary strands wound around each other in a double helix. Each DNA Strand is made up of nucleotide subunits. Each nucleotide consists of Phosphate group Pentose Sugar A nitrogenous base

5. Nitrogenous Bases There are four types of nitrogenous bases found in DNA. Adenine Guanine Thymine Cytosine Adenine always pairs with Thymine with two hydrogen bonds. Guanine always pairs with cytosine with three hydrogen bonds.

6. Histone octamer Histones are proteins that help in the coiling and packaging of DNA into structural units called nucleosomes. A single chromosome may have about a million nucleosome.

7. I wonder... How 2 meters of DNA is stretched In side 6 micrometer cell

8. Chromosome It consists of two sister chromatids attached at the point of centromere. Each chromosomes centromere is located at the particular site. It also serves to attach spindle fibres during cell division. Spindle fibers helps to detach the chromatids during cell division by pulling away from the other towards the two poles of the dividing cel l.

9. Genes Are the specific sequences of nucleotides on a chromosome, that encode particular proteins which express in the form of some particular feature of the body.

10. Need for new cells For growth For replacement For repair For reproduction

11. Cell Cycle Cells   are constantly going through the  cell cycle . Once a cell  divides , its daughter cells just repeat the cycle. Cell cycle , the ordered sequence of events that occur in a  cell  to prepare itself for  cell division and subsequent division of the cell to produce two daughter cells.

12. Cell cycle

13. Cell Cycle Phases Interphase Non- dividing phase M-phase or Mitosis Dividing phase

14. Interphase . During this period, the cell grows and prepares for  mitosis . Includes 3 Phases First growth phase Synthesis Phase Second growth Phase

15. First growth phase (G1) RNA and Protein synthesis. During this phase, the cell synthesizes various enzymes and nutrients that are needed later on for DNA replication and  cell division Volume of cytoplasm increases Mitochondria and chloroplast start to divide. In the late G1 the cells either withdraw from the cell cycle and enter resting phase or start preparing for the next cell division.

16. Synthesis Phase (S) More DNA is formed by DNA Replication Chromosomes are duplicated.

17. S econd growth Phase (G2) Shorter growth period RNA and proteins needed for cell division continues to divide. After the dividing phase gets completed, the newly formed daughter cells enter into the interphase again and the cell cycle goes on….

18. Mitosis Mitosis: Mitosis is the division of somatic cells in which two identical daughter cells are produced by the division of one parent cell. Mitosis is also known as equational division or somatic cell division because during the process daughter cells receive an equal number of chromosomes and the division occurs in the body cells or somatic cells. It is helpful for growth and development.

19. M-phase Mitosis consists of the following phases: A. Karyokinesis: It is the division of the nucleus during cell division. B. Cytokinesis: It is the division of the cytoplasm during cell division.

20. Karyokinesis is the division of nucleus. P lease sit on MAT Karyokinesis occurs in four phases: 1. P rophase 2. M etaphase 3. A naphase 4. T elophase

21. Prophase During prophase, chromatin fibres condense and thick chromosomes are visible. The nucleolus and nuclear membrane disappear. A pair of centrioles duplicates. Spindle apparatus starts forming and the duplicated chromosomes start moving towards the equator of the cell.

22. Metaphase Chromosomes are arranged on the metaphase plate or equatorial plane. Each chromosome gets attached to a spindle fibre by its centromere.

23. Anaphase The centromere divides, and the sister chromatids separate from each other. Spindle fibres contract and pull chromatids towards opposite poles.

24. Telophase Spindle apparatus disappears. Chromosomes become thin and turn into chromatin fibres. Nuclear membranes and nucleoli reappear

25. Cytokinesis The furrow continues to deepen in the cell, and it finally divides the cytoplasm forming two new daughter cells. In plant cells, a cell plate appears at the equatorial plane.

26. Meiosis Meiosis is the cell division in which each daughter cell receives half set of chromosomes. Hence, in meiosis, haploid daughter cells (n) are formed from a diploid parent cell (2n). Therefore, meiosis is also known as reductional division. Meiosis consists of two successive divisions—meiosis I and meiosis II.

27. Meiosis importance •During meiosis, homologous chromosomes (1 from each parent) pair along their lengths. The chromosomes cross over at points called chiasma. At each chiasma, the chromosomes break and rejoin, trading some of their genes. This recombination results in genetic variation. •This is how siblings, are different from each other in certain aspects.

28. Difference between Mitosis and Meiosis Occurs in somatic cells Occurs in reproductive cells Provide for growth and replacement Only for gamete formation Occurs continuously throughout life. Occurs only in reproductive age. Two daughter cells will be produced. Four daughter cells will be produced. Full set of chromosomes is passed on to each daughter cell. Only half the number of chromosomes is passed onto each daughter cells Single nuclear division Two nuclear divisions Produce identical daughter cells Results genetic variations

29. Pretend your friend was absent from class today… Write what you would say if you had to explain the lesson to your friend.