introduction: what is dna? dna (deoxyribonucleic acid)

8/14/2019 Introduction: What is DNA? DNA (Deoxyribonucleic Acid)

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DNA

Introduction: What is DNA?DNA (Deoxyribonucleic acid)

• A complex molecule containing all necessary information to build and maintainan organism.

• All living things have DNA within their cells

• It serves as the primary unit of heredity in organisms, used in reproduction of cells. A portion of the DNA is passed along to the next generation allowing for slight changes.

1.1 DNA Is a structure that encodes information – DNA or deoxyribonucleic acid contains the molecular instructions for life.

– found in nearly all living cells

– exact location within a cell depends on whether that cell possesses a specialmembrane-bound organelle called a nucleus

Organisms composed of cells that contain nuclei are classified aseukaryotes

Organisms composed of cells that lack nuclei are classified asprokaryotes.

– In eukaryotes, DNA is housed within the nucleus – In prokaryotes, DNA is located directly within the cellular cytoplasm, as there is

no nucleus available.

What is DNA exactly?

– DNA is a complex molecule that consists of many components, a portion of which are passed from parent organisms to their offspring during the process of reproduction.

– DNA is composed of the same nitrogen-based molecules

– DNA differs from organism to organism as it is simply the order in which these smaller molecules are arranged that differs among individuals.

– this pattern of arrangement ultimately determines each organism's uniquecharacteristics by another set of molecules that "read" the pattern and stimulatethe chemical and physical processes it calls for.

What components make up DNA?



– All DNA is composed of a series of smaller molecules called nucleotides.

– A nucleotide is made up of three primary components:

1. A nitrogen-containing region known as a nitrogenousbase

2. A carbon-based sugar molecule called deoxyribose,3. A phosphorus-containing region known as a phosphate

group attached to the sugar molecule

There are four different DNA nucleotide

1. adenine “A”

2. thymine "T"

3. guanine "G"

4. cytosine "C"

Adenine, thymine

– NUCLEOTIDES owe much of their structure and bondingcapabilities to the deoxyribose

– The central portion of this molecule (deox y ribose) contains five carbon atoms arranged inthe shape of a ring, and each carbon in the ring is referred to by a number followed by the prime symbol (').

– 5' carbon atom is particularly notable, it is the site at which the phosphate group is attachedto the nucleotide.

– The area surrounding this carbon atom is known as the 5' end of the nucleotide.

– Opposite the 5' carbon, on the other side of the deoxyribose ring, is the 3' carbon, which is NOT attached to a phosphate group. This portion of the nucleotide is typically referred to as

the 3' end. – Polynucleotide is when nucleotides join together in a series to form a structure.

– At each point of juncture within a polynucleotide, the 5' end of one nucleotide attaches to the3' end of the adjacent nucleotide through a connection called a phosphodiester bond. It isthis alternating sugar-phosphate arrangement that forms the "backbone" of a DNA molecule.

How is the DNA strand organized?

– Although DNA is often found as a single-stranded polynucleotide, it assumes itsmost stable form when double stranded.

– Double-stranded DNA consists of two polynucleotides that are arranged such that thenitrogenous bases within one polynucleotide are attached to the nitrogenous baseswithin another polynucleotide by way of special chemical bonds called hydrogenbonds.

Figure 1: A single nucleotide contains anitrogenous base (red) and a deoxyribose sugar molecule (grey). One side of the sugar moleculeis refered to as 3' end (dark grey) and the other 5'end (dark grey). The 5' side has a phosphategroup attached.

Figure 3: All polynucleotides containan alternating sugar-phosphate

backbone. This backbone is formedwhen the 3' end (dark gray) of onenucleotide attaches to the 5'

phosphate end (light gray) of anadjacent nucleotide by way of a

phosphodiester bond.



– Each A in one strand always pairs with a T in the other strand, and each C always pairs with a G.

– The double-stranded DNA that results from this pattern of bonding looks much like aladder with sugar-phosphate side supports and base-pair rungs.

**Note that because the two polynucleotides that make up double-stranded DNA are "upside

down" relative to each other, their sugar-phosphate ends are anti-parallel, or arranged in oppositeorientations. This means that one strand's sugar-phosphate chain runs in the 5' to 3' direction,whereas the other's runs in the 3' to 5' direction (Figure 4). The specific sequence of A, T, C, and G nucleotides within an organism's DNA is unique to that individual , and it is this sequence thatcontrols not only the operations within a particular cell, but within the organism as a whole

– Other than the ladder-like structure described above, another keycharacteristic of double-stranded DNA is its unique three-dimensional shape.

– In 1952, scientist Rosalind Franklin used a process called X-ray diffraction tocapture images of DNA molecules (Figure 5). Although the black lines in these photos look relatively sparse, Dr. Franklin interpreted them as representing distancesbetween specific molecules that were arranged in a spiral shape called a helix.

– Researchers James Watson and Francis Crick were pursuing adefinitive model for the stable structure of DNA inside cell nuclei.They used Franklin's images, along with their own evidence for thedouble-stranded nature of DNA, to argue that DNA actually takesthe form of a double helix , a ladder-like structure that is twisted along its entire length (Figure 6).

How is DNA packaged inside cells?

-DNA packaging, which is the phenomenon of fitting DNA into dense compact forms. Long pieces of double- stranded DNA are tightly looped, coiled, and folded sothat they fit easily within the cell. - Eukaryotes do this by wrapping their DNA aroundspecial proteins called histones, thereby compacting itenough to fit inside the nucleus.

Figure 4: Double-stranded DNA consists of two polynucleotide chains whosenitrogenous bases are connected byhydrogen bonds. Within this arrangement,each strand mirrors the other as a result of the anti-parallel orientation of the sugar-

phosphate backbones, as well as the

complementary nature of the A-T and C-G base pairing.

Figure 7: To better fit within the cell,long pieces of double-stranded DNA aretightly packed into structures calledchromosomes.



- Prokaryotes, compress their DNA through a twisting process called supercoiling

-In both eukaryotes and prokaryotes, this highly compacted DNA is then arranged intostructures called chromosomes.-Chromosomes take different shapes in different types of organisms.-Chromosomes exist in pairs, which means that there are two copies of each chromosomein most cells that compose these organisms' bodies. (Humans, for instance, have 23 pairsof chromosomes, for a total of 46 individual chromosomes.)

How do scientists visualize DNA?

-It is impossible to see double-stranded DNA with the naked eye - unless, that is, theyhave a large amount of it.-Scientists extract DNA from tissue samples, thereby pooling together miniscule amountsof DNA from thousands of individual cells. When this DNA is collected and purified, theresult is a whitish, sticky substance that is somewhat translucent.-To actually visualize the double-helical structure of DNA, researchers require specialimaging technology.-It is possible to see chromosomes with a standard light microscope, as long as thechromosomes are in their most condensed form.

– To see chromosomes in this way, scientists must first use a chemical process that attaches the

chromosomes to a glass slide and stains or "paints" them. Staining makes the chromosomes easier to see under the microscope. In addition, the banding patterns that appear on individual

chromosomes as a result of the staining process are unique to each pair of chromosomes, so theyallow researchers to distinguish different chromosomes from one another. Then, after a scientisthas visualized all of the chromosomes within a cell and captured images of them, he or she canarrange these images to make a composite picture called a karyotype

1.2 The Discovery of DNA Function Involved Multiple Scientists

Who first identified DNA?

– James Watson and Frances Crick are often credited with discovering DNA. – However this substance was identified nearly 90 years earlier by Swiss chemist

Friedrich Miescher. While studying white blood cells, Miescher isolated a previously unknown type of molecule that was slightly acidic and contained ahigh percentage of phosphorus. Miescher named this molecule "nuclein," whichwas later changed to "nucleic acid" and eventually to "deoxyribonucleic acid.”Interestingly, Miescher did not believe that nuclein, Miescher believed that

– Figure 8: Double-stranded DNA (grey)is wrapped aroundhistone proteins (red),and that structure isitself coiled.



proteins were responsible for heredity, because they existed in such a wide varietyof forms.

Who linked DNA to heredity?

– Most scientists believed that protein, not DNA, was the carrier of hereditaryinformation.

– This changed in 1944, when biologist Oswald Avery performed a series of groundbreaking experiments with the bacteria that cause pneumonia. At the time,scientists knew that some types of these bacteria (called "S type") had an outer layer called a capsule, but other types (called "R type") did not. Through a seriesof experiments, Avery and his colleagues found that only DNA could change R type bacteria into S type.

This meant that something about DNA allowed it to carryinstructions from one cell to another. This result highlighted DNAas the "transforming factor ," thereby making it the best candidatefor the hereditary material

Who confirmed Avery's findings?

– Avery's findings were largely unaccepted as evidence for DNA as the hereditarymaterial until separate experiments were performed by other scientists.

– Thus, eight years later, Alfred Hershey and Martha Chase further confirmed that protein was not the hereditary material through their work with bacteriophages,which are viruses that infect bacteria.

– Bacteriophages are composed of only two substances: protein and DNA. By using radioactive labelsthat would integrate specifically to either DNA or protein, but not both, Hershey and Chase were ableto show that DNA is the only material transferred directly from bacteriophages into bacteria when the

bacteria are infected by these viruses. This observation was important, because Hershey and Chaseknew that the end result of bacteriophage infection was the production of more viruses in multiple

copies

– DNA in a virus can take over a bacterial cell, causing it to replicate only the viralDNA and to create new viruses. This process is a form of hijacking, wherein theviral life-form takes over the regular machinery inside another life-form (in this case,a single bacterial cell).

– Hershey and Chase had presented experiments that clearly suggested that DNA

controls the production of more DNA, and that DNA itself was that substance thatcontrolled life-forms.

Only one year after Hershey and Chase performed these experiments, the structure of DNA was determined by James Watson and Francis Crick. This allowed investigators to put together the pieces of the story about how DNA carries hereditary information fromcell to cell. Indeed, the experimental work connecting heredity and the structure of DNAwere happening in parallel, so the next few years would be an exciting time for thediscovery of DNA function.

introduction: what is dna? dna (deoxyribonucleic acid)

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