Biology FEATURES OF A POLYNUCLEOTIDE CHAIN

### KEY TOPICS

star Elucidation of Nucleic Acid Structure
star Simple Features of the Polynucleotide chain
star Salient Features of the DNA Double Helix:

### ELUCIDATION OF NUCLEIC ACID STRUCTURE

● color{brown}"DNA" color{brown}"as an acidic substance":

star DNA as an color{Violet}"acidic substance" present in nucleus was first identified by color{Violet}"Friedrich Meischer" in 1869.

star He named it as color{Violet}"Nuclein"

star However, due to color{Violet}"technical limitation" in isolating such a long polymer intact, the color{Violet}"elucidation of structure" of DNA remained color{Violet}"elusive" for a very long period of time.

● color{brown}"Double Helix"color{brown}" Model of DNA":

star It was only in color{Violet}"1953" that
- color{Violet}"James Watson" and color{Violet}"Francis Crick", based on the color{Violet}"X-ray" color{Violet}"diffraction data"
- produced by color{Violet}"Maurice Wilkins" and color{Violet}"Rosalind Franklin",
- proposed a very simple but famous color{Violet}"Double Helix model" for the structure of DNA.

star One of the hallmarks of their proposition was color{Violet}"base pairing" between the two strands of polynucleotide chains.

● color{brown}"Chargaff’s Rule":
star The color{Violet}"base pairing proposition" was also based on the observation of color{Violet}"Erwin Chargaff".

star It was that for a color{Violet}"double stranded DNA",
- the color{Violet}"ratios" between color{Violet}"Adenine and Thymine" and color{Violet}"Guanine and Cytosine"
- are color{Violet}"constant" and equals color{Violet}"one".

### SIMPLE FEATURES OF THE POLYNULEOTIDE CHAIN

● The color{Violet}"base pairing" confers a very color{Violet}"unique property" to the polynucleotide chains.

● They are said to be color{Violet}"complementary" to each other, and therefore if the color{Violet}"sequence of bases" in one strand is known then the sequence in other strand can be predicted.

● Also, if each strand from a DNA (let us call it as a color{Violet}"parental DNA") acts as a color{Violet}"template" for synthesis of a new strand, the two double stranded DNA (let us call them as color{Violet}"daughter DNA") thus, produced would be color{Violet}"identical" to the parental DNA molecule.

● Because of this, the color{Violet}"genetic implications" of the structure of DNA became very clear.

### SALIENT FEATURES OF THE DNA DOUBLE HELIX

● It is made of color{Violet}"two polynucleotide chains", where the backbone is constituted by color{Violet}"sugar-phosphate", and the color{Violet}"bases project" inside.

● The two chains have color{Violet}"anti-parallel polarity". It means, if one chain has the polarity color{Violet}"5'-3'", the other has color{Violet}"3'-5'".

● The bases in two strands are paired through color{Violet}"hydrogen bond" (H-bonds) forming color{Violet}"base pairs" (bp).

star color{Violet}"Adenine" forms color{Violet}"two hydrogen bonds" with color{Violet}"Thymine" from opposite strand and vice-versa.
star Similarly, color{Violet}"Guanine" is bonded with color{Violet}"Cytosine" with color{Violet}"three H-bonds".
star As a result, always a color{Violet}"purine" comes color{Violet}"opposite" to a color{Violet}"pyrimidine".
star This generates approximately color{Violet}"uniform distance" between the two strands of the helix.

● The two chains are coiled in a color{Violet}"right-handed fashion".

star The color{Violet}"pitch" of the helix is color{Violet}"3.4 nm" (a nanometre is one billionth of a metre, that is 10-9 m)
star There are color{Violet}"roughly 10 bp" in each turn.
star Consequently, the color{Violet}"distance" color{Violet}"between a bp" in a helix is approximately equal to color{Violet}"0.34 nm".

● The color{Violet}"plane" of one base pair color{Violet}"stacks" over the other in double helix.

star This, in addition to color{Violet}"H-bonds", confers color{Violet}"stability" of the helical structure