1Batterjee Medical College (BMC) FAST (preparatory year) Biology Department 2012-2013
2 CELL DIVISION Objectives 1. Define types of cell division. 2. Name the stages of the cell cycle and explain what happens during each stage. 3. Differentiate between Mitosis and Meiosis The cell cycle is the series of events that take place in a cell leading to its division and duplication (replication). In cells with a nucleus (eukaryotes), the cell cycle can be divided in two periods: Interphase: during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA. The mitosis (M) phase: during which the cell splits itself into two distinct cells "daughter cells" and the final phase, cytokinesis, where the new cell is completely divided.TYPES OF CELL DIVISION:A. The type of cell division in somatic body cells of eukaryotes is known as mitosis, producing two identical cells and leaves the daughter cells capable of dividing again.B. For simple unicellular organisms such as Amoeba, one cell division known as binary fission is equivalent to reproduction-- an entire new organism is created.
3C. Another type of cell division present only in eukaryotes, called meiosis, a cell is permanently transformed into a gamete and may not divide again until fertilization. Meiosis is the process by which gametes are generated for reproduction. Some basic definitions are in order to better understand the concept of cell division,• Gene: Basic unit of heredity; codes for a specific trait.• Genome: The total hereditary content of DNA of a cell or organism.• Somatic cell: All body cells except reproductive cells.• Gamete: Reproductive cells (i.e. sperm & eggs).• Chromosome: Elongate cellular structure composed of DNA and protein - they are the vehicles which carry DNA in cells.• Diploid (2n): Cellular condition where each chromosome type is represented by two homologous chromosomes.• Haploid (n): Cellular condition where each chromosome type is represented by only one chromosome.• Homologous chromosome: chromosome of the same size and shape which carry the same type of genes.• Chromatid: One of two duplicated chromosomes connected at the centromere.
4THE CELL CYCLE: A resting phase where the cell has left the cycle Resting Gap 0 G0 and has stopped dividing. Cells increase in size in G1. This phase is marked Growth 1 G1 by synthesis of proteins and enzymes that are (Gap 1) required in S phase for DNA replication. Synthesis S DNA replication occurs during this phase. Interphase During the gap between DNA synthesis and Growth 2 mitosis, the cell will continue to grow. The G2 (Gap 2) G2control mechanism ensures that everything is ready to enter the M (mitosis) phase and divide. Cell growth stops at this stage and cellular energy Cell division Mitosis M is focused on division into two daughter cells.
5 MITOSIS (MITOTIC CELL DIVISION) Mitosis is a process of cell division which results in the production of two daughter cells from a single parent cell. The daughter cells are identical to one another and to the original parent cell. Mitosis maintains a constant amount of genetic material from cell generation to cell generation. Why mitosis is performed by somatic body cells?1. Growth. The number of cells within an organism increases by mitosis and this is the basis of growth in multicellular organisms.2. Cell Replacement. Cells are constantly sloughed off, dying and being replaced by new ones in the skin and digestive tract. When damaged tissues are repaired, the new cells must be exact copies of the cells being replaced so as to retain normal function of cells. STAGES OF MITOSIS Nuclear division: Prophase–Metaphase–Anaphase and Telophase. Cytoplasmic division: Cytokinesis.
6 Interphase (G2): Showing normal cell structure with obvious nuclear membrane and chromatin materials in the cytoplasm and duplicated DNA content.1- Prophase:- The first and longest stage of mitosis.- Chromatin condenses to chromosomes.- Each chromosome appear as two sister chromatids.-The nuclear envelope dissolves.- The nucleolus in the nucleus disappears.- Centrioles divide and migrate to opposite poles.- Aster rays and spindle fibers forms.2- Metaphase:- Chromosomes with Pairs of sister chromatids alignthemselves at the equatorial plane of the cell(metaphase plate). 3- Anaphase: - Anaphase begins when the duplicated centromeres of each pair of sister chromatids separate, and the now- daughter chromosomes begin moving toward opposite poles of the cell due to the action of the spindle.4- Telophase:- Chromosome sets assemble uncoil at opposite poles, anuclear envelope and nucleolus reforms. Cytokinesis: The division of the cytoplasm,usually is in progress before nuclear division is completeinvolves the formation of a cleavage furrow resulting inthe pinching of the cell into two.
7MEIOSIS (MEIOTIC CELL DIVISION):Meiosis is a special type of nuclear division which segregates one copy of eachhomologous chromosome into each new "gamete", the number ofchromosomes is reduced by half, and returned to the full amount when the twogametes fuse during fertilization.Meiotic division occurs only in cells of male or female sex organs andproduces 4 haploid cells. (Oogenesis in ovary & Spermatogenesis in testis).Phases of Meiosis include two successive nuclear divisions, Meiosis I(Reduction) and Meiosis II (Division).In this drawing, the parent cell (2N) is diploid. It divides in the first division(Meiosis I) into 2 daughter haploid cells (N). The daughter cells, which arealready haploid, then go through the second division (Meiosis II) into a totalof 4 daughter cells. These final daughter cells are haploid, act as gametes.
8MEIOSIS I: 1- Prophase I: Chromatin condenses into chromosomes. Chromosomes coil tightly & are visible . Nuclear membrane & nucleolus disintegrate. Centrioles separate, migrate and Spindle fiber forms. Synapsis (joining) of homologous chromosomes occurs making tetrads. Kinetochore fiber forms on each chromosome. Chromosomes in tetrad exchange fragments by a process called crossing over. 2- Metaphase I: Paired homologues chromosomes line up side by side along cell equator. 3- Anaphase I: Anaphase I is when the tetrads of homologous chromosomes (not sister chromatids) separate, and are drawn to opposite poles by the spindle fibers (reduction ½ of chromosome number). . 4- Telophase I: In telophase I only one set of (replicated) chromosomes separate (no chromatid separation) is in each "cell". Depending on species, new nuclear envelopes may or may not form. Two daughter cells are formed during telophase I. These usually go immediately into the second meiotic cell division (meiosis II).
9 MEIOSIS II:1- Prophase II: In each haploid daughter cell (called SecondarySpermatocytes or secondary oocytes ), a spindle forms.Chromatids are attached to the spindle. (Note: DNAis not copied before Meiosis II cell division).2- Metaphase II: Chromosomes become aligned in the center of the cellattached to spindle fibers. Each chromosome has two sister chromatids attached to thespindle. 3- Anaphase II: The sister chromatids separate and move to opposite poles. Two sets of chromosomes are formed at each pole of each cell (haploid=n).4- Telophase II: The chromatids, now considered individualchromosomes, arrive at the poles. Nuclear membranereforms, Nucleus reappears. Cytokinesis:The process of meiosis is completed, producingfour haploid daughter cells as a final result. • •
10 IMPORTANT TERMS FOR MEIOSIS IN HUMAN:• Reduces the number of chromosomes in new cells to half the number in the original cell• New cells have a single copy of chromosomes (23 total) but are not identical to each other or the original parent cell• Used for making gametes ( sperm and eggs) with the haploid or n number• In meiosis, cells divide twice after a single DNA duplication• Meiosis I separates homologs & the Meiosis II separates sister chromatids• Meiosis I stages are Prophase I, Metaphase I, Anaphase I, & Telophase I• Meiosis II stages are Prophase II, Metaphase II, Anaphase II, & Telophase II• Produces 4 haploid cells or gametes• When a sperm fertilizes an egg to form a zygote, the diploid number of chromosomes is restored (23 + 23 = 46)• Egg cells or ova (ovum, singular) are larger , nonmotile cells• Gametoogenesis is meiosis producing eggs & occurs in the females ovaries.• Sperms contain less cytoplasm so theyre smaller & have a flagellum to swim to the egg.• Spermatogenesis is meiosis producing sperm cells & occurs in the t e s t e s
13 HOMEWORK: CELL DIVISION 1. Mention the four stages of Mitosis in their order of occurrence. 1. ………………………………. 2. ………………………………. 3. ………………………………. 4. ………………………………. 5. Put (T) or (F) and correct the false statement: 1. In anaphase the nuclear envelope re-forms around the two sets of ( ) chromosomes. 2. Meiosis cell division goes on throughout the life in all parts of the ( ) body. 3. In prophase, the centromeres divide. At this point, each individual ( ) chromosome goes from 1 chromosome with 2 chromatids to 2 chromosomes with one chromatid each. 4. Mitosis is a reduction division. ( ) 5. In humans, each cell contains 46 pairs of chromosomes. ( ) 6. Males have two copies of the X chromosome, while females have one ( ) X and one Y chromosome. 7. Every person has two copies of each gene, one inherited from each ( ) parent. 8. Meiosis is the type of cell division by which germ cells (eggs and ( ) sperm) are produced.9. Write 3 differences between Mitosis & Meiosis. _______________________________________________________________ _____________________________________________________________ _____________________________________________________________
1410. In which phases the following events occur: 1. Formation of tetrads: 2. Separation of sister chromatids: 3. Separation of homologous chromosomes: 4. Duplication of genetic materials: 5. Formation of 2 separate cells: 6. Lining of the chromosomes at the metaphase plate: 7. Dissolving of nuclear membrane: 8. Crossing over:V. What is the difference between oogenesis and spermatogenesis?____________________________________________________________________VI. Write the title for each phase: