26. Nucleus. Structure.

Question 1

What is Nucleus? Brief

Answer 1

• The cell nucleus contains all of the information for a cell.
• It is membrane enclosed w/ DNA molecules packaged in proteins in form of chromosomes

Question 2

Components of the Nucleus?

Answer 2

The key components of the Nucleus are:

-Nuclear envelope
oA double membrane that encloses organelle
oPolyribosomes are attached to the outer membrane, indicating continuity of the envelope with the endoplasmic reticulum.
oThe nuclear envelope includes nuclear pore complexes which regulate transport b/w the nucleus and cytoplasm.

-The Nucleoskeleton
operforms the same function for the nucleus as the cytoskeleton does for the whole cell

-Chromosomes
oThe majority of the cells genetic DNA in the form of chromosomes

-Nucleolus
oForms around tandem repeats of rDNA, DNA coding for ribosomal RNA. These regions are called nucleolar organiser regions
oThe main role of the nucleolus is to synthesize rRNA and assemble ribosomes
oThe nucleolus is a highly basophilic area of the chromatin

Question 3

Lightly stained and Dark stained chromatin?

Answer 3

• Chromatin with DNA that is active in transcription stains lightly as the nuclear DNA is not as condensed.
• This light-stained nuclei is called euchromatin
• Dark-stained DNA is known as heterochromatin and indicates that the chromatin is inactive as it is more tightly wound.

Question 4

Nucleus under EM?

Answer 4

Electron microscopy shows that the nucleus is surrounded by two parallel unit membranes separated by a narrow (30-50nm) perinuclear space.

Question 5

Points about chromatin?

Answer 5

The chromatin is a complex of DNA, protein (primarily histones) and RNA, it has the following functions:
•To package DNA into a smaller volume to fit in the cell
•To reinforce the DNA macromolecule to allow mitosis
•To prevent DNA damage
•To control gene expression and DNA replication

The organisation of the chromatin can be considered as follows:
•DNA wraps around histone proteins forming nucleosomes
•Multiple histones wrap into 30nm fibre, wrapping the nucleosomes in their most compact form (heterochromatin)
•Higher-level DNA packaging of the 30nm fibre into metaphase chromosome