Cell Growth and Reproduction

Why Cells Must Divide In multi-celled organisms (like humans) cells specialize for specific functions thus the original cells must divide to produce different kinds of cells Cells can only take in nutrients and excrete waste products over the surface of the membrane that surrounds them. The surface to volume ratio decreases with the square of the volume (unless special accommodations are made)

Cell Reproduction Cell division is the process by which new cells are produced from one cell Results in two cells that are identical to the original, parent cell All organisms grow and change; worn=out tissues are repaired or are replaced by newly produced cells

Chromosomes Just before cell division, several short, stringy structures suddenly appear in the nucleus Once cell division is complete these structures seem to vanish These structures, which contain DNA and becomes darkly colored when stained, are called chromosomes

Chromosomes Chromosomes are the carriers of the genetic material that is copied and passed from generation to generation of cells Sister chromatids Centromere

Eukaryotic Chromosomes For most of a cell’s lifetime, chromosomes exist as chromatin Chromatin are long strands of DNA wrapped around proteins called histones Before cell division, chromatin must be reorganized into chromosomes

Cell Cycle The cell cycle is the sequence of growth and division of a cell Two general periods: 1.Period of growth 2.Period of division

Interphase The majority of a cell’s life is spent in the growth period known as interphase During interphase, a cell grows in size and carries on metabolism Chromosomes are duplicated in preparation for the period of division

Mitosis Following interphase, a cell enters its period of nuclear division called mitosis Mitosis is the process by which two daughter cells are formed, each containing a complete set of chromosomes Following mitosis, the cytoplasm divides, separating the two cells

Cell Cycle

Phases of Mitosis 1.Prophase 2.Metaphase 3.Anaphase 4.Telophase

Prophase During prophase, 1 st and longest phase, the long, stringy chromatin coils up into visible chromosomes Each duplicated chromosome is made up of two halves, sister chromatid, they are connected by a centromere As prophase continues, the nucleus begins to disappear as the nuclear envelope and the nucleolus disintegrate

Prophase By late prophase, these structures are completely absent (nuclear envelope and nucleolus) In animal cells, two important pairs of structures, the centrioles, begin to migrate to opposite ends of the cell The second structure that forms is the spindle, which is a football-shaped, cagelike structure In plant cells the spindle forms without the centrioles

Stages Of Mitosis Interphase Anaphase Telophase Metaphase Mitotic spindle Prophase Disappearing nuclear membrane Mother cell

Metaphase During metaphase, the doubled chromosomes become attached to the spindle fibers by their centromeres The chromosomes are pulled by the spindle fibers and begin to line up on the midline, or equator, of the spindle Each sister chomatid is attached to it’s own spindle fiber

Stages Of Mitosis Interphase Anaphase Telophase Metaphase Mitotic spindle Prophase Disappearing nuclear membrane Mother cell

Anaphase The separation of sister chromatids marks the beginning of anaphase During anaphase, the centromeres split apart and chromatid pairs from each chromosome separates from each other The chromatids are pulled apart by the shortening of the spindle fibers

Stages Of Mitosis Interphase Anaphase Telophase Metaphase Mitotic spindle Prophase Disappearing nuclear membrane Mother cell

Telophase Final phase of mitosis, begins as the chromatids reach the opposite poles of the cell Many of the changes that occurred during prophase are reversed as the new cells prepare for their own independent existence

Telophase The chromosomes, which had been tightly coiled since the end of prophase, now unwind so they can begin to direct the metabolic activities of the new cell The spindle begins to break down, the nucleolus reappears, and an new nuclear envelope forms around each set of chromosomes A new double membrane (plasma membrane0 begins to form between the two new nuclei

Stages Of Mitosis Interphase Anaphase Telophase Metaphase Mitotic spindle Prophase Disappearing nuclear membrane Mother cell

Cytokinesis Following telophase, the cell’s cytoplasm divides in a process called cytokinesis Cytokinesis differs between plants and animals In animals, the plasma membrane pinches in along the equator In plants, they form a cell plate that is laid across the equator, and a cell wall will form along this plate

Results of Mitosis Mitosis is a process that guarantees genetic continuity The new daughter cells produced will carry out the same function as the parent or original cell