Biology 3201 Unit 2A Notes Mitosis Somatic cell: refers to a body cell; anything not a germ (sex) cell Cell cycle: a continuous sequence of cell growth and division The cell cycle consists of two main stages 1. Interphase growth phase; includes G 1, S phase, and G 2 G 1 (gap 1): cell carries out metabolic activities and prepares for cell division S phase: DNA is replicated G 2 (gap 2): centrioles replicate and cell prepares for division 2. Division stage includes mitosis and cytokinesis; ( PMAT) Mitosis and cytokinesis are the two shortest events in the cell cycle. Cytokinesis - separation of the cytoplasm and the formation of two new daughter cells; cytokinesis occurs after telophase of mitosis Mitosis division of the cell s nucleus where the daughter cells receive the exact number of chromosomes and genetic makeup as the parent cell Different cells have different timing for their cells cycles; some take longer than others to go through their cycle, and they also spend different amounts of time in each stage. Parent cell the original cell that divides during mitosis to form two new daughter cells Daughter cells the cells produced during mitosis of a parent cell Why is mitosis important? In order for an organism to grow, repair, and maintain its function new cells are needed to replace old ones. Each cell that undergoes mitosis produces 2 new cells. Mitosis allows the regeneration of damaged tissue (like cuts) and to replace worn out cells (like red blood cells) - New cells are needed for growth maintenance and repair -cells can regenerate damaged tissues (cuts) -cells that do not function properly must be replaced -cells die (blood cells) -chromosome number must be maintained. eg. Humans 46 chromosomes in somatic cells
When human somatic cells undergo mitosis, a parent cell replicates to produce a daughter cell with the same number of chromosomes. Mitosis ensures that the same amount of genetic information in each type of cell. Mitosis and cell division occur in many somatic (body) cells. New cells are exact copies of previously existing cells. This occurs because of DNA stored in the nucleus. TERMS TO KNOW: Chromatin: the long strands that form chromosomes and contain DNA, RNA and various proteins. Found in the nucleus of cells. Chromosome: condensed chromatin structure formed when cells replicate (divide) (see fig. 14.6, p. 462) Chromatid: one half of a chromosome. Two sister chromatids are joined by a centromere to form a chromosome Centromere: is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact Centriole: A centriole is a barrelshaped cell structure involved in the organization of the mitotic spindle and in the completion of cytokinesis Human somatic cells have 46 chromosomes (22 pairs plus the sex chromosomes). Before mitosis, the parent cell has 46 chromosomes. After mitosis, the daughter cells each have 46 chromosomes.
The stages of mitosis : Prophase Chromatin coils and thickens forming chromosomes Chromosomes are composed of two sister chromatids held together by a centromere. Each sister chromatid is a genetic copy of the other. (Identical DNA) Because of this, each daughter cell receives a full set of parent genes The nuclear membrane disappears Centrioles made up of microtubules migrate to opposite ends of the cell. Spindle fibres (also made of microtubules) form between the two centrioles Metaphase: Spindle fibres attach to centromere of the replicated chromosome. Chromatids are guided to the cell s equator (spindle fibres from one pole attach to one chromatid and spindle fibres from the other pole attach to the other chromatid) Anaphase : Centromere splits apart and chromatids are pulled to opposite poles of the cell by spindle fibres Chromatids are pulled apart as a result of a shortening of microtubules Telophase : Chromatids have reached two opposite poles Chromatids are now a single, non-replicated chromosome Chromosome now begins to unwind and becomes less visible Spindle fibres are not needed so they break down and disappear Nucleolus reappears Nuclear membrane forms around each new set of chromosomes which are at the opposite ends of the cell Cytokinesis This is defined as the separation of the cytoplasm and the formation of two new daughter cells. cytoplasm and all its contents divides between the two halves of the cell. In animal cells an indentation of the membrane between two daughter cells forms and deepens. In plant cells, a new cell wall and membrane form and separate the newly formed nuclei. Lab: Observing the cell cycle in plant and animal cells - Pg 466-467
Mutations affecting cell division Mutation Permanent change in the DNA of an organism Can occur spontaneously or by certain compounds, radiation, etc. Mutations that occur in parent cells are passed on to daughter cells Most mutations I somatic cells are not important because those daughter cells can be replaced by normal cells However, if a mutation affects a gene (DNA) which controls cell division, cancer can result (uncontrolled rapid growth of cells) Radiation and chemotherapy: Cancer cells divide more rapidly than any other type of body cells. Therefore, anything that interferes with cell division will affect cancer cells more than healthy cells. This is the basis for radiation and chemotherapy. Radiation therapy: Directs radiation such as x-rays are gamma rays at the affected part of the body. Usually treated two to three times per week Internal radiation therapy involves placing radioactive material next to the cancerous growth Generally radiation therapy works by damaging the chromosomes in a cell. Then it cannot divide. Healthy cells are also damaged but many are able to repair themselves. Goal of radiation therapy is to focus the radiation on the diseased part of the body and avoid affecting healthy tissue. Usually used on localized cancerous tumours such as on the skin,breast, larynx, and cervix. Chemotherapy: May include one or more types of drugs depending on the patient and the cancer. May be used in conjunction with radiation or on its own. Some drugs attack dividing cells as they divide or prevent cells from dividing Chemotherapy affects the entire body and is usually used to treat cancers that are spread throughout the body such as leukemia. Unfortunately, healthy cells are affected
Side effects of radiation and chemotherapy: Radiation: Skin inflammation and fatigue Specific side effects depending on location of treatment eg: brain - hair loss testicular cancer - sterility Chemotherapy: Hair loss, nausea, diarrhea For both treatments, side effects usually last only for the duration of the treatment. However,sterility can be permanent. Treatments are particularly harmful to body cells that divide quickly, such as bone marrow cells, skin cells,hair cells, cells in the GI tract and cells of the reproductive system. What is the goal of cancer research? Find treatment that affects cancerous cells and leaves healthy cells unharmed. Meiosis A zygote (fertilized egg) contains chromosomes from both parents but does not contain double the number of chromosomes found in each body cell. This happens because of meiosis which produces haploid cells called gametes. Gametes are haploid (h) which means that they contain one copy of each type of chromosome that the diploid (2n) contains. The first part of meiosis reduces the number of chromosomes from diploid to haploid. This is referred to as reduction division. Human sperm or egg cells contain 22 autosomes and 1 sex chromosome. Autosomes - not directly involved in sex determination. Phases of meiosis : Meiosis is similar to mitosis, but there is an extra set of phases for each stage. Prophase I Each pair of homologous chromosomes (carry genes for same information) become aligned. Replicated homologous pairs are called tetrads. One copy of a homologous chromosome came from one parent (egg) and one from the other parent (sperm). Mothers chromosome - maternal origin Fathers chromosome- paternal
*note - homologous chromosomes are like a pair of shoes; same characteristics but not identical Although homologous chromosomes contain the same genes, they may have different forms of these genes called alleles. Alleles determine how a gene is expressed. (more on this in unit 3) During the pairing process, crossing over of chromatids can occur and non-sister chromatids can exchange segments of chromosomes. Each segment contains hundreds or thousands of genes and this contributes greatly to genetic variation. Result - some chromosomes will have genes from paternal origin and some from maternal. Metaphase I Spindle fibre attaches to the centromere of each chromosome Spindle fibre pulls each tetrad to center of cell Unlike mitosis, chromosomes do not line up in a single line. Instead they line up in homologous pairs so that one of each homologous pair is on a different side of the equator. chromosomes from one parent are not on one side of the cell. They are positioned randomly. Anaphase 1: Telophase I Telophase I does not occur in all cells. If it does not occur the cell goes to meiosis II. If telophase does occur : Cytoplasm is divided Nuclear membrane forms around new homologous chromosomes Second replication does not occur because each chromosome already contains two chromatids Each cell contains some maternal and paternal chromosomes and some paternal and maternal alleles. (crossing over during prophase I) Meiosis II Phases are identical to mitosis Each cell beginning meiosis II is haploid At the end of meiosis II each daughter cell is still haploid but contains a single unreplicated chromosome ( not two attached chromatids.) In animals the daughter cells develop into gametes and in plants they turn into spores or gametes
Spermatogeneis : Is the process of male gamete production in animals. Spermatogenesis begins with diploid germ cells called spermatogonia. After meiosis I, one spermatigonia divides into two cells called primary spermatocytes. Primary spermatocytes undergo meiosis II to form secondary spermatocytes, which develop into sperm. Meiosis in mature males takes place in the testes, the male reproductive organs Cell enlarges and undergoes meiosis I and II Final product is four haploid sperm cells each with the same amount of cytoplasm and number of chromosomes After meiosis II the sperm cells develop into a mature sperm. This happens when each cell loses cytoplasm and the nucleus forms a head. A flagellum is formed for locomotion. Humans produce sperm all year round but some organisms only produce sperm in breeding season Oogenesis: In females occurs in the ovaries Process begins with a diploid cell called the oogonium which enlarges and undergoes meiosis I and II. The cell that receives most of the cytoplasm is called the primary oocyte. The other cell is called a polar body and is not a viable sex cell. As the primary oocyte undergoes meiosis II, the cytoplasm is again unequally divided. Only one cell becomes an egg or an ovum and contains most of the cytoplasm. The purpose of unequal division is to provide the ovum with sufficient nutrients to support the zygote in the first few days following fertilization. Meiosis I begins in ovarian tissue before birth and does not continue past prophase I Continuation of meiosis I occurs after puberty and usually in one oogonium per month Production of ova ( two or more egg cells) in females continues from the start of puberty until menopause which usually occurs between 40 and 50 Egg cells - contain X chromosome Sperm cells - contain X or Y Sperm Egg - Small - Large - mobile - not mobile - have a cap called an acrosome which contains enzymes used to enter the egg cell - covered by a thick outer coating. After one sperm penetrates the egg, no more can enter - millions produced continuously (300 - one egg matures per month from puberty
million-500 million) to menopause - 50-100 mitochondria per cell - about 140, 000 mitochondia per cell - before ejaculation: uses fat for energy - after ejaculation: uses sugar (fructose) for energy - can only live for about a day or so with its food supply Technologies Based on Cell Division (1) Cloning make an exact copy of an organism, either an entire organism (reproductive) or parts (therapeutic) animal cloning techniques involve bypassing the meiosis step of animal reproduction Clone - identical copy of an organism Meiosis stage is bypassed Nucleus of an egg cell from a surrogate mother is removed Diploid nucleus of the animal to be cloned is placed in the empty egg cell Egg cell is implanted into the mother s uterus Cell divides and forms an embryo, which develops into an exact copy of the animal which donated the nucleus of one of its cells (2) Stem Cell Research (Core STSE) Stem cells: blank slates of the human body. They are undifferentiated (non-specialized) cells that can give rise to any other type of cell found in primarily the bone marrow but also found in blood, muscle tissue, brain, etc. Sources include aborted fetuses, unused embryos from in-vitro fertilization treatments, cord blood (from placenta) Uses: Therapeutic cloning - culturing of human cells for use in treatment of medical disorders Reproductive cloning see above Cell transplant- transplanting stem cells to replace damaged cells (eg. Pancreatic islet cells) Core STSE # 2 Stem cells Research
Modes of Reproduction Types of Reproduction There are TWO (2) ways in which organisms reproduce or create offspring. 1. Sexual Reproduction This is reproduction involving two (2) sets of DNA/Chromosomes. There is a fusion of an egg nucleus and a sperm nucleus. This results in a combining of DNA from both parents. Fertilization is the process of sperm nucleus meeting with the egg nucleus. Offspring resemble the parents but are NOT identical to them. 2. Asexual Reproduction This is reproduction involving only ONE (1) set of DNA/Chromosomes. Offspring are normally created through Mitotic divisions. Offspring are clones or identical copies of the parent. There are several different modes in which asexual reproduction occurs. Modes/Types of Asexual Reproduction (A) Budding (B) Binary Fission Reproduction where an outgrowth occurs on an organism that later breaks off and grows into an entire organism. Ex: Yeast and Hydra Reproduction where mitosis divides the DNA in an organism. The organism then splits to create two (2) new organisms. Ex: Bacteria (C) Spore Production Reproduction where mitotic cell divisions create a series of spores that grow into new organisms. Spores are released from spore sacs. Spores : small round structures containing the diploid (2n) chromosome number of a parent cell. Ex: Fungi : Rhizopus (Bread Mold) (D) Fragmentation Reproduction where a portion of an organism breaks off. The broken piece is able to grow into a full organism. Ex: cuttings or slips of house plants. (E) Parthenogenesis Reproduction where an organism produces haploid (n) unfertilized eggs that grow into adults. Ex: Insects: Balsam Wooly Aphid Wasps Whip tail lizards
Summary of Modes of Reproduction SEXUAL ASEXUAL Amount of DNA/Parents 2 sets of DNA / 2 Parents 1 Set of DNA/ 1 parent Offspring Resemble Parents Exact copies of Parent Mechanism(s) Fertilization Budding, Binary Fission, Spores, Fragmentation, Parthenogenesis Reproduction in the Flowering Plants Flowering Plants These plants are known as the angiosperms (Covered seed plants). These plants undergo sexual reproduction. These plants have evolved to be away from water, thus no dependence on water for reproduction. These flowers have evolved specialized structures for reproduction. Reproductive Structures of a Flowering Plant Perfect Flower : A flower possessing BOTH male and female reproductive structures. This type of flower is capable of undergoing self pollination (fertilization). Imperfect Flower: A flower possessing EITHER male or female reproductive structures only.
Flower Reproductive Structures Pistil: The female reproductive structure of a flower. Made up of the Stigma, Style and Ovary. 1. Stigma: Sticky portion on top of the style in a flower. Function: Trap pollen from the air for reproduction. 2. Style : Long slender tube leading from the stigma to the ovary in a flower. Function: Passage way for pollen leading to the ovary. 3. Ovary: Reproductive structure of a flower that contains eggs. Eggs are created by meiosis in this structure. Eggs contain the haploid (n) chromosome number. Function: Site of fertilization within a flower. Stamen: The male reproductive structure of a flower. Made up of the Anther and Filament. 1. Anther : Reproductive structure of a flower that contains pollen. Pollen is created by meiosis. Function: Create pollen for fertilization. Pollen: Small round structure located within the anther of a flower that contains the male sex cells (sperm). Pollen contains haploid (n) the chromosome number as the parent cell. 2. Filament : Long slender stalk that holds up the anther. Creation of Egg and Pollen within Flowering Plants NOTE: Both Eggs and pollen are created through MEIOSIS within the flowering plants. Egg formation within a flower Within the ovule of a flower, a diploid (2n) cell undergoes meiosis to create 4 haploid (n) cells. Three of these cells die. The remaining haploid cell undergoes mitosis three times to create a total of 8 haploid structures within the ovary. One of these will be the egg.
Pollen Formation within a Flower A diploid (2n) cell within the pollen sac of the anther undergoes meiosis to produce 4 haploid (n) cells called microspores. Each microspore undergoes mitosis to create two haploid nuclei known as the tube nucleus and the generative nucleus. The outer wall of the microspore hardens and is now called a pollen grain. Sexual Reproduction/Fertilization in the Flowering Plants The following events happen during reproduction in a flowering plant. Pollen grains reach the stigma and stick to the surface of the style.
The tube nucleus begins to dig a pollen tube down through the style on its way to the ovary. At the same time the generative nucleus divides by mitosis to produce two haploid sperm nuclei (sperm). The sperm nuclei make their way down through the pollen tube and into the ovule. One of the sperm fertilizes the egg and produces a diploid (2n) zygote. The other sperm joins with the two polar nuclei to form a triploid (3n) structure known as endosperm. The ovule becomes the SEED and eventually a new plant. The endosperm provides food for the developing seed. Terminology 1. Ovule : Area inside an ovary where an egg and two polar nuclei are found. The ovule is the site of fertilization within a flower. 2. Tube Nucleus: Nucleus within a pollen grain that forms the pollen tube in a flower. 3. Generative Nucleus: Nucleus within a pollen grain that produces two sperm nuclei or sperm. 4. Pollen Tube: Hollow tube leading from the stigma to an ovule within the ovary of a flower. 5. Polar Nuclei: Two haploid (n) structures within the center of an ovule. 6. 6. Zygote: A fertilized egg. A diploid (2n) structure. 7. Seed: Structure formed from the ovule of a flower. It contains the zygote (developing plant) and endosperm. 8. Endosperm: Triploid (3n) structure formed when a sperm nucleus fuses with the two polar nuclei within an ovule. This is a food source within a seed for a developing plant embryo. 9. Fruit: A swollen ovary of a plant. It surrounds and helps to protect developing seeds.