Cell nucleus - Practice Questions & MCQ

The Nucleus

• The nucleus is the most prominent and important membrane bound organelle of the eukaryotic cell. 

• It is referred to as the brain of the cell because it contains the DNA which is responsible for carrying the information of various functions of the cell. 

• It was discovered by Robert Brown in 1831. 

The structure of the nucleus shows the following details :

1. The Nuclear Envelope

• The nucleus is bound by a double-layered nuclear membrane that encloses the inner contents of the nucleus.

• The outer membrane is in continuity with the rough endoplasmic reticulum. 

• The inner membrane is directly in contact with the nucleoplasm. 

• Between the outer and inner membrane, there is a space called perinuclear space which also continues with the lumen of the endoplasmic reticulum. 

2. Nuclear Pores

• These are the channels spanning the nuclear envelope. 

• These are the sites of communication between the nucleus and the rest of the cell. 

• Proteins and RNA can be exchanged through these nuclear pores. 

3. Chromatin and Chromosome

• Chromatin is linear thread-like structure structures that are made up of nucleosomes. Nucleosomes are made up of DNA wrapped around histone (basic, positively charged) proteins. During the cell division, the chromatin condenses to form compact and discrete structures called chromosomes. 

• In eukaryotic cells, the chromosomes are linear. 

• Each chromosome has halves called the chromatids. 

• The chromatids are joined together at the centromere (primary constriction). 

• The ends of the chromosomes are called telomeres. 

Based on the position of the centromere, the chromosomes can be classified into four types:
1) Metacentric chromosome- middle centromere forming two equal arms.
2) Sub-metacentric- centromere slightly away from the centre.
3)Acrocentric- centromere is situated close to its end 
4)Telocentric- terminal centromere

4. Nucleolus

The nucleolus is that region of the chromatin that codes for ribosomal RNA. These regions stain darkly in the resting (non-dividing) nucleus. There can be more than one nucleoli in the same nucleus. 

5. Nucleoplasm

It is the jelly-like fluid present within the nuclear membrane. The chromatin is present in the nucleoplasm. It is made up of water, proteins and ions. It maintains the fluidity within the nucleus and provides structural support.

Functions of the Nucleus:

1. It controls the hereditary character of an organism. 

2. It is responsible for the protein synthesis, cell division, growth and differentiation.

3. It is a site for transcription in which messenger RNA (mRNA) is produced for protein synthesis.

4. It performs the exchange of hereditary material from one cell to the other cell.

5. It synthesizes the structural components of the ribosomes.