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Cell Reproduction
 

Cell Reproduction

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    Cell Reproduction Cell Reproduction Presentation Transcript

    • Cell Reproduction Mitosis & Meiosis
    • Why Cells Divide
      • Surface Area/ Volume Ratio
        • As the cell grows, the volume increases at a greater rate than the surface area
        • Can't take in enough nutrients, or remove wastes
        • Therefore the cell must grow or divide
      • Growth and Repair
        • Replace worn or damaged cells
        • Frequency of replacement varies:
          • bacteria ~ every 20 minutes
          • human cells ~ every 18-22 hours
        • Many cells in the body don't divide
    • Cell Division
    • Cellular Reproduction
      • When the parent cell divides, it forms new daughter cells
      • Organisms reproduce in two ways:
        • Asexual Reproduction
        • Sexual Reproduction
    • Sexual vs. Asexual Reproduction
      • Asexual Reproduction
        • production of offspring from one parent
        • therefore genetic material is identical to parent
      • Sexual Reproduction
        • formation of a new individual from the union of 2 cells
        • 2 parents, therefore offspring have some hereditary material from each
    • Types of Asexual Reproduction
      • Binary Fission
        • simplest form; the cell splits in 2
      • Spore Formation
        • Begins with replication
        • Spores can remain inactive until conditions are favorable
        • molds, fungi
      • Yeast reproduce by budding
      • Vegetative Propagation
        • Some plants, e.g. strawberries
      • Regeneration
        • planaria, star fish, etc.
    • Cell Division in Prokaryotes
      • Binary Fission:
      • The simplest form of cell division
      • The cell splits in 2
    • The Process of Binary Fission
      • First the single circular chromosome duplicates = Replication
      • Both chromosomes attach to sites on the cell membrane
      • As the cell grows, a new membrane forms between attachment sites
      • Membrane pinches off and the new cells separate
    • Sexual Reproduction
      • The joining of 2 specialized sex cells called gametes
        • male = sperm
        • female = ovum
      • Process of combining gametes = fertilization
      • Fertilization produces a zygote
        • has characteristics of both parents
    • Human Sexual Reproduction
      • Male testis produces sperm
      • Female ovary produces ova
      • Each has 23 chromosomes
      • Unite to form a zygote with 46 chromosomes
        • 23 pair
      • Develops into a fetus
    • Cell Division
      • All types of reproduction require cell division
      • 2 processes can be used to divide the cell’s nuclear material:
      • Mitosis
        • Occurs in somatic cells (body cells) in eukaryotes
        • As a result of mitosis each daughter cell receives an exact copy of the chromosomes present in the parent cell
      • Meiosis
        • Occurs in gametes (sex cells)
        • As a result each daughter cell receives 1 of each pair of chromosomes present in the parent cell
    • Mitosis
      • Cell division in eukaryotic cells involves nuclear division called mitosis
      • Occurs in somatic cells
        • body cells; not sex cells
      • As a result of mitosis, each daughter cell receives an exact copy of the chromosomes present in the parent cell
      • Chromosomes contain genetic material
        • DNA
    • Chromosomes
      • During cell division in eukaryotic cells, the DNA is coiled into chromosomes
      • Every body cell of the same type of organism has the same number of chromosomes
        • humans = 46
        • goldfish = 94
        • mosquito = 6
    • Chromosome Structure
      • Each chromosome is formed from two joined strands called chromatids
      • Each chromatid is alike
        • has a long arm & a short arm
        • joined at the centromere
      • Chromosomes contain DNA and associated proreins
    • Chromosomal Proteins
      • Each chromosome is a single DNA molecule and associated proteins
      • Histones –
        • One type of chromosomal protein
        • The DNA wraps tightly around the histones
        • Histones help maintain the shape of the chromosome
      • Nonhistone proteins -
        • control the activity of specific regions of DNA
    • Picturing Chromosome Structure
    • Visualizing Chromosomes
    • Chromosome Make-up
      • Chromosomes of somatic cells are in pairs
        • One of each pair comes from mother, one from father
      • The 2 chromosomes in a pair are homologous
        • Alike in appearance and type of genetic information carried
      • Humans have 23 pairs of chromosomes
        • 22 pairs of autosomes
          • Autosomes are all but the sex
        • 2 sex chromosomes ( X & Y)
    • Sex Chromosomes
      • Determine the sex of the organism
      • Also carry other genetic information
      • In humans, either X or Y
      • Females are XX, males are XY
      • Thus the male determines the sex of the offspring
    • Haploid vs. Diploid
      • Cells with two copies of each chromosome = diploid
      • Autosomal cells are diploid
      • Gametes (sex cells) have only one of each type of chromosome
      • Cells with one copy of each chromosome = haploid
    • Karyotypes
      • A picture of paired human chromosomes
      • Used to to detect certain genetic diseases
    •  
    • Mitosis
      • The process of dividing the nuclear material in a somatic cell in eukaryotes
      • Necessary for cell division
    • Preparation for Mitosis
      • Interphase
        • The time between the formation of a cell through mitosis and the next mitosis
      • Most of the cell cycle is interphase
      • During this phase cell prepares by:
        • replicating genetic material
        • producing organelles
        • assembling structures needed for mitosis
    • Chromosomes & Interphase
      • During interphase chromosomes cannot be distinguished under the light microscope
        • They appear as chromatin
      • At the start of mitosis, the chromatin thickens, and chromosomes become visible
    • The Cell Cycle
      • The sequence of cell growth and division
      • The cell cycle can last several hours to several days
      • Can be affected by environmental factors, like temperature
      • Has 4 stages:
        • mitosis & division of cytoplasm ( cytokinesis )
        • The other 3 are part of interphase :
          • G1
          • S
          • G2
    • Picturing the Cell Cycle
    • G1 - Growth
      • After mitosis, a period of intense cellular activity and growth
      • The cell doubles in size
      • Enzyme production is high
      • Cells that stop growing remain in G1
    • S- Synthesis
      •   Cells that divide enter S, or synthesis, phase
      •   The chromosomes replicate
    • G2 – Further Growth
      • A second period of growth
      • Structures used in mitosis are assembled
    • The Phases of Mitosis
      • Mitosis is actually a continuous process
      • But we divide it into 4 phases:
        • Prophase
        • Metaphase
        • Anaphase
        • Telophase
    • Prophase
      • 60% of the period of mitosis is prophase
      • Divided into 3 parts: early, middle, & late
      • Chromosomes begin to coil into short rods
      • Nucleoli break down & begin to disappear
      • 2 pairs of dark spots called centrosomes appear outside the nuclear membrane
        • In animal cells, the centrosomes contain centrioles , formed from microtubules
        • Plant cells have no centrioles
      • The centrosomes move to opposite sides of the cell
    • Mid Prophase
      • At the beginning of mid-prophase spindle fibers form between the centrioles
      • Additional fibers radiating out from each centriole form the aster
      • The nuclear membrane has broken down and disappeared
    • The Mitotic Spindle
      • Spindle fibers made of microtubules radiate from the centrosomes
      • This array of spindle fibers = the mitotic spindle
      • 2 types of spindle fibers:
      • Kinetechore fibers
        • Attach to a disk-shaped protein called a kinetecore
        • Found in the centromere of each chromosome
        • Extend from the kinetechore of each chromatid to one of the centrosomes
      • Polar fibers
        • Extend across the dividing cell from one centrosome to the other
    •  
    • Late Prophase
      • The centrosome pairs are at opposite ends of the cell
      • The centosomes are fully formed
      • Chromosomes are attached to the centrosomes by spindle fibers
      • Other spindle fibers stretch across the cell from one centriole to the other
    •  
    • Metaphase
      • The chromosomes are pushed and pulled by spindle fibers along cell's the midplane
        • called the equator
    • Anaphase
      • Begins with the separation of chromatids in each chromosome
      • Spindle fibers appear to shorten, pulling the chromatids apart at the centomere
      • Each chromatid is now a chromosome
      • 2 sets of separated chromosomes then move through the cytoplasm to opposite poles of the cell
    •  
    • Telophase
      • The last stage of mitosis
      • After the individual chromosomes have reached opposite poles of the cell, spindles disappear
      • A nuclear membrane forms around each set of chromosomes
      • Chromosomes return to a thread-like mass
      • Centrioles duplicate
        • 2 centrioles formed in each daughter cell
      • Nucleoli re-form within each newly formed nucleus
    •  
    • Cytokinesis
      • The division of the cytoplasm
      • Follows mitosis
      •   Cytokinesis begins during telophase
      • In animal cells, the cell membrane pinches together
      • The area that pinches in and separates is called the cleavage furrow
      • In plants, a cell plate is formed, dividing the two halves
    • Picturing Cytokinesis
    •  
    •  
    • Chromosome Number
      • Cells formed thru mitosis have the same number of chromosomes as the parent cells
      • If combined in sexual reproduction, the offspring would have 2x chromosomes!
      • Therefore gametes have only half the number of chromosomes of somatic cells
        • Gametes = sex cells
    • Meiosis
      • Gametes need another process for nuclear division
      • Meiosis reduces the number of chromosomes to 1/2 the number in somatic cells
    • Meiosis I & II
      • Forming haploid daughter cells from diploid parent cells requires two successive cell divisions:
        • First = Meiosis I –
          • homologous chromosomes separate
        • Second = Meiosis II
          • chromatids of each chromosome separate
    • Meiosis I
      • Preceded by replication of DNA that forms the chromosome
      • Synapsis = pairing of homologous chromosomes
      • Each pair of homologous chromosomes twists around each other, forming a structure called a tetrad
      • Meiosis can be divided into same 4 phases as mitosis:
        • Prophase, Metaphase, Anaphase, Telophase
    • Prophase I
      •   Chromatin begins to coil into short rods
      •   Homologous chromosomes are formed
      •   Spindle fibers appear
      •   Nucleoli break down
      • By the end, the nuclear membrane has dissolved, and tetrads are visible
    • Crossing Over
      • During synapsis , (prophase I) the chromatids of homologous pairs twist around each other
      • A portion of one chromatid may break off and reattach, “trading” with the same piece from its homologous partner
      • The exchange of genes by reciprocal segments of homologous chromosomes during meiosis = “ crossing-over ”
      • Crossing over causes exchange of genetic material between maternal & paternal chromosomes
      • Results in genetic recombination
      • Genetic recombination is less likely in genes that are closer together.
    • Chromosome Mapping
      • The likelihood that recombination will occur due to crossing-over depends on the genes’ distance from each other on the chromosome
      • Scientists can determine how frequently genes for particular traits occur together in offspring
      • This can be used to create a map of the chromosome
      • 1% recombination (crossing-over) = 1 map unit
    • Metaphase I
      • Tetrads line up along the equator of the cell
      • Each tetrad becomes attached to spindle fibers
    • Anaphase I
      • Homologous chromosomes that form each tetrad are pulled apart in pairs
      • One pair goes to one end of the cell, the other to opposite end
    • Telophase I
      • Chromosomes reach ends of the cell
      • Cell divides into 2 daughter cells
    •  
    • Independent Assortment
      • During Anaphase I, one member of each homologous chromosome pair moves to one end of the cell, the other moves to the opposite end
      • The separation of homologous chromosomes is random
      • More, or fewer maternal (or paternal) chromosomes may end up on one side or the other
      • Each separation is independent of the others
      • This is the principal of independent assortment of chromosomes
      • Results in genetic variation
    •  
    • Meiosis I Summary
      • Meiosis I is a Reductive Division
      • It reduces the number of chromosomes from diploid "2n" to haploid "n"
    • Meiosis II
      • Similar to mitosis but not preceded by replication of DNA
      • 4 Stages:
        • Prophase II
        • Metaphase II
        • Anaphase II
        • Telophase II
    • Prophase II & Metaphase II
      • Prophase II
        • A new spindle forms around paired chromatids
      • Metaphase II
        • Chromosomes line up along the equator
        • They are attached at the centromere to spindle fibers
    • Anaphase II
      • Centromeres duplicate & the chromatids separate
      • Resulting single chromatids move to opposite poles
      • Chromatids are now called chromosomes
    • Telophase II
      •   A nuclear membrane forms around each set of chromosomes
      •   The spindle breaks down and cytokinesis occurs
      •   Result: 4 haploid daughter cells
    •  
    • Males vs. Females
      • In males, all 4 daughter cells differentiate to become sperm
      •   In females, the cytoplasm divides unevenly in Meiosis I
        • The smaller cell = first polar body
        • doesn't survive
      •   In Meiosis II, the division is again unequal
        • smaller half is second polar body
      • So only 1 of 4 daughter cells survives
        • rich in cytoplasm, has many nutrients to nourish the young organism
    • Comparing Mitosis & Meiosis
      • MEIOSIS
      • 2
      • 4
      • diploid
      • haploid
      • different
      • MITOSIS
      • 1
      • 2
      • diploid
      • diploid
      • identical
      # of nuclear divisions: # of daughter cells: Parent cell type: Daughter cell type: Genetic likeness to parent:
    •  
    • Control of Cell Division
      • Timing and rate of cell division varies in different cell types
      • Control of rate of division is critical
      • Some cells require regulatory substances to begin division = growth factors
    • Effect of Growth Factors
    • Effect of Density
      • Density of cells also effects the rate of division
      • Crowding inhibits cell division
    • The Restriction Point
      • A crucial checkpoint occurs late in the G1 phase of the cell cycle
      • Point of decision to divide = restriction point
      • Cell cannot turn back after this point
      • If it is “yes,” cell goes to S phase and copies DNA
      • If “no,” it goes to non-dividing state (G 0 )
        • Most cells are in G 0
    • MPF
      • After S, the cell will enter G2
      • The “OK” signal that causes the cell to proceed from G2 to mitosis = mitosis promoting factor (MPF)
        • A complex of proteins
      • MPF is an enzyme
        • Protein kinase
    • Abnormal Cell Division
      • Cancer cells do not respond normally to the body’s control mechanisms for cell division
      • Cancer cells divide excessively
      • Can invade other body tissues
      • When a cell divides abnormally = transformed
      • Abnormal cells are usually destroyed by the immune system
    • Cancer
      • If abnormal cells are not destroyed and reproduce, they may form a mass of abnormal cells = tumor
        • Benign tumor = abnormal cells remain at the original site
        • Malignant tumor = cells spread to other parts of the body
      • Metastasis = spread of cancer cells in the body
    • Breast Cancer Cell