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:
- 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. - Nucleoplasm (Nuclear sap) (Karyolymph)
Role - Provides nutrition to the chromatin and the nucleolus. - Nucleolus is an unbounded round structure rich in RNA and protein.
Role - site of ribosome formation. - 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:
- Nucleus contains and transmits the hereditary information in the form of DNA.
- Nucleus acts as the control centre for metabolic activities of the cell.
- Ribosome formation takes place in the nucleolus present in the nucleus.
- Nucleus regulates cell cycle (cell division).
- 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. |
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 |