Question

Mitotic stages are not observed in
A. Cosmarium
B. E. coli
C. Saccharomyces
D. Chlorella

Answer 1

Hint: Mitosis is the form of division that takes place in all the cells of the body except the gametes during which the chromosomes duplicate themselves and divide into two equal cells having equal number of chromosomes.

Complete answer:
 Mitosis is also known as equational division as it equally divides one cell (mother cell) into two daughter cells. This type of division is common in the somatic cells of eukaryotes. Prokaryotes undergo nuclear division by binary fission but not mitosis as they have no nuclear membrane surrounding their DNA. Though the end results of mitosis and binary division are the same , two daughter cells are formed which are identical to each other.
The cycle that involves the duplication of the genes of the cell and other components along with the genes and dividing in two exactly similar daughter cells is known as cell cycle. The cell cycle is divided into interphase and M phase which is the mitotic phase.
Interphase – This is the phase during which the cell undergoes DNA duplication and cell growth. The cell prepares itself for the mitotic division and is said to be in a resting phase.
M phase – This phase is further divided into
-Prophase – Compaction of chromosome occurs, Chromosomes appear as two chromatids attached with centromere at the centre. Nuclear envelope, golgi apparatus, nucleolus and endoplasmic reticulum disappear.
-Metaphase – Chromosomes attach to the spindle fibres with the help of kinetochores which are present at the surface of the centromere, chromosomes get arranged on the equatorial metaphase plate.
-Anaphase – Splitting of the centromere and separation of chromatids appear after which the chromatids move to opposite poles within the cell.
-Telophase – Chromosomes assemble as clusters on the opposite pole and the nuclear envelope appears back along with golgi apparatus, endoplasmic reticulum and nucleolus.

This whole cycle appears in eukaryotes only and not in prokaryotes like bacteria and archaea. In the options given, Cosmarium is a fresh water organism which belongs to the genus Charophyta, a branch of green algae that belongs to Eukaryota and shows mitosis. E. coli is a bacteria which is prokaryote and shows no mitosis. Saccharomyces is a yeast species and yeast is a fungi and fungi are eukaryotes that show mitosis. Chlorella is a green algae and a eukaryote.

Hence, the correct answer is option (B).

Additional information:
-Each cell in our body undergo mitosis every 24 hours.
-Interphase consists of 3 phases – Gap 1, Synthesis and Gap 2 phase. Gap 1 is the phase during which the cell is metabolically active and is preparing for DNA duplication that happens in the synthesis phase. -During the gap 2 phase the proteins necessary for mitosis are produced.
-DNA duplication during the synthesis phase takes place in the nucleus.
-Plants show mitosis in both diploid and haploid cells whereas in animals only diploid cells undergo mitosis.

Note: All the cells in the human body do not undergo mitosis like the heart cells when the human enters adult age. Some cells undergo mitosis occasionally like in case of their loss during accidents or other cell injury. Such cells that do not divide stay in quiescent phase where they quit after gap 1 phase and stay as such without DNA duplication.