Question

Nucleus is absent in
A. Companion cells
B. Sieve tube cells
C. Phloem parenchyma
D. Cambium

Answer 1

Hint: In eukaryotic cells, the nucleus is responsible for containing the genetic material. It is necessary to maintain the genetic and other biological activities of the cells. It is responsible to facilitate the various processes such as replication, transcription and formation of proteins.

Complete answer:
Nucleus is a membrane -bound organelle that occurs in the eukaryotic cells. It is responsible for controlling the various processes of the cells. The nucleus in eukaryotic cells are enclosed by a membrane known as the nuclear membrane or the nuclear envelope. The nucleus contains the genetic material and helps to maintain the integrity as well as controls the various activities of the cell.
Plant cells are eukaryotic cells and nucleus is found in them. The plant cells bear several structural peculiarities such as cell wall, cell vacuole, plastids etc. The nucleus of the plant cells contains the DNA which is required to make new cells. However, in some mature plant cells, the nucleus is absent or is lost during the course of development.
The sieve tubes are the major conducting cells of the phloem of the plants. They are long, specialized cells and are responsible for transportation of organic compounds. They develop from the meristems and are elongated tube-like structures. The sieve elements do not contain nucleus and other cell organelles such as ribosomes etc.
Companion cells are present in association with the sieve tubes and are specialized parenchyma cells. As the sieve tubes lack nucleus and other cellular organelles, the companion cells are responsible for carrying out the different cell functions. The companion cells contain nucleus and large number of ribosomes and mitochondria.
Phloem parenchyma cells are present at the border of the sieve tubes and are responsible for the transportation of sugars. They bear nuclei and are living cells.
A cambium is a layer of tissue made up of partially undifferentiated cells. They contain nucleus and function in plant growth.

Hence, the correct answer is option (B).

Note: The cell nucleus is a prominent structure that is responsible for undertaking several processes such as transcription and replication. The nucleus is considered as the controlling center of the cells. It is present mostly in all eukaryotic except a few. In plant cells, the nucleus is absent in the sieve tubes. Sieve tubes cells are conducting cells of the phloem and do not contain nucleus and ribosomes.