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 non sex results in daughter cells with identical DNA identical to daughter and parent Meiosis produces non identical daughter cells gametes sperm and egg cells All the DNA in a cell constitutes the cell s genome DNA molecules in a cell are packaged into chromosomes Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Somatic 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 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 The centromere of the replicated chromosome is where the two chromatids are most closely attached Figure 12 4 Chromosome duplication and distribution during cell division Eukaryotic cell division consists of o Mitosis o Cytokinesis The cell cycle consists of the division of the nucleus the division of the cytoplasm cell 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 preparing to divide Interphase about 90 of the cell cycle can be divided into subphases o G1 phase first gap o S phase synthesis o G2 phase second gap The cell grows during all three phases but chromosomes are replicated only during the S phase Figure 12 5 The cell cycle Mitosis Mitosis is conventionally divided into five phases o Prophase o Prometaphase o Metaphase o Anaphase o Telophase Cytokinesis is well underway by late telophase Figure 12 6 The mitotic division of an animal cell Homologous chromosomes are not identical to each other one came from mom one came from dad Sister chromatids are identical S phase The mitotic spindle string DNA synthesis 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 The mitotic spindle at metaphase 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 The Cell Cycle Control System 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 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 Figure 12 14 Mechanical analogy for the cell cycle control system 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 and M phases and divide If the cell does not receive the go ahead signal it will exit the cycle switching into a nondividing state called the G0 phase Two types of regulatory proteins are involved in cell cycle control cyclins and cyclin dependent kinases Cdks The activity of cyclins and Cdks fluctuates during the cell cycle Loss of Cell Cycle Controls in Cancer Cells Cancer cells do not respond normally to the body s control mechanisms Cancer cells may not need growth factors to grow and divide o They may make their own growth factor o They may convey a growth factor s signal without the presence of the growth factor o They may have an abnormal cell cycle control system
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