1. Overview 2. Mitosis 3. Cytokinesis 4. Binary Fission Lecture 6 The Cell Cycle 1. Overview 2. Mitosis 3. Cytokinesis 4. Binary Fission Prepared by Mayssa Ghannoum

Overview The ability of organisms to reproduce their own kind is the one characteristic that best distinguishes living things from nonliving matter. The unique capacity to procreate has a cellular basis: “Where a cell exists, there must have been a preexisting cell”. The continuity of life is based on the reproduction of cells, or cell division.

The Key Roles of Cell division Cell division plays several important roles in the life of an organism. It enables sexually reproducing organisms to develop from a single cell -- the fertilized egg, or zygote. After an organism is fully grown, cell division function in renewal and repair.

The Key Roles of Cell division The Cell division process is a part of the cell cycle. Cell cycle: the life of a cell from the time it is first formed from a dividing parent cell until its own division into two cells. Passing identical material to cellular offspring is a crucial function of cell division.

The Functions of Cell Division

Most Cell division involves the distribution of identical genetic material –DNA – to two daughter cells. DNA is passed from one generation of cells to the next. A dividing cell duplicates its DNA, allocates (distributes) the two copies to opposite ends of the cell, and only then splits into daughter cells.

Cellular Organization of the Genetic Material A cell’s composition of DNA, its genetic information, is called genome. The overall length of DNA in a eukaryotic cell is enormous compared to the diameter of the cell itself but the replication and distribution of DNA is manageable because the DNA molecules are packaged into chromosomes.

Cellular Organization of the Genetic Material Eukaryotic chromosomes are made of chromatin, a complex of DNA associated with proteins molecules. Each single chromosome contains one very long, linear DNA molecule that carries several hundred genes. Genes: are the units that specify an organism’s inherited traits.

Every eukaryotic species has a characteristic number of chromosome in each cell nucleus. Somatic cells contain 46 chromosomes made up of two sets of 23, one set is inherited from each parent. Gametes ( sperm and egg) have half number of chromosomes as somatic cells, or one set of 23 chromosomes. Somatic cells: all body cells except the reproductive cells. Gametes: Reproductive cells

Distribution of chromosomes During Eukaryotic Cell Division In a non dividing cell, each chromosome is in the form of a long, thin chromatin fiber. After DNA duplication, the chromosomes condense: Each chromatin fiber becomes densely coiled and folded; the chromosomes become much shorter and so thick.

Distribution of chromosomes During Eukaryotic Cell Division Each duplicated chromosome has two sister chromatids. The two chromatids, each containing an identical DNA molecule, are initially attached all along their lengths by adhesive protein complexes called cohesions. The duplicated chromosomes has a narrow waist called centromere, where the 2 chromatids are closely attached.

The Cell cycle Mitosis: is the division of the nucleus. Mitosis is usually immediately followed by cytokinesis. Cytokinesis is the division of the cytoplasm. One parent cell is divided into two daughter cells genetically identical to their parent cell.

Phases of the Cell Cycle Mitosis is just one part of the cell cycle. In fact, the Mitotic (M) phase, which includes both: Mitosis and Cytokinesis, is the shortest part of the cell cycle. The longest stage of the cycle is called Interphase. Interphase accounts for about 90% of the cell cycle. During interphase, the cell grows and copies its chromosomes in preparation for cell division.

Interphase can be divided into three sub phases: The G1 phase (first Gap): Cell growth The S phase (synthesis): Cell copies its chromosomes The G2 phase (second Gap): Cell grow more as it completes preparation for cell division. M phase: the cell divides Daughter cells may then repeat the cycle.

Mitosis Mitosis is conventionally broken down into five stages: 1- Prophase 2- Prometaphase 3- Metaphase 4- Anaphase 5- Telophase Cytokinesis completes then the Mitotic phase.

G2 of Interphase A nuclear envelope bounds the nucleus. The nucleolus is visible. Two centrosomes have formed by replication of a single centrosome. Each centrosome features two centrioles. Chromosomes, duplicated during S phase cannot be seen individually because they have not yet condensed.

Prophase The chromatin fibers become more tightly coiled, condensing into chromosomes . The nucleolus disappear. Each duplicated chromosome appears as two identical sister chromatids joined at centromeres. The mitotic spindle begins to form. It’s composed of the centrosomes and microtubules that extend from them. Centrosomes move away from each other propelled by the lengthening microtubules between them.

Prometaphase The nuclear envelope breaks into fragments. The microtubules extending from each centrosome invade the nuclear area. Chromosomes become even more condensed. Each of the two chromatids has a kinetochore, a specialized protein located at the centromere. Some microtubules attach to the kinetochore becoming "Kinetochore microtubules”.

Metaphase It is the longest stage of Mitosis Centrosomes are at opposite poles of the cell. The chromosomes are aligned on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The Kinetochores of the sister chromatids are attached to microtubules coming form opposite poles.

Anaphase It is the shortest stage of mitosis. Anaphase begins with the cohesion proteins are cleaved, which allows the 2 sister chromatids of each pair to part suddenly. The two liberated daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. The cell elongates. By the end of anaphase, the two ends of the cell have equivalent and complete collections of chromosomes.

Telophase Two daughter nuclei form in the cell. Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope. Nucleolus reappear. The chromosomes become less condensed. Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

Cytokinesis Cytokinesis which is the division of the cytoplasm is usually well under way by late telophase, so the two daughter cells appear shortly after the end of mitosis. Cytokinesis differ between animal and plant cells.

Cytokinesis In animal cells, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface. The cleavage furrow deepens until the parent cell is pinched in two, producing two completely separated cells, each with its own nucleus and share of cytosol, and organelles.

Cytokinesis In plant cells, which have cell walls; cytokinesis occurs by a process called formation of cell plate. Cell wall materials collect in the cell plate as it grows. The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell. Two daughter cells result, each with its own plasma membrane. A new cell wall arising from the contents of the cell plate is formed between the daughter cells.

Mitosis in a plant cell

Binary Fission The asexual reproduction of single-celled eukaryotes, such as amoeba, includes mitosis and occurs by a type of cell division called binary fission. Binary fission means division in half. Prokaryotes ( bacteria and archaea) also reproduce by binary fission, but the prokaryotic process does not involve mitosis.

Binary Fission In bacteria, most genes are carried on a single bacterial chromosome that consists of a circular DNA and associated proteins. The process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at specific place called the origin of replication, producing two origins.

Binary Fission As the chromosome continue to replicate, one origin moves rapidly toward the opposite pole of the cell. Meanwhile, the cell elongates. When replication is complete and the bacterium reach about twice its initial size, its plasma membrane grows inward, dividing the parent bacteria cell into two daughter cells. Each cell inherits a complete genome.

Bacterial cell division by Binary Fission