Chromosomal theory of inheritance in relation to cell division

Cell division- a process of perpetuity. Mitosis for somatic cells and meiosis for germ cells.

1. Chromosomal theory of inheritance Bipasha Datta Genetics and Plant Breeding

2. A brief description F irst proposed by – Sutton & Boveri in 1903. Popularly known as Sutton- Boveri hypothesis. This theory states that the inheritance of characters from parents to their offspring is governed by chromosomes. or Chromosomes are considered as physical basis of inheritance.

3. They concluded that chromosomes contain genes which govern the transmission of characters from parents to their offspring. This concept was developed working with grasshoppers.

4. C oncepts which support Sutton- Bovery Hypothesis Sex chromosomes Sex linkages Structural chromosomal changes Monosomic & nullisomic analysis Bar locus Crossing over

5. Cell Division

6. The process of reproduction or formation of new cells from the pre-existing cells is called as Cell division. The cell which undergoes division is called as mother cell. The new cells which are formed by the process of cell division are termed as daughter cell.

7. In bacteria, cell division takes place by fission (cleavage) of pre-existing cell.

8. In Higher organisms (Eukaryotes) – there are two types of cell division Mitosis Meiosis

9. Cell Cycle

10. The period in which one cycle of cell division is completed is called cell cycle. A cell cycle consists of two phases A. Interphase B. Mitotic phase (Somatic cell) or Meiotic phase ( Gametic cell)

11. A. Interphase

12. It consists of following sub-phases, viz., a). G 1 – the resting phase b). S – the period of DNA replication c). G 2 – the resting phase

13. a). G 1 : It is a pre-DNA replication phase. Thus this a phase between telophase and S phase. This is the longest phase which takes 12 hours in Vicia faba . Protein and RNA syntheses take place during this place.

14. b). S: This phase comes after G 1. The chromosome and DNA replications take place during this phase. This phase takes lesser time than G 1 . In Vicia faba , it takes six hours.

15. c). G 2 : This is the post-DNA replication phase. It is the last stage of interphase. Protein and RNA syntheses occur during this stage. This phase also takes 12 hours in Vicia faba .

16. B. Mitotic phase

17. Separation of replicated DNA into two daughter nuclei without recombination. Thus the nuclei have the same chromosome as that of parent nucleus. The M phase consists of four stages ⅰ. Prophase ⅱ. Metaphase ⅲ. Anaphase ⅳ. Telophase

18. Significance of mitosis Mitosis plays an important role in the life of living organisms in various way such as- After fusion of male and female gamate zygote is formed. Mitosis is responsible for development of a zygote into adult organism. Mitosis is essential for normal growth and development of living organisms. It gives a definite shape to a specific organsm . In plants, mitosis leads to formation of new parts, viz. , roots, leaves, stems and branches. It also helps in repairing of damaged parts. In case of vegetatively propagated crops like sugarcane, sweet potato, potato etc., mitosis helps in asexual propagation. Mitosis leads to production of identical progeny in such crops. Mitosis is useful in maintaining the purity of types because it leads to production of identical daughter cells and does not allow segregation and recombination to occur. In animals, it helps in continuous replacement of old tissues with new ones, such as gut epithelium and blood cells.

19. B. Meiotic phase

20. The process of meiosis consists of two types of division 1 ). First meiotic division 2 ). Second meiotic division

21. 1). First meiotic division It results in reduction of chromosome number in each new cell to just half of the mother cell. Thus it is referred to as reductional division. It consists of four different phases, viz ., ⅰ. Prophase Ⅰ ⅱ. Metaphase Ⅰ ⅲ. Anaphase Ⅰ ⅳ. Telophase Ⅰ

22. 2). Second meiotic division It is required to reduce the number of chromatids per chromosome. It consists of four stages i.e., ⅰ. Prophase Ⅱ ⅱ. Metaphase Ⅱ ⅲ. Anaphase Ⅱ ⅳ. Telophase Ⅱ This stages are similar to mitosis stages.

23. Meiosis Ⅱ differs from mitosis in the following main aspects: the interphase prior to meiosis Ⅱ is very short. It does not have S period because each chromosome already contains two chromatids. The two chromatids in each chromosome are not sisters throughout. i.e., some chromatids have alternate segments of non-sister chromatids due to recombination. The meiosis (meiotic mitosis) Ⅱ deals haploid chromosome number, whereas normal mitosis deals with diploid chromosome number.

24. Cytokinesis The division of cytoplasm takes place either by cell plate method (in plants) or by furrow method (in animals). The cytokinesis may take place after meiosis Ⅰ and meiosis Ⅱ separately or sometimes it may take place at the end of meiosis Ⅱ only. In maize, it occurs after meiosis Ⅰ and meiosis Ⅱ. However, in Trillium cytokinesis occurs only at the end of Meiosis Ⅱ.

25. Significance of meiosis It helps in maintaining the chromosome number constant in a species. Meiosis facilitates segregation and independent assortment of chromosomes and genes. The recombination of genes also takes place during meiosis. Recombination of genes results in generation of variabilty in a biological population which is important from evolution points of view. In sexually reroducing species, meiosis is essential for the continuity of generation. Because meiosis results in the formation of male and female gametes and union of such gametes leads to the development of zygote and thereby new individual.

26. Comparison of MITOSIS and MEIOSIS MITOSIS It occurs in somatic tissues. Consists of one nuclear division. One cell cycle results in production of two daughter cells. Total DNA of nucleus replicates during S phase. There is no pairing between homologous chromosomes. MEIOSIS It occurs in reproductive tissues. Consists of two nuclear divis ions . One cell cycle results in the formation of four daughter cells. 99.7% DNA replicates during S phase and remaining 0.3% during zygotene stage. Homologous chromosomes pair during during pachytene .

27. Daughter cells are identical with mother cell in structure and chromosome composition. Segregation and recombinatio do not occur. Chromosomes are in the form of dyad at metaphase. At metaphase, centromere of each bivalent divides longitudinally. Daughter cells are different from mother cell in chromosome number and composition. Crossing over takes place during pachytene . Chromosomes are in the form of tetrad at metaphase. The centromere does not divide at metaphase. MITOSIS MEIOSIS

28. The chromosome number of daughter cells is the same as that of mother cell (2n). Maintains purity due to lack of segregation and recombination. One member of sister chromatids move to opposite pole during anaphase. Daughter cells contain half the chromosome number of mother cell (n). Generates variability due to segregation and recombination. One member of homologous chromosomes moves to opposite pole during anaphase Ⅰ. MEIOSIS MITOSIS