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In cell biology, ways in which fragmentation is useful for a cell: DNA cloning and apoptosis. DNA cloning is important in asexual reproduction or creation of identical DNA molecules, and can be performed spontaneously by the cell or intentionally by laboratory researchers. Apoptosis is the programmed destruction of cells, and the DNA molecules within them, and is a highly regulated process. These two ways in which fragmentation is used in cellular processes describe normal cellular functions and common laboratory procedures performed with cells. However, problems within a cell can sometimes cause fragmentation that results in irregularities such as red blood cell fragmentation and sperm cell DNA fragmentation. ==DNA Cloning== DNA cloning can be performed spontaneously by the cell for reproductive purposes. This is a form of asexual reproduction where an organism splits into fragments and then each of these fragments develop into mature, fully grown individuals that are clones of the original organism (See reproductive fragmentation). DNA cloning can also be performed intentionally by laboratory researchers. Here, DNA fragmentation is a molecular genetic technique that permits researchers to use recombinant DNA technology to prepare large numbers of identical DNA molecules. In order for DNA cloning to be completed, it is necessary to obtain discrete, small regions of an organism's DNA that constitute specific genes. Only relatively small DNA molecules can be cloned in any available vector. Therefore, the long DNA molecules that compose an organism's genome must be cleaved into fragments that can be inserted into the vector DNA.〔Lodish, Harvey, Arnold Berk, Chris A. Kaiser, Monty Kriger, Anthony Bretscher, Hidde Ploegh, Angelika Amon, and Matthew P. Scott. Molecular Cell Biology. 7th ed. New York: W.H. Freeman and, 2013. Print.〕 Two enzymes facilitate the production of such recombinant DNA molecules: :1. Restriction Enzymes ::Restriction enzymes are endonucleases produced by bacteria that typically recognize small base pair sequences (called restriction sites) and then cleave both strands of DNA at this site.〔Rao, Desirazu N., Swati Saha, and Vinita Krishnamurthy. "ATP-Dependent Restriction Enzymes." Progress in Nucleic Acid Research and Molecular Biology 64 (2000): 1-63. Print.〕 A restriction site is typically a palindromic sequence, which means that the restriction-site sequence is the same on each strand of DNA when read in the 5' to 3' direction. :::For each restriction enzyme, bacteria also produce a modification enzyme so that a host bacterium's own DNA is protected from cleavage. This is done by modifying the host DNA at or near each potential cleavage site. The modification enzyme adds a methyl group to one or two bases, and the presence of this methyl group prevents the restriction endonuclease from cutting the DNA.〔Rao, Desirazu N., Swati Saha, and Vinita Krishnamurthy. "ATP-Dependent Restriction Enzymes." Progress in Nucleic Acid Research and Molecular Biology 64 (2000): 1-63. Print.〕 ::Many restriction enzymes make staggered cuts in the two DNA strands at their recognition site, which generates fragments with a single stranded "tail" that overhangs at both ends, called a sticky end. Restriction enzymes can also make straight cuts in the two DNA strands at their recognition site, which generates blunt ends.〔Lodish, Harvey, Arnold Berk, Chris A. Kaiser, Monty Kriger, Anthony Bretscher, Hidde Ploegh, Angelika Amon, and Matthew P. Scott. Molecular Cell Biology. 7th ed. New York: W.H. Freeman and, 2013. Print.〕 :2. DNA ligase ::During normal DNA replication, DNA ligase catalyzes end-to-end joining (ligation) of short fragments of DNA, called Okazaki fragments. For the purposes of DNA cloning, purified DNA ligase is used to covalently join the ends of a restriction fragment and vector DNA that have complimentary ends. They are covalently ligated together through the standard 3' to 5' phosphodiester bonds of DNA.〔Tomkinson, Alan E., and Zachary B. Mackey. "Structure and Function of Mammalian DNA Ligases." Mutation Research/DNA Repair 407.1 (1998): 1-9. Print.〕 :::DNA ligase can ligate complimentary sticky and blunt ends, but blunt-end ligation is inefficient and requires a higher concentration of both DNA and DNA ligase than the ligation of sticky ends does.〔Hung, Mien-Chie, and Pieter C. Wensink. "Different Restriction Enzyme-generated Sticky DNA Ends Can Be Joined in Vitro." Nucleic Acids Research 12.4 (1984): 1863-874. Print.〕 For this reason, most restriction enzymes used in DNA cloning make staggered cuts in the DNA strands to create sticky ends. The key to cloning a DNA fragment is to link it to a vector DNA molecule that can replicate within a host cell. After a single recombinant DNA molecule (composed of a vector plus an inserted DNA fragment) is introduced into a host cell, the inserted DNA can be replicated along with the vector, generating a large number of identical DNA molecules.〔"Ch 20." Avonapbio /. N.p., n.d. Web. 20 Nov. 2012. The basic scheme for this can be summarized as follows: ::Vector + DNA Fragment :::↓ ::Recombinant DNA :::↓ ::Replication of recombinant DNA within host cell :::↓ ::Isolation, sequencing, and manipulation of purified DNA fragment There are numerous experimental variations to this scheme, but these steps are essential to DNA cloning in a laboratory.〔Lodish, Harvey, Arnold Berk, Chris A. Kaiser, Monty Kriger, Anthony Bretscher, Hidde Ploegh, Angelika Amon, and Matthew P. Scott. Molecular Cell Biology. 7th ed. New York: W.H. Freeman and, 2013. Print.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fragmentation (cell biology)」の詳細全文を読む スポンサード リンク
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