Directorate: Curriculum FET SUBJECT and GRADE TERM 1 TOPIC AIMS OF LESSON RESOURCES INTRODUCTION CONCEPTS AND SKILLS Life Sciences Grade 10 Week 7 Cell division - mitosis At the end of this lesson you should be able to know the following: • Cell cycle and mitosis • Chromosomes • Role of mitosis • Cancer (brief description) Paper based resources • Refer to: The section on mitosis in your textbook • Revise cell structure that was covered in the week 5 and 6 lessons • Cells have the ability to divide continuously • The division of somatic cells, which results in growth, is known as mitosis • Uncontrolled mitosis can result in cancer. Below are definitions of some of the terms that you will study under this section. Use the glossary in your textbook to define these new terms/words (Note: At least 2 marks are awarded if you can define a term correctly in a test or examination) Cytokinesis – Division of the cytoplasm, organelles and cell membrane Centrioles – Cell structures that form the spindle during cell division Centromere – The structure that joins two chromatids to form a chromosome Metaphase – The phase of mitosis where chromosomes arrange themselves in a single row on the equator. Cancer – The growth or tumour that forms as a result of uncontrolled mitosis Study the following information on mitosis: Chromosomes: • • • • • • Chromosomes are thread-like structures found in the nucleus of a cell. When cells are not dividing the chromosomes appear as a mass of threads, the chromatin network. A chromosome consists of the nucleic acid DNA and proteins Before a cell divides, the DNA has to be duplicated. The process during which a DNA molecule makes an identical replica (copy)of itself is known as DNA replication After replication has taken place the single - stranded chromosome consists of two strands which are called chromatids. The two chromatids of a double-stranded chromosome are joined by a centromere. The cell cycle: • • • • The cell cycle is the series of events that takes place in a cell that results in DNA replication and cell division. There are two main stages in the cell cycle. The first stage is interphase during which the cell grows and replicates its DNA. The second phase is the mitotic phase during which the cell divides and transfers one copy of its DNA to two identical cells. Interphase: • • • Interphase is the period between two consecutive cell divisions. Cell growth and DNA replication take place during this phase. After replication has taken place, the chromosome now consists of two chromatids (doublestranded) joined by a centromere. Mitosis: Four phases can be distinguished during mitosis i.e. • • • • Prophase Metaphase Anaphase Telophase Prophase • • • • • Metaphase • • Anaphase • • • • Telophase • • • • • Chromatin network shortens and thickens to form individual chromosomes Each chromosome consists of two chromatids joined by a centromere Nuclear membrane and nucleolus disappears Centrioles separate and move to opposite poles of the cell Spindle fibre forms between the centrioles Chromosomes move to the equator and arrange themselves in a single row on the equator Each chromosome is attached to the spindle fibres by its centromere The centromere of each chromosome divides into two The two chromatids separate The spindle fibre contracts and pulls the chromatids to opposite poles The two chromatids move to opposite poles The chromatids now called chromosomes arrange themselves at the poles The spindle fibres disappear The nuclear membrane forms around each group of chromosomes at each pole and a nucleolus forms. The cytoplasm divides (cytokinesis) Two identical cells with the same chromosome number as the original cell are formed The role of mitosis: • Development and growth - The number of cells increases by mitosis enabling organisms to grow from a single cell to a complex multicellular organism. • Reproduction - Some organisms use mitosis to produce genetically identical offspring. • Cell replacement - Cells are constantly replaced by new ones in the body, for example in the skin. • Replacement of damaged tissues - Some organisms use mitosis to replace body parts. Cancer: • • • • • • • • Cancer is the result of uncontrolled cell division in the body After repeated divisions a mass of tissue is formed, which is then called a tumour Some cancer cells are malignant which means they are able to invade tissues and spread to other parts of the body. Some tumours cannot spread to other tissues and are called benign tumours. Benign tumours are non-cancerous. When cancer cells are spread by the circulation of blood, new tumours form in other parts of the body. Cancer that spreads in this way is known as metastasis. An agent that can cause cancer is known as carcinogen e.g. cigarette smoke, radiation, X-rays, UV-light, dangerous chemicals etc. There are some misconceptions about cancer e.g. many people believe that cancer is a death sentence, others believe that cancer can be prevented by exercising and eating organic food and some believe that cancer is contagious etc. The conventional medical approaches to treating cancer include radiotherapy, chemotherapy and surgery. Know the meaning of instructional verbs in test and examination questions e.g. Instructional verb Name Differentiate Tabulate Describe Explain Compare Meaning Give the name of something Use differences to qualify between two or more categories Draw a table and indicate the answers as direct pairs. State in sentences the main points of a process Give your answer in a cause-effect or statement and reason sequence Give similarities and differences between concepts Answer the following questions: Question 1: 1. Study the diagrams below which represent different phases of mitosis. 1.1 Identify parts A, B, D and E. 1.2 By making use of NUMBERS ONLY (1 to 5), arrange the phases into the correct sequence. 1.3 Write down the number of chromosomes in a cell at the end of the process shown above. 1.4 Name ONE difference between plant and animal cells with regard to the process taking place at C. 1.5 State TWO reasons why mitosis is a biologically important process. Question 2: 2. The root of an onion is a rapidly growing part of the onion. Many cells will be in different stages of mitosis. A sample of an onion tip was stained and studied under a microscope. The various phases of mitosis were identified and the number of cells counted in each phase. The results are recorded in the table below. 2.1 Which phase produced the highest number of cells? 2.2 Calculate the percentage of cells produced during prophase. Show ALL calculations. 2.3 Assuming a cell takes 24 hours to complete one cycle. Calculate the duration of interphase. Show ALL calculations. 2.4 Briefly describe what happens during anaphase of mitosis. 2.5 Draw a bar graph to represent the total number of cells in each phase of the cell cycle. ACTIVITIES/ ASSESSMENT CONSOLIDATION VALUES Common errors made by learners in examinations: • Unable to answer questions based on mitosis and cancer • Unable to answer and interpret questions based on diagrams of mitosis • Complete the questions given in this lesson • Complete questions on mitosis in your textbook • Work through questions on mitosis in past examination papers • Reflect on your learning and understanding of this topic and assess your progress • By studying this section, you will develop your knowledge of key biological concepts.
