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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

describe the process of platelet formation
Joy Reply
what is parasitic helminths
Sadiya Reply
list three categories of symbiotic relationships.
mary Reply
what's the difference between microbial intoxication and infectious diseases
dranimva Reply
microbial intoxication results when a person ingests a toxin or a poisonous substance that has been produced by a microbe while infectious disease results when a pathogen colonize the body and subsequently cause disease.
mary
weighing balance is needed in lab of microbiology for weighing?
amna Reply
It is mainly used for media preparation and product testing purpose.
Nakaweesi
thank you for your joining
nuur
what is microbiology
Shamsuddeen Reply
study of living organisms that are too small to be visible with naked eye
Marah
yes
Ahmed
microbiology is the study of microscopic organisms, either unicellular(single cell), multicellular( cell colony), or acellular (lacking cell).
munachimso
is the word Atypical or a typical bacteria. .am confused pliz help
MUWANGUZI Reply
typical bacteria
Zulpha
okay thank you what does that mean
MUWANGUZI
atypical means that it has some characters from bacteria not all characters ...but tybical means that it has all the characters that bacteria have
Reham
thank you Zulpha
MUWANGUZI
some examples please
MUWANGUZI
thank you Reham
MUWANGUZI
1) typical bacteria contain a cell wall whereas atypical bacteria usually do not contain a cell . 2) typical bacteria can be either Gram-positive or Gram-negative while atypical bacteria remain colorless with Gram staining. 3) cells of typical bacteria are large ,while cells of the atypical small
Marah
Example of atypical : Mycoplasma pneumoniae , chlamydophila pneumoniae , legionella
Marah
what is micro biology
Jauharah Reply
is the study of organisms which can't be viewed by our necked eyes
Egumat
Because it preexisting causing secondary infection after collateral damage of normal microbota
Rafaa Reply
combinations of drugs that can't be taken together and why
Grace Reply
Antagonism: the combined action is less than that of the more effective agent when used alone). All these effects may be observed in vitro (particularly in terms of bactericidal rate) and in vivo والله اعلم
Lenovo
....fermentros have 1-15litre capacity
AMAR Reply
which of the following microorganisms are classified as eukaryotic?
Semugab Reply
how do I see the list?
Melissa
what are the choices?
Melissa
dear tell us the choices
MUWANGUZI
mr semugab give us the list please
Nambi
Fungi
munachimso
what are the new discoveries of microorganisms
Ezibon Reply
bacteriology,viriology,micrology
Egumat
understanding of contributed to attempts to treat contain disease
james Reply
what is a bacteria
ROSE Reply
yes tell us
Judith
a bacteria is the largest living organisms on the planet Earth
Egumat
oh, sure. we have also one big here nearby!
Maxim
Bacteria is a large group of single-cell microorganisms. some causes infections and disease in animals and humans..
munachimso
do you mean procariots?
Maxim

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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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