Part II The Cell Cell Division, Chapter 2 Outline of class notes 1 Cellular Division Overview Types of Cell Division Chromosomal Number The Cell Cycle Mitoses Cancer Cells In Vitro Fertilization Infertility Affects one in ten American couples In vitro fertilization (IVF) can help infertile couples. A sperm and an egg are joined in a petri dish. The embryo is implanted into the mother s uterus at about the 8 cell stage. IVF is one of many reproductive technologies Cell Division Cell Division Is the process whereby cells reproduce themselves Functions: Replace diseased or damaged cells Growth of the organism Production of a new organism Two types of cell division are: Somatic cell division Reproductive cell division Somatic Cell Division In somatic cell division: Cell undergoes a nuclear division called mitosis and a cytoplasmic division called cytokinesis. Results in two daughter cells genetically identical - exactly the same number and type of chromosomes as the parent cell. Reproductive Cell Division In reproductive cell division: Cells undergo a process of meiosis that produces gametes (eggs or sperm). The egg and sperm contain half the number of chromosomes as the parent cell. Occurs in the ovaries and testes. Function: The creation of offspring by the fertilization of an egg by a sperm
2 Background on Chromosomes You inherited 46 chromosomes, 23 from each parent. Each somatic cell of the human body, except for sex cells, contains 46 chromosomes (diploid number). These chromosomes are arranged in pairs, thus you have 23 pairs to give a total of 46 chromosomes. Of the 23 pairs, one pair are the sex chromosomes which consist of two X chromosomes if the person is female or an X and a Y chromosome if the person is male The remaining 22 pairs of chromosomes are called autosomes Sex cells (sperm and egg) contain 23 individual chromosomes (haploid number), or half that of somatic cells. The union of a sperm and egg during fertilization restores the chromosome number to 46 (23 pairs). A human karyotype has 22 pairs of autosomes and 1 pair of sex chromosomes (23 total pairs) Cell Cycle The cell cycle is an sequence of events in the life of a dividing cell, it lasts from the beginning of one division to the beginning of the next. Cell Cycle consists of two distinct phases: 1. Interphase 2. Mitotic (M) Phase Interphase Interphase is the time in which the cell performs its normal functions, and if appropriate, prepares for cell division. Consists of four stages: Go Phase: (G = gap) The normal functional state of a cell not dividing. G1 Phase: Cell prepares for replication. Cell produces all the enzymes required for DNA replication and duplicates most of its organelles. S Phase: (Synthesis) Chromosomes are replicated and become double stranded. The replicated chromosomes are attached together at the centromere and are known as sister chromatids. Each is an exact copy of the other and is called a chromatid. G2 Phase: Devoted to last minute protein synthesis as cell prepares to divide.
Cell Division in Somatic cells Cell division includes two overlapping processes mitoses and cytokinesis. Mitoses: Division of the nucleus (chromosomes) The chromosomes within the nucleus divide and are evenly distributed, forming two daughter nuclei Cytokinesis: Division of the cytoplasm Results in two genetically identical daughter cells Note: Mitoses is a continuum, but we can divide it into 4 stages: prophase, metaphase, anaphase, and telophase 3 Mitoses: 4 stages 1. Prophase Chromatin condenses to form visible (with a microscope) chromosomes. Centrioles move toward opposite ends of the cell Centrioles produce spindle fibers (microtubules), which extend and attach to the centromeres of each chromosome. Spindle fibers attach to the kinetochore of the centromere. Movement of chromosomes depends on the spindle fibers. Nucleolus and the nuclear membrane disappear 2. Metaphase Chromosomes (each consisting of sister chromatids) line up on the metaphase (equatorial) plate which is at the center of the cell. 3. Anaphase Sister chromatids separate at the centromere forming two sets of identical chromosomes. Note: At time of separation, each chromatid is now referred to as a chromosome. The daughter chromosomes move toward the opposite ends of the cell. Spindle fibers from the centrioles assist in this process. 4. Telophase A nuclear membrane forms around each set of chromosomes Chromosomes uncoil becoming chromatin Nucleoli reappear Cell prepares to divide Cleavage furrow begins to form
4 Cytokinesis Cytokinesis (cleavage) is the division of the cytoplasm. Typically begins during telophase. The cell membrane constricts along the plane of the metaphase (center) plate forming a cleavage furrow. Nemonic Phrase Cytoplasm divides resulting in two daughter cells each with DNA I ve Interphase that is identical to the DNA of the parent cell Probably Prophase Made Metaphase Mnemonic to Remember Mitosis All Anaphase This Telophase Correct Cytokinesis Control of the Cell Cycle Normal animal cells have a cell cycle control system. Special proteins within the cell send stop and go-ahead signals at certain key points during the cell cycle. Example: Nerve and muscle cells are in a permanent nondividing state (Go). Other cells like our skin cells, are constantly receiving signals to divide.
5 Cancer Cells: Growing Out of Control Cancer is a group of diseases characterized by uncontrolled cell proliferation - is a disease of the cell cycle. Kills one out of every 5 people in the U.S. Cancer cells do not respond normally to the cell cycle control system. They divide excessively and can invade other tissues of the body. Benign vs. Malignant Tumor Cancer cells can form tumors (neoplasms), which are abnormal growths (masses) of body cells. Two types of Tumors: 1. Benign tumors: Tumor cells remain at the original site and are often harmless (Ex: wart). Removed surgically if interferes with normal function 2. Malignant tumors: Tumor cells spread into neighboring tissue by way of the circulatory and lymphatic vessels and may form new tumors. Metastasis is the spread of cancer cells. An individual with a malignant tumor is said to have cancer. How Cancers are Named Cancers are named according to where they originate and are grouped into 4 categories: 1. Carcinomas: Originate in the external or internal coverings of the body (skin, intestine etc), lungs, breasts, pancreas, glands. Most commonly diagnosed cancers 2. Sarcomas: Originate in support tissues/structures, such as in bone, fat, and muscle 3. Leukemias: Originate in the bone marrow 4. Lymphomas: Originate in the lymph nodes Genetic Basis of Cancer Proto-oncogenes are genes that code for growth factors that regulate growth and development - stimulate cell division. When functioning normally they keep the rate of cell division at the normal level. Oncogenes are mutated forms of the proto-oncogenes and can cause cancer to develop. Oncogenes transform a normal cell into a cancerous cell. Ex: Causes excessive production of growth factors
Tumor-suppressor Genes Tumor-suppressor genes produce proteins that help prevent uncontrolled cell growth. Mutation of a Tumor-suppressor gene called p53 on chromosome 17 is the most common genetic change leading to a wide variety of tumors, including breast and colon cancers. Normal p53 protein: Arrests cells in the G1 phase, which prevents cell division. Assist in repair of damaged DNA and induces apoptosis in cell where DNA repair was not successful. Nicknamed the guardian angle of the genome 6 Oncogenic Viruses Certain viruses (oncogenic viruses) carry genes called oncogenes that can cause cancer. Viruses can insert their DNA containing oncogenes into the host chromosomes and stimulate abnormal proliferation of cells cancer cells. Human Papillomavirus (HPV) HPV causes most cervical cancer in woman Symptoms and virulence depend on the strain of virus Virus produces a protein that causes proteasomes to destroy the p53 protein. Absence of this suppressor protein causes cells to proliferate without control.
Cancer Treatment Cancer treatment can involve: Surgery: Removes the tumor or cancer. Is usually the first step Radiation therapy: High energy radiation damages DNA and disrupts cell division. Cancer cells divide more often than other cells. Can cause nausea and hair loss Chemotherapy: Uses drugs that disrupt cell division. Usual site is to disrupt spindle fiber formation or function 7 Cancer Prevention and Survival Cancer prevention includes changes in lifestyle: Not smoking Exercising adequately Avoiding exposure to the sun Eating a high-fiber, low-fat diet Visiting the doctor regularly Performing regular self- and doctor mediated examinations Breast Prostate Cervical Testicular Colon