Gill Sans Bold. Biology Preliminary Course Stage 6. Patterns in nature. Part 8: Growth and repair

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1 Gill Sans Bold Biology Preliminary Course Stage 6 Patterns in nature Part 8: Growth and repair IncorporatingOctober2002 AMENDMENTS

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3 Gill Sans Bold Contents Introduction... 2 Cell division... 3 Sites of mitosis...4 What happens during mitosis?...6 Mitosis in plant cells...7 Cytokinesis...11 Additional resources Suggested answers Exercises Part Student evaluation of module Part 8: Growth and repair 1

4 Introduction Maintenance of organisms requires growth and repair. In this part you will be given opportunities to learn to: identify mitosis as a process of nuclear division and explain its role identify the sites of mitosis in plants, insects and mammals explain the need for cytokinesis in cell division identify that nuclei, mitochondria and chloroplasts contain DNA. In this part you will be given opportunities to: perform a first hand investigation using a microscope to gather information from prepared slides to describe the sequence of changes in the nucleus of plant or animal cells undergoing mitosis Extract from Biology Stage 6 Syllabus Board of Studies NSW, originally issued The most up-to-date version can be found on the Board's website at This version November Materials required: microscope and lamp two slides and a cover slip onion with fresh roots (you need to soak onion base in water at least a week in advance) methyl green pryonin or aceto orcein stain. Alternatively use prepared slides of a root tip (if available). If you do not have access to a microscope or prepared slide, use the photographs provided. 2 Patterns in nature

5 Gill Sans Bold Cell division A multicellular organism such as a human, begins life as a single cell formed from the union of two sex cells. From this microscopic beginning the organism grows to become an adult. This is achieved by the process of cell division. One cell divides forming two cells and then each of these cells divide forming more cells to continue the process of cell division. Mitosis is a type of cell division that results in the replication of identical cells. Meiosis is another type of cell division which produces gametes or sex cells. Meiosis produces cells that have half the number of chromosomes of the parent cell. Mitosis results in growth of an organism, is involved in the healing of wounds and the replacement of cells eg. red blood cells and skin cells. With the exception of gametes (ova and sperm), all the body cells or somatic cells come from pre existing cells by mitosis. 1 Write a definition, in your own words, for mitosis. 2 How is mitosis different from meiosis? 3 What is the role of cell division in multicellular organisms? Check your answers. Part 8: Growth and repair 3

6 Sites of mitosis Mitosis occurs in areas of rapid growth in organisms. These sites are in different places in different types of organisms. This is usually due to the need for rapid replication such as growth points or sites where repair to damaged tissue is required. Multicellular organisms may also have stages in their life cycle during which mitosis may occur at a greater rate such as within a developing foetus. Plants Mitosis in plants occurs in special cells called meristematic cells and in the layer of cells in the stem called cambium. These cells are responsible for the growth in length and width. In the root there is a protective area called the root cap. Behind this area is the apical meristem where active cell division is occurring. This is followed by an area of elongation where the newly formed cells increase in size. cortex root hair phloem xylem cell elongation cell division growth zone root cap Diagram of a root tip showing the growth zone. In the stems, secondary growth occurs in the cambium. The vascular cambium forms phloem and xylem cells. At the tip of the shoots there is the apical meristem where mitosis is occurring rapidly forming new cells. Buds are another structure that contain meristematic tissue which is capable of rapid growth. 4 Patterns in nature

7 Gill Sans Bold Insects Insects have a multiple staged life cycle. During a larval stage the organism increases in size. This is due to cell enlargement and not cell division. Increased rates of mitosis occur in the epidermal cells before a moult during the pupal stage. Metamorphosis results in the breakdown of the larval tissue and the development of the adult insect. Mammals In mammals mitosis is occurring in many parts of the body. Skin, hair and nails are continually growing. Blood cells are made daily to replace those that have died. Any injury results in rapid mitosis to repair the damage. Young mammals are growing rapidly and at this stage of life mitosis rates are high. 1 Complete the following table by matching the sites of mitosis in either plant or mammal. Site of mitosis Plant/animal root tip skin digestive tract shoot tip bone marrow hair and nails stems 2 Explain why mitosis is important to insects during metamorphosis. Check your answers. Complete Exercise 8.1. Part 8: Growth and repair 5

8 What happens during mitosis? When a cell divides, a series of changes occur in the nucleus of cells. The most important parts of the nucleus involved in the process are the chromosomes. Chromosomes determine the characteristics of an organism. Genes are found along chromosomes and consist of sections of DNA (deoxyribose nucleic acid). Most of the DNA in a cell is found in the nucleus. 1 What is DNA? Why is it important? 2 The DNA part of chromosomes carries the genetic code as genes. Each time a cell divides by mitosis, new daughter cells end up with chromosomes, and hence DNA, which is identical to those of the original parent cell. The discovery of the structure of DNA and the way it is replicated during cell division, has been one of the most exciting and important events in 20th century biology. Mitosis is essentially the replication of chromosomes and their separation into daughter cells. What major developments in technology do you think assisted in the identification of DNA? Check your answers. Self replicating organelles Plastids and mitochondria are self replicating organelles. This means that when mitosis is occurring these organelles reproduce independently of the nuclear division. Chloroplasts (a type of plastid) and mitochondria both posses genetic material (DNA) that enables them to replicate. It is thought that they may be descendants of ancient procaryotic cells that have since become part of other cells. 1 List the cell organelles that contain DNA. 6 Patterns in nature

9 Gill Sans Bold 2 Most of the cell s DNA is present in the nucleus. What parts of the nucleus are made of DNA? 3 What is the role of DNA in the cell? Check your answers. Complete Exercise 8.2. Mitosis in plant cells The process of mitosis in plants is similar to that in animals. However, there are two differences: there are no centrioles in most plants the cell does not become constricted in the last stage of the process. In plant cells, the partition usually starts in the centre of the cell and grows outwards to meet the existing right cell wall. 1 Why do cells undergo mitosis? 2 What are the differences between the parent cells undergoing mitosis and the resulting daughter cells? 3 What is the significance of division after replication for a cell? Check your answers. Part 8: Growth and repair 7

10 Microscopic examination of mitosis in plant cells As you have already read the root tip is a site of rapid mitosis in plants. In this experiment you will examine a root tip for the stage of mitosis. Materials required: microscope and lamp two slides and a cover slip onion with fresh roots (you need to soak onion base in water at least a week in advance) methyl green pryonin or aceto orcein stain. Alternatively use prepared slides of a root tip (if available). If you do not have access to a microscope or prepared slide, use the photographs provided following. Procedure: 1 Remove a new root from the base of the onion and place it in the centre of a clean slide (only the top portion is required if it is very long). 2 Place another slide on top and gently squash the two slides together. This should grind the root tip. 3 Remove the top slide, ensuring the squashed material remains on the lower slide. 4 Add one drop of stain to the material and cover with the cover slip. 5 Allow to stand for about 20 minutes and then examine under the microscope. Observe the cells in the slide of the root tip. You are looking for the different stages of mitosis. 6 Identify cells that have undergone mitosis. 7 Draw diagrams showing the stages in mitosis. 8 Patterns in nature

11 Gill Sans Bold Slide of root tip showing various stages of mitosis. How many stages can you pick out? (Photo Jane West) Interphase (animal cell) Interphase plant cell Your drawing of a plant cell nucleus Part 8: Growth and repair 9

12 Prophase (animal cell) Prophase plant cell Your drawing of a plant cell Metaphase (animal cell) Metaphase plant cell Your drawing of a plant cell Anaphase (animal cell) Anaphase plant cell Your drawing of a plant cell Telophase (animal cell) Telophase plant cell Your drawing of a plant cell 10 Patterns in nature

13 Gill Sans Bold Cytokinesis Mitosis refers to the changes involving the chromosomes during cell division. Cell division, however, usually includes the division of the cytoplasm and certain organelles within the cytoplasm. The division of the cytoplasm is called cytokinesis. In animal cells cytokinesis is usually achieved by the formation of a cleavage furrow which deepens to constrict the two parts. In plant cells, a cell wall forms across the middle, separating the two parts. 1 At what point in mitosis does cytokinesis occur? 2 Why is cytokinesis important in cell division? Check your answers. Complete Exercise 8.3. You have come to the end of the module Patterns in nature. You will have come to recognise that there are patterns in living things as they adopt similar methods of solving the problems of surviving. Part 8: Growth and repair 11

14 Additional resources Phases in mitosis You do not need to learn the names of the stages of mitosis. 1 Interphase This stage is sometimes misleadingly called the resting stage. In fact, the cell is very active. It is during this stage that each chromosome becomes replicated. A cell with four chromosomes would end up with eight at this stage. Cells with 46 chromosomes (a human cell) would end up with 92 and so on. Organelles, such as mitochondria, ribosomes (and chloroplasts in plants) are also replicated although how they are replicated is not clearly understood. As well, the centrioles, which are outside the nucleus, begin to separate. In most of the more complex plants there are no centrioles. 2 Prophase During this stage, the chromosomes are visible, first as long, thin strands. As the process continues, the chromosomes become shorter and thicker. Each chromosome and its replica are held together by a structure called the centromere. The identical chromosomes at this stage are called chromatids. The centrioles move to opposite poles of the cell and spindle fibres start to form. This stage ends with the breakdown of the nuclear membrane. 3 Metaphase The spindle consists of long molecules of protein lying across the cell from pole to pole. The chromosomes move through the cytoplasm to the spindle and become fastened to it by their centromere. The centromere becomes attached along a plane about halfway between the poles. 12 Patterns in nature

15 Gill Sans Bold This plane is called the equator. At this middle stage the chromosomes are in the middle of the cell. 4 Anaphase The centromeres divide so that each chromatid has its own centromere. Each chromatid now is a daughter chromosome. The daughter chromosomes move apart, each member of a pair moving to opposite poles of the cell. Each group of daughter chromosomes forms a densely packed group at each pole. Remember: at anaphase the chromosomes are moving apart. Notice that four chromosomes move to opposite parts (poles) of the cell. Notice also that the cells start to become constricted in the centre. 5 Telophase Nuclear membranes form around each group of daughter chromosomes. The chromosomes uncoil to become slender threads. A new cell membrane forms at the equator. The cytoplasm divides and two new daughter cells now exist, where there was originally only one parent cell. Part 8: Growth and repair 13

16 1 interphase 2 early prophase 3 late prophase nucleus chromatid centromere 4 metaphase spindle 6 telophase 5 anaphase Mitosis in an animal cell with two chromosomes. 14 Patterns in nature

17 Gill Sans Bold Suggested answers Cell division 1 Mitosis is a type of cell division resulting in the replication of identical cells. 2 Meiosis produces cells that have half the number of chromosomes compared to the parent cell. This process produces the gametes or sex cells. 3 Cell division is responsible for growth, repair and reproduction of multicellular organisms. Sites of mitosis 1 Site of mitosis Plant/animal root tip skin digestive tract shoot tip bone marrow hair and nails stems plant animal animal plant animal animal plant 2 Cells need to be produced rapidly when undergoing metamorphosis compared to other stages in which no mitosis occurs. Part 8: Growth and repair 15

18 What happens during mitosis? 1 DNA makes up the material of inheritance or the genetic material in a cell. Every cell needs to have its own DNA code for its specific structure and function. 2 The development of the electron microscope and staining techniques. Self replicating organelles 1 The nucleus, chloroplasts and mitochondria all contain DNA. 2 The chromosomes in the nucleus are made of DNA. A section of DNA with specific information is called a gene. Genes are part of chromosomes. 3 DNA contains information that is transferred when cells replicate. Mitosis in plants 1 Cells undergo mitosis for growth and repair of body tissue. 2 The parent cells are usually larger than daughter cells initially. 3 Cells must divide after replication otherwise they would end up with double the amount of genetic material. Cytokinesis 1 After the chromosomes have separated into two nuclei the cytoplasm divides so that the cells are able to enter interphase. 2 Cytokinesis is important because after mitosis the nucleus has divided and separate nuclear membranes of the daughter cells have formed. Then the cytoplasm must divide (cytokinesis) to produce two new cells. 16 Patterns in nature

19 Gill Sans Bold Exercises - Part 8 Exercises 8.1 to 8.3 Name: Exercise 8.1: Mitosis Mitosis is a very significant process in any living thing. Write a short report to explain the significance of the process of mitosis for plants and animals. Your report should include reference to: the role of cell division in multicellular organisms the activities of chromosomes during mitosis (describe the sequence of change) where mitosis occurs in plants, mammals and insects. Part 8: Growth and repair 17

20 Exercise 8.2: What happens during mitosis? Identify the parts of a cell that contain DNA. Exercise 8.3: Cytokinesis Explain the importance of cytokinesis. 18 Patterns in nature