They unzip the double helix into single strands during replication. The unzipping occurs at regions called origins where replication is initiated. The protein breaks the hydrogen bonds (using energy from ATP) between the complimentary bases of the strands hence forming a replication fork.
Natural selection. The trait that is unfavorable to the baby giraffe's survival will cause the death of giraffes with that specific gene that is responsible for a short neck vs a long neck. Natural selection causes traits that help the survival of the species will be carried throughout generations while unfavorable traits will be lost.
