1 6 Cell DivisionMitosis Question Why would a

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: – G 1 – cell grows and metabolizes – S – DNA is replicated – G 2 – 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