Over the last eight segments in the Simple Cell series we have been looking at the cell nucleus and its major components. The last of those major components is the nucleolus.
The nucleolus is a non-membrane structure found in the nucleus. Represented in the feature graphic as the dark spot in the green nucleus, the nucleolus can be seen using an electron microscope. The nucleolus contains the chromatin material.
The structure of the nucleolus various depending upon the organism it is found in. For the purpose of this article, we will consider the nucleolus found in higher eukaryotic cells including those of humans.
Major components of the nucleolus are the fibrillar center (FC), the dense fibrillar component (DFC) and the granular components (GC). In plants an additional structure, the nucleolar vacuole, can be found in the nucleolus.
Like everything else we have examined in the cell and nucleus, the location of the nucleolus is not random. It is determined by what is known as the nucleolar organizing region. The nucleolar organizing region is composed of sequenced duplicates of rRNA genes located on several chromosomes.
The main function of the nucleolus, besides containing the chromatin material is the synthesis of RNA, especially mRNA and rRNA and the synthesis of ribosomes.
- RNA synthesis. Nearly 50% of RNA is synthesized in the nucleolus. Pre-mRNA and RNA polymerase II are involved in the biogenesis of mRNA in the nucleolus.
- Ribosomal Subunit Assembly. This process starts with the transcription of rRNA precursor molecule from the DNA. The rRNA precursor molecule is a long molecule that is then converted into 3 RNA molecules. These RNA molecules are combined with specific proteins to form the ribosomal subunits. The subunits are then transported out of the nucleus into the cell cytoplasm where they are used in the ribosomal synthesis of various proteins.
I don’t know how to say it any different, but with each segment of this series we have repeatedly demonstrated the complexity of the so-called simple cell. The laws of chemistry, physics and biology state that this degree of complexity cannot arise from chaos and non-complexity as evolution requires. From bacteria to man, only the hand of an omniscient Creator God could account for the complexity we see in every cell.