Nucleus

Introduction

In all living organisms made of eukaryotic cells, the nucleus plays a central and irreplaceable role. It is often referred to as the command center of the cell, because it regulates essential activities that keep the cell alive, growing, dividing, and functioning properly.

One of the most critical areas it controls is cell metabolism, the collection of all chemical reactions that occur inside the cell to maintain life.

1. What Is the Nucleus?

The nucleus is a large, membrane-bound organelle found in almost every eukaryotic cell (cells of animals, plants, fungi, and protists).

Key Features:

• It is usually spherical or oval in shape.

• Located near the center of the cell.

• Surrounded by a double membrane known as the nuclear envelope.

• Contains the cell’s genetic material (DNA).

• Appears darker than the surrounding cytoplasm under a microscope.

2. Why Is the Nucleus Important for Metabolism?

The nucleus is essential for metabolism because it:

• Stores DNA, which contains the instructions (genes) for building proteins.

• Regulates which proteins are made and when, depending on the cell’s needs.

• Controls cell growth, repair, and division — all processes tightly linked to metabolism.

• Coordinates communication between the nucleus and other organelles like mitochondria.

Without the nucleus, the cell would not be able to make proteins correctly, respond to changes in its environment, or pass on genetic information.

3. Inside the Nucleus

The nucleus is not empty — it is filled with nucleoplasm, a jelly-like substance where key structures float:

4. DNA, Chromosomes, and Genes

Inside the nucleus, DNA is stored in the form of chromatin, which coils up to form chromosomes during cell division.

• Each human body cell has 46 chromosomes, arranged in 23 pairs.

• One chromosome of each pair comes from the mother, the other from the father.

• Chromosomes contain genes – specific sequences of DNA that carry instructions for traits and protein production.

Vocabulary:

• Gene: A section of DNA that codes for a specific protein.

• Allele: Different versions of the same gene.

• Locus: The specific location of a gene on a chromosome.

5. Chromosome Types and Sex Determination

There are two types of chromosomes:

• Autosomes (body chromosomes) – determine general traits like eye color, height, etc.

• Sex chromosomes – determine biological sex (XX for females, XY for males).

The full set of chromosomes in a cell is called a karyotype. A visual diagram showing them in pairs is a karyogram.

6. Chromosomes and Cell Division

Before a cell divides, it must duplicate its DNA, a process called DNA replication.

• Each chromosome is copied, forming two identical parts called sister chromatids.

• These are joined at a region called the centromere.

• During cell division, each new cell gets an identical set of chromosomes.

7. Special Cases: Nucleus Variations

• Most cells have one nucleus.

• Some, like liver or muscle cells, may have multiple nuclei.

• Certain cells (e.g., red blood cells) lose their nucleus during maturation to create more space for carrying oxygen.

Summary

• The nucleus is the control center of eukaryotic cells.

• It holds the DNA, which contains instructions for building and regulating the cell.

• The nucleus controls metabolism by determining which proteins are made.

• DNA is organized into chromosomes, which contain genes.

• Genes come in different forms (alleles) and are inherited from both parents.

• During cell division, chromosomes are replicated to ensure genetic continuity.

Did You Know?

• The DNA in one human cell, if stretched out, would be over 2 meters long, yet it fits inside the tiny nucleus due to how tightly it is packed!

• Every cell in your body (except red blood cells) contains a complete copy of your DNA.