Mitosis

The Chromosome: an overview -Chromosome is composed of a molecule of double stranded DNA packaged in nucleoid by interactions with RNA and protein. -After replication, the chromosome contains two giant DNA molecules. -In nondividing cell, the chromosome is extended and the structure is difficult to study. -In dividing cells, the chromosomal material condenses, forming a relatively thick, rodlike structure

- The replicated chromosome is made up of two sister chromatids, two identical copies of parental chromosomes, joined together by a centromere.. -The kinetochore, a protein structure at centromere, functions in the chromosome movement during cell division. It’s position varies among different chromosomes. -The ends of the chromosomes are called telomeres.

The Cell Cycle In order for an organism to grow, whether it is single celled or multicellular, three events must take place: 1. The cell mass must increase 2.There must be a duplication of the genetic material 3. A division process must occur so that each daughter cell receives an equal and identical complement of the genetic material to ensure perpetuation of the cell line In eukaryotes, these occurrences take place in an ordered progression of events during the cell’s life span which is called cell cycle.

Five Phases of the Cell Cycle G 1 - primary growth phase S – synthesis; DNA replicated G 2 - secondary growth phase collectively these 3 stages are called interphase M - mitosis C - cytokinesis 5

The Cell Cycle 10 hr 9 hr 4 hr

Events of cell cycle 1.G1 phase (Gap phase 1): First growth phase. Growth and increase in cell mass occurs following cell division. It is involved in metabolic activities. 2. S phase (Synthesis phase): Genetic material of each and every chromosome is replicated. Thus each chromosome consists of two sister chromatids joined at the centromere. 3.G2 phase (Gap phase 2): Second growth phase. In this post- DNA replication phase, preparations for mitotic cell division takes place. 4. M phase (Mitotic phase): Division phase. This is the final part of the cell cycle. The two sister chromatids separate from each other, one goes to each of the two daughter cells. The two daughter cells are genetically identical to each other and to the parent cell.

Cell Division Eukaryotic cells divide by two main processes: Mitosis 1.It is the process by which single cells reproduce themselves and multicellular organisms grow. 2. The daughter cells are identical to each other and to the parent cell. 3.The parent and daughter cells are diploid (2n), they contain two copies of each chromosome. Meiosis 1. It is the basis of sexual reproduction in all plant cells and animals 2. It occurs during gamete formation in animals and spore formation in plants and fungi. 3.The parent cell is diploid, but the daughter cells are haploid (n), carrying one copy of each chromosome.

Interphase occurs just before Mitosis begins: DNA is replicated along with organelles and other cellular components and the cell prepares for division.

Cell Division: Mitosis Mitosis, or mitotic cell division, was first described by Walter Flemming. It consists of two interrelated processes: 1.Mitosis: The division of the nucleus. 2.Cytokinesis: The division of the cytoplasm and other nonnuclear components of the ce ll. Stages of mitosis: Interphase (consisting of G 1, G 2 and S phases), prophase, metaphase, anaphase and telophase. Prophase and telophase are usually long, whereas metaphase and anaphase are commonly brief.

Mitosis Interphase The first indications of approaching mitosis in animal cells are observed in the cytoplasm of cell at interphase. The centrosome, a centrally located organelle, is the primary microtubule organizing centre and acts as the spindle pole during mitosis in animals. The microtubules, cytoskeletal structures that radiate from the centrosome, organize and coordinate the movement of chromosomes during mitotic cell division. The centrosome is duplicated by the cell during interpahse so that each daughter cell eventually receives one. As mitosis begins, the centrosome begins splitting into two and microtubules radiates out, forming an aster. Photographs from:

Mitosis Interphase Animal cellPlant cell Photographs from:

1 st step in Mitosis: Prophase (preparation phase) In early prophase, the two centrosomes separate and move toward opposite poles of the cell. The microtubule extending from them form the ‘rails’ for chromosome movement.The chromosomes which were replicated during the S phase of the cell cycle are highly extended.They begin condensing through coiling at this phase. By late prophase, chromosomes are highly condensed.The two chromatids of each chromosome are held together at the centromere. The kinetochores complex with the centromere and functions as the sites to which microtubules attach and guide chromosomes movement. A continuous microtubule network extends around the nuclear membrane and interconnects the poles.The nuclear membrane, nucleolus become fragmented and dispersed in the cytoplasm.

Mitosis Prophase Photographs from: Animal cell Plant cell

2 nd step in Mitosis: metaphase (organizational phase) Microtubules invade the nuclear region and become attached to each chromatid at the kinetochore in the centromeric region.Each of the two sister chromatids become become attached to a different pole of the spindle, but the centromeres remain together.The chromosomes line up the middle of the cell. Metaphase chromosomes are tightly coiled and discrete.Arms of sister chromatids are extended from the centromere region, but the chromatids are held together at the centromeres until the beginning of the next phase. The diagnosis of disorders caused by the structural changes in chromosomes is commonly made by studying metaphase chromosomes.

Mitosis Metaphase Animal cellPlant cell Photographs from:

3 rd step in Mitosis: Anaphase (separation phase) The centromere separate and the sister chromatids of each chromosomes disengage and move towards opposite poles of the cell. The movement is, in part the result of microtubule shortening.After anaphase separation occurs, each chromatid has its own centromere and is now considered to be a chromosome.Anaphase chromosomes elongate somewhat by relaxation of the tight metaphase coiling and move to respective poles of spindle.

Mitosis Anaphase Animal cell Plant cell Photographs from:

4 th step in Mitosis: Telophase The chromosomes movement is completed and the microtubules disassemble.By the completion of telophase, a nuclear membrane is reconstructed around each daughter nucleus. And the nucleolus begins to reappear. The chromosomes uncoil and become more extended.Mitosis is completed.

Mitosis Telophase Animal cellPlant cell Photographs from:

After Mitosis: Cytokinesis The actual splitting of the daughter cells into two separate cells is called cytokinesis and occurs differently in both plant and animal Cells. The cytoplasmic part of the cell divides by it Beginning of cytokinesis in a plant: Beginning of cytokinesis in an animal: