CELL CYCLE How Do Cells Divide?

What you will learn…  1. Why Do Cells Divide?  2. Chromosome structure  3. Cell Division in Prokaryotes  4. Cell Cycle  5. Mitosis  6. Cytokinesis  7. Control of Cell Division and Cancer  8. Meiosis  9.Why do cells need two types of cell division?  10. Gamete Formation

1. Why Do Cells Divide?  Virchow: Cells can only come from preexisting cells  In unicellular organisms, can reproduce an entire organism  Allows multicellular organisms to reproduce asexually  Basis of sexual reproduction  sperm and egg  Allows fertilized egg, or zygote, to develop into an adult organism  Replaces worn-out or damaged cells  Enables multicellular organism to grow to adult size  m.html m.html

2. Chromosome Structure  Human cells carry about 20,000 genes to make 100,000 proteins.  Almost all genes are located in the nucleus  Very small amount found in mitochondria  Genes are found on DNA  DNA can be in the form of Chromatin:  Diffuse mass of long, thin fibers, not seen under the microscope, less tightly coiled  Combination of DNA and protein  DNA must be tightly packaged before cell division, so it can be evenly divided between the two new cells.  DNA will now be in the form of Chromosome!

2. Chromosome Structure  Chromosomes  Rod-shaped structure  Coiled up, compact forms of chromatin  Contains one long DNA molecule bearing hundreds or thousands of genes.  DNA is attached to protein molecules called histones and other non-histone proteins DNA wraps with protein like wrapping paper on a present giving it the X-shape Only found in eukaryotic cells (prokaryotes have naked, circular shaped chromosomes)

2. Chromosome Structure

 Sister chromatids  Each duplicated chromosome contains two identical copies.  Centromere  The point by which two chromatids are joined.  Chromatin  Diffuse mass of long, thin fibers, not seen under the microscope, less tightly coiled  Combination of DNA and protein

2. Chromosome Structure

3. Cell Division in Prokaryotes  Binary fission  Process by prokaryotes reproduce by cell division.  Steps: Duplication of chromosomes and separation of copies. Cell elongates Divides into two daughter cells

3. Cell Division in Prokaryotes

4. Cell Cycle  In your own body, millions of cells must divide every second to maintain the total number of about 100 trillion cells.  Some cells divide once a day, and some do not at all (mature muscle cells, brain cells)

4. Cell Cycle  Starts out with Interphase  Occurs when the cell is between cell division  Interphase stages:  G1: Cells grow to mature size  S: DNA is copied  G2: Cell prepares for division Cells exit the cell cycle via… G0: Cells do not copy DNA or prepare for mitosis, but are still alive (e.g. nervous system)

5. Mitosis  The last stage of the cell cycle when the nucleus of a cell divides to produce two new daughter cells (after cytokinesis) each with the same amount and type of chromosomes as the parent cells.  Mitosis is divided into four phases:  A.Prophase  B. Metaphase  C. Anaphase  D. Telophase

5. Mitosis  A.Prophase:  What does the cell look like? Centrioles and spindle fibers appear Nuclear envelope disappears, and chromosomes are visible  What happens to the DNA and nucleus? Chromosomes form when chromatin tightens and coils Nuclear membrane breaks down and disappears  What two things appear near where the nucleus was? Centrioles and spindle fibers

5. Mitosis  A. Prophase

5. Mitosis  B. Metaphase  What does the cell look like? Chromosomes move to the middle  Where are the chromosomes during metaphase? Middle of the cell

5. Mitosis B. Metaphase

5. Mitosis  C. Anaphase:  What does the cell look like? Chromosomes move to the end of cell  What happens to the chromosomes? Chromosome splits at centromere into 2 chromatids and moves to end of cell

5. Mitosis  C. Anaphase

5. Mitosis  D. Telophase  What does the cell look like? Cell starts to pinch in Nucleus starts to reform Chromosomes are at opposite ends  What happens to the chromosomes and nucleus? Nucleus forms back around single chromatids

5. Mitosis  D. Telophase

6. Cytokinesis  What is cytokinesis?  Cytoplasm and contents (other organelles) divide  What’s special about cytokinesis in plants?  Cell wall also divides with new cell plate in middle  What’s special about cytokinesis in animals?  Takes place when the cell membrane pinches in until the cytoplasm is pinched into two equal halfs

7. Control of Cell Division and Cancer  Cell division is a complex process that needs to be regulated.  These regulators determine when and how the cell should divide.  External Regulators  Internal Regulators

7. Control of Cell Division and Cancer  External regulators:  Various proteins produced by other cells that speed up or slow down the cycle. If the cell touches other cells, than cell division slows down. If enough space between cells and nutrients are available, growth factors and other proteins make cells divide or speed up their cell cycle.

7. Control of Cell Division and Cancer  Internal regulators:  Cyclins – proteins that regulate the timing of the cell cycle in eukaryotic cells.  Other regulator proteins (checkpoints)– they make sure that certain things happen in the cell before the cell moves to the next phase of the cell cycle 3 major checkpoints in the cell cycle.  The age of the cell.

7. Control of Cell Division and Cancer  Cancer cells  lack normal checkpoints and continue to grow without inhibition  do not respond to normal signals within the cell  are not inhibited by other cells  will divide indefinitely

7. Control of Cell Division and Cancer  Mutations in the genes of these checkpoint proteins may lead to cancer:  The uncontrolled growth of cells.  Tumor: an abnormally growing mass of body cells  Benign tumor If abnormal cells remain at original site Can be problematic if disrupt certain organs, but usually easily removed by surgery  Malignant tumor If abnormal cells spread into other tissues and body parts, interrupting organ function

7. Control of Cell Division and Cancer 1. Tumor growth 2. Blood vessels feed tumor 3. Tumor cells enter blood and lymph vessels 4. Secondary tumors form in other parts of the body Movie clips on cancer, its nature and experiments to treat it (Parts 2 and 6) ted Tumor Progression

7. Control of Cell Division and Cancer  Treatment of Cancer:  Surgical removal of tumor – Most effective when tumor is in a defined area  Chemotherapy – Medicines that disrupt the process of mitosis in rapidly growing cells  Radiation Therapy - High energy gamma radiation is aimed at the growing tumour. This damages the DNA in rapidly dividing cells and helps to destroy the tumor.

8. Meiosis  Two chromosomes composing a pair are called homologous chromosomes (or homologues) because they both carry genes controlling the same inherited characteristics.

8. Meiosis  Any cell with two homologous (the same) sets of chromosomes is called a diploid cell  the total number of chromosomes is called the diploid number (abbreviated 2n)  For humans, the diploid number is 46; that is 2n=46  Almost all human cells are diploid

8. Meiosis  The exception are the egg and sperm cells, collectively known as gametes.  A cell with a single chromosome set is called a haploid cell.  For humans, the haploid number (abbreviated n) is 23; that is n=23

8. Meiosis Many of the stages of meiosis closely resemble corresponding stages in mitosis. Type of cell division that produces haploid gametes in diploid organisms. Like mitosis, is preceded by the replication of chromosomes. However, this single replication is followed by two consecutive cell divisions, called Meiosis I and Meiosis II. These divisions result in four daughter cells, each with a single haploid set of chromosomes. Produces daughter cells with only half as many chromosomes as the parent cell.

 Prophase I –  Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. The tetrads overlap and exchange some of their genetic material – crossing-over.  Metaphase I –  Spindle fibers attach to the chromosomes.  Anaphase I –  The fibers pull the homologous chromosomes toward opposite ends of the cell.  The cells are now containing half of the genetic information from the original parent cell and are thus considered HAPLOID! 8. Meiosis

 Telophase I and cytokinesis –  Nuclear membranes reforms, the cell separates into two cells.  Prophase II –  Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.  Metaphase II –  The chromosomes line up in a similar way to the metaphase stage of mitosis. 8. Meiosis

 Anaphase II –  The sister chromatids separate and move toward opposite ends of the cell.  Telophase II and cytokinesis –  Meiosis II results in four haploid (N) daughter cells Meiosis

 Crossing over in Prophase I results in great diversity because new genetic variations can result from it. 8. Meiosis

9.Why do cells need two types of cell division?  Mitosis  Provides growth, tissue repair, and asexual reproduction  Produces daughter cells genetically identical to the parent cell  Involves one division of the nucleus, and is usually accompanied by cytokinesis, producing two diploid daughter cells.  Meiosis  Need for sexual reproduction  human egg and sperm cells  Entails two nuclear and cytoplasmic divisions  Yields four haploid daughter cells, with one member of each homologous chromosome pair.  Form tetrads; crossing over occurs.

10. Gamete Formation  In females:

 In males: 10. Gamete Formation