1.6 Cell Division/Mitosis

Question? Why would a cell want or need to divide? Growth/development of tissues/organs Repair of damaged tissue/organ Reproduction of cells/organisms

Cell Cycle The cell’s life cycle (usually just called the cell cycle) consists of two main parts: Interphase – active period in the life of a cell when many metabolic reactions occur, including protein synthesis DNA replication an increase in the number of mitochondria and/or chloroplasts Mitosis – cell divides

Cell Cycle Interphase can be divided into three phases: G1 – cell grows and metabolizes S – DNA is replicated G2 – cell prepares for division

Mitosis Mitosis can be divided into four stages: Prophase Metaphase Anaphase Telophase

Mitosis Prophase Nuclear envelope disappears With the aid of special proteins called histones, DNA supercoils into distinct chromosomes that are visible under the microscope Each pair of identical segments of DNA, called sister chromatids, bind together at the centromere to form one chromosome

Prophase Centrioles moved to opposite ends of the cell Long, tubular proteins called “spindle fibers” grow from the centrioles and attach to the centromeres of the chromosomes

Metaphase Spindle fibers pull chromosomes to the equator (center line) of the cell

Anaphase Spindle fibers shorten Sister chromatids are pulled apart and are now called chromosomes Chromosomes are pulled to opposite ends of the cell

Telophase Reverse of prophase: Spindle fibers dissapear DNA uncoils and becomes chromatin Nuclear envelope reforms

Cytokinesis Splitting of the cytoplasm Occurs when microtubule proteins pinch inward at the equator (cleavage furrow) Results in two separate but identical cells

Plant Cell Division In plants a cell plate forms instead of a cleavage furrow

Overview Mitosis produces two genetically identical nuclei During the S phase of interphase DNA is replicated to produce two identical copies During prophase the identical copies of each chromosome (homologous chromsomes) bind together Homologous chromosomes are pulled apart during anaphase and become part of two separate nuclei during telophase

Mitosis serves many purposes: Tissue/organ growth Embryonic devleopment, when the zygote divides to produce many smaller cells Tissue damage and repair Asexual reproduction (unicellular organisms)

Control of this process Cyclins Series of proteins that control cells during cell cycle.

Not always controlled Mutagens – Physical or chemical agent that changes the DNA in an organism. This increases the frequency of mutations in that organism above the natural background level. Often these mutagens can cause cancer/tumors

Oncogenes Sometimes the gene controlling cell division (called an oncogene) will become mutated The result is that cell division continues repeatedly This is how tumors are formed Tumors can form in any tissue of the body Stomach Tumor

Metastasis Greek for “Next Placement” Cells that are going through uncontrolled proliferation can sometimes slough off of a tumor and spread through the body to another area that previously was unaffected. These cells will continue their rapid mitotic division and cause the disease to spread.

Tumors Tumors are a mass of cells that are going through mitosis uncontrollably due to a mutation in the DNA Two major types: Malignant: “Badly Born” can spread to invade other cells throughout the body. Benign: lacks the ability to invade neighboring cells