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  Section: Genetics » Chemistry of the Gene » Nucleic Acids and Their Structure
 
 
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Z-DNA, a left handed DNA form

 
     
 
Content
Chemistry of the Gene 1.  Nucleic Acids and Their Structure
Nucleic acids as genetic material
Transformation experiments
Experiments with bacteriophage (T2) infection
Experiments with tobacco mosaic virus (TMV)
Structure of nucleic acids 
Bases
Nucleosides
Nucleotides
Polynucleotide
Deoxyribonucleic acid (DNA)
Alternative forms of DNA double helices
Z-DNA, a left handed DNA form
RL model
Supercoils in closed DNA
Ribonucleic acid (RNA)
Another form of DNA has been obtained artificially by synthesizing d(C - G)3 molecules which were isolated as crystals. This form of DNA has also been detected in living cells using specific antibodies raised against it. Because this form of DNA follows a zig-zag course, it has been called Z-DNA. It resembles the B form in may respects and differs from it in many other respects (Table 25.2). The differences and similarities have been described below :

Resemblances between Z-DNA and B-DNA
  1. Both are double helical.
  2. Two strands of double helix are antiparallel in both DNAs.
  3. Both forms exhibit A = T and G ≡ C pairing.

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Differences between Z-DNA and B-DNA

  1. Z-DNA has left-handed helical sense as against right handed helical sense of B-DNA (Fig. 25.16).
  2. Due to a different arrangement of molecules within Z-DNA polymer, phosphate backbone follows- a zig-zag course, while in B-DNA it is regular (Fig. 25.17).
  3. In Z-DNA, sugar residues have alternating orientation (Fig. 25.18) so that repeating unit is a dinucleotide as against B-DNA where repeating unit is a mononucleotide and the
  4. orientation of sugar molecules is not alternating.
  5. In Z-DNA, one complete helix i.e. a twist through 360°, has twelve base pairs or six repeating dinucleotide units (12 base pairs), while in B-DNA, one complete helix has only ten base pairs or ten repeating units.
  6. Because twelve base pairs are accommodated in one helix in Z-DNA, as against ten in B-DNA, the angle of twist per repeating unit (dinucleotide) is 60°, as against 36° in B-DNA.
  7. One complete helix is 45 A in Z-DNA while it is 34 A in B-DNA.
  8. Since bases get more length to spread out in Z-DNA and since the angle of tilt is 60°, they are closer to the axis and hence the diameter of Z-DNA molecule is 18 A, whereas it is 20 Å in B-DNA.
Two helices, one showing right handed sense as found in B-DNA and the other showing left handed sense as found in Z-DNA.
Fig. 25.16. Two helices, one showing right handed sense as found in B-DNA and the other showing left handed sense as found in Z-DNA.

In addition to the above, other differences include (i) differences in glycosidic torsion angle, which is anti in B-DNA and syn in Z-DNA for deoxyguanosine and (ii) differences in sugar pucker which is C2’endo for B-DNA and C3' endo for Z-DNA in deoxyguanosine. The situation is reverse for deoxycytidine. These differences are summarized in Table 25.2.
 
Side views of Z-DNA and B-DNA. Two views of Z-DNA are 30° apart. Irregularity of backbone in Z-DNA is shown by heavy lines showing the path of phosphate residues, which is quite regular and smooth in B-DNA (redrawn from Nature, Vol. 282, 1979).
Fig. 25.17. Side views of Z-DNA and B-DNA. Two views of Z-DNA are 30° apart. Irregularity of backbone in Z-DNA is shown by heavy lines showing the path of phosphate residues, which is quite regular and smooth in B-DNA (redrawn from Nature, Vol. 282, 1979).


Orientation of adjacent sugar residues in Z-DNA and B-DNA showing opposite orientations in Z-DNA and same orientation in B-DNA. This results in dinucleotide units in Z-DNA as against mononucleotide units in B-DNA.
Fig. 25.18. Orientation of adjacent sugar residues in Z-DNA and B-DNA showing opposite orientations in Z-DNA and same orientation in B-DNA. This results in dinucleotide units in Z-DNA as against mononucleotide units in B-DNA.

In vivo existence and role of Z-DNA
A few years ago, it was suggested that Z-DNA may be transient configuration of B-DNA in living cells and may thus play a role in (a) regulation of gene activity as suggested by Alexander Rich and/or in (b) genetic recombination as suggested by William Holloman and Eric Kmiec both of U.S.A. Earlier the role of Z-DNA in regulation of gene activity was suggested, since Z-DNA specific antibodies could bind specifically to interband regions of salivary gland chromosomes and not to active banded regions. This suggested that Z-DNA conformation renders the DNA inactive. These observations were later shown to be an artefact of chromosome fixation. However, sequences capable of forming Z-DNA are believed to play a role in recombination or rearrangement of DNA (e.g. immunoglobulin genes; consult Regulation of Gene Expression 3. A Variety of Mechanisms in Eukaryotes).
 
     
 
 
     




     
 
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