CELL CYCLE

1. CELL CYCLE Lecture Delivered at KMC – IC 3 October 2011

2. Format of Lecture • Introduction on Cell Cycle • Concept of Cell Division • Types of Cell Division • Role of Chromosomes in Cell Division • Mitosis: Technical Characteristics • Meiosis: Technical Characteristics • Overall Learning Outcome

3. Introduction on Cell Cycle • Distinction between Living and Non-Living matters • Living things reproduce and they have a capacity to procreate • Procreation in the living organism has a cellular basis • A great German Physician – Rudolf Virchow put this distinction very brilliantly – Where a cell exists, there must have been a preexisting Cell, just as the animal arises only from an animal and the plant only from a plant

4. Continued: • Rudolf Virchow (1821 – 1902) explained this concept of cell cycle by using a Latin axiom “Omnis cellula e cellula” – meaning every cell from a cell. • The continuity of life is based on the reproduction of cells – cell division • Virchow extended his argument on the basis of the existing cell theory proposed by Schleiden and Schwann

5. Rudolf Virchow (1821-1902)

6. Postulates of Cell Cycle • All living organisms are composed of cells and cell products • There are considerable similarities in the chemical constitution and metabolic activities of all cells • The total activities of an organism is the sum of the activities and interactions of its independent cell units

7. The Cell Cycle

8. Structural Types of Cell • Prokaryotic Cell • Eukaryotic Cell ----- Prokaryotic Cell – Greek word combination of the word Pro means first and Karyon means nucleus. The Prokaryotic cells are in the form of bacteria and blue green algae. These are very small and show primitively organised nucleus and cytoplasm ----- Eukaryotic Cell – Greek word combination of the word Eumeans true and Karyon means nucleus. The Eukaryotic cells are found in the most of the algae, fungi, plants and animals.

9. Difference Between Prokaryotic and Eukaryotic Cells • Prokaryotic Cell • Nuclear membrane absent • Nucleolus absent • Lack of clearly defined membranes • Mitotic apparatus absent • Nucleus diffused or incipient • Eukaryotic Cell • Nuclear membrane is present • Nucleolus is also present • Chromosomes are made up of DNA • Mitotic apparatus present • Nucleus is well organised

10. Biological Organisation of Protoplasm • Protoplasm is bounded by a cell membrane. It is further divisible into cytoplasm and nucleoplasm • Nucleoplasm is enclosed by the nuclear membrane • The protoplasm is highly organised and it consists of a number of organelles which are highly ordered structures performing various functions • The Protoplasm also contains a number of depositions called deutoplasmic substances and vacuoles

11. Concept and Key Roles of Cell Division • Cell Division plays several important roles in the life of an organism To explain this – one can take an example of amoeba – because it is a unicellular organism and when it divides and forms duplicate offsprings - the division of one cell reproduces an entire organism Cell division is a dynamic process by which cells multiply in number leading to the production of new cells.

12. Continued: • Without Cell division, multicellular organisms cannot grow • When cell divides, each cell is a complete cell in itself. • Immediately following division, the daughter cells may be smaller than the parent cell but they grow rapidly and soon reach the size of the original cell. • In real sense, cell division is a process of duplication or multiplication rather than the division in the usual sense.

13. Continued: • Cell division continues to function in renewal and repair • It replaces cells that die from normal wear and tear or accidents. • Cell division process is an integral part of the cell cycle • How cell division distributes identical genetic material to daughter cells • Understanding of the cellular mechanics of cell division is very essential for a scholar

14. Continued: • Knowledge on the molecular control systems that regulates progress through the cell cycle will also be necessary • What happens when the control system malfunctions will also be useful to understand. • Relevance of Cell Cycle regulation – an active area of research across the globe.

15. Role of Chromosomes in Cell Division • A chromosome is a body having special organisation, individuality and functional qualities • It is capable of reproducing itself through successive cell divisions • There are two basic types of chromosomes found in living organisms - a) those found in Prokaryotes – single circular DNA not associated with basic protein (histone); b) true chromosomes characteristics of eukaryotes, this contains DNA, RNA associated with histones and other proteins

16. Chromosomes • The chromosomes exhibit great variations in their shape, number and length. • The shape of the chromosomes depends on the location of the centromere – the centromere is the driver of the chromosome because it is responsible for the movement of chromosomes in mitotic spindle. • The longitudinal half of chromosome is known as Chromatid. • The backbone of the chromosomal fibre is made up of DNA molecule

17. Number of Chromosomes in Some Organisms

18. Types of Cell Division • Cell division involves the distribution of identical genetic material – DNA – to two daughter cells • It is well known that the DNA is passed along from one generation of cells to the next • A dividing cell duplicates its DNA - allocates the two copies to opposite ends of the cell and afterwards split into daughter cells.

19. Cell Division • Cell division may occur by four methods: • Amitosis or direct cell division • Endomitosis • Mitosis • Meiosis

20. Amitosis or Direct Cell Division • Amitosis is the means of asexual reproduction • It happens in unicellular organisms like bacteria, yeast etc. • In this type the splitting of nucleus is followed by cytoplasmic constriction

21. Endomitosis • In this type, chromosomes duplicate but cytoplasmic division is prevented. • This results in the duplication of chromosome number.

22. The Cell Cycle