They must go through the security check at the front entrance.

They must go through security check at the front entrance because they make sure the person isn't armed, make sure you don't carry / have something that isn't allowed etc.

Okay, biology isn't exactly my strongest subject (it never has been), but i'm pretty sure the answer is nucleotides. dna is made up of nucleotides. each nucleotide contains a phosphate group, a sugar group, and a nitrogen base. there are four different nitrogen bases: adenine (a), thymine (t), guanine (g), and cytosine (c). a always pairs with t, and g always pairs with c.