Cytology

Nucleus:
>site of DNA replication and transcription of DNA.
>contains all of the enzymes required for replication,repair for DNA,transcription and processing of precursor RNA molecules.

>enclosed by NUCLEAR ENVELOP and contains NUCLEAR LAMINA,NUCLEOLUS and CHROMATIN.

Nuclear Envelop:
>double membrane containing pores(90nm.
>outer membrane is continuous with endoplasmic reticulum.


Nuclear Lamina:
>lattice network of proteins lamins.
>lamins attach chromatin to inner membrane of nuclear envelop.
>lamins attach chromatin to inner membrane of nuclear envelop.
>participate in breakdown and reformation of  nuclear envelop during cell cycle.
>phosphorylation of the lamin during prophase of mitosis initiates nuclear disassembly into small vesicles.


Nucleolus:
>responsible for rRNA synthesis and ribosome assembly.
>has three zones:
Granular zone-found at periphery,contains ribosomal precursor particles.
Fibrillar zone-centrally located.contains ribonuclear protein fibrils.
Fibrillar center-contains DNA that is not transcribed.

Chromatin:
>complex of histones proteins,DNA,nonhistone proteins.
Forms of chromatin;
1)Heterochromatin:highly condensed and transcriptionally inactive.in a typical eukaryotic cell ~10% of the chromatin is heterochromatin.e.g barr body.
2)Euchromatin:extended form of DNA and transcriptionlly active.in atypical cell ~90% of total chromatin.