Question

What is Karyotheca?

Answer 1

The nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus in each cell, but some cell types, such as mammalian red blood cells have no nuclei, and a few other cells including osteoclasts have many nuclei. The nucleus is the largest and most prominent organelle inside the cell. The nucleus houses the genetic material (DNA) and it is the site of RNA production, which helps regulate cell metabolism by generating various enzymes.

Complete answer:
Karyotheca: The nuclear envelope is called karyotheca, it is a double membrane that encloses the entire organelle and isolates its contents from the cellular matrix. It is made up of two lipid bilayer membranes-

an inner nuclear membrane and an outer nuclear membrane, the space between the membranes is called the perinuclear space. It is usually about 20–40 nm wide.

The outer nuclear membrane is continuous with the endoplasmic reticulum membrane.

The inner nuclear membrane encloses the nucleoplasm and is covered by the nuclear lamina; it is a mesh of intermediate filaments which gives structural support to the nucleus and stabilizes the nuclear membrane as well involved in chromatin function and its entire expression.

The nuclear envelope has many nuclear pores that allow different materials to pass through inside and outside between the cytosol and the nucleus.

The inner and outer membranes are connected to each other by nuclear pores. A nuclear membrane has thousands of nuclear pores. Nuclear pores are large hollow protein complexes that link the inner and outer membrane.

The nucleus contains all of the cell's genome, except for the small amount of mitochondrial DNA and in plant cells, plastid DNA.

Nuclear DNA is wrapped around proteins called histones to form chromosomes. The genes are located within these chromosomes which promotes cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression hence the nucleus is the control center of the cell.

The nuclear envelope is impermeable to large molecules and nuclear pores regulate the nuclear transport of molecules across the envelope. The pores cross both nuclear membranes providing a channel through which larger molecules are actively transported by carrier proteins while small molecules and ions can move freely.

Note:
The interior of the nucleus does not contain any membrane-bound organelle, still its contents are not uniform, and contains a number of nuclear bodies made up of unique proteins, RNA molecules, and parts of the chromosomes. The best-known of these is the nucleolus, which is involved in the assembly of ribosomes, and after being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate messenger RNA.