Eukaryotic Cell Division: Mitosis

Pre-Assessment 1. Examine the picture of the cell. What phase is the cell in? A. prophase B. anaphase C. metaphase D. telophase 2. What is the structure labeled "X" on the picture? A. centriole B. spindle C. chromosome D. chromatid 3. During which phase does the DNA make a copy of itself? A. prophase B. metaphase C. interphase D. anaphase 4. During which phase do chromosome first become visible? A. interphase B. telophase C. metaphase D. prophase

What is the purpose of cell division? Growth (example: baby  adult) Reproduction (asexual reproduction in single celled organisms) Replacement of dead/damaged/infected (example: skin/red blood cells/bone cells) Gamete formation in multi-cellular organisms (meiosis)

Chromosome Structure In eukaryotes: made up of DNA and proteins At different times, proteins cause the DNA to: be spread out like spaghetti in a bowl be tightly condensed into the X-shaped (these we can see in the microscope) Central constricted region called centromere that serves as an attachment point for the spindle fibers during mitosis.

Chromosome Structure Chromosomes exist in 2 different states: before DNA replication, chromosomes have one chromatid. after DNA replication, chromosomes have 2 sister chromatids, held together at the centromere. Each chromatid is one piece of DNA with its supporting proteins. Why is DNA replication necessary?

Chromosomes

**Note: Two chromatids make up a chromosome

Cell Cycle The cell cycle describes the behavior of cells as they grow and divide We will study the cycle which includes Mitosis and Cytokinesis Tumors (cancer) are the result of uncontrolled cell division and these can occur in any organ/tissue http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.html

For a typical rapidly proliferating human cell with a total cycle time of 24 hours, the G1 phase might last about 11 hours, S phase about 8 hours, G2 about 4 hours, and M about 1 hour.

Cell Cycle Interphase: Longest part of the cell cycle Includes G1, S and G2 G1: cell growth S: cell growth; replication of cell’s DNA G2: cell growth; organelles replicate DNA begins to condense into chromosomes

Mitosis Mitosis is the division of the eukaryote nucleus, which goes on throughout life in all parts of the body. Organelles can be randomly separated into the daughter cells but chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA. Every human cell has the same 46 chromosomes Mitosis is usually divided into 4 phases: Prophase (P) Metaphase (M) Anaphase (A) Telophase (T) PMAT

Prophase

Phases of Mitosis: 1. Prophase Chromatin finishes condensing into chromosomes (visible under light microscope) Nucleolus/nuclear envelope broken down Spindle fibers form from centrosomes/centrioles with microtubules extending out Chromosomes appear as 2 identical sister chromatids joined together at centromeres

Metaphase

2. Metaphase Longest stage of mitosis Chromosomes move to middle of cell (metaphase plate) Chromosome’s centromeres are on metaphase plate with sister chromatids each facing opposite sides of cell Centrosomes at opposite sides of cell

Anaphase

3. Anaphase: Shortest stage of mitosis Sister chromatids separate and begin moving towards opposite ends of cell (spindle fibers pull sister chromatids in via the centromere) and each one is now a “chromosome” Cell elongates At end of phase, each end of the cell contains complete and identical chromosomes

Telophase and Cytokinesis Cytokinesis: cell divides in two

4. Telophase: -Chromosomes are at each side of cell and nuclear envelope begins to re-form around -Chromosomes elongate to form chromatin -Spindle fibers disintegrate -Cell is elongated and ready for cytokinesis

Animal Cell Cytokinesis inward pinching of plasma membrane to form cleavage furrows

Plant Cell Cytokinesis cell plate forms and moves outward towards the sides of the cell from central region

Prophase Prometaphase Mid-prometaphase Metaphase Anaphase Telophase

What happens when there is no control of cell division? http://learn.genetics.utah.edu/content/begin/cells/signals/ Tumors = result of uncontrolled cell division The genetic checks that stop cells from reproducing fail to work and they grow out of control Oncogene = gene that turns a normal cell into a cancer cell Tumors can occur in any organ or tissue, though are most common after exposure to carcinogens (e.g. tobacco smoke) or in particularly active tissues (e.g. breast, skin) Angiogenesis: tumor recruits blood vessels and grows larger Metastasis: part of the tumor invades the blood vessel, travels through the blood and starts to forma a tumor in another part of the body

Where is the lack of control? Normal cells are controlled by several factors: Normal cells stay in the G1 stage of the cell cycle until they are given a specific signal. Cancer cells enter the S phase without waiting for a signal. Normal cells are mortal. This means that they can divide about 50 times and then they lose the ability to die. This “clock” gets re-set during the formation of the gametes. Cancer cells escape this process of mortality: they are immortal and can divide endlessly. Normal cells that suffer significant chromosome damage destroy themselves due to the action of a gene called “p53”. Cancer cells either lose the p53 gene or ignore its message and fail to kill themselves (process known as apoptosis)