Biomedical 06

Nucleus

The nucleus occurs only in eukaryotes. It has a role in controlling the shape and features of the cell. When a cell has grown to a certain size it divides into two cells. It is composed of: 1- Nuclear membrane (nuclear envelope), 2- Nuclear sap, 3- Nucleoli and 4- Chromatin network.

Nuclear sap

1- The nuclear envelope It appears as a double membrane (outer and inner); each is similar in structure to the plasma membrane. Numerous nuclear pores occur in it, allowing RNA and other chemicals to pass while DNA can not go out through it.

Functions:

Structure of the nuclear envelope and nuclear pores

It was used to protect DNA (genetic material that found in the nucleus forming the chromosomes) from reactions that occur in the cytoplasm which could damage it.

2- The nuclear sap (nucleoplasm): It is a colloidal clear medium in which all the contents of the nucleus are embedded It contains lipoproteins, ions, enzymes … etc.

3- Nucleolus

There are one or more nucleoli in each nucleus. It is involved in the formation of ribosomal RNA (rRNA), which is responsible for protein synthesis in ribosomes.

4- Chromatin network: The material of chromatin network is formed mainly of DNA (deoxyribonucleic acid) as a double helix around a core of protein called histone. DNA contains the code of genetic information forming chromosomes. Chromatin network is found in two forms which are:

1- Euchromatin: They are called active chromatin because they found in active cells. They are involved in protein synthesis. They are called extended chromatin because they appear as thin threads.

2- Heterochromatin:

They are called inactive chromatin. They are not involved in protein synthesis. They are called condensed chromatin.

Heterochromatin appears as: 1- Peripheral chromatin: when they are attached to the inner nuclear membrane (nuclear envelope).

2- Chromatin islands: when they are scattered as granules in the nuclear sap.

Functions of chromatin network: 1) It carries genetic information. 2) It directs protein synthesis by coding the DNA bases to form mRNA.

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