FUNBIO 6 - Cell Ultrastructure: Nucleus

Question 1

Describe the Nuclear Envelope.

Answer 1

The entire contents of the nucleus is isolated and surrounded by a double-layered membrane called a NUCLEAR ENVELOPE. Between the inner layer of the nuclear envelope and the outer layer of the nuclear envelope is a 20-40nm space called the PERINUCLEAR SPACE. The nuclear envelope runs continuously with the rough endoplasmic reticulum that surrounds the entire nucleus.

Question 2

Describe the Nuclear Lamina.

Answer 2

The inner layer of the nuclear envelope is lined with the NUCLEAR LAMINA, which is lined with a network of protein filaments. The nuclear lamina provides structure to the nuclear envelope and helps with the positioning of the nuclear pores.

Question 3

Describe Nuclear Pores.

Answer 3

NUCLEAR PORES are ring-like octagonal structures that are 50nm in diameter that span from the outer layer of the nuclear envelope to the inner layer of the nuclear envelope, therefore they have a cytoplasmic face and a nuclear face. They are made of 8 protein subunits that surround the PORE COMPLEX which is 9nm in diameter and allows for the transport of water-soluble particles through the nuclear envelope. Nuclear pores take up 10-30% of the nuclear envelop surface.

Question 4

What is the cytoplasm in the nucleus called?

Answer 4

The cytoplasm in the nucleus is called the NUCLEOPLASM. It is a highly viscous liquid that surrounds ribosomes and nucleoli. Nucleotides, nucleosides, and nuclear matrix are all dissolved in the nucleoplasm.

Question 5

Describe chromatin.

Answer 5

When a cell is in interphase and is not dividing, CHROMATIN is visible under a light microscope in the nucleoplasm. Chromatin are 2nm in diameter. There are two types of chromatin:

1. Euchromatin
   - lightly packed chromatin
   - DNA is transcribed from euchromatin
2. Heterochromatin
   - tightly packed form of DNA
   - found at peripheral regions of nucleus
   - DNA is not transcribed from heterochromatin
   - used for gene regulation and protection of the integrity of chromosomes

Question 6

Describe Nucleoli.

Answer 6

There are one to four nucleoli per cell. They all disappear during cell division and reappear post-division. Chromosomes, with nucleolar organizer regions, determine the location of nucleoli. The size of nucleoli is a good indicator of the activity in the cell, so they can be small or take up 25% of the nucleoplasm.

Question 7

Describe ribosome formation.

Answer 7

Within the nucleolus, chromosomal DNA transcribes rRNA which then associates with proteins to form ribosomal particles:

1. The nucleolar organizer DNA transcribes a 45S precursor rRNA molecule, which contains 28S, 5.8S, and 18S rRNA molecules.
2. 5S rRNA molecule is assembled in the cytoplasm of the cell.
3. The 45S precursor rRNA molecule complexes with proteins made in the cytoplasm of the cell and forms the 80S pre-ribosomal particle.
4. This then breaks down into the 40S and 60S subunits. The 60S subunit takes 1hr to assemble.
5. Finally, the 40S and 60S subunits assemble in the cytoplasm of the cell as a ribosomal particle.