Question

Briefly, how do viruses enter animal cells?

Answer 1

Virus are obligate intracellular parasite. a virus must have to enter in a living cell to replicate. after coping its genome virus find various way to escape the host cell . viruses find their host cell (permissive cell) by the surface receptors of the host cell . these receptors are known as viral receptor ,on which virus could attach . viral entry is the first stage of their life cycle .

the way of viral entry are as follow -

Membrane fusion(for enveloped virus) : first of all, viruses find the way to move near host cell . the receptors presented on viral envelop ( covering of viruses which are made of phospholipid and proteins , similar to host cell membrane thus help to avoid host's immune system) are complementary receptor to the host cell receptor .  it is the most well known way of enveloped virus entry . envelop receptors then attach to the cell membrane complementary receptor. then viral envelop fuses with cell membrane and release its content i.e viral genome . HIV ,herpes simplex virus uses membrane fusion method to enter in cells .

Endocytosis : non enveloped virus or naked virus usually enter in the host cell by endocytosis ( it is a cellular process by which cell engulf something by its membrane ) naked virus attach to the host cell membrane receptor and pretend to be nutrients or food particles . host's cell membrane engulf the virus as food particles and cover them within vesicle and transport them(within vesicle) to the cytoplasm .after that virus breaks out the vesicle and affect the cell . Polio virus , Hepatitis C virus enter in host cell by endocytosis

Entry via pore creating : It is more specific method especially for bacteriophages   .in this way viruses attach  to host cell and create pores on cell surface via chemicals (enzyme etc) and then inject their genetic material,toxins leaving rest part of the virus on host cell surface . it is also called genetic injection . for example T2 phage virus attaches on a cell then pierce the cell membrane and inject its DNA .