Lecture 6: Cell division The continuity of life from one cell to another is based on بناءاً على the reproduction of cells via بواسطة cell division. This division process occurs as part of the cell cycle (the life of a cell from its origin in the division of a parent cell until its own division into two). Unicellular وحيد الخلية organisms (e.g. Amoeba) divide to reproduce an entire organism, increasing the population (asexual reproduction). Multicellular عديد الخلية organisms divide to develop a fertilized cell, grow or repair the damaged cells. Fig. 12.1, Page 216

Division differs among cells: Cell division distributes the genetic material (DNA) to two daughter cells. Division differs among cells: Skin cells divide frequently. Liver cells divide when needed (damage repair). Nerve cells and muscle cells do not divide at all.

Cell division distributes identical sets of chromosomes to daughter cells. A cell’s genetic information (genome البنك الـجينى) is packaged as DNA. In prokaryotes, the genome is often a single long DNA molecule. In eukaryotes, the genome consists of several DNA molecules. A human cell must duplicate about 3 m of DNA and separate the two copies such that each daughter cell ends up with a complete genome. DNA molecules are packaged into chromosomes. Human gametes أمشاج (sperm or eggs) have 23 chromosomes, half the number in a somatic cell الخلية الجسدية. Each eukaryotic chromosome consists of a long, linear DNA molecule.

Each chromosome has hundreds or thousands of genes. This DNA-protein complex (chromatin) is organized into a long thin fiber. After the DNA duplication, chromatin condenses to form chromosome. Each duplicated chromosome consists of two sister chromatids which contains identical copies of the chromosome’s DNA. The narrow region where the chromosomal strands connect is called centromere. The process of the formation of the two daughter nuclei called mitosis.

Types of cell divisions 1- Mitosis الانقسام غير المباشر أو الميتوزي: Mitosis is a continuous process and occurs only in eukaryotic cells in non-sex cells (somatic cells). Prokaryotic cells divide by a different process called binary fission. Mitosis has two divisions which are karyokinesis (division of the nucleus) is followed by cytokinesis (division of the cytoplasm). Mitosis occurs in either haploid أحادي المجموعة الكروموسومية or diploid ثنائي المجموعة الكروموسومية cells. 2- Meiosis الانقسام الاختزالي أو الميوزي: Meiosis occurs only in germ (sex) cells and is restricted to diploid cells. Meiosis takes place solely in the testes (males) and ovaries (females) to produce haploid sex cells or gametes.

Homologous Chromosome Centromere Chromatid Chromatin + DNA Sister chromatid Homologous Chromosome Centromere Chromosome الكروموسوم أو الصبغ

The mitotic cell cycle The cell cycle consists of two basic stages: a) interphase and b) mitotic (M) phase. The M phase includes Karyokinesis الانقسام النووي and cytokinesis الانقسام السيتوبلازمي . Interphase accounts for 90% of the cell cycle. During interphase the cell prepares for division by producing cytoplasmic organelles and copying its chromosomes. Interphase has three subphases: The G1 phase (first gap): the cell grows. The S phase (DNA synthesis): where the DNA replication occurs The G2 phase (second gap): the cell completes preparations for cell division. Mitotic phase (M). The cell starts the division process. The resulting daughter cells may then repeat the cycle again.

The phases of Mitotic division: Prophase الطور التمهيدي Metaphase الطور الإستوائي Anaphase الإنفصالي الطور Telophase الطور الإنتهائي By late interphase (G2), the chromosomes have been duplicated تضاعفت but are loosely packed. The centrosomes have been duplicated and begin to organize microtubules into an aster (star).

Prophase: The first stage of mitosis begins with the shorting and thickening of the chromosomes. The chromosomes are tightly coiled, with sister chromatids joined together. The nuclear membrane fragments and the nucleolus disappears. 2) Metaphase: The mitotic spindle fibers begins to form and appears to push the centromere away from each other towards opposite ends (poles) of the cell. Microtubules from one pole attach to one of two kinetochores (special regions of the centromere) while microtubules from the other pole attach to the other kinetochore. The chromosomes align along the metaphase plate.

3) Anaphase: The centromeres divide, result in separating and moving of the two sister chromatids toward opposite poles on the spindle fibers . By the end, the two poles have equivalent collections of chromosomes. 4) Telophase: Two nuclei begin to form, surrounded by the nuclear membrane. Chromatin becomes less tightly coiled. Cytokinesis takes place. In animals, cytokinesis is first apparent by contraction between the two poles of the cell. In plants, a cell plate grows in the approximate location of the metaphase plate.

Summary of the Cell Cycle Interphase G1 S G2 Prophase Metaphase Anaphase Telophase Mitotic phase (M) karyokinesis Cytokinesis