Implantation occurs in the uterus. The vagina serves as the birth canal. The capsids of all viruses are alike. A virus uses either DNA or RNA as its core, but not both. Viruses can be replicated only after they enter a living cell.
Most viruses have a protein coat or covering.
A virus may not kill a host cell but instead may become inactive for a period of time. Sperm usually survive in the female reproductive tract for about 24-48 hours. The first step in the birth process is signaled by the fetal pituitary gland. Oncogenes are frequently derived from mutations in a gene controlling normal development. Up to now it has been impossible to successfully clone any vertebrate organisms.