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The Fundamental Unit of Life

Tutormate > CBSE Syllabus-Class 9th Biology > The Fundamental Unit of Life

06 Nucleus

Anatomy of a nucleus

  • The nucleus is a large, centrally located spherical cellular component.
  • It is bounded by two nuclear membranes, both forming a nuclear envelope.
  • The nucleus is an integral component of the cell.
  • Its plural is nuclei, derived from a Latin word meaning kernel.
  • This is because its structure is a membrane-enclosed organelle.
  • It is found in eukaryotic cells.
  • Eukaryotes usually have a single nucleus, but a few cell types have no nuclei and a few others have many.


  • It is generally the most prominent organelle in the cell that is surrounded by a structure called the nuclear envelope that separates the contents of the nucleus from the cytoplasm.
  • The cell’s chromosomes are also enclosed within it and contain DNA.
  • This provides the genetic information necessary for the production of other cell components and for reproduction of life.
  • So, the nucleus contains the cell’s hereditary information.
  • It also controls the cell’s growth and reproduction.


The following essential cellular processes take place in the nucleus:

  • DNA can be duplicated in the nucleus. This process is called replication and creates an identical copy of the DNA. The first step in cell division is creating two identical copies of the host or body. Each new cell gets its own set of instructions.
  • The nucleus is the spot of transcription which is the process of creating different types of RNA from DNA. Transcription is similar to making copies of individual pages of the human body’s instructions that can then be passed out and read by the rest of the cell. According to the central rule of biology, DNA is copied into RNA. RNA is then turned into protein.

Differences between nucleus and nucleoid:

Nucleus Nucleoid
Nucleus is larger in size. Nucleoid is comparatively smaller in size.
It is covered by a double membrane envelope A covering membrane is absent. It lies free in the cytoplasm.
Nucleolus is present in it. Nucleolus is absent in it.
Its DNA content is equal to two or several DNA molecules. Its DNA content is equal to a single DNA molecule.
Its DNA is associated with histone proteins to form chromatin. In it histones are absent. DNA of a nucleoid is often naked.

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