An eukaryotic cell is covered by a two layer plasma membrane. The region inside the plasma membrane can be divided into fluid like cytoplasm and Nucleus. The nucleus is an organelle found in all eukaryotic cells with the exception of Red blood cells. It happens to be the largest organelle in animal cells and also one of easily identifiable by light microscopy.
Composition of Nucleus:
Nucleus is regarded as the master controller of the cell as it contains the cell’s hereditary information. It also regulates cells growth and division. So this is really an important part of the cell and needs to be protected. The nucleus has a double membrane covering called as nuclear membrane or envelope, which separates the nucleus from the cytoplasm. The organization of this envelope is similar to plasma membrane surrounding the cell and it runs connected with the rough endoplasmic reticulum. In spite of being separated from cytoplasm by means of nuclear membrane, nucleus remains functionality connected with cytoplasm through minute apertures in membrane called as nuclear pores.These pores allows passage of materials such as water , proteins and RNA molecules from nucleus to cytoplasm and sometimes in reverse direction. Adjacent to the nuclear membrane there lies a network of intermediate filaments known as the nuclear lamina. This layer is made up of lamin proteins and provides mechanical strength to the nucleus.
Nucleus contains genetic material – DNA
The cell’s hereditary information which accounts for inheritance of characters from parents to next generation is present in the form of nucleic acid Deoxyribonucleic Acid (DNA). DNA contains all the information required for synthesis of all the proteins and regulatory molecules (non-coding RNA’s) which are vital for constructing, organizing and function of our body. Some regions of DNA may not be coding for any functional protein but might be important for controlling expression of the genes (segments of DNA which contains message for synthesis of proteins) in different cells by switching them into ON or OFF states. This process of gene regulation depends on the cells environment and where they are present in the body.
DNA in the nucleus can be present in two forms depending the state of the cell. In a cell which is not in a dividing state, it is visible as entangled mass of thread called as chromatin (DNA plus proteins). At the time of cell division, the chromatin material gets organized into rob shaped structures called as Chromosomes. Every eukaryotic species has a precise set of chromosomes in the nucleus of each cell. For example, in humans, 23 pairs =46 chromosomes and Drosophila contains only 4 pairs=8 chromosomes. `
Nucleolus and other nuclear bodies:
One of the most prominent sub domain in the nucleus happens to be nucleolus. It is seen as a dense body and lacks a membrane . The nucleolus is a region filled with tandem repeats of DNA region which codes ribosomal RNA (rRNA). This indicates main function of nucleolus is to synthesize rRNA and to assemble these subunits of rRNA into ribosomes, a major component for synthesis of proteins in the cell.
Apart from nucleolus there lies few other smaller bodies in the nucleus such as cajal bodies, promyelocytic leukemia bodies, nuclear speckles, etc. The role played by these tiny organelles are not completely known, but some evidence suggests a possible role in transcription and RNA processing.
Nucleus is filled with gelatinous substance called as “Nucleoplasm”. In composition, it is similar to cytoplasm in having water, many dissolved ions and organic molecules. Nucleoplasm offers protection to the contents of nucleus ( DNA and nucleolus) by acting as a cushion to absorb shocks. It also provides a medium for all DNA-dependent functions and transportation of materials such as enzymes, nucleotide, RNA sub-units and transcripts from the nucleus into cytoplasm.
Functions of nucleus:
- The most important function of nucleus is to store genetic material DNA.
- The other important function of nucleus is to provide medium to participate and regulate in all DNA-dependent functions, such as transcription, RNA splicing, RNP assembly, DNA repair, and replication.
- Nucleus plays a vital role in protein synthesis which generally occurs in cytoplasm. The messenger RNA (mRNA) or RNA transcript of a gene segment, required for synthesis of proteins is formed in nucleus. RNA transcription is a process where information necessary for protein synthesis, originally present in the gene / DNA, is copied on to a RNA molecule called as mRNA. Since DNA has to remain in nucleus and protein synthesis can occur only in cytoplasm, this mRNA transcript with the message from DNA travels to cytoplasm, where translation of protein (protein production) occurs with the help of ribosomes and another RNA molecule called transfer RNA. In this process, RNA acts as a messenger of DNA and carries information from nucleus to the cytoplasm, hence the name “messenger” RNA.
- Nucleolus has many repeats of DNA region which codes ribosomal RNA (rRNA) which is crucial for formation of ribosomes).
Image credit: OpenStax.org