- The nucleus is a prominent organelle in eukaryotic cells.
- It was first discovered by Robert Brown in 1831, and the material within it, which stains with basic dyes, is known as chromatin.
- Components of the Nucleus:
- The interphase nucleus contains highly extended nucleoprotein fibers called chromatin.
- Chromatin is made up of DNA, histone and non-histone proteins, and RNA.
- It is responsible for encoding genetic information and is present in a loose and indistinct network in the interphase.
2. Nuclear Matrix:
- The nuclear matrix or nucleoplasm contains nucleoli and chromatin.
- It's a semi-liquid substance within the nucleus.
- Nucleoli are spherical structures present in the nucleoplasm.
- These structures are not membrane-bound and are involved in active ribosomal RNA synthesis.
- Cells actively engaged in protein synthesis have larger and more numerous nucleoli.
- Nuclear Envelope:
- The nucleus is enclosed by a nuclear envelope consisting of two parallel membranes with a space between them, known as the perinuclear space.
- The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum and often bears ribosomes.
- Nuclear pores are present in the envelope, allowing the movement of RNA and protein molecules in both directions between the nucleus and cytoplasm.
- Number of Nuclei:
- Typically, a cell contains one nucleus.
- However, variations in the number of nuclei can occur, such as in multinucleated muscle cells or in some unicellular organisms like amoeba.
- Nucleus in Mature Cells:
- Some mature cells, like erythrocytes (red blood cells) in many mammals and sieve tube cells in vascular plants, lack a nucleus.
- Despite this, they are still considered "living" cells because they carry out important functions in the organism, although they are non-dividing cells.
- DNA Packaging:
- In different stages of cell division, the loose and indistinct chromatin in the interphase nucleus condenses into structured chromosomes.
- Each chromosome has a primary constriction called the centromere, which holds two chromatids.
- Disc-shaped structures called kinetochores are found on either side of the centromere.
Types of Chromosome
- Chromosomes play a pivotal role in genetics and cell division.
- They can be classified based on the position of their centromere:
1. Metacentric Chromosome:
- Centromere is situated in the middle, dividing the chromosome into two equal arms.
- Both arms are of approximately the same length.
- Metacentric chromosomes exhibit a symmetrical appearance.
2. Sub-metacentric Chromosome:
- The centromere is positioned slightly away from the center, causing one arm to be shorter than the other.
- One arm is noticeably longer than the other.
- Sub-metacentric chromosomes display asymmetry due to the unequal arm lengths.
3. Acrocentric Chromosome:
- The centromere is located close to one end of the chromosome.
- This results in one extremely short arm and one very long arm.
- Acrocentric chromosomes are characterized by an elongated appearance with a small, almost non-existent, short arm.
4. Telocentric Chromosome:
- The centromere is positioned at the terminal end of the chromosome.
- These chromosomes have one long arm and appear to lack a short arm.
- Telocentric chromosomes are elongated with an extended single arm.
- Secondary Constrictions and Satellites:
- Some chromosomes feature additional non-staining constrictions at fixed locations.
- These secondary constrictions create the appearance of small fragments attached to the main chromosome, and these fragments are called satellites.
- Secondary constrictions are essential for various genetic processes and are associated with the formation of specific parts of chromosomes, such as nucleolar organizing regions.
- Microbodies are membrane-bound organelles found in both plant and animal cells.
- These minute vesicles play a crucial role in various cellular functions.
- Microbodies contain a variety of enzymes that are essential for specific metabolic processes. These enzymes are enclosed within the membrane of the microbody.
- Types of Microbodies:
There are two main types of microbodies:
- Peroxisomes are a type of microbody that contain enzymes involved in various metabolic reactions.
- One of their essential functions is the breakdown of fatty acids, which provides energy for the cell.
- Additionally, peroxisomes are involved in detoxification processes.
- Glyoxysomes are a specific type of microbody found in plant cells.
- They are responsible for converting stored fats into carbohydrates during seed germination.
- This conversion process provides an energy source for the growing seedling.
Microbodies have distinct functions, which vary depending on the type of microbody:
- These organelles are essential for fatty acid metabolism and detoxification processes.
- They play a role in breaking down long-chain fatty acids and detoxifying harmful substances, such as hydrogen peroxide.
- Glyoxysomes are primarily involved in the conversion of fats into carbohydrates.
- This process is crucial for providing energy to the growing plant embryo during seed germination.
- Microbodies contribute to the overall metabolic and cellular functions of the cell.
- They help in maintaining energy balance, lipid metabolism, and detoxification, which are vital processes for cell survival and growth.