You already know that purines bond with pyrimidines, but why can’t purines bond with purines or pyrimidines bond with pyrimidines? It’s because there is not enough space for two purines to fit within the helix and too much space for two pyrimidines to get close enough together for hydrogen bonds to form between them.

The rules of base pairing (or nucleotide pairing) are: A with T: the purine adenine (A) always pairs with the pyrimidine thymine (T) C with G: the pyrimidine cytosine (C) always pairs with the purine guanine (G)

Because purines always bind with pyrimidines – known as complementary pairing – the ratio of the two will always be constant within a DNA molecule. There are two main types of purine: Adenine and Guanine. Both of these occur in both DNA and RNA.

Base pairing takes place between a purine and pyrimidine. In DNA, adenine (A) and thymine (T) are complementary base pairs, and cytosine (C) and guanine (G) are also complementary base pairs, explaining Chargaff’s rules (Figure 7).

Explanation: Pairing of a specific purine to a pyrimidine is due to the structure and properties of these bases. Matching base pairs ( purines and pyrimidines ) form hydrogen bonds. A and T have two sites where they form hydrogen bonds to each other.

Nitrogenous bases present in the DNA can be grouped into two categories: purines (Adenine (A) and Guanine (G)), and pyrimidine (Cytosine (C) and Thymine (T)).

Notable purines They include the nucleobases adenine (2) and guanine (3). In DNA, these bases form hydrogen bonds with their complementary pyrimidines, thymine and cytosine, respectively. This is called complementary base pairing. In RNA, the complement of adenine is uracil instead of thymine.

Therefore the correct pair of pyrimidine bases is thymine and cytosine, so the correct answer is option C. Note: The two types of nitrogenous bases are purines and pyrimidine. The purines include adenine and guanine nitrogen bases while pyrimidine includes cytosine and thymine or uracil nitrogen bases.

“These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine).”

The similarities include the following: (1) both bases require glutamine amide for their synthesis; (2) an amino acid is incorporated as the “core” of the purine and pyrimidine base to be synthesized. In the formation of the purine ring, glycine provides two carbon atoms and one nitrogen atom.

a)They are hydrophobic and relatively insoluble in water at the near-neutral pH of the cellb)At acidic or alkaline pH the bases become charged and their solubility in water increasesc)Purines have two rings in their structure, but pyrimidine bases have only one ringd)At acidic or alkaline pH the bases become charged …

d) It contains linker regions which are arranged into loops and coiled to form a helix. 28. _____ Which property is shared by the five histone proteins? a) They are acidic proteins with relatively large molecular weights.

Differences between purines and pyrimidines:

Cytosine and thymine are the two major pyrimidine bases in DNA and base pair (see Watson–Crick Pairing) with guanine and adenine (see Purine Bases), respectively.

Adenine and guanine have a fused-ring skeletal structure derived of purine, hence they are called purine bases. Similarly, the simple-ring structure of cytosine, uracil, and thymine is derived of pyrimidine, so those three bases are called the pyrimidine bases.

Unlike the case for pyrimidines, the purine bases are assembled already attached to the ribose ring. Alternatively, purine bases, released by the hydrolytic degradation of nucleic acids and nucleotides, can be salvaged and recycled.

Note that the purine bases (adenine and guanine) have a double ring structure while the pyrimidine bases (thymine and cytosine) have only a single ring.