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A photograph of a wire model in a museum.
In 1953, James Watson and Francis Crick built this model of the structure of DNA, shown here on display at the Science Museum in London.
  • Which scientists are given most of the credit for describing the molecular structure of DNA?

Dna structure

Watson and Crick proposed that DNA is made up of two strands that are twisted around each other to form a right-handed helix. The two DNA strands are antiparallel , such that the 3ʹ end of one strand faces the 5ʹ end of the other ( [link] ). The 3ʹ end of each strand has a free hydroxyl group, while the 5ʹ end of each strand has a free phosphate group. The sugar and phosphate of the polymerized nucleotides form the backbone of the structure, whereas the nitrogenous bases are stacked inside. These nitrogenous bases on the interior of the molecule interact with each other, base pairing.

Analysis of the diffraction patterns of DNA has determined that there are approximately 10 bases per turn in DNA. The asymmetrical spacing of the sugar-phosphate backbones generates major grooves (where the backbone is far apart) and minor grooves (where the backbone is close together) ( [link] ). These grooves are locations where proteins can bind to DNA. The binding of these proteins can alter the structure of DNA, regulate replication , or regulate transcription of DNA into RNA.

a) A diagram of DNA shown as a double helix (a twisted ladder). The outside of the ladder is a blue ribbon labeled “sugar phosphate backbone”. The rungs of the ladder are labeled “base pair” and are either red and yellow or green and blue. Red indicates the nitrogenous base adenine. Yellow indicates the nitrogenous base thymine. Blue indicates the nitrogenous base guanine. Green indicates the nitrogenous base cytosine. The ladder twists so that there are wide spaces (called major grooves) and narrow spaces (called minor grooves) between the twists. B) A different diagram of DNA showing it as a straight ladder. This makes it easier to see the bases (which can now be labeled with the letters A, T, C or G directly on the image. The left strand has a 3-prime at the top and a 5-prime at the bottom. The right strand has a 5-prime at the top and a 3-prime at the bottom. C) Another diagram of DNA showing a much shorter segment which allows the chemical structures to be seen more clearly. The strands show that the phosphate group is always between carbon 3 of one nucleotide and carbon 5 of the next. The two strands are connected with dotted lines indicating hydrogen bonds. The A-T bond has 2 hydrogen bonds and C-G has 3 hydrogen bonds. The negative charge of the phosphates is also apparent.
Watson and Crick proposed the double helix model for DNA. (a) The sugar-phosphate backbones are on the outside of the double helix and purines and pyrimidines form the “rungs” of the DNA helix ladder. (b) The two DNA strands are antiparallel to each other. (c) The direction of each strand is identified by numbering the carbons (1 through 5) in each sugar molecule. The 5ʹ end is the one where carbon #5 is not bound to another nucleotide; the 3ʹ end is the one where carbon #3 is not bound to another nucleotide.

Base pairing takes place between a purine and pyrimidine. In DNA, adenine (A) and thymine (T) are complementary base pairs , and cytosine (C) and guanine (G) are also complementary base pairs, explaining Chargaff’s rules ( [link] ). The base pairs are stabilized by hydrogen bonds; adenine and thymine form two hydrogen bonds between them, whereas cytosine and guanine form three hydrogen bonds between them.

A diagram of DNA showing a short segment which allows the chemical structures to be seen more clearly. The strands show that the phosphate group is always between carbon 3 of one nucleotide and carbon 5 of the next. The two strands are connected with dotted lines indicating hydrogen bonds. The A-T bond has 2 hydrogen bonds and C-G has 3 hydrogen bonds. The negative charge of the phosphates is also apparent.
Hydrogen bonds form between complementary nitrogenous bases on the interior of DNA.

In the laboratory, exposing the two DNA strands of the double helix to high temperatures or to certain chemicals can break the hydrogen bonds between complementary bases, thus separating the strands into two separate single strands of DNA (single-stranded DNA [ ssDNA ]). This process is called DNA denaturation and is analogous to protein denaturation, as described in Proteins . The ssDNA strands can also be put back together as double-stranded DNA ( dsDNA ), through reannealing or renaturing by cooling or removing the chemical denaturants, allowing these hydrogen bonds to reform. The ability to artificially manipulate DNA in this way is the basis for several important techniques in biotechnology ( [link] ). Because of the additional hydrogen bonding between the C = G base pair, DNA with a high GC content is more difficult to denature than DNA with a lower GC content.

Questions & Answers

what is mutation
Cynthia Reply
alteration in genetic makeup
UDEME
Hi, I'm new here. I'm Bello Abdul Hakeem from Nigeria.
BELLO Reply
welcome on board
Kamaluddeen
Thanks brother I'm an undergraduate. I hope to study for MBBS.
BELLO
pls guys help me out
Linda
what is limitation of plate
Linda
limitation of plate load test . there are some that should be considered while performing load test which are given below, this test is usually performed on relatively similar plate ,usually 1 or 2 square foot area the reason is that the plate of greater are the economically not feasible
yemi
Mechanism of bacterial pathogenicity
rashida Reply
Toxicity
Ibrahim
write a short note on Algea
najaatu Reply
green minutes plants organisms that are produced in turf.
Prince
Algae is a kind of a photosynthetic organism, which is usually grown in the moist areas. These are usually the simple plants that grow near to the water bodies. It contains a kind of chlorophyll pigments that act as a primary coloring agent.
Avi
they are eukaryotic and most lived in fresh water. they are photosynthetic that's, they contain chlorophyll and store starch in discrete chloroplast.
Prince
are bacteria important to man
just Reply
yes, depending on the type of bacteria .eg the normal florals, the latic acid batteries etc are important to man
Lois
I mean latic acid bacterial
Lois
I don't understand this topic
Jane Reply
what topic is that
Rose
sahi kon hai bhosri ke
saurabh Reply
define metabolism of carbohydrates with example
Thavasi Reply
what is sterilization a
Sani
the process of keep equipment free from bacteria
shar
is it only bacteria?
Lois
no undesirable fungi and contamination also.
Khushbu
what is streak plate method
Offikwu
what is the biofilm
Dimingu
metabolism is the sum of all the biochemical reaction required for energy generation and use of that energy to synthesize cell materials from small molecules in environment.
Alex
boclenia
Pooja
what are granulocytes
Shawnitta Reply
e.coli
Sukhdeep
granulocytes are type of WBCs which contains granules in the cytoplasm
Owili
what is mutation
Cynthia
it is the interchange of genes from their normal sequence
Esther
is an heritable change of the base-pair sequence of genetic material
Elizabeth
suitable example for prokaryotes
Suvetha Reply
one of the possible early sources of energy was
Suvetha
uv radiation and lighting
Anisha
e coli is the example of prokaryotes
Sukhdeep
archaea too
Noel
which is the specific virus causing typhoid
Jeremiah Reply
it's caused by a virulent bacteria called Salmonella Typhi
Sarah
write the life cycle of HIV
Firomsa Reply
describe the internal and external structure of prokaryotic cell in terms of there appearance and functions
Lenia Reply
compare and contrast similar structures found in prokaryotic and eukaryotic cells
Lenia
control of microorganisms
ISTEFAZUL Reply
what is sterilization
Sani
how can a doctor treat a person affected by endospore forming bacteria in his/her wound?
Mambo Reply
define disinfectant
Sani
the process of killing
shar
the process of killing microorganisms
shar

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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