Lecture 05

Nucleus In cell biology, the nucleus (pl. nuclei; from Latin nucleus- inner part), also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. Nucleus is the most conspicuous and the largest organelle in the cell. It contains most of the cell's genetic material, organized with multiple, long, linear, DNA molecules in complex including a large variety of proteins, such as histones, to form chromosomes.

In young cell In long cell In mature cell Single nucleus, multiple nucleus Absent in lower bacteria & BGA

Structure Nucleus is bound by a double layer membrane, called nuclear membrane or nuclear envelope. It is discontinuous unlike the cell membrane and shows numerous pores called nuclear pores. Some of these pores are connected with branches of endoplasmic reticulum.

The membrane encloses a clear fluid matrix called nucleoplasm or karyolymph. It encloses a fine network of thread-like structures, called chromatin network. It is composed of chromatins that contain DNA and histones arranged in a specific manner. The chromatin condenses to form distinct structures called chromosomes at the time of cell division.  

The nucleus also shows a round body called nucleolus in between the chromatin network. The large nucleolus consists of intermingled granular and fibrillar components. It is rich in- RNA Ribosome Synthesis of proteins

Composition Nucleoproteins- Inorganic salts-

Function The main function of the cell nucleus is to control gene expression and mediate the replication of DNA during the cell cycle. Nucleus maintains the cell and brings about its growth by directing the synthesis of structural proteins. Nucleus regulates the metabolic processes in the cell by directing the synthesis of functional proteins such as enzymes.

Nucleus distributes the genetic material equally through the process of replication. Nucleus is involved in the formation of ribosome. Nucleus contains the genetic information necessary for reproduction, development and metabolism of the organism as a whole.

Vacuole Most mature plant cells have one or several vacuoles that typically occupy more than 30% of the cell's volume, and that can occupy as much as 90% of the volume for certain cell types and conditions. A vacuole is surrounded by a membrane called the tonoplast. Strands of the cytoplasm often run through the vacuole.

Function Storing foods (e.g., proteins in seeds) Storing wastes Storing malic acid in CAM plants Storing various ions (e.g., calcium, sodium, iron) Maintain turgor in the cell.