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Cell Divison Mitosis and Meiosis Cell Division The ability of an organism to procreate and survive is dependent upon cell division. Some organisms are exact copies of their parents, while others are unique and different from their parents. There are 2 types of cell division to accomplish this, each with different functions. Types of Cell Division Mitosis – production of identical daughter cells, usually for the purpose of growth or replacement of body cells. This form of cell division is also found in asexual reproduction. Meiosis – production of the gametes (sex cells). These cells (sperm and egg) contain only one half of the total genetic information from the parent. The combination of gametes during sexual reproduction gives rise to individuals with unique characteristics. Some required terms: Chromatin – threadlike network in a cells nucleus that is the combination of all DNA (genes) and proteins (not visible with a light microscope) Chromosome – is a single thread-like structure in the nucleus of a dividing cell that consists of chromatin coiled up. Humans have 46 chromosomes (23 pairs) in every cell except the gametes, which contain only 23 chromosomes. (We count chromosomes by counting centromeres.) Chromatids – the two identical parts of a chromosome following replication of DNA. Usually two sister chromatids are held together by a centromere. Gene – is the basic unit of heredity that is composed of a specific sequence of nucleotide bases in DNA. This code is translated into specific proteins and structures within the cell. Haploid – (N) the number of distinct chromosomes an organism possesses in the gametes (sperm or egg cells) Diploid – (2N) having twice the number of chromosomes that are found in the gametes Homologous chromosomes – are chromosomes that are the same size and shape and carry the genes for the same traits, but have different details Autosomes – are chromosomes that do not influence biological sex Sex Chromosomes – X and Y chromosomes that determine biological sex in humans The Cell Cycle Most cells only spend a short percentage of their time (10%) dividing. Most of the time (90%), cells are busy carrying on with other cell activities, and preparing to divide. The cell cycle can be divided up into phases: Interphase – refers to the time when cells are not actively dividing. This accounts for about 90% of the cell’s time. This phase involves a number of sub-phases: G1 – gap phase - the cell manufactures proteins and amino acids needed for both cell processes and cell division. (8 hours of the cell cycle) S – synthesis phase – all nuclear DNA replicates. (6-8 hours of the cell cycle) This stage is critical for cell division to occur. G2 – gap phase – the cell increases the rate of protein synthesis and completes all processes necessary for cell division. The cell also grows larger. (4.5 hours of the cell cycle) Mitosis/ Meiosis – refers to the time when cells are actively dividing. This process takes up about 10% of the cell cycle. This can be divided up into a series of sub-stages. Cells do not remain functional for an indefinite length of time. Cells replace themselves on a regular basis, though cell division. There is a limit on the number of times a cell can undergo cell division. On average it is believed that a cell can divide only about 50 times on average. To ensure that the cells in a tissue are healthy, cells will undergo a form of cell suicide called apoptosis. This programmed cell death ensures that the cells remaining in a tissue are functional, healthy and active. The cell cycle really consists of a pattern of development of cells that involves both cell division and cell death. Cell Death Mitosis – Asexual cell division Mitosis involves the replication of daughter cells for the purpose of growth and replacement of dead cells. This process produces diploid daughter cells that are genetically identical to the parent (original cell). Mitosis is usually divided up into 4 major stages: prophase, metaphase, anaphase and telophase (PMAT). Prophase chromatin condenses into chromosomes, nucleolus and cell membrane begin to break down, microtubules form from spindle fibres in plants, centrioles separate to form asters in animals and migrate to the poles Metaphase chromosomes move towards the middle of the cell (equatorial plate), each chromosome is attached to a separate spindle fibre Anaphase chromatids begin to separate with one member of each pair being pulled to opposite poles Telophase nucleolus re-appears and the nuclear membrane reforms around each set of chromosomes, the chromosomes unravel to form a loose mass of chromatin, cells undergo cytokinesis. Plant vs Animal Cells a cell plate forms between the new daughter cells the cell membrane pinches in between the two daughter cells This is called cytokinesis Mitosis Overall http://www.loci.wisc.edu/outreach/bioclips/CDBio.html Some Methods of Asexual Reproduction 1. Binary fission - equal division of both the organism cytoplasm and nucleus to form two identical organisms ex: Protist - amoeba 2. Budding - one parent dividing its nucleus (genetic material) equally, but cytoplasm unequally ex: Fungi- yeast 3. Sporulation (spore formation) --is reproduction involving specialized single cells coming from one parent ex. Fungi -mold spores Meiosis – Sexual Cell Division Meiotic cell division only occurs in the sex cells and is responsible for the production of haploid gametes (sperm and eggs). Unlike mitosis, the goal of meiosis is to produce variability. Half of the DNA from the parent cell is passed down to one gamete. Each gamete will be different depending what combination of the 23 chromosomes it receives. To increase variability, crossing over occurs, which causes a mixing of genetic information on one chromosome. The stages of meiosis are very similar to the stages of mitosis, except that there are 2 cell divisions, with only one replication of the DNA. Meiosis I – Reduction Division Prophase I – homologous chromosomes undergo synapsis (the pairing of homologous chromosomes) so that information can be exchanged by crossing over. Metaphase I – homologous pairs line up along the equatorial plate. Anaphase I – homologous chromosomes separate and are pulled to opposite poles by centromeric spindle fibers. Telophase I – daughter cells separate, each has one chromosome from each homologous pair. Synapsis / Crossing Over Meiosis II – no replication of DNA Prophase II – cells have one chromosome from each homologous pair Metaphase II – double stranded chromosomes align at the equatorial plate Anaphase II – the daughter chromatids separate and move towards separate poles Telophase II – spindle fibers disappear, nuclei reform and cytokinesis takes place. All 4 daughter cells are haploid. NOVA Online | 18 Ways to Make a Baby | How Cells Divide: Mitosis vs. Meiosis (Flash) Successful cell division relies on 2 things: 1. accurate replication of the chromosomes 2. exact separation / distribution of the chromosomes Oogenesis vs Spermatogenesis Oogenesis Spermatogenesis meiosis II completes only if egg is fertilized ootid receives most of the cytoplasm and becomes the egg polar bodies have less cytoplasm and degenerate one egg cell is produced meiosis I and II are both completed before fertilization each sperm cell produced receives an equal share of cytoplasm flagellum forms from one of the centrioles four spermatids are produced Oogenesis Spermatogenesis Abnormal Meiosis Non-disjunction – occurs during meiosis when two homologous chromosomes move to the same pole during either meiosis I or meiosis II. Normal Division Non-disjunction Trisomic Zygote When the daughter cell with too many or too few chromosomes is fertilized, the chromosome number becomes either one too many or one too few. Trisomy – a condition where three chromosomes replace a normal pair (47 chromosomes in humans) Monosomy – a condition where one chromosome replaces a normal pair (45 chromosomes in humans) Biological Sex – in humans is determined by the presence / absence of the Y chromosome. Female is XX Male is XY (This varies with other organisms) The process of crossing over during meiosis can also result in a number of mutations (alterations to the genetic code). Inversions – occur when a segment of a chromosome is turned 180 degrees and reconnected in the wrong orientation Ex. Toothpaste - Tophtoaste Translocations – occur when a chromosomal segment breaks off and re-attaches to another chromosome Ex. Swimming Elephants - Swimm Ingelephants Deletions - occur when a chromosomal segment breaks off and does not re-attach anywhere else Ex. Superstitious - Suitiious Duplications – occur when a chromosomal segment is present more than once on the chromosome because extra copies of that segment were made. Ex. Bubblegum - Bubblebleblebleblegum Chromosomal abnormalities can be detected by doing a karyotype chart. Fetal cells are collected through amniocentesis or chorionic villi sampling, and the chromosomes are arranged in homologous pairs according to size and banding patterns. Chromosomes can be counted to determine whether a non-disjunction has occurred. Human Karyotype Chart Some Common Genetic Disorders Down’s syndrome – trisomy 21 more common in children born to women over 40 characterized by short stature, folds to the eyelids, stubby fingers, wide gap between 1st and 3rd toes, large fissured tongue, round head, palm creases, mild to severe mental retardation. Edward’s syndrome – trisomy 18 results in severe overall defects with a life expectance of only 10 weeks Patau’s syndrome – trisomy 13 results in non-functioning eyes, severe deficits and limited life expectancy. Turner’s syndrome – XO female females with short stature, broad chest, heart defects, lack of breasts and absence of sexual maturation and menstruation Klinefelter’s syndrome – XXY male sterile males with underdeveloped testes, overdeveloped breast tissue and sub-normal intelligence Metafemale – XXX female no obvious deficits however menstrual irregularities and early menopause are common XYY male taller than normal, recurrent acne, barely normal intelligence Cri du Chat syndrome deletion of a portion of one copy of chromosome number 5, malformed face and head, short life Societal Issues of Cell Division Cloning – is a process in which identical offspring are formed from a single cell or tissue of the parent. Cloning occurs in nature when plants send out runners, when bacteria divide and in identical twins. Cloning as a technology is more complicated. How to Clone: (the quick and easy, yet not so successful way) Take a haploid egg cell from an adult female and remove the nucleus (enucleation). Replace the nucleus with the nucleus from a body cell of the individual to be cloned. (the cell must be toti-potent, or have its identity genes turned off) (you can use stem cells here) Zap it!!! (a little magic is required here) Implant the zygote or blastula into the mother’s uterus. Wait until the gestational period is up and voila! Click and Clone Problems with cloning: some think it is wrong!!! artificially cloned organisms seem to age faster it is very time and resource consuming and is not guaranteed to work Ageing is a normal part of life. Many believe that we are preprogrammed to age, since repeated divisions and a long cellular life could lead to more genetic mutations. There is a limited number of times (about 50) that cells divide before they undergo a form of cell suicide called apoptosis. Scientists believe that the ends of the chromosomal arms, called telomeres, shorten each time a cell divides, and when they become critically short, the cell dies. Cancer involves abnormal, uncontrolled cell division. Each time a cell divides, start and stop genes are activated to tell the cell when it is appropriate to replicate, and when to stop. In cancer, one of these genetic switches quits working. Cancer is dangerous for a number of reasons: Cancer cells do not differentiate, so they are not effective and do not participate in helping the body to function. Cancer cells require lots of nutrients, and take nutrients away from functional cells preventing other cells from doing their jobs. Cancer cells can break away (metastasis) from the tumour mass and spread to other parts of the body. NOVA Online | Cancer Warrior | How Cancer Grows Since many substances that are known to cause mutations also cause cancer, it is believed that cancer is a mutation of specific genes. Tumour suppressor genes – suppress cell division. (mutation turns these off) Proto-oncogenes – stimulate cell division. (mutation turns these on)