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Class 9, Science, Chapter-5, Lecture-4, Nucleus (Notes)

NUCLEUS:

Nucleus is a dense protoplasmic cell organelle containing hereditary information which control cell activities.

Position:

  • In animal cells - at the centre
  • In young plant cells - at the centre
  • In mature plant cells - towards the periphery

Structure:

  1. Nuclear Envelope is a bi-layered discontinuous membrane containing nuclear pores.
    Role - Separates chromatin from the cytoplasm
    The pores allow exchange of material between the nucleus and the cytoplasm.
  2. Nucleoplasm (Nuclear sap) (Karyolymph)
    Role - Provides nutrition to the chromatin and the nucleolus.
  3. Nucleolus is an unbounded round structure rich in RNA and protein.
    Role - site of ribosome formation.
  4. Chromatin Material (Chromatin Reticulum) is an inter wined mass of thread like structure made of DNA and protein.
    Role - Contains hereditary information in the form of DNA.

Role of Nucleus:

  1. Nucleus contains and transmits the hereditary information in the form of DNA.
  2. Nucleus acts as the control centre for metabolic activities of the cell.
  3. Ribosome formation takes place in the nucleolus present in the nucleus.
  4. Nucleus regulates cell cycle (cell division).
  5. Nucleus determines the specific structure and function of a cell during Differentiation.
  • B.C. in mammals and Sieve element in plants do not contain nucleus.
  • Paramecium contains two nuclei

NUCLEOID:

An undefined nuclear region containing nucleic acid without any membranous covering in prokaryotic cells is termed as nucleoid.

CHROMOSOMES:

Rod-shaped structure which are formed by condensation of chromatin fibres in a cell nucleus during cell division are called chromosomes.

PROKARYOTES:

The organisms in which a nucleus membrane does not bind the nuclear material in the cells are called prokaryotes.

PROKARYOTIC CELLS:

The cells in which a nucleus membrane does not bind the nuclear material are called prokaryotic cells.

EUKARYOTES:

The organisms in which a nucleus membrane binds the nuclear material in the cells are called eukaryotes.

EUKARYOTIC CELLS:

The cells in which a nucleus membrane binds the nuclear material are called eukaryotic cells.

Differences between:

Feature

Prokaryotic Cell

Eukaryotic Cell

Size

1-10 m

5-100 m

Cell organelles

Membrane bound cell organelles are absent

Membrane bound cell organelles are present

Nucleus

Membrane bound Nucleus is absent.
Nucleoid is not surrounded by nuclear membrane

Membrane bound Nucleus is present

Chromosome

Single DNA molecule is present

More than one DNA molecules are present

Nucleolus

Absent

Present

Cell division

By fission or budding (amitosis)

Mitotic or meiotic