Unit 2: Reproduction and Development. The Cell Cycle

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1 PAGE : 1 The Cell Cycle Cell Cycle: A continuous series of cell growth and division for a cell. All cells go through a cell cycle of some sort. The cell cycle consists of two stages. a. Growth Phase Diagram of the Cell Cycle b. Division Phase A. Growth Phase (longest part of cell cycle) Also known as Interphase. Consists of three segments. o G1 or Gap 1 phase Cell undergoes rapid growth and normal metabolic activities. (A Cell spends most of its time in this phase). o S Phase The cell replicates(duplicates) the DNA (Chromosomes) o G2 or Gap 2 Phase The centrioles replicate in preparation for cell division. B. Division Phase (Shortest part of the cell cycle) This is the stage where the cell divides into two new cells. Consists of two Stages. i. Mitosis The process of dividing the nucleus. ii. Cytokinesis The process of dividing the cytoplasm.

2 PAGE : 2 How long is the cell cycle? The cell cycle varies in length depending on the type of cell and its environment. Some cell cycles last for a few hours whereas others last for a number of years. Ex: Cells in stomach lining cell lasts for about 2 days before dividing. Brain cells lasts for up to 50 years.

3 PAGE : 3 Mitosis Mitosis: The series of stages a cell goes through in order to divide the nucleus (DNA material). Purpose of Mitosis: 1. To ensure genetic continuity by creating a copy of the DNA. I.e. make sure the DNA is carried on by maintaining the same number of chromosomes from one cell to the next. 2. To create new cells for growth and repair Where does mitosis occur? Mitosis occurs within the body (somatic) cells of an organism. Ex: skin, muscle, bone, etc. All cells EXCEPT the Gonads (Ovary and testes) Results of Mitosis when finished Two New cells called Daughter cells. Each Cell has the same number of chromosomes as the parent cell. Chromosomes Strands of DNA that carry out the genetic code. The genetic code determines your traits such as hair colour, height etc. Chromosomes are found within the nucleus of a cell. Chromosomes are normally Single Stranded (SS) but become Double Stranded (DS) during interphase when they are replicated.

4 PAGE : 4 The Stages of Mitosis There are four stages to Mitosis. In order, they are; A. Prophase B. Metaphase C. Anaphase D. Telophase Prophase The following happens during this stage. < The Chromatin (Fine threads of DNA in the nucleus) condenses to reveal Double Stranded (DS) Chromosomes. < The Centrioles begin to move to the opposite ends of the cell. < The nuclear membrane breaks down and disappears. < The centrioles grow filaments called spindles and are now called asters. Spindles Disappearing Nuclear Envelope Centriole DS Chromosomes

5 PAGE : 5 Metaphase During Metaphase, the following events happen. < The Chromosomes line up at the middle of the cell. < The spindles attach to the chromosomes at the Centromeres. Centromere: The middle portion of a chromosome that holds the chromatids together. Centromere Spindles

6 PAGE : 6 Anaphase During Anaphase, the following events happen. < The Spindles pull the chromosomes apart and to the opposite ends of the cell. < There are now Single Stranded (SS) chromosomes at each end of the cell. Single Stranded Chromosomes

7 PAGE : 7 Telophase During Telophase, the following events happen. < The chromosomes uncoil and become chromatin again. < The nuclear membrane begins to form around each set of chromosomes. < The Spindle and Aster disappear < Cytokinesis begins. Cytokinesis: The process of dividing the Cytoplasm of a cell to create two new cells. The cytoplasm is divided equally among the two new daughter cells. This occurs when the cell membrane in animal cells and the cell plate (plant cells) pinches together in the middle and eventually closes off from each other. Nuclear membrane reappearing Chromosomes uncoiling Cytokinesis occurring

8 PAGE : 8 Definitions: 1. Chromatid: In a DS chromosome, an exact copy of DNA. 2. Nuclear Membrane: The membrane that surrounds the nucleus of a cell. Keeps its contents together. 3. Spindle: Long fibre created from the aster that attaches to the centromere of chromosomes. Pulls chromosomes apart. 4. Aster: The star-like structure at each end of a dividing cell that helps pull chromosomes apart. 5. Cell Plate: A plate of tissue that grows between dividing plant cells during telophase. The cell plate separates the two new plant cells. 6. Cytoplasm: The fluid of the cell. Important in giving structure to the cell. 7. Chromosome: A long strand of DNA (Deoxyribonucleic Acid), a protein, which carries genes to express traits. 8. Gene: A small segment of DNA on a chromosome that is responsible for the expression of traits. 9. Centrioles: Cylindrical shaped bodies near the nucleus of a cell that becomes the aster during cell division. 10. Chromatin: Fine strands of DNA within the nucleus of a cell that makes up the chromosomes.

9 PAGE : 9 Cell Division and Cancer Mutation: A permanent change in the DNA molecule resulting in a change to genes. Causes of Mutations: Mutations are normally caused by either of the following: Implications of Mutations i)chemicals ii)radiation iii)viruses If a mutation occurs within a gene responsible for cell division within somatic cells, cancer may result. CANCER: The uncontrolled, rapid growth and division of cells. How does cancer operate? Within cells are certain genes that control the rate at which mitosis occurs. These genes are like switches. When a cell finishes mitosis certain genes are turned on that produces proteins that stop mitosis. When the genes are off mitosis can continue and the cell can divide to produce new cells. Mutations can permanently inactivate the off switch or gene resulting in no stop to mitosis. The result is uncontrollable growth and division of cells. Other genes start mitosis and if these are switched on permanently, then cancer or uncontrollable cell division results. Oncogenes: These are genes that when mutated can cause cancer in a cell.

10 PAGE : 10 TREATMENTS FOR CANCER Cancer cells divide much more quickly than do normal cells so anything that interferes with cell division will harm these cells more readily than normal cells. Cancer treatments are based on this fact. 1. Radiation Therapy Radiations such as X-rays or gamma rays are directed at the affected area of the body. Radiation works by damaging the chromosomes within a cell making it unable to grow or divide. Radiation affects normal cells too, but they are usually able to repair themselves. Radiation is often directed at a very specific target to minimize damage in other areas. Radiation therapy is often used to treat: Skin, Breast, larynx and cervical tumors. Side affects of Radiation Therapy: 2. Chemotherapy i) Fatigue ii) Skin inflammation iii) Hair loss (if treatment on the brain) iv) Sterility in males (if treatment on testicles) The use of drugs to treat cancer. Sometimes used in conjuction with radiation therapy. Used to treat cancers found throughout the body. Ex: Leukemia Chemotherapy durgs work by attacking cells that are dividing as well as preventing others from dividing. Chemotherapy affects both cancer and normal cells.

11 PAGE : 11 Side effects of Chemotherapy 1. Hair loss 2. Nausea 3. diarrhea

12 PAGE : 12 Meiosis Meiosis: A type of cell division that results in gametes (sperm and egg) being created with half the number of chromosomes (haploid n) as the parent cell. Purpose of Meiosis: 1. Genetic continuity 2. Genetic variability Where does meiosis occur: Meiosis occurs in the gonads (Testes and Ovaries). There are two types of Meiosis. 1. Ôogenesis: (See below for explanation) 2. Spermatogenesis (See below for explanation) The Stages of Meiosis NOTE: Meiosis occurs in two stages. Stage 1 is called Meiosis 1 and Stage 2 is called Meiosis 2. MEIOSIS 1 Has 4 stages a) Prophase 1 b) Metaphase 1 c) Anaphase 1 d) Telophase 1 Meiosis 2 Has 4 stages a) Prophase 2 b) Metaphase 2 c) Anaphase 2 d) Telophase 2

13 PAGE : 13 Stages of Meiosis 1 Meiosis 1 is also known as Reduction Division Reduction Division: The process of reducing the number of chromosomes in a cell by half (1/2). In other words, going from diploid (2n) to haploid (n) Ex: 46 chromosomes 23 chromosomes Prophase 1 During Prophase 1 of meiosis, homologous chromosomes (similar chromosomes) line up next to each other as pairs. Pairs of homologous chromosomes are called TETRADS. Crossing over of non-sister chromatids occurs. Crossing Over: The overlapping of non-sister chromatids resulting in an exchange of genes. Homologous Chromosomes Tetrad (pair of homologous chromosomes)

14 PAGE : 14 Crossing over explained Metaphase 1 During Metaphase 1 Homologous chromosomes line up at the middle of the cell. The spindle from one end of the cell attaches to one pair of sister chromatids while a spindle from the other end attaches to the other pair of sister chromatids.

15 PAGE : 15 Anaphase 1 During Anaphase 1, the spindle pulls homologous chromosomes to separate ends of the cell. Telophase 1 During Telophase 1, the following events happen. Nuclear membrane reappears around each set of separated chromosomes. Spindle disappears Cytokinesis (division of the cytoplasm) occurs. Two new cells are created with ½ (haploid) the chromosome number of the original cell. o i.e 46 DS chromosomes 23 DS chromosomes per cell. Or 96 chromatids 46 chromatids per cell.

16 PAGE : 16 Stages of Meiosis II NOTE: The stages of Meiosis 2 are identical to the stages of Mitosis. Prophase 2 During prophase 2 the following events occur. Nuclear membrane disappears. Spindle forms. Chromsomes become visible. Metaphase 2 During Metaphase 2 the following events occur. Chromosomes line up at the middle of the cell. Spindle attaches to centromere of each chromosome. Anaphase 2 During Anaphase 2 the following events occur. Spindle pulls one chromatid from each chromosome to opposite ends of cell. Telophase 2 During Telophase 2 the following events occur. Nuclear membrane reappears around each group of chromatids (now called SS chromosomes) Spindle disappears. Cytokinesis occurs. Two new cells are created, each having the haploid number of chromosomes.

17 PAGE : 17 Ôogenesis This is meiosis that occurs in the ovaries of females. Result: The creation of an egg with ½ the normal number of chromosomes. Explanation of Oogenesis Within the ovaries of females, a diploid (2n) cell called a Oogonium undergoes meiosis. Before Meiosis begins, the oogonium replicates (doubles) its chromosomes so that it has 46 DS chromosomes or 92 sister Chromatids. During the first Meiotic division, the Homologous (similar) chromosomes are separated. During this stage cytokinesis occurs unevenly and as a result one large and one small cell are created. The large cell is called a Primary Oocyte and the small cell is called a Polar Body. Each of these cells will have 23 DS chromosomes or 46 SS chromosomes (chromatids). During the second meiotic division, the cells divide again creating 4 cells, but this time the new cells created each have 23 SS chromosomes. Again, cytokinesis has resulted in an unequal distribution of cytoplasm. NOTE: The three polar bodies will die because they do not have enough cytoplasm to keep them running.

18 PAGE : 18 Spermatogenesis This is meiosis that occurs in the testes of males. Result: The creation of FOUR Sperm, each with ½ (haploid) the number of chromosomes as the original cell. Explanation of Spermatogenesis Within the testicles of a male, a spermatogonium replicates its chromosomes to contain 46 DS chromosomes or 92 chromatids. During the first meiotic division, the cells divide and the homologous chromosomes separate. This results in 2 new cells, each with 23 DS chromosomes or 46 chromatids each. During the second division, two new cells are created, each with 23 SS chromosomes. These cells, called primary spermatids, will mature into sperm. NOTE: Each spermatid matures into sperm in the Epididymis of the testicles.

19 PAGE : 19 Meiosis Definitions 1. Diploid (2n): This is the normal number of chromosomes an organism has in each of its cells. Ex: humans - Diploid(2n) = Haploid/Monoploid (n): This is ½ the normal number of chromosomes an organism has in each cell. Ex: humans - monoploid (n) = Tetrad: A term meaning a group of 2 homologous DS chromosomes. A term meaning a group of 4 chromatids 4. Synapsis: The pairing of the homologous chromosomes. 5. Crossing Over: The exchange of genes between non-sister chromatids in homologous pairs of chromosomes. 6. Nondisjunction: The failure of homologous chromosomes to separate during meiosis. 7. Reduction Division: The stage where the number of chromosomes in each cell is reduced by ½.

20 PAGE : 20 The end result of Meiosis is as follows: End Result of Meiosis 4 new cells created. Each cell has ½ or Haploid (n) the number of chromosomes as the parent/original cell. Crossing over has resulted in a new combination of genes on chromosomes leading to variety in organisms. MITOSIS/MEIOSIS COMPARED Event Mitosis Meiosis Where does it occur? Somatic Cells Gonads Number of Cells Created per complete cycle 2 4 Number of Divisions 1 2 Number of Chromosomes per new cell created Does Crossing over happen? Do Homologous Chromosomes line up? Same as parent cell No No ½ of parent cell Yes, possibly Yes Purpose Genetic Continuity, growth Genetic variability