Why does adenine only pair with thymine in dna? why does cytosine only pair with guanine?

Adenine,Thymine,Guanine,Cytosine are nitrogenous base pairs of DNA

Explanation:

Due to base pairing rule Adenine binds with thymine and guanine binds with Cytosine.1 purine+1 pyrimidine. The imidazole ring of purine attaches with pyrimidine ring.Two purines never fit between the two strands of DNA.Therefore hydrogen bond (H-bond) connects these two bases by means of N1 and amino acid. These bases make up a nucleic acid.Adenine binds with thymine with 2 (H-bonds) and similarly guanine binds with cytosine with 3 (H-bonds). This binding follows the Erwin Chargaff's Rule

1 and 6 for apex

you are certainly talking about this picture:

a: is the label of the white square, it is showing the  replication fork, formed by helicase (d) ==> so it is the site of dna separation and replication 

b: is a new dna brand that is following directly the replication fork, so it is continuous ==> built continuously from 5' to 3

c: dna polymerase cannot replicate dna on its own, it needs the of their proteins, and one of them plays the role of a clamp which fixes  the dna with the dna template and increases  the processivity of the enzyme. ==>   stops polymerase from falling off the  template.

d: is the enzyme responsible for forming the replication fork (helicase) ==> unwinds and separates dna into two strands

e: is the delayed strand, it is built  in the opposite sense of the helicase, so dna will be built in fragments ==> built  fragments from 5' to 3 joins

and finally f: is the enzyme that builds  the new dna strand (dna polymerase) ==>   adds nucleotides to make a new dna helix.

credit: gradman555p98x9p

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