Nucleus

NUCLEUS

  • 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:

1. Chromatin:

- 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. 

3. Nucleoli:

- 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

  • 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:

1. Peroxisomes:

- 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. 

2. Glyoxysomes:

- 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.

  • Functions:

Microbodies have distinct functions, which vary depending on the type of microbody:

Peroxisomes:

- 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:

- 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. 

  • Importance:

- 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.