Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1/9/2015 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. LIMITS TO CELL GROWTH 2. Exchanging materials – materials enter and exit a cell through the cell membrane. 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 Ratio of Surface Area to Volume Surface area – L x W x number of sides Volume – L x W x H LIMITS TO CELL GROWTH 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 cm2 its volume would be 1 cm3 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. 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 1/9/2015 Prevention from getting too big Before a cell gets too big, it will split or divide in half. Genes 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. DNA contains the information needed to direct a cell’s activities. DNA is composed of genes – segments of DNA that encode a protein. 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 1/1000th 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. Chromosome structure Eukaryotes have even more DNA so the cell had to have a way to fit it all in the nucleus. Are transmitted info from parent to offspring. 1 DNA strand contains 1000’s of genes. DNA molecules are extremely long. 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. Chromosomes contain both DNA and proteins. Chromatin – DNA that is tightly coiled around proteins called histones. 2 1/9/2015 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. PROKARYOTIC CELLS SIMPLY SPLIT DNA unzips making 2 strands and each strand is copied giving 2 identical copies of DNA. 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. Bacterial cell grows and then splits into equal halves. This is called binary fission. Product of binary fission – 2 identical bacterial cells. 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 1/9/2015 INTERPHASE CELL CYCLE - MITOSIS 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. CYTOKINESIS MITOSIS 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. 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. 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! Biologists divide the events of mitosis into four phases: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase 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 1/9/2015 TELOPHASE 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. 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 product is two identical cells. Following cytokinesis, the cell re-enters interphase at the G1 phase, and the cell cycle continues. The plate is formed from secretions of the golgi. 5