00:01
Hello students in this question this has been informed that the dna model the dna double helix model has a ladder like structure and this leddolized structures look something like this look at this structure this is the led like structure of the double elix model of the dna and we have to find out that these rungs of the letter are made up of what so these are basically made up of nitrogenous bases made up of nitrogenous basis these nitrogenous bases are of two types namely purines and the other ones are pyrimidines.
00:33
So purines include two types of nitrogenous bases which are adenine and vanine and the pyrimidines include three nitrogenous bases which are respectively cytosine, thymein and uresil.
00:54
Remember that urexyl is always present in rna only whereas thymine is present in only in dnas.
01:00
Now look at the structure over here.
01:03
In this structure supposedly the purple ones are adenine, the purple ones are adenine and the green ones are thiamine, the green ones here are thymine, the pink ones over here are cytosine and let's say the blue ones are vanine.
01:28
According to the rules of the double -allical model of the dna, it has been informed to us that 1 fluene always binds with 1, 3midein, also that adenine always binds with thiamine via only 2 hydrogen bonds, whereas guanine binds with cytosine via 3 hydrogen bonds.
01:48
There are 3 hydrogen bonds present between guanine and cytosine and 2 in between adenine and thymine.
01:53
So we can say that there will be equal number of adenine and thiamine and equal number of guanine and cytosine.
01:58
Present in a dna molecule and therefore the ratio of a by g will be is equal to the ratio of t by c and also because there are equal number of adenine and thymine and equal number of guanine and cytosine we can also say that a by t will be equal to g by c and with this will be equals to 1.
02:16
Now let us look at our options in the question we were asked that the lungs of this ladder like structure are formed by and the options are a purine binds with a in the next option it has been told us that adenine binds to guanine and the cytosine binds to thymine.
02:34
Third option says that the sugar phosphate of one nucleotide binds with the sugar phosphate of another nucleotide.
02:40
For the last option that says the 5 -dash end of a dna molecule is paired with the 3 -dash end of another dna molecule.
02:48
Let's go to the options one by one.
02:50
So as we have already seen in the discussion, a purine always binds to a pyrimidine...