Chapter10.DNA Structure and Analysis

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

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

Three characteristics for material responsible for hereditarThree characteristics for material responsible for hereditary information:y information: Must contain the Must contain the informationinformation in stable form for an org in stable form for an org

anism’s cell structure, function, development, & reprodanism’s cell structure, function, development, & reproduction.uction.

Must Must replicate accuratelyreplicate accurately so progeny cells have same so progeny cells have same information as parental cell.information as parental cell.

Must be capable of Must be capable of changechange, so adaptations to the envir, so adaptations to the environment & variation can occur.onment & variation can occur.

In 1890 Weismann proposed that there was a substance in In 1890 Weismann proposed that there was a substance in cell nuclei that controlled characteristics of entire organiscell nuclei that controlled characteristics of entire organism.m.

10.1 Search for the Genetic Material

In the early 1900’s, the In the early 1900’s, the Chromosome Theory of InheriChromosome Theory of Inheritancetance was formulated by Morgan & his students. It st was formulated by Morgan & his students. It states that chromosomes are the carriers of hereditary ates that chromosomes are the carriers of hereditary material.material.

Chemical analysis of chromosomes over the next 40 yChemical analysis of chromosomes over the next 40 years revealed that chromosomes contain only protein ears revealed that chromosomes contain only protein & nucleic acids.& nucleic acids.

A series of experiments beginning in the late 1920’s dA series of experiments beginning in the late 1920’s determined that DNA is the hereditary material.etermined that DNA is the hereditary material.

Experiments include Griffith transformation experimeExperiments include Griffith transformation experiment, Avery nt, Avery et alet al. transformation experiments, and Hers. transformation experiments, and Hershey-Chase bacteriophage experiments.hey-Chase bacteriophage experiments.

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

10.2 Evidence favoring DNA in bacteria and phages

These arguments for DNA as the genetic material These arguments for DNA as the genetic material were unconvincing, however, because crude were unconvincing, however, because crude chemical analyses had suggested (erroneously, as chemical analyses had suggested (erroneously, as it turned out) that DNA lacks the chemical it turned out) that DNA lacks the chemical diversity needed in a genetic substance. diversity needed in a genetic substance.

The favored candidate for the genetic material The favored candidate for the genetic material was protein, because proteins were known to be was protein, because proteins were known to be an exceedingly diverse collection of molecules. an exceedingly diverse collection of molecules.

Proteins therefore became widely accepted as the Proteins therefore became widely accepted as the genetic material, and DNA was assumed to genetic material, and DNA was assumed to function merely as the structural framework of function merely as the structural framework of the chromosomes. The experiments described the chromosomes. The experiments described below finally demonstrated that DNA is the below finally demonstrated that DNA is the genetic material.genetic material.

Transformation Studies of Transformation Studies of Streptococcus pneuStreptococcus pneumoniae (moniae ( 肺炎双球菌肺炎双球菌 ))

An important first step was taken by Frederick GriAn important first step was taken by Frederick Griffith in 1928ffith in 1928

Streptococcus pneumoniae Streptococcus pneumoniae identified as S and R. identified as S and R. The S type of The S type of S. pneumoniae S. pneumoniae synthesizes a gelatisynthesizes a gelati

nousnous Mice injected with living S cells get pneumonia. MiMice injected with living S cells get pneumonia. Mi

ce injected either with living R cells or with heat-kce injected either with living R cells or with heat-killed S cells remain healthy. Here is Griffith’s critiilled S cells remain healthy. Here is Griffith’s critical finding: mice injected with a cal finding: mice injected with a mixture mixture of living of living R cells and heat-killed S cells contract the diseaseR cells and heat-killed S cells contract the disease—they often die of pneumonia (—they often die of pneumonia (Figure 1Figure 1).).

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

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.

1944 Avery

The Hershey-Chase Experiment Transfection of phages

10.3 indirect and direct evidences favoring DNA in eukaryotes

Indirect Evidence: Indirect Evidence: Distribution of DNADistribution of DNA Because it had been established earlier that chromosoBecause it had been established earlier that chromoso

mes within the nucleus contain the genetic material, a mes within the nucleus contain the genetic material, a correlation was expected between the ploidy (correlation was expected between the ploidy (n, n, 2n, 2n, etetc.) of cells and the quantity of the molecule that functic.) of cells and the quantity of the molecule that functions as the genetic material. Meaningful comparisons cons as the genetic material. Meaningful comparisons can be made between gametes (sperm and eggs) and soan be made between gametes (sperm and eggs) and somatic or body cells. The latter are recognized as being matic or body cells. The latter are recognized as being diploid diploid (In) (In) and containing twice the number of chromand containing twice the number of chromosomes as gametes, which are haploid osomes as gametes, which are haploid (n).(n).

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.

Indirect Evidence: Indirect Evidence: Mutagenesis Mutagenesis Ultraviolet (UV) light is one of a number of agents Ultraviolet (UV) light is one of a number of agents

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

(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

19521952 年， 年， WilkinsWilkins 和和FranklinFranklin 用高度定向用高度定向的的 DNADNA 纤维作出高质纤维作出高质量的量的 X-X- 光衍射照片光衍射照片

Eukaryotic cells can acquire a new phenotype as the result of transfection by added DNA.

Direct :DNA is the genetic material

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.

1956 年 A.Gierer 和 G.Schraman 发现 烟草花叶病毒（ tobacco mosaic vir

us,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。

TMV RNA 病毒单链 RNA

蛋白质外壳

6%RNA94% 蛋白质

用水和苯酚处理RNA 蛋白质

感染烟草 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

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

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

Different DNA structures By synthesizing short pieces of DNA (known as olBy synthesizing short pieces of DNA (known as ol

igomers), crystallization and analysis of structure iigomers), crystallization and analysis of structure is possible.s possible.

These studies should that DNA exists in several diThese studies should that DNA exists in several different forms: A-DNA, B-DNA, and Z-DNA are tfferent forms: A-DNA, B-DNA, and Z-DNA are the most common types detected.he most common types detected. B-DNAB-DNA is the Watson & Crick model. is the Watson & Crick model. A-DNAA-DNA is also a right-handed helix, is also a right-handed helix, while while Z-DNAZ-DNA is a left-handed helix with a zigz is a left-handed helix with a zigz

ag backbone.ag backbone.

10.5 Alternative form of DNA

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。

Different DNA structures-3

A-DNA is found only when DNA is A-DNA is found only when DNA is dehydrated.dehydrated.

Z-DNA is still debated as to whether or Z-DNA is still debated as to whether or not it is found in living cells.not it is found in living cells.

The role of Z-DNA is still not clearly The role of Z-DNA is still not clearly defined.defined.

Some organisms are found to have Z-Some organisms are found to have Z-DNA-binding proteins in their nuclei.DNA-binding proteins in their nuclei.

存在的条件 沟 型双螺旋类型

每螺旋内碱基对 数

每对碱基的转角

每碱基对的间

距(

A)

直径（nm）相对湿度盐的种类大沟小沟

A 11 34.7°(右 旋)

2.56 2.375%Na+,K+, Cs+窄深宽深

B 10 34.0°(右 旋)

3.38 1.992%Na+低盐宽中等深窄中等深

C 9.3338.6°(右 旋)

3.32 1.966%Li+ 宽中等深窄中等深

Z 12 -30°(左 旋)

3.71 1.843%Na+, Mg++高盐平 浅窄深

10.6 Molecular hybridazition•Denaturation and renaturation of DNA (Denaturation and renaturation of DNA ( 变变性与复性性与复性 )-)- The technique that is used to determinThe technique that is used to determine the sequence complexity of any genomee the sequence complexity of any genome

•The DNA is denatured by heating which melts the The DNA is denatured by heating which melts the H-bonds and renders the DNA single-stranded. H-bonds and renders the DNA single-stranded. •The DNA is allowed to cool slowly,and sequences thThe DNA is allowed to cool slowly,and sequences that are complementary will find each other and eventat are complementary will find each other and eventually base pair again. ually base pair again. •The rate at which the DNA reanneals (another term The rate at which the DNA reanneals (another term for renature) is a function of the species from which for renature) is a function of the species from which the DNA was isolated. 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.

10.7 Electrophoresis of Nucleic acids

Analysis of DNA Sequences in Eukaryotic Genomes

The technique that is used to determine the sequThe technique that is used to determine the sequence complexity of any genome involves the ence complexity of any genome involves the dendenaturation and renaturationaturation and renaturation of DNA. DNA is dena of DNA. DNA is denatured by heating which melts the H-bonds and rentured by heating which melts the H-bonds and renders the DNA single-stranded. If the DNA is rapidlders the DNA single-stranded. If the DNA is rapidly cooled, the DNA remains single-stranded. But if y cooled, the DNA remains single-stranded. But if the DNA is allowed to cool slowly, sequences that the DNA is allowed to cool slowly, sequences that are complementary will find each other and eventare complementary will find each other and eventually base pair again. The rate at which the DNA rually base pair again. The rate at which the DNA reanneals (another term for renature) is a function eanneals (another term for renature) is a function of the species from which the DNA was isolated. of the species from which the DNA was isolated. Below is a curve that is obtained from a simple geBelow is a curve that is obtained from a simple genome. nome.

The Y-axis is the percent of the DNA that remainThe Y-axis is the percent of the DNA that remains single stranded. This is expressed as a ratio of s single stranded. This is expressed as a ratio of the concentration of single-stranded DNA (the concentration of single-stranded DNA (CC) to t) to the total concentration of the starting DNA (he total concentration of the starting DNA (CoCo). T). The X-axis is a log-scale of the product of the initiahe X-axis is a log-scale of the product of the initial concentration of DNA (in moles/liter) multiplied l concentration of DNA (in moles/liter) multiplied by length of time the reaction proceeded (in secoby length of time the reaction proceeded (in seconds). The designation for this value is nds). The designation for this value is CotCot and is and is called the "Cot" value. The curve itself is called acalled the "Cot" value. The curve itself is called a "Cot" curve. As can be seen the curve is rather s "Cot" curve. As can be seen the curve is rather smooth which indicates that reannealing occurs slmooth which indicates that reannealing occurs slowing but gradually over a period of time. One paowing but gradually over a period of time. One particular value that is useful is rticular value that is useful is Cot½Cot½ , the , the CotCot valu value where half of the DNA has reannealed. e where half of the DNA has reannealed.

Steps Involved in DNA Denaturation and RenaSteps Involved in DNA Denaturation and Renaturation Experimentsturation Experiments

1. Shear the DNA to a size of about 400 bp.1. Shear the DNA to a size of about 400 bp.2. Denature the DNA by heating to 1002. Denature the DNA by heating to 100ooC.C.3. Slowly cool and take samples at different time 3. Slowly cool and take samples at different time intervals.intervals.4. Determine the % single-stranded DNA at each 4. Determine the % single-stranded DNA at each time point. time point.

The shape of a "Cot" curve for a given species is The shape of a "Cot" curve for a given species is a function of two factors: a function of two factors:

1.1. the size or complexity of the genome; and the size or complexity of the genome; and

2.2. the amount of repetitive DNA within the genome the amount of repetitive DNA within the genome

If we plot the "Cot" curves of the genome of threIf we plot the "Cot" curves of the genome of three species such as bacteriophage lambda, e species such as bacteriophage lambda, E. coliE. coli and yeast we will see that they have the same sand yeast we will see that they have the same shape, but the hape, but the Cot½Cot½ of the yeast will be largest, of the yeast will be largest, E.E. coli coli next and lambda smallest. Physically, the la next and lambda smallest. Physically, the larger the genome size the longer it will take for anrger the genome size the longer it will take for any one sequence to encounter its complementary y one sequence to encounter its complementary sequence in the solution. This is because two cosequence in the solution. This is because two complementary sequences must encounter each otmplementary sequences must encounter each other before they can pair. The more complex the her before they can pair. The more complex the genome, that is the more unique sequences that genome, that is the more unique sequences that are available, the longer it will take for any two care available, the longer it will take for any two complementary sequences to encounter each othomplementary sequences to encounter each other and pair. Given similar concentrations in solutier and pair. Given similar concentrations in solution, it will then take a more complex species longon, it will then take a more complex species longer to reach Cot½ . er to reach Cot½ .

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

Eukaryotic genomes actually have a wide array Eukaryotic genomes actually have a wide array of sequences that are represented at different lof sequences that are represented at different levels of repetition. evels of repetition. Single copy sequences arSingle copy sequences are found once or a few times in the genome.e found once or a few times in the genome. Many of the sequences which encode functionaMany of the sequences which encode functional genes fall into this class. l genes fall into this class. Middle repetitive DMiddle repetitive DNA are found from 10s - 1000 times in the geNA are found from 10s - 1000 times in the genome.nome. Examples of these would include rRNA Examples of these would include rRNA and tRNA genes and storage proteins in plants and tRNA genes and storage proteins in plants such as corn. Middle repetitive DNA can vary frsuch as corn. Middle repetitive DNA can vary from 100-300 bp to 5000 bp and can be disperseom 100-300 bp to 5000 bp and can be dispersed throughout the genome. The most abundant sd throughout the genome. The most abundant sequences are found in the equences are found in the highly repetitive Dhighly repetitive DNANA class. class.

These sequences are found from 100,000 to These sequences are found from 100,000 to 1 million times in the genome and can range i1 million times in the genome and can range in size from a few to several hundred bases in n size from a few to several hundred bases in length. These sequences are found in regions length. These sequences are found in regions of the chromosome such as heterochromatin, of the chromosome such as heterochromatin, centromeres and telomeres and tend to be arrcentromeres and telomeres and tend to be arranged as a tandem repeats. The following is anged as a tandem repeats. The following is an example of a tandemly repeated sequence:an example of a tandemly repeated sequence:

ATTATAATTATA ATTATAATTATA ATTATAATTATA // // ATTATAATTATA

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

Sequence InterspersionSequence Interspersion Even though the genomes of higher organisms contain singEven though the genomes of higher organisms contain sing

le copy, middle repetitive and highly repetitive DNA sequenle copy, middle repetitive and highly repetitive DNA sequences, these sequences are not arranged similarly in all specices, these sequences are not arranged similarly in all species. The prominent arrangement is called es. The prominent arrangement is called short period intershort period interspersionspersion. This arrangement is characterized by repeated s. This arrangement is characterized by repeated sequences 100-200 bp in length interspersed among single cequences 100-200 bp in length interspersed among single copy sequences that are 1000-2000 bp in length. This arrangopy sequences that are 1000-2000 bp in length. This arrangement is found in animals, fungi and plants. ement is found in animals, fungi and plants.

The second type of arrangement is The second type of arrangement is long-period intersperslong-period interspersionion. This is characterized by 5000 bp stretches of repeated . This is characterized by 5000 bp stretches of repeated sequences interspersed within regions of 35,000 bp of singlsequences interspersed within regions of 35,000 bp of single copy DNA. e copy DNA. DrosophilaDrosophila is an example of a species with thi is an example of a species with this uncommon sequence arrangement. In both cases, the reps uncommon sequence arrangement. In both cases, the repeated sequences are usually from the middle repetitive claseated sequences are usually from the middle repetitive class. We discussed above where highly repetitive sequences as. We discussed above where highly repetitive sequences are found. re found.

Eukaryotic Chromosome KaryotypeEukaryotic Chromosome Karyotype Whereas bacteria only have a single chromosWhereas bacteria only have a single chromos

ome, eukaryotic species have at least one pair ome, eukaryotic species have at least one pair of chromosomes. Most have more than one paof chromosomes. Most have more than one pair. Another relevant point is that eukaryotic chrir. Another relevant point is that eukaryotic chromosomes are detected only occur during cell omosomes are detected only occur during cell division and not during all stages of the cell cydivision and not during all stages of the cell cycle. They are in their most condensed form durcle. They are in their most condensed form during metaphase when the sister chromatids are ing metaphase when the sister chromatids are attached. This is the primary stage when cytogattached. This is the primary stage when cytogenetic analysis is performed. enetic analysis is performed.

Each species is characterized by a Each species is characterized by a karyotypekaryotype. Th. The karyotype is a description of the number of chroe karyotype is a description of the number of chromosomes in the normal diploid cell, as well as theimosomes in the normal diploid cell, as well as their size distribution. For example, the human chromr size distribution. For example, the human chromosome has 23 pairs of chromosome, 22 somatic posome has 23 pairs of chromosome, 22 somatic pairs and one pair of sex chromosomes. One importairs and one pair of sex chromosomes. One important aspect of genetic research is correlating changant aspect of genetic research is correlating changes in the karyotype with changes in the phenotype es in the karyotype with changes in the phenotype of the individual. of the individual.

One important aspect of genetics is correlating chOne important aspect of genetics is correlating changes in karyotype with changes in phenotype. Foanges in karyotype with changes in phenotype. For example, humans that have an extra chromosomr example, humans that have an extra chromosome 21 have Down's syndrome. Insertions, deletions e 21 have Down's syndrome. Insertions, deletions and changes in chromosome number can be deteand changes in chromosome number can be detected by the skilled cytogeneticist, but correlating thcted by the skilled cytogeneticist, but correlating these with specific phenotypes is difficult. ese with specific phenotypes is difficult.

The first discriminating parameter when develThe first discriminating parameter when developing a karyotype is the size and number of thoping a karyotype is the size and number of the chromosomes. Although this is useful, it doee chromosomes. Although this is useful, it does not provide enough detail to be begin the des not provide enough detail to be begin the development of a correlation between structure velopment of a correlation between structure and function (phenotype). To further distinguisand function (phenotype). To further distinguish among chromosomes, they are treated with h among chromosomes, they are treated with a dye that stains the DNA in a reproducible ma dye that stains the DNA in a reproducible manner. After staining, some of the regions are lanner. After staining, some of the regions are lightly stained and others are heavily stained. ightly stained and others are heavily stained. As described above, the lightly stained regionAs described above, the lightly stained regions are called s are called euchromatineuchromatin, and the dark staine, and the dark stained region is called d region is called heterochromatinheterochromatin. The curre. The current dye of chose is the nt dye of chose is the Giemsa stainGiemsa stain, and the r, and the resulting pattern is called the esulting pattern is called the G-banding patteG-banding patternrn. .

C-Value ParadoxC-Value Paradox In addition to describing the genome of an organism by its In addition to describing the genome of an organism by its

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

A dramatic example of the range of C values A dramatic example of the range of C values can be seen in the plant kingdom where can be seen in the plant kingdom where ArabArabidopsis idopsis represents the low end and lily (1.0 x represents the low end and lily (1.0 x 10^8 kb/haploid genome) the high end of co10^8 kb/haploid genome) the high end of complexity. In weight terms this is 0.07 picogramplexity. In weight terms this is 0.07 picograms per haploid ms per haploid Arabidopsis Arabidopsis genome and 100 genome and 100 picograms per haploid lily genome. picograms per haploid lily genome.

GenomeGenome - the complete set of chromosomes - the complete set of chromosomes inherited from a single parent; the complete inherited from a single parent; the complete DNA component of an individual; the definitioDNA component of an individual; the definition often excludes organelles n often excludes organelles

Species Sequence Distribution Bacteria 99.7% Single Copy Mouse 60% Single Copy

25% Middle Repetitive10% Highly Repetitive

Human 70% Single Copy13% Middle Repetitive8% Highly Repetitive

Cotton 61% Single Copy27% Middle Repetitive

8% Highly Repetitive Corn 30% Single Copy

40% Middle Repetitive20% Highly Repetitive

Wheat 10% Single Copy83% Middle Repetitive4% Highly Repetitive

Arabidopsis 55% Single Copy27% Middle Repetitive

10% Highly Repetitive

The following table gives the sequence distribution for selected species.

C Values of Organisms Used in Genetic Studies

Species Kilobases/haploid genome

E. coli 4.5 x 103

Human 3.0 x 106

Drosophila 1.7 x 105

Maize 2.0 x 106

Aribidopsis 7.0 x 104