LIMITS TO CELL GROWTH Mitosis and Meiosis Cell growth and division The larger the cell, the more trouble the cell has moving nutrients and waste across the cell membrane. LIMITS TO CELL GROWTH 1. DNA/information overload As a cell increases in size, it does not make extra copies of its DNA. Cell size is limited by its DNA, if the cell gets too big the DNA would no longer be able to serve the needs of the cell. LIMITS TO CELL GROWTH 2. Exchanging materials materials enter and exit a cell through the cell membrane. Surface area the total area of the cell membrane The rate at which materials can be exchanged depends on the surface area. The rate at which materials are used up and waste is produced depends on the cell s volume. LIMITS TO CELL GROWTH Ratio of Surface Area to Volume Surface area L x W x number of sides Volume L x W x H If a cell has a length of 1 cm, width of 1 cm, height of 1 cm, and 6 sides, its surface area would be 6 cm 2 its volume would be 1 cm 3 LIMITS TO CELL GROWTH The ratio of surface area to volume in this example cell would be 6:1. If the cell increase in size, its volume increases faster than the surface area. This makes it more difficult for the cell to move needed materials in and waste products out. Goal of the cell: have a large ratio of surface area to volume. 1
Prevention from getting too big Before a cell gets too big, it will split or divide in half. A parent cell forms two new daughter cells. A cell must copy all its DNA before division to ensure that each cell is the same, thus each daughter cell has their own copy of DNA, both identical to the other. Genes DNA contains the information needed to direct a cell s activities. DNA is composed of genes segments of DNA that encode a protein. Are transmitted info from parent to offspring. 1 DNA strand contains 1000 s of genes. Prokaryotic chromosomes Most prokaryotes have a single circular DNA molecule, or chromosome. Chromosomes structures that contain the genetic information that is passed on from one generation of cells to the next. Found in the cytoplasm of prokaryotes. Folded into a space about 1/1000 th smaller than a prokaryote s length. Eukaryotic chromosomes Most eukaryotes have as much as 1,000 times the amount of DNA as prokaryotes. Eukaryotic DNA is primarily located in the nucleus in the form of multiple chromosomes. DNA molecules are extremely long. Chromosome structure Eukaryotes have even more DNA so the cell had to have a way to fit it all in the nucleus. Chromosomes contain both DNA and proteins. Chromatin DNA that is tightly coiled around proteins called histones. Chromosome structure Nucleosomes Beaded structure composed of multiple histones with associated DNA. During mitosis the histones cause the fibers of the chromosome to coil up and packed into the structures that you can see. 2
Chromosome structure Chromosomes are 40% DNA and 60% protein. Chromosomes copy themselves during DNA replication forming sister chromatids. Chromosomes must copy themselves so that the new cell gets the same info as the old cell. Sister chromatids are attached by a centromere. PROKARYOTIC CELLS SIMPLY SPLIT Bacteria are prokaryotes lacking nuclei. Bacterial DNA is a circular chromosome. DNA unzips making 2 strands and each strand is copied giving 2 identical copies of DNA. PROKARYOTIC CELLS SIMPLY SPLIT Bacterial cell grows and then splits into equal halves. This is called binary fission. Product of binary fission 2 identical bacterial cells. EUKARYOTIC CELL CYCLE Eukaryotic cells have a nucleus, so they must undergo nuclear division. Cell cycle repeating sequences of growth and division through which many kinds of eukaryotic cells pass. INTERPHASE G1 rapid cell growth, a cell is in this part of the cell cycle for the longest period of time between cell divisions. S DNA is copied, chromosome replicated is now two sister chromatids joined at centromere. 3
INTERPHASE G2 Organelles replicate, microtubules are reassembled to form spindle apparatus that will move chromosomes, cell is now prepared for mitosis. This is the shortest phase of interphase. An eukaryotic cell spends most of its time in Interphase. CELL CYCLE - MITOSIS Mitosis the process by which the nucleus of a cell is divided into two nuclei, each with the same number of chromosomes. Misconception mitosis IS NOT cell division, it is a part of cell division! CYTOKINESIS Cytokinesis the division of the cytoplasm. This is the last step of cell division. After cytokinesis is complete, the cell will be in Interphase again. MITOSIS Biologists divide the events of mitosis into four phases: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase PROPHASE Chromosomes condense and become visible. Centrioles form and take up positions on opposite ends of the nucleus. Spindle becomes visible. Nuclear membrane breaks down, and the nucleolus disappears. METAPHASE Spindle fibers assist in moving the chromosomes to the equator (middle) of the cell. The centromeres of all the sister chromatids line up. The imaginary line that bisects each of the chromatids through the centromere is called the metaphase plate. 4
ANAPHASE Sister chromatids separate from each other at the centromere. The spindle now pulls each chromosome to opposite ends of the cell (toward the centrioles). The spindle is taken apart as the chromosomes move. Each pole now has one complete set of chromosomes. TELOPHASE Chromosomes uncoil, spindle fibers disappear, and the nuclear membrane reforms. Mitosis is complete. CYTOKINESIS Cytoplasm of original cell is split in half. Cell membrane grows to enclose both cells. Animal cells pinch in the membranes forming a cleavage furrow. Plant cells form a cell plate to split the cytoplasm. CYTOKINESIS (continued) Plant cells form a cell plate at the equator of the cell where new cell wall forms on both sides of the plate. The plate is formed from secretions of the golgi. The product is two identical cells. Following cytokinesis, the cell re-enters interphase at the G1 phase, and the cell cycle continues. Mitosis and Meiosis The two types of cell division with two different purposes! and two different outcomes! 5
Mitosis occurs during the mitotic phase (M) of the cell cycle Mitosis is how somatic cells grow and divide (cell division)! Animal cell mitosis How do drawings of mitotic phases and pictures of actual cells undergoing mitosis compare? How do you identify the mitotic phases of onion root tip cells? What does mitosis look like in an actual cell? Mitosis in epithelial cell Meiosis is different than mitosis in several ways! Purpose is reproduction, NOT growth! 2 rounds of cell division. Four daughter cells produced. Haploid cells produced. 6
Meiosis I movie (actual spermatocyte cells in a fly (video recorded over 4 hours)) Can you identify???? 1. Centrosomes at prophase I 2. Homologous chromosomes at metaphase plate in metaphase I 3. Kinetochore microtubules at anaphase I 4. Contractile ring at telophase I/ cytokinesis Meiosis II movie (actual spermatocyte cells in a fly (video recorded over 3 hours)) Can you identify???? 1. Nucleus in prophase II 2. Sister chromatids at metaphase plates in metaphase II 3. Meiotic spindles in anaphase II Now, lets review the differences between mitosis and meiosis! Work individually to fill in the chart while listening to the meiosis rap! (Hint: listen to the words for clues!) # rounds DNA replication # rounds cell division # daughter cells Final chromosome number Makeup of daughters Function Mitosis 1 1 2 2n (diploid) same as parent growth Meiosis 1 2 4 n (haploid) different reproduction MEIOSIS Making gametes SEXUAL REPRODUCTION Combines genetic material from 2 parents (sperm & egg) so offspring are genetically DIFFERENT from parents 7
ADVANTAGES OF SEXUAL REPRODUCTION Allows for variation in population Individuals can be different EGG + SPERM Provides foundation for EVOLUTION Allow species adapt to changes in their environment If egg and sperm had same number of chromosomes as other body cells... baby would have too many chromosomes! MEIOSIS is the way to make cells with ½ the number of chromosomes for sexual reproduction DIPLOID & HAPLOID Most cells have 2 copies of each chromosome = DIPLOID 2n (one from mom; one from dad) HOMOLOGOUS CHROMOSOMES All BODY ( ) = SOMATIC cells are diploid DIPLOID & HAPLOID Some cells have only one copy of each chromosome = HAPLOID 1n All sperm and egg cells are haploid MITOSIS Makes 2 cells genetically identical to parent cell & to each other Makes 2n cells Makes SOMATIC (body) Used by organisms to: increase size of organism, repair injuries, replace worn out cells 8
MEIOSIS Makes 4 cells genetically different from parent cell & from each other Makes 1n cells Makes Germ cells OR Gametes (sperm & eggs) Used for sexual reproduction WHAT MAKES MEIOSIS DIFFERENT? 1. SYNAPSIS & CROSSING OVER (PROPHASE I) 2. SEGREGATION & INDEPENDENT ASSORTMENT (ANAPHASE I) 3. Skip INTERPHASE II (NO S) CELL DIVIDES TWICE, BUT ONLY COPIES DNA ONCE 1. WHAT MAKES MEIOSIS DIFFERENT? Homologous chromosomes pair up during PROPHASE I = SYNAPSIS This group of FOUR (4) chromatids is called a TETRAD 1. WHAT MAKES MEIOSIS DIFFERENT? Exchange of DNA between homologous pairs = CROSSING OVER during PROPHASE I Allows shuffling of genetic material 1. HOMOLOGOUS CHROMOSOMES Image modified by Riedell ONE FROM EACH PARENT SAME SIZE SAME SHAPE CARRY GENES for the SAME TRAITS NOT IDENTICAL BUT! (Don t have to have the SAME CHOICES) 1. CROSSING OVER Allows for rearranging of DNA in different combinations After crossing over, chromatid arms are NOT IDENTICAL anymore 9
WHAT MAKES MEIOSIS DIFFERENT? 2.Separation during ANAPHASE I 2. SEGREGATION (Anaphase I) SEGREGATION & INDEPENDENT ASSORTMENT Separates gene choices and allows shuffling of genetic material 2. SEGREGATION & CROSSING OVER together make even more combinations 2.INDEPENDENT ASSORTMENT 2.INDEPENDENT ASSORTMENT at ANAPHASE I Lots of different combinations are possible! This is why you don t look exactly like your brothers and sisters even though you share the same parents! WHAT MAKES MEIOSIS DIFFERENT? Crossing over Segregation Independent assortment are ALL ways MEIOSIS results in = GENETIC RECOMBINATION So daughter cells are different from parents and from each other 10
WHAT MAKES MEIOSIS DIFFERENT? 3. Skip INTERPHASE II (No S) CELL DIVIDES TWICE, BUT ONLY COPIES ITS DNA ONCE MITOSIS: G 1 S G 2 P M A T C MEIOSIS: G 1 S G 2 P M A T C P M A T C ( I ) ( II ) Ways Meiosis is different? Homologous pairs match up & trade DNA (SYNAPSIS & CROSSING OVER) in PROPHASE I SEGREGATION & INDEPENDENT ASSORTMENT in Anaphase I create genetic recombination Skipping INTERPHASE II- (Dividing TWICE but copying DNA once) produces 1n cells 11