Nucleus – Structure and Functions (A-Level Biology)

The nucleus is a membrane-bound organelle present in eukaryotic cells. It houses genetic material in the form of chromatin and plays a crucial role in gene expression and DNA replication. The nucleus has a complex structure, including the nuclear envelope, nucleoplasm, nucleolus, and chromatin, each with specialised functions vital for cell growth, development, and differentiation.

Nucleus Structure: The nucleus, a membrane-bound structure, contains hereditary information and controls cell growth and reproduction. It is present in all eukaryotic cells, it may be absent in certain cells like mammalian red blood cells. The nucleus typically has a round shape, though it can be oval or disc-shaped depending on the cell type. It serves as the command centre of a eukaryotic cell, often being the most prominent organelle.


The nuclear envelope, a double membrane, separates the nucleus from the cytoplasm. Nuclear pores on the envelope facilitate the passage of molecules into and out of the nucleus. Inbound traffic includes nuclear and ribosomal proteins, while outbound traffic consists of mRNAs and ribosomal subunits. The nuclear envelope, composed of phospholipids in a lipid bilayer, helps maintain the nucleus's shape and regulates molecule flow through nuclear pores. It is connected to the endoplasmic reticulum (ER), creating continuity between the internal compartment of the nuclear envelope and the ER's lumen.

Chromosomes, composed of DNA, carry hereditary information and instructions for cell functions. During the cell's "resting" state, chromosomes organize into long, entangled structures called chromatin, not individual chromosomes.

Nucleoplasm is the gelatinous substance within the nuclear envelope, and the nucleolus, a densely stained structure within the nucleus, lacks a membrane.


Functions of the Nucleus:

  1. It controls the hereditary characteristics of an organism.
  2. It regulates cellular metabolism by controlling the synthesis of specific enzymes.
  3. The nucleus is responsible for protein synthesis, cell division, growth, and differentiation.
  4. It stores hereditary material in the form of DNA strands, as well as proteins and RNA in the nucleolus.
  5. The nucleus is a site for transcription, producing messenger RNA (mRNA) for protein synthesis.
  6. It facilitates the exchange of DNA and RNA between the nucleus and the rest of the cell.
  7. The nucleolus produces ribosomes, acting as protein factories.
  8. It regulates the integrity of genes and gene expression.


Test Yourself 

1.) What is the primary function of the nucleus in a eukaryotic cell?

a) Energy production
b) Protein synthesis
c) Genetic information storage and control
d) Cell movement

2.) What is the role of the nuclear envelope in the nucleus?

a) Maintaining cell shape
b) Controlling protein synthesis
c) Separating the nucleus from the cytoplasm
d) Facilitating cell division

3.) What is the primary component of chromatin in the nucleus?

a) Lipids
b) DNA
c) Proteins
d) Carbohydrates

4.) What happens during the "resting" state of a cell regarding chromosomes?

a) They disappear
b) Organise into long, entangled structures called chromatin
c) Divide into individual chromosomes
d) Move to the cell membrane

5.) Which structure connects the nuclear envelope to the endoplasmic reticulum (ER)?

a) Nucleoplasm
b) Nucleolus
c) Nuclear envelope
d) Chromatin

6.) What is the primary function of nuclear pores in the nuclear envelope?

a) Synthesising proteins
b) Controlling cell growth
c) Regulating the flow of molecules into and out of the nucleus
d) Organising chromosomes

7.) In which part of the cell does the nucleolus produce ribosomes?

a) Cytoplasm
b) Nucleolus
c) Endoplasmic reticulum
d) Nuclear envelope

8.) What is the primary shape of the nucleus in eukaryotic cells?

a) Square
b) Rectangular
c) Round
d) Triangular

9.)  molecule is responsible for carrying energy within cells?

a) DNA
b) ATP
c) RNA
d) Proteins

10.) What does phosphorylation involve in a cell?

a) Energy release
b) Adding a phosphate group to a molecule
c) Breaking down nucleotides
d) Stopping cell growth

11.) Which organelle is commonly the most prominent in a eukaryotic cell?

a) Golgi apparatus
b) Nucleus
c) Mitochondrion
d) Endoplasmic reticulum

12.) What does the nucleoplasm refer to in the nucleus?

a) Gelatinous substance
b) Membrane
c) Gelatinous substance within the nuclear envelope
d) Nucleolus

13.) Which type of sugar is part of the structure of RNA?

a) Glucose
b) Ribose
c) Fructose
d) Sucrose

14.) What is the primary function of nucleic acids in living organisms?

a) Energy production
b) Cell movement
c) Storage and transmission of genetic information
d) Protein synthesis

15.) What does chromatin consist of in the nucleus?

a) Proteins
b) DNA
c) Lipids
d) Carbohydrates

16.) What is the shape of chromosomes when a cell is not dividing?

a) Circular
b) Square
c) Long entangled structures called chromatin
d) Oval

17.) What is the nucleolus's role in the nucleus?

a) Separating genetic material
b) Controlling cell growth
c) Producing ribosomes
d) Synthesising proteins

18.) What is the connection between the nuclear envelope and the endoplasmic reticulum?

a) They are not connected
b) The nuclear envelope is inside the ER
c) The internal compartment of the nuclear envelope is continuous with the lumen of the ER
d) The ER surrounds the nuclear envelope

19.) What is the primary function of RNA in a cell?

a) Energy storage
b) Carrying genetic instructions from DNA to protein synthesis machinery
c) Storing hereditary material
d) Regulating cell growth

20.) What is the role of ATP in a cell?

a) Storing genetic information
b) Forming the nuclear envelope
c) Carrying energy within cells
d) Regulating protein synthesis


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