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'''Nucleic acid structure''' refers to the [[Biomolecular structure|structure]] of [[nucleic acid]]s such as [[DNA]] and [[RNA]] It is often divided into four different levels:
'''Nucleic acid structure''' refers to the [[Biomolecular structure|structure]] of [[nucleic acid]]s such as [[DNA]] and [[RNA]]. Chemically speaking, DNA and RNA are very similar. Nucleic acid structure is often divided into four different levels primary, secondary, tertiary and quaternary.


== Primary structure ==
* [[Nucleic acid primary structure|Primary structure]]—the raw sequence of [[nucleobase]]s of each of the component DNA strands;
{{Main|Nucleic acid sequence}}
* [[Nucleic acid secondary structure|Secondary structure]]—the set of interactions between bases, i.e., which parts of which strands are bound to each other;
[[File:New nucleotide.JPG|thumb|right|Nucleotide]]
* [[Nucleic acid tertiary structure|Tertiary structure]]—the locations of the atoms in three-dimensional space, taking into consideration geometrical and [[steric]] constraints; and

* [[Nucleic acid quaternary structure|Quaternary structure]]—the higher-level organization of DNA in chromatin, or to the interactions between separate RNA units in the [[ribosome]] or [[spliceosome]].
Primary structure consists of a linear sequence of nucleotides that are linked together by phosphodiester bonds. It is this linear sequence of nucleotides that make up the primary structure of [[DNA]] or [[RNA]]. Nucleotides consist of 3 things:
# Nitrogenous base
## [[Adenine]]
## [[Guanine]]
## [[Cytosine]]
## [[Thymine]](present in [[DNA]] only)
## [[Uracil]] (present in [[RNA]] only)
# 5-carbon sugar which is called deoxyribose (found in DNA) and ribose (found in RNA).
# One or more phosphate groups.<ref name=Krieger_2004>{{cite book | author = Krieger M, Scott MP, Matsudaira PT, Lodish HF, Darnell JE, Lawrence Z, Kaiser C, Berk A| title = Molecular cell biology | publisher = W.H. Freeman and CO | location = New York | year = 2004 | pages = | isbn = 0-7167-4366-3 | chapter = Section 4.1: Structure of Nucleic Acids | chapterurl = http://www.ncbi.nlm.nih.gov/books/NBK21514/ }}</ref>

The nitrogen bases are [[pyrimidine]] in structure and form a [[glycosidic bond]] between their 1' nitrogen and the 1' -OH group of the deoxyribose. The phosphate group forms a bond with the deoxyribose sugar through an ester bond between one of its negatively charged oxygen groups and the 5' -OH of the sugar.<ref name=Sparknotes>{{cite web | url = http://www.sparknotes.com/biology/molecular/structureofnucleicacids/ | title = Structure of Nucleic Acids | format = | work = SparkNotes }}</ref> The polarity in DNA and RNA is derived from the oxygen and nitrogen atoms in the backbone. Nucleic acids are formed when nucleotides come together through phosphodiester linkages between the 5' and 3' carbon atoms.<ref name=Anthony-Cahill_2012>{{cite book | author = Anthony-Cahill SJ; Mathews CK, van Holde KE, Appling DR | title = Biochemistry (4th Edition) | publisher = Prentice Hall | location = Englewood Cliffs, N.J | year = 2012 | pages = | isbn = 0-13-800464-1 }}</ref>
A [[Nucleic acid sequence]] is the order of nucleotides within a DNA (GACT) or RNA (GACU) molecule that is determined by a series of letters. Sequences are presented from the 5' to 3' end and determine the covalent structure of the entire molecule. Sequences can be [[complementary]] to another sequence in that the base on each position is complementary as well as in the reverse order. An example of a complementary sequence to AGCT is TCGA. DNA is double-stranded containing both a [[sense]] strand and an [[antisense]] strand. Therefore, the complementary sequence will be to the sense strand. <ref name=Alberts_20022>{{cite book | author = Alberts B, Johnson A, Lewis J, Raff M, Roberts K & Wlater P | title = Molecular Biology of the Cell (4th ed.)| | publisher = Garland Science | location = New York NY | year = 2002 | | isbn = 0-8153-3218-1 }}</ref>

[[Image:Mao-4armjunction-schematic.png|thumb|250px|Nucleic acid design can be used to create nucleic acid complexes with complicated [[Nucleic acid secondary structure|secondary structures]] such as this four-arm junction. These four strands associate into this structure because it maximizes the number of correct [[base pair]]s, with [[Adenine|A]]'s matched to [[Thymine|T]]'s and [[Cytosine|C]]'s matched to [[Guanine|G]]'s. Image from Mao, 2004.<ref name="Mao04">{{cite journal |last=Mao |first=Chengde |authorlink= |year=2004 |month=December |title=The Emergence of Complexity: Lessons from DNA |journal=[[PLoS Biology]] |volume=2 |issue=12 |pages=2036–2038 |issn=1544-9173|doi=10.1371/journal.pbio.0020431 |pmc=535573 |pmid=15597116}}</ref>]]

== Secondary structure ==
{{Main|Nucleic acid secondary structure}}
[[File:RNA secondary structure.png|thumb|right|RNA secondary structure]]

Secondary structure is the set of interactions between bases, i.e., parts of which is strands are bound to each other. In DNA double helix, the two strands of DNA are held together by [[hydrogen bond]]s. The [[nucleotide]]s on one strand [[base pair]]s with the nucleotide on the other strand. The secondary structure is responsible for the shape that the nucleic acid assumes. The bases in the DNA are classified as [[Purines]] and [[Pyrimidines]]. The purines are [[Adenine]] and [[Guanine]]. Purines consist of a double ring structure, a six membered and a five membered ring containing nitrogen. The pyrimidine are [[Cytosine]] and [[Thymine]]. It has a single ringed structure, a six membered ring containing nitrogen. A purine base always pairs with a pyrimidine base (Guanosine (G) pairs with Cytosine(C)and Adenine(A) pairs with Thymine (T) or [[Uracil]] (U). DNA's secondary structure is predominantly determined by [[base-pairing]] of the the two polynucleotide strands wrapped around each other to form a [[double helix]]. There is also a [[major groove]] and a [[minor groove]] on the double helix.

The secondary structure of RNA consists of a single polynucleotide. Base pairing in RNA occurs when RNA folds between complementarity regions. Both single- and double-stranded regions are often found in RNA molecules. The antiparallel strands form a helical shape.<ref name=Anthony-Cahill_2012/> The four basic elements in the secondary structure of RNA are helices, loops, bulges, and junctions. [[Stem-loop]] or hairpin loop is the most common element of RNA secondary structure.<ref>{{cite journal|last=Tinoco I|first=Jr|coauthors=Bustamante, C|title=How RNA folds.|journal=Journal of molecular biology|date=1999 Oct 22|volume=293|issue=2|pages=271-81|pmid=10550208}}</ref> [[Stem-loop]] is formed when the RNA chains fold back on themselves to form a double helical tract called the stem, the unpaired nucleotides forms single stranded region called the loop.<ref name="RNA structure">{{cite web|title=RNA structure (Molecular Biology)|url=http://what-when-how.com/molecular-biology/rna-structure-molecular-biology/}}</ref> Secondary structure of RNA can be predicted by experimental data on the secondary structure elements, helices, loops and bulges. Bulges and internal loops are formed by separation of the double helical tract on either one strand (bulge) or on both strands (internal loops) by unpaired nucleotides. A [[Tetraloop]] is a four-base pairs hairpin RNA structure. There are three common families of tetraloop in ribosomal RNA: UNCG, GNRA, and CUUG (N is one of the four nucleotides and R is a purine).UNCG is the most stable tetraloop.<ref>{{cite journal|last=Hollyfield|first=JG|coauthors=Besharse, JC; Rayborn, ME|title=The effect of light on the quantity of phagosomes in the pigment epithelium.|journal=Experimental eye research|date=1976 Dec|volume=23|issue=6|pages=623-35|pmid=1087245}}</ref> [[Pseudoknot]] is a RNA secondary structure first identified in turnip yellow mosaic virus.<ref>{{cite journal|last=Rietveld|first=K|coauthors=Van Poelgeest, R; Pleij, CW; Van Boom, JH; Bosch, L|title=The tRNA-like structure at the 3' terminus of turnip yellow mosaic virus RNA. Differences and similarities with canonical tRNA.|journal=Nucleic acids research|date=1982 Mar 25|volume=10|issue=6|pages=1929-46|pmid=7079175}}</ref> Pseudoknots are formed when nucleotides from the hairpin loop pairs with a single stranded region outside of the hairpin to form a helical segment. H-type fold pseudoknots are best characterized. In H-type fold, nucleotides in the hairpin loop pairs with the bases outside the hairpin stem forming second stem and loop. This causes formation of pseudoknots with two stems and two loops.<ref>{{cite journal|last=Staple|first=DW|coauthors=Butcher, SE|title=Pseudoknots: RNA structures with diverse functions.|journal=PLoS biology|date=2005 Jun|volume=3|issue=6|pages=e213|pmid=15941360}}</ref> Pseudoknots are functional elements in RNA structure having diverse function and found in most classes of RNA. DotKnot-PW method is used for comparative pseudoknots prediction .The main points in the DotKnot-PW method is scoring the similarities found in stems, secondary elements and H-type pseudoknots.<ref>{{cite journal|last=Sperschneider|first=J|coauthors=Datta, A; Wise, MJ|title=Predicting pseudoknotted structures across two RNA sequences.|journal=Bioinformatics (Oxford, England)|date=2012 Dec 1|volume=28|issue=23|pages=3058-65|pmid=23044552}}</ref>

== Tertiary structure ==
{{main|Nucleic acid tertiary structure}}
[[File:DNA structure and bases.svg|thumb|right|DNA structure and bases]]
[[File:A-B-Z-DNA Side View.png|thumb|right|A-B-Z-DNA Side View]]

Tertiary structure is the locations of the atoms in three-dimensional space, taking into consideration geometrical and [[steric]] constraints. A higher order than the secondary structure in which large-scale folding in a linear polymer occurs and the entire chain is folded into a specific 3-dimensional shape. There are 4 areas in which the structural forms of DNA can differ.
#Handedness - right or left
#Length of the helix turn
#Number of base pairs per turn
#Difference in size between the major and minor grooves<ref name=Anthony-Cahill_2012/>

The tertiary arrangement of DNA's [[double helix]] in space includes [[B-DNA]], [[A-DNA]] and [[Z-DNA]].

[[B-DNA]] is the most commons form of DNA in vivo and is more narrow, elongated helix than A-DNA. Its wide major groove makes it more accessible to proteins. On the other hand, it has a narrow minor groove. B-DNAs favored conformations occurs at high water concentrations and the hydration of the minor groove appears to favor B-DNA. B-DNA base pairs nearly perpendicular to helix axis. The sugar pucker which determines the shape of the a-helix, whether the helix will exist in the A-form or in the B-form occurs at the C2'-endo.<ref name=Dickerson_1982>{{cite journal | author = Dickerson RE, Drew HR, Conner BN, Wing RM, Fratini AV, Kopka ML | title = The anatomy of A-, B-, and Z-DNA | journal = Science | volume = 216 | issue = 4545 | pages = 475–85 | year = 1982 | month = April | pmid = 7071593 | doi = }}</ref>

[[A-DNA]] is shorter and wider than helix B. Most RNA and RNA-DNA duplex in this form. A-DNA has a deep, narrow major groove which does not make it easily accessible to proteins. On the other hand, its wide, shallow minor groove makes it accessible to proteins but with lower information content than the major groove. Its favored conformation is at low water concentrations. A-DNAs base pairs tilt to helix axis and are displaced from axis. The sugar pucker ocurrs at the C3'-endo and in RNA 2'-OH inhibits C2'-endo conformation.<ref name=Dickerson_1982/>

[[Z-DNA]] is a relatively rare left-handed double-helix. Given the proper sequence and superhelical tension, it can be formed in vivo but its function is unclear. It has a more narrow, more elongated helix than A or B. Z-DNA's major groove is not really groove and it is has a narrow minor groove. The most favored conformation ocurs when there are high salt concentrations. There are some base subsitutions but requires an alternating purine-pyrimidine sequence. The N2-amino of G H-bonds to 5' PO which explains the slow exchange of protons and the need for the G purine. Z-DNA base pairs nearly perpendicular to the helix axis. Z-DNA does not contain single base-pairs but rather a GpC repeat with P-P distances varying for GpC and CpG. On the GpC stack there is good base overlap whereas on the CpG stack there is less overlap. Z-DNA's zigzag backbone is due to the C sugar conformation compensating for G glycosidic bond conformation. The conformation of G is syn, C2'-endo
and for C it is anti, C3'-endo.<ref name=Dickerson_1982/>

Linear DNA molecule having free ends can rotate to adjust to changes of various dynamic processes in the cell by changing the number of times two chains of the double helix twist around each other. Some DNA molecules are circular and are topologically constrained. A covalently closed, circular DNA also known as cccDNA is topologically constrained as the number of times the chains coiled around one other cannot change. This cccDNA can be supercoiled which is the tertiary structure of DNA. Supercoiling is characterized by the linking number, twist and writhe. The Linking number Lk for circular DNA is defined as the number of times one strand would have to pass through the other strand to completely separate the two strands. The linking number for circular DNA can only be changed by breaking of a covalent bond in one of the two strands. Linking number is always an integer. The Linking number of a cccDNA is sum of two components twists (Tw) and writhes (Wr).<ref name=Mirkin_2001>{{cite journal | author = Mirkin SM | title = DNA Topology: Fundamentals | journal = Encyclopedia of Life Sciences | year = 2001 | doi = 10.1038/npg.els.0001038 }}</ref>
Lk = Tw +Wr
Twists are the number of times the two strands of DNA are twisted around each other. Writhes are number of times the DNA helix crosses over itself. DNA in cell is negatively supercoiled and has the tendency to unwind. Hence the separation of strand is easier in negatively supercoiled DNA than the relaxed DNA. The two component of supercoiled DNA, are [[solenoid]] and plectonemic. The plectonemic supercoil is found in prokaryotes and the solenoidal supercoiling is mostly seen in eukaryotes.

== Quaternary structure ==
{{main|Nucleic acid quaternary structure}}

[[File:DNA to Chromatin Formation.jpg|thumbnail|right|DNA to Chromatin ]]

The quaternary structure of nucleic acids is similar to that of [[protein quaternary structure]]. Although some of the concepts are not exactly the same, the quaternary structure refers to a higher-level of organization of nucleic acids. Moreover, it refers to interactions of the nucleic acids with other molecules. The most commonly seen form of higher-level organization of nucleic acids is seen in the form of [[chromatin]] which leads to its interactions with the small proteins [[histones]]. Also, the quaternary structure refers to the interactions between separate RNA units in the [[ribosome]] or [[spliceosome]].<ref>{{cite web|title=Strucual Biochemistry/Nucleic Acid/DNA/DNA Structure|url=http://en.wikibooks.org/wiki/Structural_Biochemistry/Nucleic_Acid/DNA/DNA_structure|accessdate=11 December 2012}}</ref>


==See also==
==See also==
Line 14: Line 67:
* [[Nucleic acid thermodynamics]]
* [[Nucleic acid thermodynamics]]
* [[Crosslinking of DNA]]
* [[Crosslinking of DNA]]
* [[Non-helical models of DNA structure]]
* [[DNA supercoil]]
* [[DNA supercoil]]

==References==
{{Reflist}}

{{refbegin}}

{{refend}}
== Further reading ==
{{No footnotes|date=November 2012}}



{{molecular-biology-stub}}
{{molecular-biology-stub}}

Revision as of 12:59, 15 December 2012

Nucleic acid structure refers to the structure of nucleic acids such as DNA and RNA. Chemically speaking, DNA and RNA are very similar. Nucleic acid structure is often divided into four different levels primary, secondary, tertiary and quaternary.

Primary structure

Main article: Nucleic acid sequence
File:New nucleotide.JPG
Nucleotide

Primary structure consists of a linear sequence of nucleotides that are linked together by phosphodiester bonds. It is this linear sequence of nucleotides that make up the primary structure of DNA or RNA. Nucleotides consist of 3 things:

  1. Nitrogenous base
    1. Adenine
    2. Guanine
    3. Cytosine
    4. Thymine(present in DNA only)
    5. Uracil (present in RNA only)
  2. 5-carbon sugar which is called deoxyribose (found in DNA) and ribose (found in RNA).
  3. One or more phosphate groups.[1]

The nitrogen bases are pyrimidine in structure and form a glycosidic bond between their 1' nitrogen and the 1' -OH group of the deoxyribose. The phosphate group forms a bond with the deoxyribose sugar through an ester bond between one of its negatively charged oxygen groups and the 5' -OH of the sugar.[2] The polarity in DNA and RNA is derived from the oxygen and nitrogen atoms in the backbone. Nucleic acids are formed when nucleotides come together through phosphodiester linkages between the 5' and 3' carbon atoms.[3] A Nucleic acid sequence is the order of nucleotides within a DNA (GACT) or RNA (GACU) molecule that is determined by a series of letters. Sequences are presented from the 5' to 3' end and determine the covalent structure of the entire molecule. Sequences can be complementary to another sequence in that the base on each position is complementary as well as in the reverse order. An example of a complementary sequence to AGCT is TCGA. DNA is double-stranded containing both a sense strand and an antisense strand. Therefore, the complementary sequence will be to the sense strand. [4]

Nucleic acid design can be used to create nucleic acid complexes with complicated secondary structures such as this four-arm junction. These four strands associate into this structure because it maximizes the number of correct base pairs, with A's matched to T's and C's matched to G's. Image from Mao, 2004.[5]

Secondary structure

Main article: Nucleic acid secondary structure
File:RNA secondary structure.png
RNA secondary structure

Secondary structure is the set of interactions between bases, i.e., parts of which is strands are bound to each other. In DNA double helix, the two strands of DNA are held together by hydrogen bonds. The nucleotides on one strand base pairs with the nucleotide on the other strand. The secondary structure is responsible for the shape that the nucleic acid assumes. The bases in the DNA are classified as Purines and Pyrimidines. The purines are Adenine and Guanine. Purines consist of a double ring structure, a six membered and a five membered ring containing nitrogen. The pyrimidine are Cytosine and Thymine. It has a single ringed structure, a six membered ring containing nitrogen. A purine base always pairs with a pyrimidine base (Guanosine (G) pairs with Cytosine(C)and Adenine(A) pairs with Thymine (T) or Uracil (U). DNA's secondary structure is predominantly determined by base-pairing of the the two polynucleotide strands wrapped around each other to form a double helix. There is also a major groove and a minor groove on the double helix.

The secondary structure of RNA consists of a single polynucleotide. Base pairing in RNA occurs when RNA folds between complementarity regions. Both single- and double-stranded regions are often found in RNA molecules. The antiparallel strands form a helical shape.[3] The four basic elements in the secondary structure of RNA are helices, loops, bulges, and junctions. Stem-loop or hairpin loop is the most common element of RNA secondary structure.[6] Stem-loop is formed when the RNA chains fold back on themselves to form a double helical tract called the stem, the unpaired nucleotides forms single stranded region called the loop.[7] Secondary structure of RNA can be predicted by experimental data on the secondary structure elements, helices, loops and bulges. Bulges and internal loops are formed by separation of the double helical tract on either one strand (bulge) or on both strands (internal loops) by unpaired nucleotides. A Tetraloop is a four-base pairs hairpin RNA structure. There are three common families of tetraloop in ribosomal RNA: UNCG, GNRA, and CUUG (N is one of the four nucleotides and R is a purine).UNCG is the most stable tetraloop.[8] Pseudoknot is a RNA secondary structure first identified in turnip yellow mosaic virus.[9] Pseudoknots are formed when nucleotides from the hairpin loop pairs with a single stranded region outside of the hairpin to form a helical segment. H-type fold pseudoknots are best characterized. In H-type fold, nucleotides in the hairpin loop pairs with the bases outside the hairpin stem forming second stem and loop. This causes formation of pseudoknots with two stems and two loops.[10] Pseudoknots are functional elements in RNA structure having diverse function and found in most classes of RNA. DotKnot-PW method is used for comparative pseudoknots prediction .The main points in the DotKnot-PW method is scoring the similarities found in stems, secondary elements and H-type pseudoknots.[11]

Tertiary structure

Main article: Nucleic acid tertiary structure
DNA structure and bases
A-B-Z-DNA Side View

Tertiary structure is the locations of the atoms in three-dimensional space, taking into consideration geometrical and steric constraints. A higher order than the secondary structure in which large-scale folding in a linear polymer occurs and the entire chain is folded into a specific 3-dimensional shape. There are 4 areas in which the structural forms of DNA can differ.

  1. Handedness - right or left
  2. Length of the helix turn
  3. Number of base pairs per turn
  4. Difference in size between the major and minor grooves[3]

The tertiary arrangement of DNA's double helix in space includes B-DNA, A-DNA and Z-DNA.

B-DNA is the most commons form of DNA in vivo and is more narrow, elongated helix than A-DNA. Its wide major groove makes it more accessible to proteins. On the other hand, it has a narrow minor groove. B-DNAs favored conformations occurs at high water concentrations and the hydration of the minor groove appears to favor B-DNA. B-DNA base pairs nearly perpendicular to helix axis. The sugar pucker which determines the shape of the a-helix, whether the helix will exist in the A-form or in the B-form occurs at the C2'-endo.[12]

A-DNA is shorter and wider than helix B. Most RNA and RNA-DNA duplex in this form. A-DNA has a deep, narrow major groove which does not make it easily accessible to proteins. On the other hand, its wide, shallow minor groove makes it accessible to proteins but with lower information content than the major groove. Its favored conformation is at low water concentrations. A-DNAs base pairs tilt to helix axis and are displaced from axis. The sugar pucker ocurrs at the C3'-endo and in RNA 2'-OH inhibits C2'-endo conformation.[12]

Z-DNA is a relatively rare left-handed double-helix. Given the proper sequence and superhelical tension, it can be formed in vivo but its function is unclear. It has a more narrow, more elongated helix than A or B. Z-DNA's major groove is not really groove and it is has a narrow minor groove. The most favored conformation ocurs when there are high salt concentrations. There are some base subsitutions but requires an alternating purine-pyrimidine sequence. The N2-amino of G H-bonds to 5' PO which explains the slow exchange of protons and the need for the G purine. Z-DNA base pairs nearly perpendicular to the helix axis. Z-DNA does not contain single base-pairs but rather a GpC repeat with P-P distances varying for GpC and CpG. On the GpC stack there is good base overlap whereas on the CpG stack there is less overlap. Z-DNA's zigzag backbone is due to the C sugar conformation compensating for G glycosidic bond conformation. The conformation of G is syn, C2'-endo and for C it is anti, C3'-endo.[12]

Linear DNA molecule having free ends can rotate to adjust to changes of various dynamic processes in the cell by changing the number of times two chains of the double helix twist around each other. Some DNA molecules are circular and are topologically constrained. A covalently closed, circular DNA also known as cccDNA is topologically constrained as the number of times the chains coiled around one other cannot change. This cccDNA can be supercoiled which is the tertiary structure of DNA. Supercoiling is characterized by the linking number, twist and writhe. The Linking number Lk for circular DNA is defined as the number of times one strand would have to pass through the other strand to completely separate the two strands. The linking number for circular DNA can only be changed by breaking of a covalent bond in one of the two strands. Linking number is always an integer. The Linking number of a cccDNA is sum of two components twists (Tw) and writhes (Wr).[13] 

                                               Lk = Tw +Wr

Twists are the number of times the two strands of DNA are twisted around each other. Writhes are number of times the DNA helix crosses over itself. DNA in cell is negatively supercoiled and has the tendency to unwind. Hence the separation of strand is easier in negatively supercoiled DNA than the relaxed DNA. The two component of supercoiled DNA, are solenoid and plectonemic. The plectonemic supercoil is found in prokaryotes and the solenoidal supercoiling is mostly seen in eukaryotes.

Quaternary structure

Main article: Nucleic acid quaternary structure
File:DNA to Chromatin Formation.jpg
DNA to Chromatin

The quaternary structure of nucleic acids is similar to that of protein quaternary structure. Although some of the concepts are not exactly the same, the quaternary structure refers to a higher-level of organization of nucleic acids. Moreover, it refers to interactions of the nucleic acids with other molecules. The most commonly seen form of higher-level organization of nucleic acids is seen in the form of chromatin which leads to its interactions with the small proteins histones. Also, the quaternary structure refers to the interactions between separate RNA units in the ribosome or spliceosome.[14]

See also

References

  1. ^ Krieger M, Scott MP, Matsudaira PT, Lodish HF, Darnell JE, Lawrence Z, Kaiser C, Berk A (2004). "Section 4.1: Structure of Nucleic Acids". Molecular cell biology. New York: W.H. Freeman and CO. ISBN 0-7167-4366-3. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)CS1 maint: multiple names: authors list (link)
  2. ^ "Structure of Nucleic Acids". SparkNotes.
  3. ^ a b c Anthony-Cahill SJ; Mathews CK, van Holde KE, Appling DR (2012). Biochemistry (4th Edition). Englewood Cliffs, N.J: Prentice Hall. ISBN 0-13-800464-1.{{cite book}}: CS1 maint: multiple names: authors list (link)
  4. ^ Alberts B, Johnson A, Lewis J, Raff M, Roberts K & Wlater P (2002). Molecular Biology of the Cell (4th ed.). New York NY: Garland Science. ISBN 0-8153-3218-1. {{cite book}}: Cite has empty unknown parameters: |1= and |2= (help)CS1 maint: multiple names: authors list (link)
  5. ^ Mao, Chengde (2004). "The Emergence of Complexity: Lessons from DNA". PLoS Biology. 2 (12): 2036–2038. doi:10.1371/journal.pbio.0020431. ISSN 1544-9173. PMC 535573. PMID 15597116. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: unflagged free DOI (link)
  6. ^ Tinoco I, Jr (1999 Oct 22). "How RNA folds". Journal of molecular biology. 293 (2): 271–81. PMID 10550208. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ "RNA structure (Molecular Biology)".
  8. ^ Hollyfield, JG (1976 Dec). "The effect of light on the quantity of phagosomes in the pigment epithelium". Experimental eye research. 23 (6): 623–35. PMID 1087245. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ Rietveld, K (1982 Mar 25). "The tRNA-like structure at the 3' terminus of turnip yellow mosaic virus RNA. Differences and similarities with canonical tRNA". Nucleic acids research. 10 (6): 1929–46. PMID 7079175. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ Staple, DW (2005 Jun). "Pseudoknots: RNA structures with diverse functions". PLoS biology. 3 (6): e213. PMID 15941360. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ Sperschneider, J (2012 Dec 1). "Predicting pseudoknotted structures across two RNA sequences". Bioinformatics (Oxford, England). 28 (23): 3058–65. PMID 23044552. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ a b c Dickerson RE, Drew HR, Conner BN, Wing RM, Fratini AV, Kopka ML (1982). "The anatomy of A-, B-, and Z-DNA". Science. 216 (4545): 475–85. PMID 7071593. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  13. ^ Mirkin SM (2001). "DNA Topology: Fundamentals". Encyclopedia of Life Sciences. doi:10.1038/npg.els.0001038.
  14. ^ "Strucual Biochemistry/Nucleic Acid/DNA/DNA Structure". Retrieved 11 December 2012.

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