The Cell Cycle Chapter 12
Why are cells small? As cells get bigger they don t work as well WHY? Difficulties Larger Cells Have: More demands on its DNA Less efficient in moving nutrients/waste across its cell membrane à Diffusion
Function of Cell Division Unicellular organisms reproduction of organism, increase in population Multicellular organisms development, growth, repair Involves distribution of identical genetic material to two daughter cells
The Cell Cycle Interphase cell grows by producing proteins & organelles, replicates DNA, prepares for cell division. Accounts for 90% of cell cycle G1 phase growth, cell performs it s normal function This phase varies most in length among different types of cells S phase DNA synthesis chromosomes are duplicated G2 phase growth and preparation for cell division Mitotic phase: includes mitosis & cytokinesis Mitosis = the formation of two daughter nuclei Cytokinesis = division of the cytoplasm
Replication of DNA during the S phase DNA molecules are packaged into chromosomes Prokaryotes genome is a single circular chromosome Eukaryotes genome consists of several chromosomes Every species has a characteristic number of chromosomes in the nucleus of each cell. Human somatic (body) cells have 46 chromosomes or two sets of 23. Human gametes (sperm or egg) have 23 chromosomes
Learning Check A single gamete can grow into a zygote through the process of mitosis: True False Eggs and sperm are what type of cells? Somatic Cells Gametes Germ Cells
Learning Check A zygote grows into an organism through what process? Mitosis Meiosis Gametes are produced from what type of cells? Somatic Cells Gametes Germ Cells
Learning Check A skin cell is an example of: Gamete Germ Cell Zygote Somatic Cell A skin cell would divide to make more skin cells using the process of: Mitosis Meiosis Fertilization
Learning Check Why must somatic cells be produced through mitosis? Because mitosis produces four genetically distinct daughter cells. Because somatic cells are used for sexual reproduction To ensure that old, damaged, or growing cells are replaced with cells that are different from the parent cell To ensure that old, damaged, or growing cells are replaced with the cells that are the same type and can perform the same function
Replication of DNA during the S phase Eukaryotic chromosomes are made of chromatin a complex of DNA & protein. Proteins maintain the structure and help control gene activity In a non-dividing cell, each chromosome is in the form of a long thin chromatin fiber.
DNA replication (continued) Before cell division, chromatin condenses Each duplicated chromosome consists of two sister chromatids = identical copies of DNA Centromere = region where chromatids connect During mitosis, the sister chromatids are pulled apart.
Chromosomes
Subphases of Mitosis Prophase Prometaphase Metaphase Anaphase Telophase
Learning Check During the S phase of the cell cycle, DNA is replicated. What form would you expect the DNA to be in? Tightly wound around histone proteins as chromosomes Separate nucleotides just floating around the nucleus In a loose pile During the M phase of the cell cycle, what form would you expect the DNA to be in? Tightly wound around histone proteins as chromosomes Separate nucleotides just floating around the nucleus In a loose pile
Learning Check In which phase of the cell cycle does DNA replication take place? G1 S G2 M Interphase is the part of the cell cycle when the cell: Grows and duplicates its DNA Undergoes mitosis Ceases to function Divides its cytoplasm
Learning Check In which phase of the cell cycle are new daughter cells formed? G1 S G2 M Cytokinesis In which stage of the cell cycle would you expect the growing of the cell to occur? G1 S G2 M Cytokinesis In which stage of the cell cycle would checking for DNA integrity occur? G1 S G2 M Cytokinesis
Learning Check In the above diagram, what do the yellow circles represent? Chromosomes Homology Chromatids centromeres
Learning Check What do these two chromosomes together represent? Homologous Chromosomes Histones Chromatids centromeres
Learning Check How many chromatids are shown in the diagram above? Centromeres connect how many chromatids?
Role of mitotic spindle Mitotic spindle - originates from centrosomes, composed of microtubules and associated proteins (tubulin) In animal cells each centrosome has a pair of centrioles at the center. As mitosis starts, the two centrosomes are located near the nucleus As the spindle fibers grow from them the centrioles are pushed apart towards opposite ends of the cell
Role of spindle fibers (cont.) Each sister chromatid has a kinetochore a region of proteins and DNA at the centromere. During prometaphase, some spindle microtubules attach to the kinetochores. the chromosome will move toward the pole from which the microtubules come. Eventually the chromosomes settle along the metaphase plate. Nonkinetochore microtubules from opposite poles overlap and interact with each other
Role of spindle fibers (cont.) During anaphase the proteins that have held the sister chromatids together are inactivated and each chromatid becomes a full fledged chromosome Chromosomes move toward opposite poles of the cell Hypothesis: motor proteins on the kinetochore walk the attached chromosome along the microtubule. Excess microtubule sections depolymerize at the end of the kinetochore
Cytokinesis Division of the cytoplasm; typically follows mitosis Animal cells the process involves cleavage First sign of cleavage = cleavage furrow Cleavage furrow is formed by a ring of contractile actin microfilaments + molecules of the motor protein myosin Contraction of the ring pinches the cell in two.
Cytokinesis in Plant Cells Plant cells cell wall necessitates a different mechanism cell plate During telophase, vesicles from the Golgi apparatus organize at the metaphase plate forming the cell plate The plate enlarges until its membranes fuse with the plasma membrane The contents of the vesicles form new cell wall material between the daughter cells
Cell Division in Prokaryotes Mitosis in eukaryotes may have evolved from binary fission in prokaryotes Bacterial chromosome is a circular DNA molecule with associated proteins. In binary fission, chromosome replication begins at one point (the origin of replication) and proceeds in both directions around the circle
The Evolution of Mitosis Since prokaryotes preceded eukaryotes by billions of years It is likely that mitosis evolved from bacterial cell division Certain protists Exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells
Prokaryotes. During binary fission, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor A hypothetical daughter chromosomes sequence to specific for the sites evolution on the of plasma membrane. mitosis Dinoflagellates. In unicellular protists called dinoflagellates, the nuclear envelope remains intact during cell division, and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a fission process reminiscent of bacterial division. Diatoms. In another group of unicellular protists, the diatoms, the nuclear envelope also remains intact during cell division. But in these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei. Most eukaryotes. In most other eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope then re-forms. Bacterial chromosome Chromosomes Microtubules Intact nuclear envelope Kinetochore microtubules Intact nuclear envelope Kinetochore microtubules Centrosome Fragments of nuclear envelope