When the mRNA strand is being formed, what base matches up to adenine?

Uracil

Explanation:

Transcription is the first stage of gene expression in a living cell and it involves the synthesis of a mRNA molecule using a DNA template. The process of transcription is carried out by an enzyme called RNA polymerase, which initially binds to the PROMOTER region of a DNA, unwind its double-helix and begins to add nucleotide bases to the growing RNA strand based on the nucleotides it reads on the DNA.

The basis of nucleotide addition by RNA polymerase is COMPLEMENTARY BASE PAIRING RULE, which states that Adenine (A) always pairs with Thymine (T) and Guanine (G) with Cytosine (C). RNA polymerase adds to the RNA strand, nucleotides complementary to the ones it reads on the DNA. Since, Uracil base replaced Thymine in the RNA molecule, RNA polymerase adds Uracil to the RNA strand, for every Adenine it reads on the DNA template.

rain is the best answer

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.