Biology Unit 7 Cell Division
Why would a cell want to divide?
Reasons Cells Divide To avoid getting too big (cells not the organism) To help an organism grow and organize To replace old cells
Cell Size Limitations Some of the smallest cells are no larger than 8 nanometers while some of the largest can reach up to 8cm. Cell size can be limited by several things: Diffusion DNA Surface area to volume ratio
Diffusion Limits Cell Size You should know that the plasma membrane allows nutrients to enter cells while wastes leave the cell. Diffusion is fast over short distances but becomes inefficient over larger distances. The organelles on the inside of giant cells would die waiting for nutrients to get to them. If we keep cells small, we can keep the rate of diffusion fast and allow nutrients to reach all parts of the cell quickly.
DNA Limits Cell Size We know that the nucleus contains blueprints for the cell s proteins, which are used by almost all organelles. There is a limit to how quickly these blueprints can be copied. If a single nucleus is in charge of one giant cell, proteins will not be able to be made quickly enough for the cell to survive.
What adaptation might a giant single celled organism have to make sure its proteins get made efficiently? Have more than one nucleus!
Surface Area to Volume Limits Cell Size As a cell s size increases, its volume increases much faster than its surface area. So what s the problem? If a cell doubles its size, it requires about 8 times more nutrients and produces 8 times more waste. But, the surface area would only increase by a factor of 4. Since waste and nutrients are brought in through the membrane (surface), this means that the cell would either starve or be poisoned by excess waste.
So How Do Cells Divide? Because cells are replacing other cells that do a specific job, they need to be able to divide and create exact copies of themselves. This is done by cells copying their chromosomes and passing them to the new cells. Chromosomes are the combination of protein and DNA that contain the genetic info in each cell.
The Cell Cycle The cell cycle is the sequence of growth and division in a cell. Consists of 2 general parts: Interphase: Most of a cell s life, normal cell function Mitosis: The time during which cells divide forming two daughter cells with identical genetic information.
Interphase Part 1: Cells grow; protein production is high. Part 2: Chromosomes are copied in preparation for cell division. Part 3: Mitochondria as well as other cell parts needed for division are manufactured.
Mitosis Division in all types of cells except for sex cells. Used to help organisms grow and become organized. Also used for asexual reproduction.
Mitosis: 4 Stages PMAT Prophase Metaphase Anaphase Telophase
Prophase Longest phase of mitosis. Chromosomes become visible. Nucleus begins to disappear. Centrioles begin to migrate to opposite ends of the cell. Spindle fibers begin to form between centrioles.
Structure of a Chromosome
Prophase
Metaphase Middle Phase Chromosomes become attached to spindle fibers and are pulled toward the middle of the cell. Each sister chromatid is attached to its own spindle fiber. Chromosomes line up directly down the center of the cell.
Metaphase
Anaphase Apart Phase Sister chromatids are separated. This happens when the centromere splits and chromatids are pulled apart by shortening spindle fibers.
Anaphase
Telophase Two Phase Chromosomes begin to unwind. Spindle fibers break down. Nucleus reappears. New membrane begins to form between the two new cells. CELLS HAVE NOT YET SEPARATED!!!!!!
Telophase
Cytokinesis Not technically part of mitosis Cytoplasm actually separates. 2 identical cells are formed.
Cytokinesis
Recap Question Now that we have made 2 new cells, what part of the cell cycle will they be in?
Looking into Interphase Interphase can be broken down into three specific phases. G1 S G2
G1: Growth Phase 1 Cell grows to maturity Produces many ribosomes/proteins Builds up sufficient nutrients to undergo cell division When this is complete, the cell passes through G1 checkpoint. Checks for perfect DNA, enough protein/nutrients for division.
S Phase: Synthesis Phase If the cell passes through the checkpoint, it enters S phase. DNA and the proteins to carry it are doubled during S phase
G2: Growth Phase 2 Organelles are doubled Organelles specific to cell division are made Even more protein is made Cell goes through one more checkpoint before committing to mitosis How many chromosomes are in the cell during G2? 92
Cancer: Scary Facts Cancer is the 2 nd leading cause of death in the United States after heart disease. It is estimated that there will be 1,695,000 new cases in 2016. Approximately 40% of Americans will be diagnosed with cancer in their lives. Currently, the survival rate is approximately 70%.
What is cancer? The malignant growth caused by uncontrolled cell division. May be caused by environmental changes or a change in enzyme production. Enzymes that control the cell cycle are called cyclins. Scientists believe that the one cause of cancer may be errors in cyclins. These errors are caused by mutated genes. What type of macromolecules are cyclins? Proteins
Environmental Causes There are many environmental causes of cancer. Air Pollution Sun Tobacco Smoke Bacteria/Viruses Diet Cancer can also be genetic! No matter the cause, the thing that leads to cancer is mutated genes.