Nucleus

Introduction

All biological cells have different structures with specialised functions. These structures are called the cell's organelles. According to the structure of a cell's organelle nucleus, all cells are divided into two main groups: prokaryotes and eukaryotes. This chapter looks at the structure of the most important cell organelle - the nucleus.

Genetic information

All biological cells store their genetic information in the form of DNA. DNA is an abbreviation of deoxyribonucleic acid. DNA molecules determine the types of protein synthesised within the cell. Eukaryotes store their DNA in their nucleus and prokaryotes store their DNA in their nucleoid.

Compared to other cell organelles, the nucleus is usually the largest. Besides containing a cell's genetic information, it also controls all biochemical reactions within the cell. Sometimes the nucleus is called the 'cell's brain'. See image 1.

Structure of a nucleus

A nucleus is roughly spherical, membrane-bound organelle responsible for a cell's growth and reproduction. It is surrounded by a cell's cytoplasm. Compared with other organelles, it is usually the largest. See image 2.

1. Nuclear envelope - a double, porous membrane that is very similar to the cell's outer membrane. Nuclear pores are also known as the nuclear pore complex. The nuclear envelope separates the nucleus from the rest of the cell components, maintains its shape and regulates molecular transport between the nucleus and cytoplasm. A cell's endoplasmic reticulum (see chapter one of this topic) is attached to the nuclear envelope.

2. Nucleoplasm - liquid substance inside the nucleus which is similar to cell cytoplasm. Structural units of DNA and RNA (ribonucleic acid) and different enzymes are in nucleoplasm. Nucleoplasm is also referred to as karyoplasm.

3. Nucleolus - can be called 'the nucleus of the nucleus'. As it is a part of the cell's organelle, it falls in to the category of sub-organelles. Nucleolus is responsible for the production of ribosomes - the protein-making devices of the cell. Ribosomes are cytoplasmic granules composed of RNA and protein. No membrane separates the nucleolus from the nucleoplasm. Most plant and animal cells have one or more nucleoli.

4. Chromatin - is made of DNA, RNA and associated proteins. Under a microscope chromatin resembles a pile of tangled threads. Chromatin is divided into two categories: euchromatin and heterochromatin. During cell division chromatin forms more defined and visible structures called chromosomes. Chromosomes carry genetic information in the form of genes. Different types of chromatin have different molecular structures and therefore they stain differently when dyed for scientific research purposes. Euchromatin forms the active parts of chromosomes and heterochromatin forms the genetically inactive parts. It is believed that heterochromatin serves mostly structural purposes.

Nucleoid

Most bacteria are unicellular prokaryotes. Prokaryotic cells do not have a defined nucleus. Their DNA freely 'floats' in a more or less specified area of the cytoplasm called the nucleoid. A bacterial nucleoid usually consists of a single molecule of DNA. In order to fit within the cell, long molecules of DNA have to be tightly packed. An enzyme called topoisomerase, coils and twists the DNA molecule upon itself so when packed, it resembles something like a tangled spring wire. In most bacteria, two ends of the DNA bond together, forming a circle, so most bacterial DNA is circular. All bacterial genetic information is stored in this single, circular DNA. Most bacteria have only one chromosome and they reproduce only asexually.

Bacterial cells also have other small circular DNA structures called plasmids. They are much smaller in size than the main chromosome and usually contain only genes that are not essential for a cell's survival. The antibiotic resistance of some bacteria is believed to be encoded in their plasmids. Plasmids can replicate independently from the chromosomal DNA. See image 3.