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72 Cards in this Set
- Front
- Back
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who published the double helix model? and when?
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1953, James Watson and Francis Crick published the double helix model of DNA.
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Nucleic acids are unique among molecules in their ability to . .
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direct their own replication
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The resemblance of offpsring to their parents depends on
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the precise replication of DNA and it's tranmission from one generation to the next
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T.H Morgan
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showed that genes are located on chromomes and chromsomes contain proteins and DNA, thus proteins and DNA were the candidates for the genetic matieral.
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Chromosomes contain
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proteins and DNA
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who showed that genes are located on chromsomes and that proteins and DNA were the candidates for the gentic material
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T.H. Morgan
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James Watson
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in 1953 he published the double helix model for DNA
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Until 1940s and T.H. Morgan people thought ______ were the genetic material instead of DNA
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protiens, however this was not consistent with experiemnts with microorganisms like bacteria and viruses.
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Frederick Griffin
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1929, examined a bacterium that causes pneumonia in mammals. He discovered transformation, the change in genotype and phenotype due to the assimilation of foreign substance (DNA) by a cell. But he did not know that the substance was DNA
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Break down Frederick Griffin's experiment process
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He examined a bacterium that causes pneumonia in mammals.
One strain (R strain) was harmless but the other was pathogenic (S strain). Griffin mixed heat-killed S strain with live R strain bacteria and injected this into a mouse. The mouse died and Griffin recovered the pathogenic S strain from the mouse's blood. He called this phenomenon, transformation (change in genotype and phenotype due to the assimilation of a foreign substance by a cell (DNA) |
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Who later discovered what the foreign substance was in Frederick Griffin's experiment?
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Oswald Avery and Maclyn McCarty and Colin MacLeod
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Alfred Hershey and Martha Chase
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showed that DNA was the genetic material of the phage T2.
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Further evidence that DNA was the genetic material was derived from
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studies that tracked infection of bacteria by viruses.
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Explain what a virus consists of and how it replicates itself
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DNA enclosed by a protective coat of protein. To replicate itself, a virus must infect a host cell and take over the cell's metabolic machinery.
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Viruses that specifically attack bacteria are called
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bacteriophages or phages
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T2 phage
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a virus that attacks bacteria, consists almost entirely of DNA and protein, attacks E. coli ( a common intestinal bacteria of mammals) This phage can quickly turn an E.coli cell into a T2 producing factory that releases phages when the cell ruptures.
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Explain Alfred Hershey and Martha Chase's experiment
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they showed the DNA was the genetic mateiral of the phage T2.
To determine the source of genetic material in the phage they designed the experiment where they could label protein or DNA and then track which entered the E. coli cell during infection. They grew one batch of T2 phage in the presence of radioactive sulfur makring the proteins but not DNA They grew another batch in the presense of radioactive phophorus marking the DNA but not the proteins They allowed each batch to infect separate E. coli cultures shortly after the infection they placed the cultured infected cells in a blender, shaking loose any parts of the phage that remained outside the bacteria. The mixtures were spun in a centrifuge which separated the heavier bacterial cells int he pellet, from lighter free phages and part of phage in the liquid supernatant. They then tested the pellet and supernatant of teh separate treatments for the presence of radioactivity. They found that when the bacteria had been infected with T2 phages that contained radio-labeled sulfur proteins most of the radioactivity was in the supernatant not in th epellet. When they examined the bacterial cultures with T2 phage that had radio-labeled phosphorus DNA most of the radioactivity was in the pellet with the bacteria. They concluded that the injected DNA of the phage provides the genetic information that makes the infected cells produce new viral DNA and proteins which assemble into new viruses |
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Erwin Chargaff
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developed a series of rules based on a survey of DNA composition in organisms
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Explain Erwin Chargaff's experiment
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scientists knew that DNA was a polymer of nucleotides consisting of a nitrogenous base, deoxyribose and a phosphate group.
Chargaff noted that the DNA composition varies from species to species. In any one species the four bases are found in characteristic, but not necessarily equal ratios. He also found a peculiar regulatriy in the ratios of nucleotide bases whcih are known as Chargaff's rules |
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Chargaff's rules
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the number of adenines was approximately equal to the number of thymines (A=T) the number of guanines was approximately equal to the number of cytosines (G=C)
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was is the percent of all the bases in human DNA?
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A=30.9%
T=29.4% G=19.9% C=19.8% |
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Watson and Crick
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discovered the double helix by huilding models to conform to X-ray data.
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Explain Watson and Crick's experiment
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they built models from x-ray data.
Using wire and paper models, first tried to place the sugar-phosphate chains on the inside but this did not fit the xray measurements. Watson put the sugar-phosphate chain on the outside and the nitrogen bases on the inside of the doubel helix. Pairing like nucleotides did not fit the uniform diamter indicated byt he x-ray data. A purine purine pair would be too wide (ex. adenine and guanine, purines) A pyrimidine pyrimidine pairing would be too short (ex. thiymine and cytosine, pyrimidines only a pyrimidine puring pairing would produce the 2 nm diameter indicated by the x-ray. Watson and Crick also determined that chemical side groups off the nitrogen bases would form hydrogen bonds, connecting the two strands |
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Adenine would form ____ hydrogen bonds only with _____
Guanine would form ____ hydrogen bonds only with _____ This explains who's ideas? |
two hydrogen bonds only with thymine
3 hydrogen bonds only with cytosine Chargaff's rules |
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Linus Pauling and Maruice Wilkins and Rosalind Franklin
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were also trying to determine structure of DNA
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Explain Linus Pauling and Maurice Wilkins and Rosalind Franklin's experiment
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Wilkins and Franklin used x-ray crystallography to study the structure of DNA.
X-rays are diffracted as they passed through aligned fibers of purifed DNA. The diffraction pattern can be used to deduce the 3D shape of molecules. Franklin's data suggested a helix Pauling had shown the role of hydrogen bonding but she hypothesized a triple helix which was incorrect. |
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who learned from Franklin's and Wilkin's research what did they learn?
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Watson, learned that DNA was helicial in shape and it was a double helix
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DNA replication
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base pairing enables existing DNA strands to serve as template sfor new complimentary strands. Because each strand is complementary to each other, each forms a template strand when separated.
The order of bases on one strand can be used to add in complementary bases and therefore duplicate the pairs of bases exactly. One at a time, nucleotides line up along the template strand according to the base-pairing rules. They are then linked to form new strands |
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semiconservative replication model
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watson and crick's hypothesis, predicts that when a double helix replicates each of the daughter molecules will have one old strand and one newly made strand
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Matthew Meselson and Franklin Stahl
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conducted experiments that supported the semiconservative hypothesis.
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Explain Matthew Meselson and Franklin Stahl's experiments
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they labeled the nucleotides of the old strands with a heavy isotope of nitrogen while any new nucleotides would be indicated by a lighter isotope of nitrogen
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What are the 3 predictions on the density of replicated DNA strands?
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conservative, semiconservative and dispersive
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conservative:
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after one replication of the DNA, there would be one heavy and one light DNA molecule. After two replications there would be three light molecules and one heavy molecule.
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semiconservative
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after one replication there would be two intermediate molecules. After two replications, there would be two intermediate molecules and two light molecules
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dispersive
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after one replication there would be two intermediate molecules after two replications there would be four intermediate molecules
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replication of DNA in bacteria
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replication of DNA molecules begin at special sites called origins of replication. This origin of replication is a single specific sequence of nucleotides that is recognized by the replication enxymes
These enxymes separate the strands, forming a replication bubble replication proceeds in both directions until the entire molecule is copied. |
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replication of DNA in eukaryotes
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there maybe be hundreds or thousands of origin sites per chromosome.
At the origin sight the DNA strands separate forming a replication bubble with replication forks at each end. the replication bubbles elongate as the DNA is replicated and eventually fuse. DNA polymerase catalyzse the elongation of new DNa at a replication fork. As nucleotides align with complementary bases along the template strand, they are added to the growing end of the new strand by the polymerase. The rate of elongation is about 500 nucleotides pr second in bacteria and 50 per second in human cells. The raw nucleotides are nucleotide triphosphates (ATP, CTP, GTP, TTP) Each has a nitrogen base, deoxyribose and a triphosphate tail as each nucleotide is added the last two phosphate groups are hydrolyzed to form pyrophosphate. The exergonic hydrolysis of pyrophosphate to two inorganic phosphate molecules drives the polymerization of the nucleotide to the new strand. |
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strands in the double helix are
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antiparallel meaning, the sugar phosphate backbones runin opposite directions
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explain the how the DNA strands in a double helix are antiparallel
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Each DNA strand has a 3' end with a free hydroxyl group attached to the deoxyribose and a 5' end with a free phosphate group attached to the deoxyribose so it looks like this 3'------->5' and the other strand runs counter that 5'-------->3'
DNA polymerases can only add nucelotides to the free 3' end of a growing DNA strand A new DNA strand can only elongate in the 5'---->3' direction. |
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A new DNA strand can only elongate in what direction?
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5----3
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What problem is created by the antiparallel double helix strands and how is taken care of?
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the problem is at the replication fork, one parental strand is oriented 3---5 into the fork while the other strand is oriented 5----3 into the fork
3---5 is the leading strand and can be used by polymerases as a template for a continuous complimentary strand. The other parental strand 5----3 is the lagging strand and is copied away from the fork in short segments called Okazaki fragments. |
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Okazaki fragments
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the short segments that are copied away from the lagging strand. Each are about 100 to 200 nucleotides in length and are joined by DNA ligase to form the sugar phosphate backbone of a single DNA strand
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To start a new DNA strand. . .
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requires a short segment of RNA called a primer.
The RNA primer is about 10 ribonucleotides long Primase, an RNA polymerase, links ribonucleotides that are complementary to the DNA template into the primer. After the formation of the primer, DNA polymerases then can add deoxyribonucleotides to the 3' end of the ribonucleotide chain. After the primer is formed, DNA polymerase can add new nucleotides away from the fork until it runs into th eprevious Okazaki fragment. The leading strand requires the formation of only a single primer as the replication fork continues to separate. The lagging strand requires formation of new primers as the replication fork progresses. Another DNA polymerase later replaces the primer RNA nucleotides with DNA nucleotides complimentary to the template. After RNA primers are converted to DNA, the enzyme DNA ligase joins the DNA fragments togehter |
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primer
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a short segment of RNA that is sometimes used to start a new DNA strand
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primase
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RNA polymerase that links ribonucleotides that are complementary to the DNA template into the primer.
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simplified version of starting a new DNA strand is
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at the replication fork, the leading stand is copied continuousl yinto the fork from a single RNA primer. The lagging strand is copied away from the fork in short segemnts, with each segment requiring a new RNa primer.
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helicase
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untwists and separates the template DNA strands at the replication fork
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single-strand binding proteins
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keep the unpaired template strands apart during replication
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Topoisomerase
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relieves the stress caused by the untwisting of the double helix that causes physical stress for the DNA molecule ahead of the replication fork. It relives this pressure by breaking, sweiveling and rejoining the DNA ahead of the replication fork.
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How is damage repaired in existing DNA?
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by enzymes that proofread DNA during it's replication. Mistakes during the initial pairing of template nucleotides and complementary nucleotides occurs at a rate of about one error per 10,000 base pairs. DNA polymerase proofreads each new nucleotide against the template nucleotide as soon as the nucleotide is added. If there is incorrect pairing the enzyme removes the wrong nucleotide, replaces it with the correct one and resumes synthesis.
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what can change nucleotides in ways that can affect the encoded genetic information?
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reactive chemicals, radioactive emissions, x-rays and ultraviolet light
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mismatch repair
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special repair enzymes fix incorrectly paired nucleotides
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speical repair enzymes fix incorrectly paired nucleotides
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mismatch repair
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nucleotide excision repair
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a nuclease cuts out a segment of a damaged strand. The gap is then filled in by DNA polymerase and ligase
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a cuclease cuts out a segemnt of a damaged strand. the gap is then filled in by DNA polymerase and ligase.
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nucleotide excision repari
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what disorder reflects the importance of proper fuction of repair enzymes?
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xeroderma pigmentosum: makes individuals hypersensitive to sunlight.
Ultroviolet light can produce thymine dimers between adjacent thymine nucleotides on a DNA strand. These dimers buckles the DNA double helix and interferes with DNA replication. For individuals with this disorder mutations in their skin cells are left uncorrected. |
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telomeres
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ends of chromosomes that are repetitive sequences that help postpone the loss of genes by the loss of the ends.
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ends of chromsomes
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telomeres
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The ends of eukaryotic chromsomes get shorter with each round of replications how does this happen?
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as the last 5' end is removed, there is not free 3' end available for DNa polymerase. Over time, the chromsomes get shorter.
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telomerase
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an enzyme that helps to elongate the telomers back to their original lengths in germ cells.
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Normal shortening of telomeres may be an adaptation to help. .
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prevent cancers by limiting the number of divisions that somatic cells can undergo.
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There are two major difference in gene expression between eukaryotes and prokaryotes
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The typical multicellular eukaryotic genome is much larger than that of a bacterium
Cell sepcialization limits the expression of many genes in specific cells |
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What happens with the extra amount of DNA that does not code for the synthesis of RNA or protein?
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it is elaborately organized. Not only is DNA associated with protein to form chromatin, but the chromatin is organized in higher organizational levels
the level of packing is one way that gene expression is regulated |
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What is one way that gene expression is regulated?
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level of packing
Densley packed areas are inactivated, thus little or no transcirption occurs there Genes in loosely packed arease can be actively transcribed |
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What type of structure is based on successive levels of DNA packing?
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chromatin
During interphase of the cell cycle, chromatin fibers are usually highly extended within the nucleus. However, during mitosis the chromatin coils and condenses to form short, thickchromsomes. |
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Eukaryotic DNA is precisely combined with large amounts of
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protein
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Histone
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proteins that are responsbile for the first level of DNA packaging
Have a number of positively charged amino acids that bind tightly to negative charged DNA. |
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nucleosome
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unfolded chromatin that has an appearance of a beaded string in which DNA winds around a core of histone proteins
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Steps in Chromatin Packing in a Eukaryotic chromosome are
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DNA double helix
histones nucleosomes 30 nm fiber looped domains Metaphase chromsome |
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30 nm fibers
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As chromsomes enter mitosis the DNA undergoes higher-order packing
The beaded string regions of DNA coils to for the 30 nm choromatin fiber. This chromatin fiber in turn forms looped domains attached to a scaffold of nonhistone proteins ina mitotic chromosome, the looped domains coil and fold to produce the characteristic metaphase chromsome. |
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heterochromatin
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interphase chromsomes that have areas that remain highly condensed
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euchromatin
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interphase chromsomes that have areas that remain highly compacted
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