Cell nucleus Nucleus A membrane bound organelle containing

Cell nucleus

Nucleus • A membrane bound organelle containing most DNA of the cell [mitochondria is the other organelle containing DNA] • DNA in the nucleus is complexed with histone to form chromatin • Nucleolus is a dense nonmembrane bound area within the nucleus Function • It has two main functions: 1 - During cell division : its DNA is replicated to give each daughter cell complete copy of the genetic material 2 - During the rest of cell life: its DNA is transcripted to RNA that direct then translated into protein

chromatin Chromatin = DNA + histone It consists of double-stranded DNA complexed with histones (histone is a globular protein)

Chromatin is present in 2 forms 1 – Euochromatin (the active extended form) 2 - Heterochromatin (the inactive condensed form) • At the periphery of the nucleus • At site of the nucleolus • Scattered in the nucleus NOTES 1 – The function of DNA is to be copied, either copied to another DNA (replication), or copied to RNA (transcription). So, Only extended chromatin is active because DNA can be copied only when extended 2 - Euchromatin/heterochromatin ratio is higher in malignant cells than in normal cells [Actively dividing means more chromatin needs to be extended for DNA replication] 3 - Females have 2 X chromosomes. One is active (extended), the other is inactive (condensed) and against nuclear membrane and known as Barr body

Chromosome • Chromosomes consist of extensively folded chromatin • The d-chromosome consists of 2 sister chromatids(=2 s-chromosomes) connected I a point called a centromere • Chromosomes are only seen in actively dividing cells. During the rest of cell life, the genetic content is present as euochromatin and heterochromatin

Chromosome • s-chromosomes (single) = chromatid • d-chromosomes (double)

• Non dividing cell: Euochromatin and heterochromatin • Dividing cell: chromosomes

Nuclear envelope • The nuclear envelope surrounds the nuclear material and consists of two parallel membranes separated from each other by a narrow space • Nuclear pores are openings in the nuclear envelop, that allow passage of RNA synthesized in the nucleus to the cytoplasm Nucleolus • Dense part of the nucleus not surrounded by a membrane • It is the site of manufacturing r. RNA, so it contains DNA, and the r. RNA • It is well-marked in cells actively synthesizing protein (need for more r. RNA) • It disappears during cell division (DNA is condensed in chromosomes)

Signs of cell death Three signs may appear in the nucleus indicating that the cell is dying : 1. Pyknosis: the nucleus becomes smaller in size and dark and eccenteric. 2. Karyorrhexis: the nucleus breaks down into smaller fragments. . It usually follows pyknosis 3. Karyolysis: the nucleus dissolves and finally disappears. It usually follows karyorrhexis

Types of cell death 1. Necrosis : Induced cell death [due to external factor eg: infection, toxins, or trauma] 2. Apoptosis : Programmed cell death (cell suicide, self destruction) : by the cell itself (e. g. monthly removal of excess oocytes)