Tag: caskets of life

Viruses are an exception to cell theory as it is an acellular organism that is living inside the host and nonliving outside the host. In viruses, only one nucleic acid (DNA or RNA) is present. Thus, a virus can be either a DNA virus or an RNA virus. Viruses infecting animals usually have DNA as genetic material. Viruses infecting plants usually have RNA as genetic material. All eukaryotic cells have both DNA and RNA in their cells.

Nucleus is double membrane covered protoplasmic body that contains hereditary information. A nucleus is absent in some mature cells like mammalian RBCs and sieve tube cells in plants. They are called as enucleate cells. In mammals, erythrocytes also lose all other cellular organelles such as their mitochondria, Golgi apparatus and endoplasmic reticulum. As a result of not containing mitochondria, these cells use none of the oxygen they transport; instead they produce energy by glycolysis of glucose and lactic acid fermentation of the resulting pyruvic acid.

All the multicellular organisms start their life with a single cell (zygote). Later, it is divided and differentiated into different cell types in multicellular organisms. Cells are capable of dividing to form their own kind of cells. Thus, all cells of the body come from the pre-existing cells. Thus, the correct answer is option C.

The average diameter of bacteria is 0.2 to 2 micrometer. 
Mitochondrion size is 0.5 to 10 micrometer. 
The average diameter of a mammalian cell is 20 micrometer. 
Virus diameter ranges from 20 nanometers to 250-400 nanometer. 1000 nanometer = 1 micrometer. Hence, the virus particle is the smallest of all.

Cells of unicellular organisms are usually spherical due to surface tension. 
Cells of the multicellular organism are usually polyhedral. Because polyhedral cells have more neighbors which means more intercellular exchange can occur. Also, the polyhedral group of cells resists stretching to a more extent. 

In unicellular, reproduction occurs asexually and two new identical daughter cells that are produced. All multicellular start their life from a single cell i.e., zygote which is a product of the fusion (fertilization) of usually two haploid gametes, a female and a male gamete. The zygote (diploid) divide to give rise huge variety of different types of cell, specialized for different functions which in turn forms multicellular organism.

Cell size in unicellular organisms is 0.5-10 micrometer whereas, in multicellular organisms, cell size is 10-100 micrometers.