Cell Growth and Reproduction Robert Hooke was the first person to describe cells, in the year 1665. He was looking through his microscope at a piece of cork when he noticed a lot of repeating honeycomb shaped structures. He used the term "cell" to describe them. A few years later living blood cells, bacteria, and tiny single celled organisms were observed in a drop of water using a simple microscope (microscope with 1 lens. As microscopes improved cells could be examined more closely. It wasn't until 1820 that Robert Brown examined plant cells and identified the tiny sphere called the nucleus (plural: nuclei). Not long after Brown identified the nucleus of a plant cell, two scientists discovered that plant and animal tissues are made up of cells. This discovery provided the foundation for cell theory! Cell theory is based on 3 important principles. 1. All living things are composed of one or more cells. 2. The cell is the functional unit of life. 3. All cells come from pre-existing cells. Microscopes: What is the purpose of a microscope? Microscopes allow us to look very closely at things, or to magnify things that are very small. The power of a microscope is described with a number followed be the letter "x". For example, if you can see a substance 25 times larger than actual size, its magnification power is 25x. Microscopes allow you to change lenses to increase or decrease magnification power. The simple compound light microscope has only one lens between the object and the eye.
Microscope Breakthrough = The addition of a second lens to the compound light microscope! An image that was magnified only 10 times by the first lens, would now be multiplied by 10 again. Therefore the object would be viewed 100x larger! Even with the most sophisticated microcopes we can only reach 2000x magnification. In order to see very tiny viruses or detailed human cells, we need greater magnification...the electron microscope provides us with MORE POWER! Transmission Electron Microscope The first electron microscope was invented in 1932 and provided an image of 400x magnification. Five years later at the University of Toronto, an electron microscope capable of 7000x magnification was unveiled. Today, electron microscopes are capable of 2000000x magnification! Instead of light (like the simple microscope), electron Microscopes use beams of electrons to produce their images. The electron microscope is ideal for studying the structures within the cell. These powerful machines do have 2 major limitations. Thick specimens will absorb the electrons and produce blackened images, so only very thin sections of cells (encased in plastic) can be viewed. The cells die when placed in plastic, therefore only dead cells can be examined which does not allow you to view cells as they divide. Scanning Electron Microscope The scanning electron microscope allows us to examine the thicker specimens that the transmission electron microscope could not. It also produces 3D images on a TV screen for us to view. However, it does not have the magnification and high resolution of the transmission electron microscope.
The Importance of Cell Division! Have you ever had a sunburn that caused your skin to peel? Had a cut on your hand? How is this related to cell division? What would have happened if that skin had never been replaced? You will rely on cell division throughout your life to replace dead or damaged cells in your body! Functions of Cell Division: Healing and Tissue Repair: You don't go through life with the same cells you had at birth. In fact, every second millions of your body cells are injured or die. Growth: An obvious function of cell division is to increase the number of cells. As the number of cells increase, so does the size of the organism. Therefore growth depends on cell division! Reproduction of Organisms: Cell division perpetuates life. In one celled organisms, like bacteria, it creates two new organisms. Cell division is also fundamental to reproduction of multi-cellular organisms (like us!). Cell Division Cells alternate between stages (or phases) of dividing and not dividing. Moving from one stage to another is called the Cell Cycle. The cell division phase is a small part of this cycle. The stage between cell divisions is called interphase. During interphase, the cell takes in nutrients, such as sugars, and produces building materials, such as proteins. After this growth, the cell duplicates (makes an identical copy) its chromosomes (blueprints of the cell!) in the nucleus. This is very important as the new cell will need a copy! We then are able to witness cell division! Most cells divide in the same way. The initial "mother" cell divides into two identical "daughter" cells.
Mitosis and Cytokinesis Cell division involves the division of nuclear materials (genetic material) and the sharing of cytoplasm (including the organelles). During cell division, the duplicated chromosomes copied during interphase, split apart and move to opposite ends of the cell. This is known as mitosis! Cell division continues with the separation of the cytoplasm and organelles into equal parts. This is known as cytokinesis.
Reproduction and Cell Division Organisms of all species reproduce. They may reproduce sexually or asexually. In sexual reproduction, genetic information from two cells is combined to produce a new organism. Usually, sexual reproduction occurs when two specialized sex cells unite to form a fertilized egg called a zygote. In asexual reproduction a single organism produces offspring with identical genetic information. - Most of the cells of the human body reproduce asexually by mitosis. - Most single-cell organisms, such as bacteria, and some multi-cellular organisms use asexual reproduction to produce offspring. Types of Asexual Reproduction 1. In binary fission, the organism splits directly into two equal-sized offspring, each with a copy of the parent's genetic material. Binary fission is a common type of reproduction in single-celled organisms. 2. In budding, the offspring begins as a small outgrowth from the parent. Eventually, the bud breaks off from the parent, becoming an organism on its own. Budding occurs in some single-cell organisms (yeast), and in some multicellular organisms. 3. In fragmentation, a new organism is formed from a part that breaks off from the parent. Many types of algae and some plants and animals can reproduce this way. 4. In spore formation, the organism undergoes frequent cell division to produce many smaller, identical cells called spores. The spores are usually housed within the parent cell. Many spores have a tough, resistant coating that allows them to survive after the parent cell dies. Penicillium mold reproduces by forming spores. (develop into mature organisms). 5. Many plants, such as spider plants, strawberries, and the quaking aspen make use of vegetative reproduction. They produce runners that can develop into another plant with identical genetic material. Growth Hormones in Animals! Animals (including us) have hormones that affect division and growth of our cells as well. Growth Hormone (GH) is carried through the blood and affect bone, muscle, and cartilage cells which make organisms grow larger. Effects of human growth hormone are extremely noticeable when it its produced in abnormal amounts. Low production of GH during childhood can result in dwarfism, while high productions can result in gigantism.
DNA, Chromosomes, Genes Remember that cells are shaped by the genetic information found in chromosomes. All chromosomes are comprised of the same chemical, DNA. DNA provides the directions that guide the repairing of cells and the construction of new ones. DNA sends information from the nucleus to the various organelles in the cytoplasm using chemical messengers. Understanding DNA, Genes, Chromosomes Can we compare the relationship between DNA, genes, and chromosomes to a library? A library is full of books. What makes up books? Words! And what makes up words? Letters! In the DNA analogy, the chromosomes would represent the library. (Dogs have 76 chromosomes, mice have 40, humans have 46...) Chromosomes are made up of strands of DNA called genes. These genes represent the books inside the library. (Genes tell us eye colour, nose, skin colour of individuals) Each gene (or book...) is made up of nitrogen bases (or letters). There are 4 types of nitrogen bases: 1. Adenine 2. Thymine 3. Cytosine 4. Guanine Different combinations of these bases are like different combinations of letters which can form different words. The nitrogen bases always form the rungs in pairs. Adenine always pairs with Thymine. Cytosine always pairs with Guanine. If everybody s DNA is made up of the same stuff (nitrogen bases), why are we all different?
Even though there are only 4 nitrogen bases, we are all different because of the sequence, or order, that the bases appear! This makes each DNA molecule different. Every person has 23 pairs of chromosomes, for a total of 46. One chromosome in each pair comes from your mother and the other from your father. DNA Mutations As we discussed earlier, when cells divide, two new cells that are identical to the mother cell are usually created. What happens if the DNA of the original cell is exposed to radiation from the sun, to viruses or to some type of chemical that is foreign to the cell nucleus? The genetic code of the DNA may be altered or changed. These changes are referred to as mutations. Some mutations may be beneficial but most are not. The most notorious type of mutation is cancer. Cancer can be described as cell division out of control. Cancer is actually a group of diseases, each associated with uncontrolled, unregulated cell division. While many diseases attempt to prevent cells from dividing, cancer cells divide more quickly than they should. All cancers are caused by mutations in the genes that regulate cell division. Anything that causes such mutations is known as a carcinogen. There are 3 known types of carcinogens: Viruses Radiation Hazardous Material Leukemia Skin Cancer Lung Cancer
Cancer Cells Since cancer cells cannot perform some of the functions of normal cells, they are inefficient. They use up the energy and resources of the other cells to reproduce, but they do not do the same work as normal cells. Rapid cell growth or cancer can result in a mass of cells called a tumor. Tumors are classified as either benign or malignant. Benign tumors are harmless. They remain in a confined area, causing little or no damage. Malignant tumors are dangerous.