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Chapter10. DNA Structure and Analysis With few exceptions, the nucleic acid DNA serves as the genetic material in every living thing. The structure of.

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Presentation on theme: "Chapter10. DNA Structure and Analysis With few exceptions, the nucleic acid DNA serves as the genetic material in every living thing. The structure of."— Presentation transcript:

1 Chapter10. DNA Structure and Analysis With few exceptions, the nucleic acid DNA serves as the genetic material in every living thing. The structure of DNA allows genetic information to be storied and expressed chemically within cells, as well as transmitting it to future generations.The molecule is a double stranded helix united by hydrogen bonds formed between complementary nucleotides. In some viruses, RNA serves as the genetic material With few exceptions, the nucleic acid DNA serves as the genetic material in every living thing. The structure of DNA allows genetic information to be storied and expressed chemically within cells, as well as transmitting it to future generations.The molecule is a double stranded helix united by hydrogen bonds formed between complementary nucleotides. In some viruses, RNA serves as the genetic material

2 Topics for This chapter 10.1 Search for the Genetic Material 10.2 Evidence favoring DNA in bacteria and phages 10.3 indirect and direct evidences favoring DNA in eukaryotes 10.4 RNA as genetic material in some viruses 10.5 Alternative form of DNA 10.6 Molecular hybridazition 10.7 Electrophoresis of Nucleic acids

3 Three characteristics for material responsible for hereditary information: Three characteristics for material responsible for hereditary information:  Must contain the information in stable form for an organism ’ s cell structure, function, development, & reproduction.  Must replicate accurately so progeny cells have same information as parental cell.  Must be capable of change, so adaptations to the environment & variation can occur. In 1890 Weismann proposed that there was a substance in cell nuclei that controlled characteristics of entire organism. In 1890 Weismann proposed that there was a substance in cell nuclei that controlled characteristics of entire organism Search for the Genetic Material

4 In the early 1900 ’ s, the Chromosome Theory of Inheritance was formulated by Morgan & his students. It states that chromosomes are the carriers of hereditary material. In the early 1900 ’ s, the Chromosome Theory of Inheritance was formulated by Morgan & his students. It states that chromosomes are the carriers of hereditary material. Chemical analysis of chromosomes over the next 40 years revealed that chromosomes contain only protein & nucleic acids. Chemical analysis of chromosomes over the next 40 years revealed that chromosomes contain only protein & nucleic acids. A series of experiments beginning in the late 1920 ’ s determined that DNA is the hereditary material. A series of experiments beginning in the late 1920 ’ s determined that DNA is the hereditary material. Experiments include Griffith transformation experiment, Avery et al. transformation experiments, and Hershey-Chase bacteriophage experiments. Experiments include Griffith transformation experiment, Avery et al. transformation experiments, and Hershey-Chase bacteriophage experiments.

5 By the 1920s, several lines of indirect evidence began to suggest a close relationship between chromosomes and DNA. Microscopic studies with special stains showed that DNA is present in chromosomes. Chromosomes also contain various types of proteins, but the amount and kinds of chromosomal proteins differ greatly from one cell type to another, whereas the amount of DNA per cell is constant. Furthermore, nearly all of the DNA present in cells of higher organisms is present in the chromosomes. By the 1920s, several lines of indirect evidence began to suggest a close relationship between chromosomes and DNA. Microscopic studies with special stains showed that DNA is present in chromosomes. Chromosomes also contain various types of proteins, but the amount and kinds of chromosomal proteins differ greatly from one cell type to another, whereas the amount of DNA per cell is constant. Furthermore, nearly all of the DNA present in cells of higher organisms is present in the chromosomes Evidence favoring DNA in bacteria and phages

6 These arguments for DNA as the genetic material were unconvincing, however, because crude chemical analyses had suggested (erroneously, as it turned out) that DNA lacks the chemical diversity needed in a genetic substance. These arguments for DNA as the genetic material were unconvincing, however, because crude chemical analyses had suggested (erroneously, as it turned out) that DNA lacks the chemical diversity needed in a genetic substance. The favored candidate for the genetic material was protein, because proteins were known to be an exceedingly diverse collection of molecules. The favored candidate for the genetic material was protein, because proteins were known to be an exceedingly diverse collection of molecules. Proteins therefore became widely accepted as the genetic material, and DNA was assumed to function merely as the structural framework of the chromosomes. The experiments described below finally demonstrated that DNA is the genetic material. Proteins therefore became widely accepted as the genetic material, and DNA was assumed to function merely as the structural framework of the chromosomes. The experiments described below finally demonstrated that DNA is the genetic material.

7 Transformation Studies of Streptococcus pneumoniae ( 肺炎双球菌 ) Transformation Studies of Streptococcus pneumoniae ( 肺炎双球菌 ) An important first step was taken by Frederick Griffith in 1928 An important first step was taken by Frederick Griffith in 1928 Streptococcus pneumoniae identified as S and R. Streptococcus pneumoniae identified as S and R. The S type of S. pneumoniae synthesizes a gelatinous The S type of S. pneumoniae synthesizes a gelatinous Mice injected with living S cells get pneumonia. Mice injected either with living R cells or with heat-killed S cells remain healthy. Here is Griffith ’ s critical finding: mice injected with a mixture of living R cells and heat-killed S cells contract the disease — they often die of pneumonia (Figure 1). Mice injected with living S cells get pneumonia. Mice injected either with living R cells or with heat-killed S cells remain healthy. Here is Griffith ’ s critical finding: mice injected with a mixture of living R cells and heat-killed S cells contract the disease — they often die of pneumonia (Figure 1).

8 TABLE 10.1 Strains of Diplococcus pneumoniae Used by Frederick Griffith in His Original Transformation Experiments

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10 Figure 10.2 The Griffith's experiment demonstrating bacterial transformation. A mouse remains healthy if injected with either the nonvirulent R strain of S. pneumoniae or heat- killed cell fragments of the usually virulent S strain. R cells in the presence of heat- killed S cells are transformed into the virulent S strain, causing pneumonia in the mouse.

11 1944 Avery

12 The Hershey-Chase Experiment Transfection of phages

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17 10.3 indirect and direct evidences favoring DNA in eukaryotes Indirect Evidence: Distribution of DNA Indirect Evidence: Distribution of DNA Because it had been established earlier that chromosomes within the nucleus contain the genetic material, a correlation was expected between the ploidy (n, 2n, etc.) of cells and the quantity of the molecule that functions as the genetic material. Meaningful comparisons can be made between gametes (sperm and eggs) and somatic or body cells. The latter are recognized as being diploid (In) and containing twice the number of chromosomes as gametes, which are haploid (n). Because it had been established earlier that chromosomes within the nucleus contain the genetic material, a correlation was expected between the ploidy (n, 2n, etc.) of cells and the quantity of the molecule that functions as the genetic material. Meaningful comparisons can be made between gametes (sperm and eggs) and somatic or body cells. The latter are recognized as being diploid (In) and containing twice the number of chromosomes as gametes, which are haploid (n).

18 Table 10.2 compares the amount of DNA found in haploid sperm and the diploid nucleated precursors of red blood cells from a variety of organisms.

19 Indirect Evidence: Mutagenesis Indirect Evidence: Mutagenesis Ultraviolet (UV) light is one of a number of agents capable of inducing mutations in the genetic material. Simple organisms such as yeast and other fungi can be irradiated with various wavelengths of UV light, and the effectiveness of each wavelength can be measured by the number of mutations it induces. When the data are plotted, an action spectrum of UV light as a mutagenic agent is obtained. This action spectrum can then be compared with the absorption spectrum of any molecule suspected to be the genetic material (Figure 10-6). The molecule serving as the genetic material is expected to absorb at the wavelengths found to be mutagenic. Ultraviolet (UV) light is one of a number of agents capable of inducing mutations in the genetic material. Simple organisms such as yeast and other fungi can be irradiated with various wavelengths of UV light, and the effectiveness of each wavelength can be measured by the number of mutations it induces. When the data are plotted, an action spectrum of UV light as a mutagenic agent is obtained. This action spectrum can then be compared with the absorption spectrum of any molecule suspected to be the genetic material (Figure 10-6). The molecule serving as the genetic material is expected to absorb at the wavelengths found to be mutagenic.

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21 (1)DNA 通常只存在染色体 (2) 每个细胞中含量基本相同, 精 细胞半 (3) 细胞中恒定, 蛋白质不恒定 (4)DNA 结构改变, 引起突变 (5) A=T G=C, A+G=T+C, A+T≠G+C,Chargaff law

22 1952 年, Wilkins 和 Franklin 用高度定向的 DNA 纤维作出高质量 的 X- 光衍射照片 1952 年, Wilkins 和 Franklin 用高度定向的 DNA 纤维作出高质量 的 X- 光衍射照片

23 Eukaryotic cells can acquire a new phenotype as the result of transfection by added DNA. Direct :DNA is the genetic material

24 10.4 RNA as genetic material in some viruses Some viruses contain an RNA core rather than one composed of DNA. In these viruses, it appears that RNA serves as the genetic material—an exception to the general rule that DNA performs this function. In 1956, it was demonstrated that when purified RNA from tobacco mosaic virus (TMV) is spread on tobacco leaves, the characteristic lesions caused by viral infection subsequently appear on the leaves. It was concluded that RNA is the genetic material of this virus.

25 1956 年 A.Gierer 和 G.Schraman 发现 烟草花叶病毒( tobacco mosaic virus,TMV ),其遗传物质是 RNA 。 1957 年美国的 Heinz Fraenkel-Conrat 和 B.Singre 用重建实验证实了这一结论。

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27 TMV RNA 病毒 单链 RNA 蛋白质外壳 6%RNA 94% 蛋白质 用水和苯 酚处理 RNA 蛋白质 感染烟草 感染不能感染 S 株系 -His.Met HR 株系 - S 株系 Pro HR 株系 Pro S 株系 RNA HR 株系 RNA

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30 In 1965 and 1966, Norman Pace and Sol Spiegelman demonstrated further that RNA from the phage QB can be isolated and replicated in vitro. Replication depends on an enzyme, RNA replicase, which is isolated from host E. coli cells following normal infection. When the RNA replicated in vitro is added to E. coli protoplasts, infection and viral multiplication (transfection) occurs. Thus, RNA synthesized in a test tube serves as the genetic material in these phages by directing the production of all components necessary for viral replication

31 Finally, one other group of RNA-containing viruses bears mentioning. These are the retroviruses, which replicate in an unusual way. Their RNA serves as a template for the synthesis of the complementary DNA molecule! The process, reverse transcription, occurs under the direction of an RNA-dependent DNA polymerase enzyme called reverse transcriptase. This DNA intermediate can be incorporated into the genome of the host cell, and when the host DNA is transcribed, copies of the original reiroviral RNA chromosomes are produced. Retroviruses include the human immunodeficiency virus (HIV), which causes AIDS, as well as the RNA tumor viruses

32 Different DNA structures By synthesizing short pieces of DNA (known as oligomers), crystallization and analysis of structure is possible. By synthesizing short pieces of DNA (known as oligomers), crystallization and analysis of structure is possible. These studies should that DNA exists in several different forms: A-DNA, B-DNA, and Z-DNA are the most common types detected. These studies should that DNA exists in several different forms: A-DNA, B-DNA, and Z-DNA are the most common types detected.  B-DNA is the Watson & Crick model.  A-DNA is also a right-handed helix,  while Z-DNA is a left-handed helix with a zigzag backbone Alternative form of DNA

33 DNA 的构象现已知有 A , B , C , D , E , T , Z 7 种。 引起 DNA 双链构象改变有以下因素: ( 1 )核苷酸顺序; ( 2 )碱基组成; ( 3 )盐的种类; ( 4 )相对湿度。

34 Different DNA structures-3 A-DNA is found only when DNA is dehydrated. A-DNA is found only when DNA is dehydrated. Z-DNA is still debated as to whether or not it is found in living cells. Z-DNA is still debated as to whether or not it is found in living cells. The role of Z-DNA is still not clearly defined. The role of Z-DNA is still not clearly defined. Some organisms are found to have Z- DNA-binding proteins in their nuclei. Some organisms are found to have Z- DNA-binding proteins in their nuclei.

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37 10.6 Molecular hybridazition Denaturation and renaturation of DNA ( 变性 与复性 )- The technique that is used to determine the sequence complexity of any genome Denaturation and renaturation of DNA ( 变性 与复性 )- The technique that is used to determine the sequence complexity of any genome The DNA is denatured by heating which melts the H-bonds and renders the DNA single-stranded. The DNA is denatured by heating which melts the H-bonds and renders the DNA single-stranded. The DNA is allowed to cool slowly,and sequences that are complementary will find each other and eventually base pair again. The DNA is allowed to cool slowly,and sequences that are complementary will find each other and eventually base pair again. The rate at which the DNA reanneals (another term for renature) is a function of the species from which the DNA was isolated. The rate at which the DNA reanneals (another term for renature) is a function of the species from which the DNA was isolated. This is a plot of the reannealing of a simple genome. This is a plot of the reannealing of a simple genome.

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44 10.7 Electrophoresis of Nucleic acids

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46 Analysis of DNA Sequences in Eukaryotic Genomes The technique that is used to determine the sequence complexity of any genome involves the denaturation and renaturation of DNA. DNA is denatured by heating which melts the H-bonds and renders the DNA single-stranded. If the DNA is rapidly cooled, the DNA remains single- stranded. But if the DNA is allowed to cool slowly, sequences that are complementary will find each other and eventually base pair again. The rate at which the DNA reanneals (another term for renature) is a function of the species from which the DNA was isolated. Below is a curve that is obtained from a simple genome. The technique that is used to determine the sequence complexity of any genome involves the denaturation and renaturation of DNA. DNA is denatured by heating which melts the H-bonds and renders the DNA single-stranded. If the DNA is rapidly cooled, the DNA remains single- stranded. But if the DNA is allowed to cool slowly, sequences that are complementary will find each other and eventually base pair again. The rate at which the DNA reanneals (another term for renature) is a function of the species from which the DNA was isolated. Below is a curve that is obtained from a simple genome.

47 The Y-axis is the percent of the DNA that remains single stranded. This is expressed as a ratio of the concentration of single-stranded DNA (C) to the total concentration of the starting DNA (Co). The X-axis is a log-scale of the product of the initial concentration of DNA (in moles/liter) multiplied by length of time the reaction proceeded (in seconds). The designation for this value is Cot and is called the "Cot" value. The curve itself is called a "Cot" curve. As can be seen the curve is rather smooth which indicates that reannealing occurs slowing but gradually over a period of time. One particular value that is useful is Cot ½, the Cot value where half of the DNA has reannealed. The Y-axis is the percent of the DNA that remains single stranded. This is expressed as a ratio of the concentration of single-stranded DNA (C) to the total concentration of the starting DNA (Co). The X-axis is a log-scale of the product of the initial concentration of DNA (in moles/liter) multiplied by length of time the reaction proceeded (in seconds). The designation for this value is Cot and is called the "Cot" value. The curve itself is called a "Cot" curve. As can be seen the curve is rather smooth which indicates that reannealing occurs slowing but gradually over a period of time. One particular value that is useful is Cot ½, the Cot value where half of the DNA has reannealed.

48 Steps Involved in DNA Denaturation and Renaturation Experiments Steps Involved in DNA Denaturation and Renaturation Experiments 1. Shear the DNA to a size of about 400 bp. 2. Denature the DNA by heating to 100 o C. 3. Slowly cool and take samples at different time intervals. 4. Determine the % single-stranded DNA at each time point. 1. Shear the DNA to a size of about 400 bp. 2. Denature the DNA by heating to 100 o C. 3. Slowly cool and take samples at different time intervals. 4. Determine the % single-stranded DNA at each time point. The shape of a "Cot" curve for a given species is a function of two factors: The shape of a "Cot" curve for a given species is a function of two factors: 1. the size or complexity of the genome; and 2. the amount of repetitive DNA within the genome

49 If we plot the "Cot" curves of the genome of three species such as bacteriophage lambda, E. coli and yeast we will see that they have the same shape, but the Cot ½ of the yeast will be largest, E. coli next and lambda smallest. Physically, the larger the genome size the longer it will take for any one sequence to encounter its complementary sequence in the solution. This is because two complementary sequences must encounter each other before they can pair. The more complex the genome, that is the more unique sequences that are available, the longer it will take for any two complementary sequences to encounter each other and pair. Given similar concentrations in solution, it will then take a more complex species longer to reach Cot ½. If we plot the "Cot" curves of the genome of three species such as bacteriophage lambda, E. coli and yeast we will see that they have the same shape, but the Cot ½ of the yeast will be largest, E. coli next and lambda smallest. Physically, the larger the genome size the longer it will take for any one sequence to encounter its complementary sequence in the solution. This is because two complementary sequences must encounter each other before they can pair. The more complex the genome, that is the more unique sequences that are available, the longer it will take for any two complementary sequences to encounter each other and pair. Given similar concentrations in solution, it will then take a more complex species longer to reach Cot ½.

50 Repeated DNA sequences, DNA sequences that are found more than once in the genome of the species, have distinctive effects on "Cot" curves. If a specific sequence is represented twice in the genome it will have two complementary sequences to pair with and as such will have a Cot value half as large as a sequence represented only once in the genome. Repeated DNA sequences, DNA sequences that are found more than once in the genome of the species, have distinctive effects on "Cot" curves. If a specific sequence is represented twice in the genome it will have two complementary sequences to pair with and as such will have a Cot value half as large as a sequence represented only once in the genome.

51 Eukaryotic genomes actually have a wide array of sequences that are represented at different levels of repetition. Single copy sequences are found once or a few times in the genome. Many of the sequences which encode functional genes fall into this class. Middle repetitive DNA are found from 10s times in the genome. Examples of these would include rRNA and tRNA genes and storage proteins in plants such as corn. Middle repetitive DNA can vary from bp to 5000 bp and can be dispersed throughout the genome. The most abundant sequences are found in the highly repetitive DNA class. Eukaryotic genomes actually have a wide array of sequences that are represented at different levels of repetition. Single copy sequences are found once or a few times in the genome. Many of the sequences which encode functional genes fall into this class. Middle repetitive DNA are found from 10s times in the genome. Examples of these would include rRNA and tRNA genes and storage proteins in plants such as corn. Middle repetitive DNA can vary from bp to 5000 bp and can be dispersed throughout the genome. The most abundant sequences are found in the highly repetitive DNA class.

52 These sequences are found from 100,000 to 1 million times in the genome and can range in size from a few to several hundred bases in length. These sequences are found in regions of the chromosome such as heterochromatin, centromeres and telomeres and tend to be arranged as a tandem repeats. The following is an example of a tandemly repeated sequence: These sequences are found from 100,000 to 1 million times in the genome and can range in size from a few to several hundred bases in length. These sequences are found in regions of the chromosome such as heterochromatin, centromeres and telomeres and tend to be arranged as a tandem repeats. The following is an example of a tandemly repeated sequence: ATTATA ATTATA ATTATA // ATTATA ATTATA ATTATA ATTATA // ATTATA

53 Genomes that contain these different classes of sequences reanneal in a different manner than genomes with only single copy sequences. Instead of having a single smooth "Cot" curve, three distinct curves can be seen, each representing a different repetition class. The first sequences to reanneal are the highly repetitive sequences because so many copies of them exist in the genome, and because they have a low sequence complexity. The second portion of the genome to reanneal is the middle repetitive DNA, and the final portion to reanneal is the single copy DNA. The following diagram depicts the "Cot" curve for a "typical" eukaryotic genome Genomes that contain these different classes of sequences reanneal in a different manner than genomes with only single copy sequences. Instead of having a single smooth "Cot" curve, three distinct curves can be seen, each representing a different repetition class. The first sequences to reanneal are the highly repetitive sequences because so many copies of them exist in the genome, and because they have a low sequence complexity. The second portion of the genome to reanneal is the middle repetitive DNA, and the final portion to reanneal is the single copy DNA. The following diagram depicts the "Cot" curve for a "typical" eukaryotic genome

54 Sequence Interspersion Sequence Interspersion Even though the genomes of higher organisms contain single copy, middle repetitive and highly repetitive DNA sequences, these sequences are not arranged similarly in all species. The prominent arrangement is called short period interspersion. This arrangement is characterized by repeated sequences bp in length interspersed among single copy sequences that are bp in length. This arrangement is found in animals, fungi and plants. Even though the genomes of higher organisms contain single copy, middle repetitive and highly repetitive DNA sequences, these sequences are not arranged similarly in all species. The prominent arrangement is called short period interspersion. This arrangement is characterized by repeated sequences bp in length interspersed among single copy sequences that are bp in length. This arrangement is found in animals, fungi and plants. The second type of arrangement is long-period interspersion. This is characterized by 5000 bp stretches of repeated sequences interspersed within regions of 35,000 bp of single copy DNA. Drosophila is an example of a species with this uncommon sequence arrangement. In both cases, the repeated sequences are usually from the middle repetitive class. We discussed above where highly repetitive sequences are found. The second type of arrangement is long-period interspersion. This is characterized by 5000 bp stretches of repeated sequences interspersed within regions of 35,000 bp of single copy DNA. Drosophila is an example of a species with this uncommon sequence arrangement. In both cases, the repeated sequences are usually from the middle repetitive class. We discussed above where highly repetitive sequences are found.

55 Eukaryotic Chromosome Karyotype Eukaryotic Chromosome Karyotype Whereas bacteria only have a single chromosome, eukaryotic species have at least one pair of chromosomes. Most have more than one pair. Another relevant point is that eukaryotic chromosomes are detected only occur during cell division and not during all stages of the cell cycle. They are in their most condensed form during metaphase when the sister chromatids are attached. This is the primary stage when cytogenetic analysis is performed. Whereas bacteria only have a single chromosome, eukaryotic species have at least one pair of chromosomes. Most have more than one pair. Another relevant point is that eukaryotic chromosomes are detected only occur during cell division and not during all stages of the cell cycle. They are in their most condensed form during metaphase when the sister chromatids are attached. This is the primary stage when cytogenetic analysis is performed.

56 Each species is characterized by a karyotype. The karyotype is a description of the number of chromosomes in the normal diploid cell, as well as their size distribution. For example, the human chromosome has 23 pairs of chromosome, 22 somatic pairs and one pair of sex chromosomes. One important aspect of genetic research is correlating changes in the karyotype with changes in the phenotype of the individual. Each species is characterized by a karyotype. The karyotype is a description of the number of chromosomes in the normal diploid cell, as well as their size distribution. For example, the human chromosome has 23 pairs of chromosome, 22 somatic pairs and one pair of sex chromosomes. One important aspect of genetic research is correlating changes in the karyotype with changes in the phenotype of the individual. One important aspect of genetics is correlating changes in karyotype with changes in phenotype. For example, humans that have an extra chromosome 21 have Down's syndrome. Insertions, deletions and changes in chromosome number can be detected by the skilled cytogeneticist, but correlating these with specific phenotypes is difficult. One important aspect of genetics is correlating changes in karyotype with changes in phenotype. For example, humans that have an extra chromosome 21 have Down's syndrome. Insertions, deletions and changes in chromosome number can be detected by the skilled cytogeneticist, but correlating these with specific phenotypes is difficult.

57 The first discriminating parameter when developing a karyotype is the size and number of the chromosomes. Although this is useful, it does not provide enough detail to be begin the development of a correlation between structure and function (phenotype). To further distinguish among chromosomes, they are treated with a dye that stains the DNA in a reproducible manner. After staining, some of the regions are lightly stained and others are heavily stained. As described above, the lightly stained regions are called euchromatin, and the dark stained region is called heterochromatin. The current dye of chose is the Giemsa stain, and the resulting pattern is called the G-banding pattern. The first discriminating parameter when developing a karyotype is the size and number of the chromosomes. Although this is useful, it does not provide enough detail to be begin the development of a correlation between structure and function (phenotype). To further distinguish among chromosomes, they are treated with a dye that stains the DNA in a reproducible manner. After staining, some of the regions are lightly stained and others are heavily stained. As described above, the lightly stained regions are called euchromatin, and the dark stained region is called heterochromatin. The current dye of chose is the Giemsa stain, and the resulting pattern is called the G-banding pattern.

58 C-Value Paradox C-Value Paradox In addition to describing the genome of an organism by its number of chromosomes, it is also described by the amount of DNA in a haploid cell. This is usually expressed as the amount of DNA per haploid cell (usually expressed as picograms) or the number of kilobases per haploid cell and is called the C value. One immediate feature of eukaryotic organisms highlights a specific anomaly that was detected early in molecular research. Even though eukaryotic organisms appear to have 2-10 times as many genes as prokaryotes, they have many orders of magnitude more DNA in the cell. Furthermore, the amount of DNA per genome is correlated not with the presumed evolutionary complexity of a species. This is stated as the C value paradox: the amount of DNA in the haploid cell of an organism is not related to its evolutionary complexity. (Another important point to keep in mind is that there is no relationship between the number of chromosomes and the presumed evolutionary complexity of an organism.) In addition to describing the genome of an organism by its number of chromosomes, it is also described by the amount of DNA in a haploid cell. This is usually expressed as the amount of DNA per haploid cell (usually expressed as picograms) or the number of kilobases per haploid cell and is called the C value. One immediate feature of eukaryotic organisms highlights a specific anomaly that was detected early in molecular research. Even though eukaryotic organisms appear to have 2-10 times as many genes as prokaryotes, they have many orders of magnitude more DNA in the cell. Furthermore, the amount of DNA per genome is correlated not with the presumed evolutionary complexity of a species. This is stated as the C value paradox: the amount of DNA in the haploid cell of an organism is not related to its evolutionary complexity. (Another important point to keep in mind is that there is no relationship between the number of chromosomes and the presumed evolutionary complexity of an organism.)

59 A dramatic example of the range of C values can be seen in the plant kingdom where Arabidopsis represents the low end and lily (1.0 x 10^8 kb/haploid genome) the high end of complexity. In weight terms this is 0.07 picograms per haploid Arabidopsis genome and 100 picograms per haploid lily genome. A dramatic example of the range of C values can be seen in the plant kingdom where Arabidopsis represents the low end and lily (1.0 x 10^8 kb/haploid genome) the high end of complexity. In weight terms this is 0.07 picograms per haploid Arabidopsis genome and 100 picograms per haploid lily genome. Genome - the complete set of chromosomes inherited from a single parent; the complete DNA component of an individual; the definition often excludes organelles Genome - the complete set of chromosomes inherited from a single parent; the complete DNA component of an individual; the definition often excludes organelles

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61 SpeciesSequence Distribution Bacteria99.7% Single Copy Mouse 60% Single Copy 25% Middle Repetitive 10% Highly Repetitive Human 70% Single Copy 13% Middle Repetitive 8% Highly Repetitive Cotton 61% Single Copy 27% Middle Repetitive 8% Highly Repetitive Corn 30% Single Copy 40% Middle Repetitive 20% Highly Repetitive Wheat 10% Single Copy 83% Middle Repetitive 4% Highly Repetitive Arabidopsis 55% Single Copy 27% Middle Repetitive 10% Highly Repetitive The following table gives the sequence distributio n for selected species. Sequence Interspersi on

62 C Values of Organisms Used in Genetic Studies SpeciesKilobases/haploid genome E. coli4.5 x 10 3 Human3.0 x 10 6 Drosophila1.7 x 10 5 Maize2.0 x 10 6 Aribidopsis 7.0 x 10 4


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