Chapter 12 The Cell Cycle Multicellular organisms depend on cell division for o Development from a zygote o Growth o Repair Cell division is an integral part of the cell cycle the life of a cell from formation to its own division Most cell division mitosis results in daughter cells with identical DNA The daughter cells are identical to each other and the parent cells they came from Meiosis produces non identical daughter cells gametes sperm and egg cells All of the DNA in a cell constitutes the cell s genome Genome refers to all of the DNA in a cell DNA molecules in a cell are packaged into chromosomes Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Stomatic cells non reproductive cells have two sets of chromosomes 1 from each parent Gametes reproductive cells sperm and eggs have half as many chromosomes as somatic cells Gametes only have one copy of each chromosome because they combine and join with another cell sperms and eggs combine their DNA Eukaryotic chromosomes consist of chromatin a complex of DNA and protein that condenses during cell division In preparation for cell division DNA is replicated and the chromosomes condense Each replicated chromosome has two sister chromatids which separate during cell division o The term sister chromatid refers to replicated chromosomes so they are identical The centromere of the replicated chromosome is where the two chromatids are most closely attached Figure 12 4 Eukaryotic cell division consists of o Mitosis the division of the nucleus o Cytokinesis the division of the cytoplasm The cell cycle lifespan of the cell consists of o Mitotic M phase the dividing phase mitosis and cytokinesis o Interphase the non dividing phase cell growth and copying of chromosomes in preparation for cell division Interphase about 90 of the cell cycle can be divided into subphases o G1 Phase first gap the cell is getting bigger in size o S Phase synthesis when the cell is getting replicated o G2 Phase second gap The cell grows during all three phases but chromosomes are replicated only during the S phase Figure 12 5 o Most of the cell s lifespan is spent in interphase Very little time is actually spent dividing Mitosis is conventionally divided into five phases o Prophase o Prometaphase o Metaphase o Anaphase o Telophase Homologous chromosomes have the same genes that control the same Cytokinesis is well underway by late telophase characteristics but they are not identical They re a homologous pair because they have the same genes they just have different versions of the genes Before mitosis can occur each chromosome must be copied and produce identical sister chromatids Sister chromatids are identical to each other but homologous chromosomes are not What process produces sister chromatids S phase Figure 12 6b o Kinetochores where the kinetochore microtubules attach to the o During prophase and prometaphase kinetochore microtubules are attaching chromosomes to the chromosomes Figure 12 6d o During metaphase chromosomes line up at the metaphase plate with sister chromatids facing opposite ends of the cell o During anaphase the microtubules from the spindle that attached the to kinetochore pull the sister chromatids apart o During telophase the cleavage furrow forms and the cell begins to divide The nuclear envelope forms and the cells begin to split apart The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis During prophase assembly of spindle microtubules begins in the centrosome the microtubule organizing center The centrosome replicates forming two centrosomes that migrate to opposite ends of the cell as spindle microtubules grow out from them During prometaphase some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes At metaphase the chromosomes are all lined up at the metaphase plate the midway point between the spindle s two poles Figure 12 7 In anaphase sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell Non kinetochore microtubules from opposite poles overlap and push against each other elongating the cell In telophase genetically identical daughter nuclei form at opposite ends of the cell In animal cells cytokinesis occurs by a process known as cleavage forming a cleavage furrow In plant cells a cell plate forms during cytokinesis which will form the cell walls of the daughter cells Binary Fission Prokaryotes bacteria and archaea reproduce by a type of cell division called binary fission In binary fission the chromosome replicates and the two daughter chromosomes actively move apart Binary fission is asexual reproduction The bacteria and archaea reproduce themselves so the only way for DNA to change is if a mutation occurs The Cell Cycle Control System The sequential events of the cell cycle are directed by a distinct cell cycle control system which is similar to a clock The clock has specific checkpoints where the cell cycle stops until a go ahead signal is received If cells get damaged the DNA is mutated etc the best case scenario is that the checkpoint will notice the difference and send the cell to cell arrest where it is not allowed to divide thus preventing the replication of the mutation Figure 12 14 o There is a checkpoint in the G1 Phase This is an important checkpoint because DNA is copied in the S phase so the mistakes need to be noticed by the S Phase o Following the S phase is the G2 checkpoint that makes sure the S phase did not miscopy any DNA If there is a mutation the cell is not allowed for division o Before mitosis is a checkpoint that makes sure the replications are okay and that the cell should be allowed to divide For many cells the G1 checkpoint seems to be the most important one If a cell receives a go ahead signal at the G1 checkpoint it will usually complete the S G2 or M phases and divide If a cell does not receive the go ahead signal it will exit the cycle switching into a non dividing state called the G0 Phase o Examples of a G0 phase cell that doesn t divide in adults are brain cells and muscle cells If either of these types of cells dies they do not get replaced o Neurons and muscle cells are in the G0 phase permanently Figure 12 15 Two types of regulatory proteins are involved in cell cycle control cyclins and cyclin dependent kinases Cdks o The activity of cyclins and Cdks fluctuates during the cell cycle o All kinases
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