Nuclear Structure & Functions BY NNM.pptx

Description about cell and cell cycle, different stages of cell cycle, in detail description of mitosis and meiosis, figures of mitosis and meiosis , importance of cell division in human life,

1. Nuclear Structure & Functions Nazratun Noor Maria B. Pharm , M. Pharm (CU) Lecturer, Department of Pharmacy, USTC.

2. Why do cells divide? •Growth •Repair and replace dead cells •asexual reproduction Cell Cycle: A cell cycle is a series of events that a cell passes through from the time until it reproduces its replica. - It is the growth and division of single cell into daughter cells and duplication (replication). - In prokaryotic cells, the cell cycle occurs via a process termed binary fission. In eukaryotic cells, the cell cycle can be divided in two periods a)Interphase During this phase the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA. b) Mitosis/Miosis During which the cell splits itself into two/four distinct cells.

3. Interphase: During this phase the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA. It is the longest phase. In a typical human cell, out of the 90h, interphase lasts for 89h. Characters Of Interphase: - It is the resting phase of the cell. Resting refers to the rest from division. But, the cells in the interphase are metabolically active. The metabolic activities are high in this phase. - The cell grows during phase. During this phase mRNA and rRNA are synthesized. - The chromosomes duplicates into two chromatids. - The centrioles duplicates into two. Thus two centrioles are formed. - The centrospheres of centrioles, microtubules arise. These microtubules form asters. Stages Of Interphase: Interphase consists of 3 sub-stages. They are 1. G1 phase 2. S phase 3. G2 phase

4. G1 phase It is the post mitotic phase and takes place at the end of cell division the newly formed cells accumulate the energy and prepares themselves for the synthesis of DNA . During this , active synthesis of RNA and protein takes place. During G1, a lot of protein synthesis happens in order to increase the amount of cytosol in the cell. The increase in cell size happens in this stages. S phase It is the synthesis phase during this phase duplication(replication) of DNA and centriole takes places. The duplication of DNA results in the duplication of chromosomes . G2 phase It is the pre- mitotic gap phase (invisible phase) the synthesis of RNA and protein continues in this phase. The formation of macro molecules for spindle formation takes place and the cell prepare itself to go into the mitotic phase . Cell continues to grow and if a problem occurs in DNA replication, it will be repaired

5. M Phase (Mitosis): Mitosis is the distribution of the two sets of chromosomes into two separate and equal nuclei. - This is the division phase. During this phase the cell divides. This phase has a short duration. A typical human cell cycle has duration of 90h. Of these the M phase has duration of 45 to 60min. This phase has two sub-phases called karyokinesis and cytokinesis. - Karyokinesis refers to the cell division of nucleus into two daughter nuclei. It has 4 sub-stages, namely prophase, metaphase, anaphase and telophase. - Cytokinesis refers to the cell division of the cytoplasm resulting in two daughter cells

6. Prophase • Nucleus disappears • Spindle fibers form in the cytoplasm • Spindle fibers attach to sister chromatids Metaphase • The sister chromatids are pulled to the center of the cell • They line up in the middle of the cell Anaphase - Spindle fibers begin to shorten - The sister chromatids are pulled to the opposite ends of the cell Telophase - The sister chromatids arrive at the opposite poles of the cell and begin to unravel - New nucleus begins to form

7. Cytokinesis - Cytokinesis is the division of the cytoplasm - Results in two separate daughter cells with identical nuclei

8. Almost all of the body's cells divide by mitosis. Meiosis is used to produce only one type of cell, and those are the gametes. During meiosis, a diploid cell divides to produce four, non-identical haploid daughter cells, each containing a single set of chromosomes. In humans, these are sperm and egg cells.

9. Meiosis: the process of cell division that produces haploid gametes (half the number of chromosomes: humans: 23) Unique Features of Meiosis: 1. Synapsis: Following chromosome replication, the homologous chromosomes pair all along their length. This process is called synapsis. 2. Crossing Over: While the homologous chromosomes are joined, crossing over occurs. Crossing over is the exchange of genetic material from homologous chromosomes. This causes genetic variations. 3. Reduction Division: The chromosomes are not copied between the two divisions. At the end of meiosis, each cell contains one-half the genetic material. (haploid or “n”)

10. Meiosis I: - Preceded by Interphase- chromosomes are replicated to form sister chromatids - Sister chromatids are genetically identical and joined at the centromere - Single centrosome replicates, forming 2 centrosomes 1. Prophase I - Individual chromosomes first become visible homologous chromosomes become closely associated in synapsis forming a tetrad (four chromatids). - Crossing over is a complex series of events in which DNA segments are exchanged between non-sister or sister chromatids - Nuclear membrane disappears and the centrioles move to the opposite ends.

11. 2. Metaphase I The homologous chromosomes line up in the center of the cell and are still held together through spindle fibers. 3. Anaphase I • Spindle fibers shorten • The homologous chromosomes are separated (the sister chromatids are still paired) 4. Telophase I -The nuclear membrane reforms around each daughter nucleus - Each new cell now contains two sister chromatids that are NOT identical due to crossing over At the end of Meiosis I… - Have 2 cells - Each cell contains a haploid number of chromosomes – 1 copy of each chromosome (for humans, each haploid cell has 23 chromosomes)

12. Meiosis II Meiosis II resembles normal, mitotic division 1. Prophase II The nuclear membrane breaks down again 2. Metaphase II The chromosomes line up in the middle of the cell 3. Anaphase II The spindle fibers shorten and the sister chromatids move to opposite poles. 4. Telophase II The nuclear envelope re-forms around the four sets of daughter chromosomes. At the end of Meiosis II… there are 4 haploid cells. (only 1 copy of each chromosome) (for humans, each haploid cell has 23 chromosomes) No two of these haploid cells are alike due to crossing over. This is why you and your siblings are genetically unique