Identify two structural features of purines and pyrimidines. Purines contain only three ring nitrogen atoms. contain only two ring nitrogen atoms. contain four ring nitrogen atoms. contain two heterocyclic rings. contain one heterocyclic ring.

1) Purines contain only four ring nitrogen atoms while Pyrimidines contain only two nitrogen ring atoms

2) Purines contain two heterocyclic rings while Pyrimidines contain one heterocyclic ring

Explanation:

Purines and Pyrimidines are two classes of nitrogenous bases that make up nucleic acids. They are both heterocyclic aromatic compounds which contains nitrogen atoms in their structure. Although, they are similar but have distinct features namely:

Purines are ring molecules which consists of four (4) nitrogen atoms in their aromatic structure. They are structures with two heterocyclic rings consisting of five (5) carbon and four (4) nitrogen atoms. Adenine and Guanine are the purines in nature.

On the other hand, Pyrimidines are another ring molecules consisting of two (2) nitrogen atoms in their aromatic structure. Their only ring i.e. 1, is also heterocyclic in nature and contains four (4) carbon and two (2) nitrogen atoms. Cytosine, Thymine and Uracil are the pyrimidines.

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.