Nucleus

There are two types of organelles in our body, one which are membrane bound and the other which are non-membrane bound.
The Nucleus is a membrane bound organelle present inside a cell. Its is the site of DNA replication and transcription into precursor RNA molecules. All the enzymes neccessary for the replication and transcription proccesses are present within the nucleus. The nucleus is enclosed by a nuclear envelope that contains lamina, nucleolus and chromatin.

The Nuclear Envelope: it is a double membrane containing pores that are approximately 90nm in diameter.The outer nuclear membrane is continuous with the endoplasmic reticulum.

Nuclear Lamina: a network that contains protiens including lamins. These attach chromatin to the inner membrane of the nuclear envelope and participate in the breakdown and reformation of the nuclear envelope during the cell cycle.

Nucleolus: responsible for ribosomal RNA synthesis and ribosome assembly. it has three zones:

1. Granular zone: peripheral containing ribosomal precursor particles.
2. Fibrillar zone: central, containing ribonuclear protein fibrils.
3. Fibrillar center: containing DNA not being transcribed.

Chromatin: a complex of DNA, histone proteins and nonhistone proteins.

Forms of chromatin: two forms:

1. Heterochromatin: highly condensed, transcriptionally inactive. approximately 10% of chromatin
2. Euchromatin: extended form of DNA, transcriptionally active. approximately 90% of chromatin.