Which of the following lacks a well-defined nucleus?

Which of the following is responsible for causing diseases in the cardiovascular system?

The cell wall of Gram-negative bacteria is:

The nuclear membrane is absent in .....

The given characters are seen in which of the following group?
$(i)$ Unicellular,colonial,filamentous,symbiotic or free living.
$(ii)$ The colonies are surrounded by a gelatinous sheath.
$(iii)$ Some can fix atmospheric nitrogen in specialized cells called heterocysts.
$(iv)$ They often form blooms in water bodies.

Assertion: Bacteria have three basic shapes,i.e.,round,rod,spiral.
Reason: Cocci and Bacilli may form clusters or chains of a definite length.