Answer:
I think you're referring to the Metaphase...
Explanation:
During Cell Division (or, rather before!) the DNA needs to be replicated. This normally is done by DNA Polymerase, and results in two identical double strands (the "Sister Chromatids".
These chromatids are held together at the Centromere, which will be the heart of the "X"-shape. After that the DNA is packed into Chromosomes by winding it around proteins called Histones. If I remember correctly there are 5 different Histones.
This "Zipping Up" makes the chromosomes more visible.
As the two sister chromatids are still connected, the result is the X-shaped Metaphase Chromosome....
Region A, Hope this helps.
Answer:
Exposure to a virus is closely related to the development of immunity, since the immune system is responsible for creating antibodies in response to the presence of potentially harmful biological agents, such as these viruses.
Explanation:
Viruses are biological agents that can produce diseases when they enter the body of a living being.<u> In response to contact with a virus, the</u><u> immune system</u><u> synthesizes specific </u><u>antibodies</u> to combat the foreign biological agent.
Antibody immunity can be of two types. It is fast when it is mediated by immunoglobulins M (IgM), which attack the virus anywhere, while the late and prolonged response is due to the development of immunoglobulins G (IgG).
An example of immunity to a virus occurs with the exhibition to the virus of the Dengue, that creates permanent immunity to the viral serotype to which an individual is exposed.
The acquired immunity, mediated by antibodies, is the base for the development of vaccines.
B. Cells reproduce independently and viruses require a host to reproduce
