What Does The Nucleus Do?

DNA is the most important molecule for life. DNA contains the genetic code unique to each individual; the genetic code that is the blueprint for the organism and the information that allows cells to perform their functions. As such, the body needs to keep its DNA safe from external threat, as even a slight change in DNA can affect the entire organism. Luckily, cells have specialized structures, meant to do exactly that.

In eukaryotic cells, the nucleus is a membrane-bound organelle that contains the organism’s genetic information in the form of DNA. Inside the nucleus, DNA is organized into tightly coiled linear structures called chromosomes. These chromosomes contain all the genomic information relevant for organizing the body and constructing the proteins necessary for it to function. The main 2 functions of the nucleus are to protect DNA and control the activity of the cell by regulating gene expression. The nucleus regulates gene expression by controlling the rate of RNA transcription. The nucleus can be likened to the main control center of the cell. The nucleus plays a central role in deciding which genes get expressed and when.

Eukaryotic cells generally have a single nucleus located near the center of the cell. Some eukaryotic cells, however, do not have a nucleus, like red blood cells, and others have multiple nuclei, like osteoclasts (bone cells). Prokaryotic cells, on the other hand, lack a nucleus entirely. Their genetic material instead exists freely floating in the intracellular cytoplasm.

Nucleus Structure

Credit: WikiCommons CC0 1.0

The nucleus is the largest organelle, measuring about 6 μm in diameter (1 μm = 10−6 m) in mammalian cells. and taking up 10% of the volume of the cell. Inside the nucleus is a viscous liquid called the nucleoplasm. The composition of the nucleoplasm is similar to the composition of the cytosol. In general, the nucleus takes on a roughly spherical shape though this shape can differ depending on the cell.

Surrounding the nucleus is the nuclear envelope—a complex of two phospholipid membranes arranged parallel to each other. The intermembrane space between the two layers of the envelope is directly connected to the endoplasmic reticulum. Like the larger cell membrane, the nuclear envelope regulates the flow of substances in and out of the nucleus. Small particles like oxygen pass through easily, but larger molecules are kept out. Dotting the exterior of the nuclear envelope are several channels called nuclear pores. Nuclear pores contain proteins that facilitate the transport of larger materials through nuclear envelope.  The typical mammalian nucleus has about 3,000-4,000 nuclear pores.

Inside the nucleus is a dense interlocking network of fibers called the nuclear lamina. The nuclear lamina functions analogously to the cytoskeleton of the larger cell; it gives the nucleus its mechanical strength and support. The lamina is composed of tough fibrous proteins called lamina.

In the center space of the nucleus lies the most complex structure found in the nucleus, the chromosomes. Chromosomes are tightly condensed chains of DNA. The average mammalian cell contains about 2 meters of DNA wrapped into chromosomes. Chromosomes contain the bulk of the genetic information of the organism. They are also the entities responsible for hereditary. During sexual reproduction, a zygote inherits sets of chromosomes from both parents, which combine to form a unique genome.

A stain of the nucleolus. Credit: Orlov I via WikiCommons CC BY-SA 4.0

The last major structure in the nucleus is the nucleolus. The main function of the nucleolus is to construct ribosomes, the cellular structures that physically assemble the proteins encoded in DNA and RNA. The nucleolus is a complex of proteins, DNA, and RNA that form around specific regions of the chromosomes. These chromosomal regions encode for ribosomal RNA (rRNA) a special kind of RNA that conglomerates with ribosomal proteins to make the ribosomes. The newly created ribosomes are then shuttled out of the nucleolus into the cytoplasm to do their job. On stains, the nucleolus appears as a darkened blob near the center of the nucleus.

Nucleus Function

The nucleus serves two major functions. First, the nuclear envelope protects DNA from external threats and maintains the integrity of the genome. Chromosomes are extraordinarily complex and even slight damage to them can cause many problems in the cell. The nuclear envelope protects chromosomes by controlling what can go in and out of the nucleus. Small molecules like oxygen can freely diffuse through the membrane but larger proteins and RNA molecules require special transport proteins to enter and exit. For example, the nuclear envelope keeps DNA and RNA viruses out of the nucleus, so they can’t use the cell’s machinery to reproduce.

Second, the nucleus regulates the process of gene expression. During gene expression, information encoded in DNA is extracted and copied into the form of messenger RNA (mRNA) in a process called transcription. The sequence of nucleotide bases in mRNA encodes the structure of proteins. The mRNA strand is exported to the cytoplasm and then fed into ribosomes, which construct the encoded proteins This process is called translation.

The nucleus regulates transcription by keeping proteins that initiate transcription out of physical range of DNA until they are called for by certain signaling pathways. An example of this regulatory feedback mechanism is seen in glycolysis, the first step of cellular respiration. During glycolysis, the enzyme hexokinase stimulates the breakdown of glucose by first bonding to glucose to make an intermediary product called fructose 6-phosphate. When fructose 6-phosphate concentrations are high, hexokinase is isolated in the nucleus where it interacts with transcription mechanisms to slow the transcription of genes involved in glycolysis. This results in slowing the rate of glycolysis.

Regulation of gene expression via hormone signaling pathways. Credit: A Zifan via WikiCommons CC BY-SA 4.0

The nucleus regulates transcription in the presence of hormones too. Hormones attach to receptor proteins which shuttle them inside the cell nucleus. Once there, the protein-hormone complex initiates or slows the transcription of genes related to hormone production, depending on the hormone.

The nucleus is also the site of post-transcription modification. In eukaryotes, when mRNA is transcribed, it is not immediately ready for translation. It must first go through some modifications to alter its structure. One kind of post-transcription modification found only in eukaryotes is RNA splicing. During RNA splicing, sequences of mRNA that do not code for proteins (called introns) are cut out of the strand and it is rejoined into a continuous molecule. Other post-transcription modifications include 5′ capping and 3′ polyadenylation, both of which involve slightly altering the nucleotide structure of RNA.

The nucleus plays a role in cell division. Cells reproduce by binary fission, i.e. splitting into two copies. Before undergoing mitosis, the cell makes a complete copy of its chromosomes. Once copied, the nuclear envelope begins to dissolves, freeing the chromosomes in the cytoplasm. The two sets of chromosomes are pulled to the poles of the cell. and fission occurs, splitting the cell into two. The nuclear envelope in each cell regenerates, forming two distinct cells.

Origin Of The Nucleus

As with many fundamental biological entities, the evolutionary origin of the nucleus is not clear. The nucleus is considered the defining characteristic of eukaryotic cells, so it is thought that the story of the development of the nucleus and the story of the emergence of eukaryotic life is one and the same.

Currently, there are 4 major hypotheses regarding the emergence of the nucleus. According to the first hypothesis, the nucleus evolved as a result of the symbiosis of archaea and bacteria. Ancient archaea invaded and began to inhabit bacterial cells, eventually developing into the modern nucleus. This hypothesis is similar to the accepted explanation for the existence of eukaryotic mitochondria and chloroplasts, which are thought to have evolved from symbiotic bacteria engulfed by early pre-eukaryotic cells.

Second, it has been proposed the nucleus evolved independently in bacteria without the need for symbiosis. Evidence for this hypothesis consists of the presence of modern bacteria that contain proto-nuclear structures and internal membranes. These membranes contain porin similar to the nuclear pores.

Third, some biologists have argued that the nucleus originally stemmed from bacteria that developed a secondary cell wall. Over time, the outer cell wall grew larger and the inner cell wall became the nuclear envelope.

Lastly, it has been hypothesized that the nucleus came from the viral infection of a prokaryote. According to the theory of viral eukaryogenesis, the membrane-bound nucleus is the result of a DNA virus that infected ancient archaebacteria. Some evidence for this theory includes the fact that viral DNA and eukaryotic DNA share similarities like a linear structure and that some DNA viruses are capable of protein biosynthesis.

To sum up, the nucleus is an organelle that contains the DNA of the cell. It is composed out of double layer nuclear envelope which envelops the chromosomes and the nucleolus. The nucleus functions to protect the chromosomes from damage and to regulate the transcription of genes.  The nucleus protects DNA by regulating the flow of materials in and out of the cell and regulates transcription by adding or removing transcription factors. During cellular reproduction, the nucleus dissolves then later regenerates.