Bs 1 Dna Ekstrakromosomal

DNA EKSTRAKROMOSOMAL, SIFAT, DAN KEGUNAANNYA

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Pengertian  DNA ekstrakromosomal : DNA lain yang terdapat dalam sel di luar nukleus, yaitu :  - DNA mitokondria, DNA kloroplas, DNA plasmid The Genome Defines the Program of Multicellular Development  The cells in an individual animal or plant are extraordinarily varied. Fat cells, skin cells, bone cells, nervecells—they seem as dissimilar as any cells could be. Yet all these cell types are generated during embryonic development from a single fertilized egg cell, and all (with minor exceptions) contain identical copies of thegenome of the species.  The explanation lies in the way in which these cells make selective use of their genetic instructions according to the cues they get from their surroundings. The DNA is not just a shopping list specifying the molecules that every cell must have, and the cell is not an assembly of all the items on the list. Rather, the cell behaves as a multipurpose machine, with sensors to receive environmental signals and highly developed abilities to call different sets of genes into action according to the sequences of signals to which the cell has been exposed. A large fraction of the genes in the eucaryotic genome code for proteins that serve to regulate the activities of other genes. Most of these gene regulatory proteins act by binding, directly or indirectly, to the regulatory DNA adjacent to the genes that are to be controlled (Figure 140), or by interfering with the abilities of other proteins to do so. The expanded genome of eucaryotes therefore serves not only to specify the hardware of the cell, but also to store the software that controls how that hardware is used. Eucaryotic Genomes Are Rich in Regulatory DNA

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Much of our noncoding DNA is almost certainly dispensable junk, retained like a mass of old papers because, when there is little pressure to keep an archive small, it is easier to retain everything than to sort out the valuable information and discard the rest. Certain exceptional eucaryotic species, such as the puffer fish (Figure 1-39), bear witness to the profligacy of their relatives; they have somehow managed either to rid themselves of large quantities of noncoding DNA, or to have avoided acquiring it in the first place. Yet they appear similar in structure, behavior, and fitness to related species that have vastly more such DNA. DNA  inheritance blueprint  consist the genes (functional units)  covered into chromosome  Chromosomal aberration include the genes (multi genes Cells cultured

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Kromosom manusia

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Human chromosom

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Karyotyping – dr masa ke masa

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Chromosome Abnormalities

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Abnormalitas struktur  Translokasi  Resiprok

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 robertison Delesi

 Makro, mikro  Terminal, intersisal  Inversi  Perisentrik  parasentrik  Inversi  Ring dan isochromosome Perbedaan cell line  mosaik  kimerisme The Mouse Serves as a Model for Mammals  Mammals have typically three or four times as many genes as Drosophila, a genome that is 20 times larger, and millions or billions of times as many cells in their adult bodies. In terms of genome size and function, cell biology, and molecular mechanisms, mammals are nevertheless a highly uniform group of organisms. Even anatomically, the differences among mammals are chiefly a matter of size and proportions; it is hard to think of a human body part that does not have a counterpart in elephants and mice, and vice versa. Evolution plays freely with quantitative features, but it does not readily change the logic of the structure.  To get a more exact measure of how closely mammalian species resemble one another genetically, we can compare the nucleotide sequences of corresponding (orthologous) genes, or the amino acid sequences of the proteins that these genes encode. The results for individual genes and proteins vary widely. But typically, if we line up the amino acid sequence of a human protein with that of the orthologous protein from, say, an elephant, about 85% of the amino acids are identical. A similar comparison between human and bird shows an amino acid identity of about 70%—twice as many differences, because the bird and the mammalian lineages have had twice as long to diverge as those of the elephant and the human Kelainan genetik

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Eucaryotic cells by definition, keep their DNA in a separate membrane-bounded compartment, the nucleus. They have, in addition, a cytoskeleton for movement, elaborate intracellular compartments for digestion and secretion, the capacity (in many species) to engulf other cells, and a metabolism that depends on the oxidation of organic molecules by mitochondria. These properties suggest that eucaryotes originated as predators on other cells. Mitochondria—and, in plants, chloroplasts—contain their own genetic material, and evidently evolved from bacteria that were taken up into the cytoplasm of the eucaryotic cell and survived as symbionts. Eucaryotic cells have typically 3–30 times as many genes as procaryotes, and often thousands of times more noncoding DNA. The noncoding DNA allows for complex regulation of gene expression, as required for the construction of complex multicellular organisms. Many eucaryotes are, however, unicellular, among them the yeast Saccharomyces cerevisiae, which serves as a simple model organism for eucaryotic cell biology, revealing the molecular basis of conserved fundamental processes such as the eucaryotic cell division cycle. A small number of other organisms have been chosen as primary models for multicellular plants and animals, and the sequencing of their entire genomes has opened the way to systematic and comprehensive analysis of gene functions, gene regulation, and genetic diversity. As a result of gene duplications during vertebrate evolution, vertebrate genomes contain multiple closely related homologs of most genes. This genetic redundancy has allowed diversification and specialization of genes for new purposes, but it also makes gene functions harder to decipher. There is less

genetic redundancy in the nematode Caenorhabditis elegans and the fly Drosophila melanogaster, which have thus played a key part in revealing universal genetic mechanisms of animal development Eucaryotic DNA Is Packaged into a Set of Chromosomes  In eucaryotes, the DNA in the nucleus is divided between a set of different chromosomes. For example, the human genome— approximately 3.2 × 109 nucleotides —is distributed over 24 different chromosomes. Eachchromosome consists of a single, enormously long linear DNA molecule associated with proteins that fold and pack the fine DNA thread into a more compact structure. The complex of DNA and protein is called chromatin(from the Greek chroma, “color,” because of its staining properties). In addition to the proteins involved in packaging the DNA, chromosomes are also associated with many proteins required for the processes of geneexpression, DNA replication, and DNA repair.  Bacteria carry their genes on a single DNA molecule, which is usually circular (see Figure 1-30). This DNA is associated with proteins that package and condense the DNA, but they are different from the proteins that perform these functions in eucaryotes. Although often called the bacterial “chromosome,” it does not have the same structure as eucaryotic chromosomes, and less is known about how the bacterial DNA is packaged. Even less is known about how DNA is compacted in archaea. Therefore, our discussion of chromosome structure will focus almost entirely on eucaryotic chromosomes.  With the exception of the germ cells, and a few highly specialized cell types that cannot multiply and lack DNAaltogether (for example, red blood cells), each human cell contains two copies of each chromosome, one inherited from the mother and one from the father. The maternal and paternal chromosomes of a pair are

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calledhomologous chromosomes (homologs). The only nonhomologous chromosome pairs are the sex chromosomes in males, where a Y chromosome is inherited from the father and an X chromosome from the mother. Thus, each human cell contains a total of 46 chromosomes—22 pairs common to both males and females, plus two so-called sex chromosomes (X and Y in males, two Xs in females). DNA hybridization(describe d in detail in Chapter 8) can be used to distinguish these human chromosomes by “painting” each one a different color (Figure 4-10). Chromosome painting is typically done at the stage in the cell cycle when chromosomes are especially compacted and easy to visualize Nucleosomes Are the Basic Unit of Eucaryotic Chromosome Structure  The proteins that bind to the DNA to form eucaryotic chromosomes are traditionally divided into two general classes: the histones and the nonhistone chromosomal proteins. The complex of both classes of protein with the nuclear DNA of eucaryotic cells is known as chromatin. Histones are present in such enormous quantities in the cell (about 60 million molecules of each type per human cell) that their total mass in chromatin is about equal to that of the DNA.  Histones are responsible for the first and most basic level of chromosome organization, the nucleosome, which was discovered in 1974. When interphase nuclei are broken open very gently and their contents examined under the electron microscope, most of the chromatin is in the form of a fiber with a diameter of about 30 nm(Figure 4-23A). If this chromatin is subjected to treatments that cause it to unfold partially, it can be seen under the electron microscope as a series of “beads on a string” (Figure 4-23B). The string is DNA, and each bead is a “nucleosome core particle” that consists of DNA wound around a protein core formed from histones. The beads on a string represent the

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first level of chromosomal DNA packing. nucleosome Structural organization of the nucleosome. A nucleosome contains a protein core made of eight histone molecules. As indicated, the nucleosome core particle is released from chromatin by digestion of the linker DNA with a nuclease, an enzyme that breaks

Each nucleosome core particle is separated from the next by a region of linker DNA, which can vary in length from a few nucleotide pairs up to about 80. (The term nucleosome technically refers to a nucleosome core particle plus one of its adjacent DNA linkers, but it is often used synonymously with nucleosome core particle.) On average, therefore, nucleosomes repeat at intervals of about 200 nucleotide pairs. For example, a diploidhuman cell with 6.4 × 109 nucleotide pairs contains approximately 30 million nucleosomes. The formation of nucleosomes converts a DNA molecule into a chromatin thread about one-third of its initial length, and this provides the first level of DNA packing.

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Genes carry biological information that must be copied accurately for transmission to the next generation each time a cell divides to form two daughter cells. Two central biological questions arise from these requirements: how can the information for specifying an organism be carried in chemical form, and how is it accurately copied? The

discovery of the structure of the DNA double helix was a landmark in twentieth-century biology because it immediately suggested answers to both questions, thereby resolving at the molecular level the problem of heredity. We discuss briefly the answers to these questions in this section, and we shall examine them in more detail in subsequent chapters. DNA encodes information through the order, or sequence, of the nucleotides along each strand. Each base—A, C, T, or G—can be considered as a letter in a four-letter alphabet that spells out biological messages in the chemical structure of the DNA. As we saw in Chapter 1, organisms differ from one another because their respective DNA molecules have different nucleotide sequences and, consequently, carry different biological messages. But how is the nucleotide alphabet used to make messages, and what do they spell out? As discussed above, it was known well before the structure of DNA was determine d that genes contain the instructions for producing proteins. The DNA messages must therefore somehow encode proteins (Figure 46). This relationship immediately makes the problem easier to understand, because of the chemical character of proteins. As discussed in Chapter 3, the properties of a protein, which are responsible for its biologicalfunction, are determined by its three-dimensional structure, and its structure is determined in turn by the linear sequence of the amino acids of which it is composed. The linear sequence of nucleotides in a gene must therefore somehow spell out the linear sequence of amino acids in a protein. The exact correspondence between the fourletter nucleotide alphabet of DNA and the twenty-letter amino acid alphabet of proteins—thegenetic code—is not obvious from the DNA structure, and it took over a decade after the discovery of thedouble helix before it was worked

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out. In Chapter 6 we describe this code in detail in the course of elaborating the process, known as gene expression, through which a cell translates the nucleotide sequence of a gene into the amino acid sequence of a protein. The Structure of DNA Provides a Mechanism for Heredity

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In Eucaryotes, DNA Is Enclosed in a Cell Nucleus Nearly all the DNA in a eucaryotic cell is sequestered in a nucleus, which occupies about 10% of the total cell volume. This compartment is delimited by a nuclear envelope formed by two concentric lipid bilayer membranes that are punctured at intervals by large nuclear pores, which transport molecules between the nucleus and thecytosol. The nuclear envelope is directly connected to the extensive membranes of the endoplasmic reticulum. It is mechanically supported by two networks of intermediate filaments: one, called the nuclear lamina, forms a thin sheetlike meshwork inside the nucleus, just beneath the inner nuclear membrane; the other surrounds theouter nuclear membrane and is less regularly organized