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In this article we will discuss about the types and parts of nucleus in plants.
Types of Nucleus:
1. Uninucleate cell:
It is also referred to as monokaryotic cell, mostly plant cell which contain single nucleus.
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2. Bi-nucleate cell:
It is also called as dikaryotic cell, which contains 2 nucleus at a time. Example is (i) Paramecium (mega and micronucleus) (ii) Balantidium (iii) Liver cells and cartilage cells.
3. Multinucleate cells:
It is also known as polynucleate cell which contains more than 2 nucleus at a time e.g., in, plants latex cells and latex vessels. In animals, striated muscle cell and bone narrow cells.
4. Enucleate cells:
Cell without nucleus are called enucleate cells. However, some living cells like mature sieve tubes of phloem and RBCs of mature mammals lack nuclei.
Parts of Nucleus:
Nucleus has four parts:
1. Nuclear membrane or Nuclear envelope or Karyotheca.
2. Chromatin threads or Nuclear reticulum.
3. Nuclear sap or Nucleoplasm or Karyolymph.
4. Nucleolus.
(1) Nuclear membrane:
The nuclear envelope is made up of outer and inner membrane (2 unit membranes), made up of lipoproteins, perinuclear space, pores, annuli material and inner dense lamella. The outer membrane is continuous with ER. The exchange of different substances between nucleus and cytoplasm takes place through minute pores already present in nuclear membrane.
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(2) Chromatin threads:
This term was proposed by W. Flemming. Chromatin threads are intermixed with one another and form a network called chromatin reticulum. At the time of cell division the chromatin threads separate from one another and become thicker and smaller and are now termed as chromosomes. It is basically nucleoprotein, made up of nucleic acid and basic protein histone. Nucleic acid contains sugar, phosphate, nitrogenous bases and are very complex organic acids.
Nucleic acids are of 2 types:
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(1) DNA (Deoxyribonucleic acid)—especially found in chromatin, also present in mitochondria and chloroplast.
(2) RNA (Ribonucleic acid)—especially found in cytoplasm in soluble form and is called soluble RNA or £RNA, also present to some amount in ribosomes of nucleus, chromatin and nucleolus. It is synthesized from DNA and is accumulated in nucleolus. Later, it travels to cytoplasm and gets attached to ribosomes.
Chromatin is basophilic in nature and most of the chromatin material is transformed into a specific number of chromosomes during cell division. The chromatin material may be heterochromatin, euchromatin and sex chromatin. Heterochromatin and Euchromatin terms were given by Emil Heitz.
(3) Nuclear Sap (= Nucleoplasm):
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The nuclear membrane encloses the clear, homogenous, transparent, colloidal liquid of variable consistency. It is chiefly composed of nucleoproteins, a small amount of organic and inorganic substances like nucleic acids, proteins dissolved phosphorus, minerals, ribose sugars, enzymes and nucleotides.
(4) Nucleolus:
It was first observed by Wagner and the term was proposed by Browman, subsequently described by Fontana.
Characteristics:
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(i) One or more nucleoli may be present within a nucleus. Four nucleoli are present in each nucleus in onion.
(ii) Nucleolus disappears in late prophase stage.
(iii) Reappears in telophase stage.
(iv) It is store house of RNA.
