Ressources pour les SVT, les BPH et la DNL SVT-Anglais

Theme 1 - DNA
Article published on 18 September 2023
last modification on 26 September 2023

by F. Touihrat

September 26th, 2023

Task : create an animated video [1] to explain the differences between DNA and XNA.


  • In groups of 2 students
  • 2 min long
  • Both students should talk equally
  • Any material can be used in the animated video
  • You are not to shoot your video at home. If you are not finished today, the task will be continued next week.


Abstract 1 [2]:

Synthetic biologists try to engineer useful biological systems that do not exist in nature. One of their goals is to design an orthogonal chromosome different from DNA and RNA, termed XNA for xeno nucleic acids. XNA exhibits a variety of structural chemical changes relative to its natural counterparts. These changes make this novel information-storing biopolymer “invisible” to natural biological systems. The lack of cognition to the natural world, however, is seen as an opportunity to implement a genetic firewall that impedes exchange of genetic information with the natural world, which means it could be the ultimate biosafety tool.

Abstract 2 [3]:

Those who believe in the beauty of naturally evolved DNA might be surprised by recent efforts to free-up life (as we know it) from its evolutionary constraints. One can gaze at the biological diversity on our planet and still be stunned about the chemical uniformity of present biological life. SB includes biologists and chemists who are trying to produce unnatural molecules and architectures in order, eventually, to create xenobiological systems. To come up with an orthogonal chromosome, it is necessary to focus on the nucleotides. The genetic code of all living organisms does not know more than eight nucleoside triphosphates, four in RNA and four in DNA. Synthetic biologists have now altered these canonical nucleotides to the effect that natural biological organisms and systems cannot read and interpret them any more. Experiments replacing or enlarging the genetic alphabet of DNA with unnatural base pairs led for example to a genetic code that instead of four bases ATGC had six bases ATGCPZ. In a recent study, 60 candidate bases (that means 3,600 base pairs) were tested for possible incorporation in the DNA. These unnatural bases are not recognized by natural polymerases,

NB : in this context, orthogonal means that any change in the said orthogonal system would have no effect on the natural biological system : the systems are totally independant.

Abstract 3 [4]:

Another attempt to come up with unnatural nucleotides focuses on the backbone or the outgoing motif of the DNA. Originally this research was driven by the question of how life evolved on earth and why RNA and DNA were selected by (chemical) evolution over other possible nucleic acid structures.1 Systematic experimental studies aiming at the diversification of the chemical structure of nucleic acids resulted in completely novel informational biopolymers (see Table 11 and Fig. 22)
Although the genetic information is still stored in the four canonical base pairs, natural DNA polymerases cannot read and duplicate this information. In other words the genetic information stored in XNA is “invisible” and therefore useless to natural DNA-based organisms.

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