lesson 2: mitosisIt’s very likely that each time you go to the doctor, a nurse measures your height and mass. This activity gives the medical profession an idea of how people grow. Much of the growth happens because the number of cells in your body increases as you develop.
Most of the kinds of cells in your body are constantly replacing themselves. Bone cells make new bone cells. Muscle cells make new muscle cells. The fact is that you are constantly changing. You aren’t the same now as you were a year ago or even a few hours ago. At this very moment, as you read this page, groups of cells throughout your body are growing, dividing, and dying. Worn out cells on the palms of your hands are being replaced. Cuts and bruises are healing. Other organisms undergo similar processes. How does this happen? There are two ways by which cells increase in number: § Mitosis – is the process in which a cell nucleus divides into two new nuclei, each of which contains the same number of chromosomes as the parent cell. § Meiosis – is the process in which there are two divisions of the nucleus: meiosis I and meiosis II. the cell cycle |
OBJECTIVES OF THE LESSON:
At the end of this lesson, learners are expected to demonstrate an understanding of: (1) determine the stages in the cell cycle; (2) differentiate the phases of cell division. video lessons |
mitosis
Cell division is a cycle of events. The process of cell division has been divided into five phases. These are:
§ Interphase: the state of a eukaryotic cell when not undergoing division. Every time a cell divides, it must first replicate all of its DNA. Because chromosomes are simply DNA wrapped around protein, the cell replicates its chromosomes also. These two chromosomes, positioned side by side, are called sister chromatids and are identical copies of one another. Before this cell can divide, it must separate these sister chromatids from one another.
§ Prophase: The chromatin (The total collection of DNA and proteins in a chromosome.), diffuses in interphase, condenses into chromosomes. Each chromosome has duplicated and now consists of two sister chromatids. The centrioles form asters (ray-like structures) and move toward the opposite sides of the cell. At the end of prophase, the nuclear envelope breaks down into vesicles.
§ Metaphase: The chromosomes align at the equatorial plate (center of the cell) and are held in place by microtubules attached to the mitotic spindle and to part of the centromere.
§ Anaphase: The centromeres divide. Sister chromatids separate and move toward the corresponding poles.
§ Telophase: Daughter chromosomes arrive at the poles and the microtubules disappear. The condensed chromatin expands and the nuclear envelope reappears. The cytoplasm divides, the cell membrane pinches inward ultimately producing two daughter cells (phase: Cytokinesis).
§ Interphase: the state of a eukaryotic cell when not undergoing division. Every time a cell divides, it must first replicate all of its DNA. Because chromosomes are simply DNA wrapped around protein, the cell replicates its chromosomes also. These two chromosomes, positioned side by side, are called sister chromatids and are identical copies of one another. Before this cell can divide, it must separate these sister chromatids from one another.
§ Prophase: The chromatin (The total collection of DNA and proteins in a chromosome.), diffuses in interphase, condenses into chromosomes. Each chromosome has duplicated and now consists of two sister chromatids. The centrioles form asters (ray-like structures) and move toward the opposite sides of the cell. At the end of prophase, the nuclear envelope breaks down into vesicles.
§ Metaphase: The chromosomes align at the equatorial plate (center of the cell) and are held in place by microtubules attached to the mitotic spindle and to part of the centromere.
§ Anaphase: The centromeres divide. Sister chromatids separate and move toward the corresponding poles.
§ Telophase: Daughter chromosomes arrive at the poles and the microtubules disappear. The condensed chromatin expands and the nuclear envelope reappears. The cytoplasm divides, the cell membrane pinches inward ultimately producing two daughter cells (phase: Cytokinesis).
meiosis
Four stages can be described for each nuclear division.
§ First division of meiosis- REDUCTIONAL DIVISION
ú Prophase 1: Each chromosome duplicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage.
ú Metaphase 1: Homologous chromosomes align at the equatorial plate.
ú Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
ú Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
§ Second division of meiosis: EQUATIONAL DIVISION
ú Prophase 2: DNA does not replicate.
ú Metaphase 2: Chromosomes align at the equatorial plate.
ú Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.
ú Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.
One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.
Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.
§ First division of meiosis- REDUCTIONAL DIVISION
ú Prophase 1: Each chromosome duplicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage.
ú Metaphase 1: Homologous chromosomes align at the equatorial plate.
ú Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
ú Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
§ Second division of meiosis: EQUATIONAL DIVISION
ú Prophase 2: DNA does not replicate.
ú Metaphase 2: Chromosomes align at the equatorial plate.
ú Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.
ú Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.
One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.
Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.