Chapter 5: Cell Growth and Division

Section 5.1: The Cell Cycle To this point we have learned what a cell is and what parts make up a cell. This chapter will explain how cells duplicate themselves. The duplication of cells is called the Cell Cycle.

Section 5.1: The Cell Cycle The cell cycle is a regular pattern of growth, DNA replication, and cell division that occurs in eukaryotic cells.

Section 5.1: The Cell Cycle There are 4 main stages to the cell cycle: Gap 1 Stage (G1)- cells grow, carry out normal functions and start replicating its organelles. Most of a cell’s life is spent in (G1) Synthesis- Cell’s start to copy the DNA code. Gap 2 Stage (G2)- cell continues normal growth patterns, this is also one of the final check points to see if everything is “ok”. Mitosis (M-Phase)-this is when the nucleus and the cell is actively dividing. We will talk more about this later

Section 5.1: The Cell Cycle Although most all cells go through the cell cycle, all cells go through at different rates. The rate of cell division is dependent upon your body’s need for certain cells Cell Type Approximate Life Span Skin Cells 2 weeks Red Blood Cells 4 months Liver Cells 300-500 Days Intestine-internal lining 4-5 Days Intestine- muscle and other tissue 16 years

Section 5.1: The Cell Cycle Cell Type Approximate Life Span Skin Cells 2 weeks Red Blood Cells 4 months Liver Cells 300-500 Days Intestine-internal lining 4-5 Days Intestine- muscle and other tissue 16 years Remember this is just the life span we are talking about, not the amount of time spent in the cell cycle. For most human cells (G1, S, G2 &M) takes 12 hours.

Section 5.1: The Cell Cycle Cells that divide very rarely enter a stage that some scientists call G0 State. In G0, cells are unlikely to divide, although they continue to carry out all the normal functions. Some cells, such as neurons, seem to be in stage G0 their entire lifespan and never divide. Recently, scientists have discovered that neurons can divide, but scientists don’t know why they don’t. When we find how to make neurons divide, conditions like paralysis and strokes would be temporary.

Section 5.1: The Cell Cycle Scientists have often wondered why cells are the size that they are and why do they divide when they do. Studying these two ideas have led scientist to some basic understandings about cells: Cells have upper and lower size limits! -If cells were too small, then they would not be able to fit all of the necessary organelles and molecules into the cell. -For example, a cell with too few mitochondria would not be able to function. -If cells get to big, than the ratio of surface area to volume gets to far out of wack!

Section 5.1: The Cell Cycle When a cell increases in size, the volume of that cell increases at a much faster rate than the size. Because many substances like oxygen, nutrients, and waste must enter and leave the cell, if the cell gets too big it will expend more energy getting materials into and out of the cell than the amount of ATP’s it can make. If a cell gets to big, it becomes to inefficient to survive and will eventually die. Instead, it divides to put the surface area to volume ration back in balance.

Section 5.1: The Cell Cycle Generally speaking, cells divide for 2 reasons: Keep surface area to volume ratio. Repair and replace old cells.

Section 5.2: Mitosis & Cytokinesis In order for this whole process of the cell cycle to work, the genetic material must be duplicated. DNA is located on chromosomes, which are long threads of DNA. The amount of DNA in just one of your cells is about 10 feet long. So, How does it fit?

Section 5.2: Mitosis & Cytokinesis DNA goes through many phases to get prepared to divide. During interphase, DNA and the chromosome it is on are loosely organized and it looks like spaghetti! At this point, the genetic material is called Chromatin.

Section 5.2: Mitosis & Cytokinesis Chromatin is very unorganized and is tangled together. Before mitosis and cell division, the DNA must organize so it can get duplicated properly. Chromatin starts condensing and coiling around proteins called Histones. These proteins organize DNA into specific chromosomes. Remember, each of these chromosomes have been duplicated in the S-stage, so there are two of each.

Section 5.2: Mitosis & Cytokinesis It would be impossible for the cell to separate chromatin equally, so the genetic information must organize.

Section 5.2: Mitosis & Cytokinesis Remember, the chromosomes have already been duplicated so there are two of them and they form an “X” shape. Each “leg” is called a chromatid. Each chromatid is held together by a centromere. Each end of a chromatid is called a Telomere. Telomere

Section 5.2: Mitosis & Cytokinesis Telomeres do not code for genes! they simply keep the chromosome from unwrapping or connecting with other chromosomes. Once a duplicated chromosome has organized, mitosis and cytokinesis can occur. Mitosis has 4 phases and each can be indentified by what the chromosomes are doing.

Section 5.2: Mitosis & Cytokinesis Prophase: chromatin chromosomes, nuclear membrane breaks down, spindle fibers start to appear, centrioles migrate to the poles.

Section 5.2: Mitosis & Cytokinesis 2. Metaphase: shortest phase… spindle attaches to centromere as duplicated chromosomes line up on cell equator.

Section 5.2: Mitosis & Cytokinesis 3. Anaphase: sister chromatids separate… spindle shortens…chromosomes move to the poles.

Section 5.2: Mitosis & Cytokinesis 4. Telophase: The opposite of prophase. Nuclear membrane reforms, spindle disappears, chromosomes uncoil.

Section 5.2: Mitosis & Cytokinesis After telophase starts, cytokinesis usually starts as well. Cytokinesis is the splitting of the cell into two daughter cells. Cytokinesis, is an example of what happens in animal cells. In plant cells the cell membrane can not pinch in because of the cell wall. Instead, a cell plate forms between the two nuclei.

Section 5.3: Cell Cycle Regulation Many factors affect the cell cycle. These factors control the process of cell division. External Factors: Cell to cell contact: When cells come in contact with each other they stop dividing. Cells release chemicals: This signals other cells to divide. Internal Factors: Enzymes called Cyclin-Dependent Kinases: Carry cells through many check points to make sure the cell gets through the whole cell cycle.

Section 5.3: Cell Cycle Regulation When cell division is not controlled, cancer is the end product. Cancer is just uncontrolled cell growth. Cells usually grow and divide until they come in contact with another cell and then stop. If the division doesn’t stop, tumors are formed. Tumors are big piles of rapidly dividing cells. There are two types of tumors

Section 5.3: Cell Cycle Regulation There are two types of tumors: Benign: cancer cells remain clustered together. Malignant: cancer cells break away and spread to other parts of the body (metastasize) So what makes cancer cells harmful?

Section 5.3: Cell Cycle Regulation So what makes cancer cells harmful? Cancer cells do not carry out the functions that certain type of cell is supposed to. For example, lung cells have to exchange oxygen and carbon dioxide. Cancer cells in the lungs don’t do this, so while these cancer cells are taking up space, they are not functioning. Eventually the good cells die out and you are left with all cancer cells.

Section 5.4: Other Types of Cell Reproduction Not all cells reproduce by mitosis! Prokaryotes no not because they have no nucleus. Most prokaryotes reproduce asexually by a process called binary fission. The cell simply duplicates its DNA and divides into roughly two equal cells.

Section 5.4: Other Types of Cell Reproduction Some eukaryotes also reproduce asexually. Mitotic reproduction is common in simpler plants and animals. Mitotic reproduction can take several forms: Budding- a small projection grows on the surface of the parent organism, forming a separate new individual. Fragmentation- a parent organism splits into pieces, each of which can grow into a new organism.

Section 5.5: Multicellular Life In multicellular organisms, cells communicate and work together in groups that form complex organisms. Cells Tissues  Organs  Organ Systems  Organisms In multicellular organisms, cells go through the process of cell differentiation.

Section 5.5: Multicellular Life Cell Differentiation: Early cells of organisms are called stem cells. Stem Cells: These are cells that do not have a function As the organism grows, the stem cells turn into all of the cells your body needs.