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Grade 9 Science –summary notes for unit test 2012 Unit 1 – Reproduction Key Concepts How the cell theory was developed The important role that mitosis and cell division play in the cell cycle The various types of asexual reproduction in living organisms Some advantages and disadvantages to asexual reproduction How knowledge of asexual reproduction is used in plant production Key Skills Update your skills in using a microscope to observe the process of cell division Predict the number of cell divisions required to produce a certain number of organisms Investigate the phases of mitosis and cell division Key Terms Cell Theory Organelles Nucleus DNA Chromatin Nucleolus Ribosomes Cell Membrane Cytoplasm Golgi Bodies Pistil stigma ovum prophase anaphase helix clone Cell Wall Chloroplasts Mitosis Replication Chromosomes Centromere Prophase Metaphase Vacuoles Ovary stamen filament metaphase cytokinesis variation Mitochondria Interphase Cell Cycle Centrioles Asexual Reproduction Binary Fission Fragmentation Pollination Genes or traits Meiosis style ovary anther telophase base pairs somatic cells The Cell Theory The cell theory states: All living organisms are composed of one or more cells. Cells are the basic units of structure and function in all organisms. All cells come from previously existing cells. The activity of an entire organism depends on the total activity of its independent cells. 1 Grade 9 Science –summary notes for unit test 2012 The Cell: The Basic Unit of Life Cells have numerous membrane-bound organelles. Organelles are structures that perform specific functions (or have certain jobs) in the cell. The nucleus is an organelle bounded by a double-layered porous membrane and it acts as the control center. It coordinates, controls and manages cell functions and it is the storage center for all information and instructions for the organelles. Chromosomes are thread-like structures found within the nucleus that carry genetic information. Each chromosome contains many different genes. Genes are units of genetic information that determine the specific characteristics of an individual. The nucleolus is a darker area within the nucleus. It is believed that the nucleolus is involved in the making of proteins. Ribosomes are organelles that build proteins essential for cell growth and reproduction. The cell membrane encloses or covers the cell contents. This membrane acts like a gatekeeper; it controls the movement of materials in and out of the cell. The cytoplasm is a jelly-like material enclosed by the cell membrane. The cytoplasm supports the nucleus and the other organelles. This is the area where work is done within the cell; nutrients are absorbed, transported and processed here. The endoplasmic reticulum is a folded membrane that forms a series of canals that transports materials to different parts of the cell. Mitochondria are tiny, oval-shaped organelles that provides cells with energy. Golgi bodies store proteins until they are needed either inside or outside of the cell. Centriole is a small protein structure critical to cell division (found only in animal cells). Vacuoles are fluid-filled storehouses that contain water, food, wastes and other materials. 2 Grade 9 Science –summary notes for unit test 2012 Differences Between Animal and Plant Cells Plant cells have a structure called a cell wall. It lies outside the cell membrane. This fibrous wall provides structure and support for the cell. Multicellular plants rely on the strength of their cell walls to provide support for the entire organism. The cell wall is composed of a material called cellulose. Plant cells also contain chloroplasts. This organelle allows plant cells to make their own food through the process of photosynthesis. Animal Cells Cell Membrane Centrioles Vacuoles are very small Plant Cells Rigid Cell Wall Chloroplasts Vacuoles are very large The Importance of Cell Division Cell division is when one cell divides into two identical cells. Cell division occurs for three main reasons: Healing and Tissue Repair Growth Reproduction The Cell Cycle The cell cycle is the sequence of events that occurs from one cell division to the next cell division. Cell division is only a small phase in the entire process of the cell cycle. Getting Ready for Mitosis Before mitosis can begin, replication must occur. Replication occurs when the nucleus makes an identical copy of the chromatin. Now there are two complete sets of DNA. The replicated chromatin coils up to form double-stranded chromosomes, which are joined in the middle by a centromere. Now the nucleus is ready for mitosis to begin. The Phases of Mitosis Mitosis involves four major phases that always take place in the same order: prophase, metaphase, anaphase and telophase. 3 Grade 9 Science –summary notes for unit test 2012 Phase 1: Prophase During this phase, the nucleolus and the nuclear membrane disappear. In animal cells, spindle fibres begin to form and stretch across the cell from the centrioles that have moved to opposite ends of the cell. The spindle fibres attach to one side of each centromere. A similar process in the cells of plants, fungi and some protests occurs. They also form spindle fibres, but not centrioles. Phase 2: Metaphase In metaphase, the spindle fibres pull the double-stranded chromosomes into a line across the middle of the cell. Phase 3: Anaphase In anaphase, the spindle fibres begin to contract and shorten. This action pulls the centromere apart, allowing one of each of the replicated strands to move to opposite ends (poles) of the cell. Phase 4: Telophase One complete set of chromosomes is now at each pole of the cell. The spindle fibres begin to disappear and a nuclear membrane forms around each set of chromosomes. A nucleolus appears within each new nucleus. Single-stranded chromosomes start to uncoil into thin strands of chromatin. Now there are two nuclei in one cell, and the cell itself is ready to divide. Cell Division At the end of mitosis, there is still only one cell, but it now has two identical nuclei. The next event in a cell’s life is cell division. In animal cells, the cell membrane pinches in near the middle of the cell, dividing the cytoplasm into two cells. In plant cells, a cell plate develops across the center of the cell, forming a new cell wall between the two new cells. In both plant and animal cells, each new cell after division is an exact copy of the original. 4 Grade 9 Science –summary notes for unit test 2012 Interphase Mitosis and cell division take up only a small fraction of the cell’s life. Interphase is the stage at which the cell spends most of its time. During interphase, the cells grow, replicate their DNA, and prepare for the first phase of mitosis. The cell cycle is the continuous process of mitosis, cell division and interphase. Asexual Reproduction Mitosis and cell division are the basis for asexual reproduction in many organisms. Asexual reproduction is the formation of a new individual that has identical genetic information to its parent. Asexual Reproduction in Bacteria Bacteria are unicellular organisms that do not contain a true nucleus. Instead, bacteria contain a single chromosome that is not surrounded by a nuclear membrane. Bacteria reproduce asexually through a process called binary fission. In binary fission, a parent cell divides so that each new cell contains a single chromosome carrying a complete set of DNA that is identical to its parent cell. Asexual Reproduction in Protists Unicelluar organisms in the kingdom Protista, such as the amoeba, also rely on asexual reproduction to increase their population. The DNA of protist is contained within a true nucleus. Mitotic cell division in these unicellular organisms results in the formation of two identical offspring. Asexual Reproduction in Fungi Moulds, yeast and mushrooms are members of the kingdom Fungi. The bodies of these organisms are composed of many thin filaments called hyphae. The hyphae grow over the surface of and into the bodies of other organisms to obtain food. Members from this kingdom can use one of three methods of reproducing asexually: fragmentation, budding and spores. Fragmentation occurs when a small piece breaks away from the main mass of hyphae and grows into a new individual. 5 Grade 9 Science –summary notes for unit test 2012 Budding is the way yeast reproduces asexually. First, a copy of the nucleus is made and then a tiny bud begins to form on the cell wall. The new nucleus continues to grow larger and eventually it breaks away to become a single, independent cell. Asexual Reproduction in Plants The ability of plants to reproduce asexually and repair themselves is, as in all other kingdoms, the result of mitotic cell division. Asexual reproduction in plants ensures that each new plant will have the same DNA as its parent. Some plants have the ability to reproduce asexually from their roots (asparagus) tubers. In other plants, the stem divides to produce cells that will become a new plant (strawberries). These plants have special stems called runners. Pieces may be broken off in a process called fragmentation. 7.2 Meiosis Meiosis is the process that ensures that each gamete contains only one-half set of chromosomes. This process also ensures that each gamete has a different combination of the chromosomes. Once the chromosome number has been halved a sperm cell and a egg cell combine during fertilization to form a diploid zygote. Only a zygote can function as the offspring’s first body cell. Organisms that reproduce sexually show greater variance, meaning differences in characteristics, than those that reproduce asexually. An offspring receives a set to 23 chromosomes from each parent. The chromosomes that are similar in shape, size and gene arrangement are combined together to form a pair of chromosomes. The appearance of the organism is determined by the way the genes from this pair interact. (colour of hair, eyes, etc…) Phases of Meiosis Organisms that reproduce sexually contain two types of cells. Cells that reproduce by normal cell division and mitosis are called somatic cells (skin cells and muscle cells). Reproductive cells are the second type of cell. These cells produce sex cells that contain only half the number of chromosomes through the process of meiosis. 6 Grade 9 Science –summary notes for unit test 2012 Meiosis involves two cell divisions that produce four cells with single sets of chromosomes. Meiosis I Meiosis I is that phase where the first cell division occurs. In this phase, homologous chromosomes move to opposite poles of the cell. During this division, a pair(2n) becomes two single cells (n). Meiosis II –Final stages where four daughter cells are produced. All daughter cells are single sets of chromosomes. Mitosis Asexual- one cell produces two daughter cells that are identical to the parent Meiosis Sexual-one cell produces 4 daughter cells that have half the chromosomes of the original cell 7.3 Reproduction in Flowering Plants The reproductive cells of plants are found within their flowers. The male sex cells of the flower, the pollen, are produced in the anthers, which are at the tips of the stamens. Female sex cells, called eggs, are located in a structure call the ovary, which is at the base of the pistil. Pollination is the process of fertilization in plants. This occurs when the pollen moves from the anther and reaches the egg cells of the female. 7 Grade 9 Science –summary notes for unit test 2012 Pollen from one plant can sometime fertilize the same plant. However, more commonly, the pollen of one plant must reach the egg of a different plant. The pollen can be transported by wind, insects and animals, and humans (artificially). The fertilized eggs of the flower become the seeds. As the seeds are developing, the petals slowly shrivel up and fall off the plant. In some plants, the ovary surrounding the seeds develops into a fruit (to help protect and disperse the seeds). Sepal - green, leaf-like structure that protects the budding flower. Petal - colorful and often scented part of the flower that attracts insects. Stamen - the part of the flower that produces pollen. Consists of a filament and an anther. Anther - sac located at the tip of the filament that contains pollen. Filament - stalk that connects to and holds up the anther. Pistal - consists of the stigma, style, and ovary. Stigma - the tip of the carpel that is sticky in order to collect pollen. Style - the slender, neck-like portion of the carpel that leads to the ovary. Ovary - structure at the base of the carpel that houses the ovule or egg. DNA: The Genetic Material The chromosomes in all living beings are made up of a chemical known as deoxyribonucleic acid (DNA). This chemical gives directions to your cells to repair damaged cell parts or to make new cells. DNA also tells cells how to respond to changes in their environment and how to respond to messages sent by other cells. The Structure of DNA DNA molecules are shaped like a twisted ladder. Sugars and phosphate molecules make up the sides of the ladder to give DNA its shape. The rungs (or middle) of the ladder are made up of nitrogen bases. The order (or sequence) that the nitrogen bases are placed in the strands of DNA is known as the genetic code. The genetic code is arranged in three character sequences (of nitrogen bases) or “words,” and all these words combined tell “stories” or information about an individual’s genes (eye colour, hair colour, artistic ability, etc.). The nitrogen bases are combined in pairs. Adenine always pairs with thymine and cytosine always pairs with guanine. 8