Biology Help-->Genetic Control!

[XPFMember]

Assalamoalaikum!!
Can somebody please put down the explanation why purines always pair with pyrimidines...ii hope somebody takes the pain to put that here..
Jazak Allah Khair!!

 

[larina]

Walaikumusalam.
purines are double ringed structures while pyrimidines are single ringed.
In a DNA double helix molecule....there should be one big(purine) and one smaller(pyrimidine) molecules.
this pairing cause the DNA molecule to be in a helix shape.

Also, purines are thymine and guanine
and pyrimidines adenine and cytosine.

Thymine always always always pair up with adenine
Guanine always pair up with cytosine.
this forms complementary base pairings.

 

[XPFMember]

actually my teacher said smthng abt maintaining the width...and that's what i want...anybody..?thanks larina by the way...Jazak Allah!

 

[ahmed t]

adenine and cytosin are small (1 aromatic ring)
guanine and thymine are big (2 aromatic rings)
so that the width between the two dna strands are to be equal the big has to bond to a small
(u cannot have a big and a big or a small and a small cause that will cause irregularity in the width of dna strands)
so that is why purines bond with pyrimidines
hope this helps, cause its kind of hard to explain
just try to imagine it!!!!!!!!!!!!

 

[hassam]

well below is a part of text from a book...b4 dat guyx.....purines(double-ringed) are adenine and guanine
Because the AT and GC pairs are of equal length and fit
identically into the double helix (like rungs on a ladder), the
diameter of the helix is uniform. The base pairs are flat (see
Figure 11.6), and their stacking in the center of the molecule
is stabilized by hydrophobic interactions

well.......hope it helps....for further clearance i ll be uploading one picture cos at the moment i CNt find it

 

[beacon_of_light]

There is a constant size of gap b/w the two strands of DNA molecule. Only a purine and a pyrimidine can span the gap. two purines would be too long and two pyrimidines would be too short ...