The nucleus is a protoplasmic body that is covered with a double membrane containing all the genetic information required to control cellular metabolism and transmission to the progeny. It controls the activities of all the other organelles of the cell. For example, the nucleus tells the ribosomes when to start protein synthesis and decides when the cell must undergo division. Even autonomous cell organelles like mitochondria and chloroplast are dependent on the nucleus to some extent. As the nucleus controls the activities of all the cell organelles, it is known as the brain of the cell or the director of the cell.
Occurrence of the nucleus
The nucleus is absent in prokaryotic cells. Their genetic material lies haphazardly in the cell in the form of the nucleoid. The nucleus is found in all eukaryotic cells, except mammalian RBCs and mature sieve cells of vascular plants. This point is very important from the NEET 2022 point of view. If you need more information regarding the NEET 2022 Exam Pattern and NEET 2022 Important Chapters, check NEET Exam Pattern.
Number of nuclei present in a cell
- Usually, a single nucleus is present per cell (uninucleate). However, some cells may contain more than one nucleus.
- Binucleate: The cells which contain two nuclei are called binucleate cells. An example of this is the protozoan paramecium caudatum. Paramecium contains a macronucleus and a micronucleus. The macronucleus controls the metabolic activities of the organism, and the micronucleus contains genetic information.
- Polynucleate: The cells which contain multiple nuclei are called multinucleate cells. You can see the multinucleate or polynucleated condition in some cells of bone marrow, latex vessels, striated muscles, several fungi and algae. Multinucleate animals or protistan cells are also referred to as syncytial cells (e.g., the epidermis of Ascaris), while in plants and fungi, they are called coenocytic cells (e.g. rhizopus, vaucheria). Acellular slime moulds have a multinucleate protoplasmic body called plasmodium.
The shape of the nucleus
The nucleus varies in shape from cell to cell. For example, epithelial cells of the intestine contain an oval-shaped nucleus, and monocytes contain a bean-shaped nucleus; some other types of WBCs have horseshoe-shaped nuclei WBC and neutrophils have a multi-lobed nucleus. These points can be asked in NEET 2022. If you want to learn more about NEET 2022 preparation tips, check NEET Previous Year Question Paper here NEET 2022 Exam.
Structure of the nucleus
The nucleus is made of the following structures:
- Nuclear membrane
- Nucleoplasm
- Chromatin
- Nucleolus
Let us have a look at the ultrastructure of the nucleus in detail:
Nuclear membrane or nuclear envelope: The nucleus is enclosed by a double membrane called the nuclear membrane. A nuclear membrane is also called a karyotheca or nuclear envelope. There is an outer and an inner membrane. The inner membrane is smooth. The outer membrane may be smooth or has ribosomes attached to it, and it is continuous with the rough endoplasmic reticulum. The space between the outer and the inner membrane is called perinuclear space.
It is about 10-50 nm in size. The outer and inner membranes fuse, but the fusion is not continuous. The nuclear membrane acts as a barrier to prevent the passage of macromolecules like proteins to and from the nucleus. The nuclear membrane has holes called nucleopores, which are around 3000-4000 in number. Nucleopores are formed of proteins called nucleoporins. Nuclear pores have a complex structure. They may have a diaphragm, septum, plug of electron-dense material or nucleoplasmin, blebs or annuli.
The pores and their annuli form a pore complex called an annulated pore. The nuclear membrane encloses all the structures of the nucleus. It separates the nucleoplasm from the cytoplasm. If you find this discussion a little tricky, checking NCERT Solutions for Class 12 will be helpful.
- Nucleoplasm: It is the transparent, semi-solid fluid present inside the nucleus. It has chromatin material, nucleolus, and certain enzymes like DNA polymerase, RNA polymerase, and nucleoside phosphorylase suspended in it. These enzymes catalyse all the activities that occur inside the nucleus. The nucleoplasm also contains nucleotides which help in DNA replication. The nucleoplasm and cytoplasm are collectively called protoplasm. This term protoplasm was coined by J. E. Purkinje in the year 1839. This point may be helpful in NEET UG 2022.
- Chromatin: It is a hereditary DNA-protein fibrillar complex that occurs in a haphazard entangled fashion in the nuclear matrix. They are differentiated into two regions- euchromatin and heterochromatin. Euchromatin is the narrow, lightly stained and diffused fibrous part which forms the bulk of chromatin. Heterochromatin is a wider, darkly stained and condensed granular part attached here and there to the euchromatin.
Chromatin has two parts- DNA ( acid) and histone proteins. The presence of hereditary material inside the nucleus was proved by the work of Joachim Hammerling in 1953 on single-celled algae, Acetabularia. The whole chromatin is not functional. Generally, only a portion of euchromatin associated with acid proteins takes part in the transcription or formation of RNAs. During the prophase of nuclear division, the chromatin fibres condense to form a definite number of thread-like structures called chromosomes.
- Nucleolus: It is present in the centre of the cell and produces ribosomes. The nucleolus is also called the little nucleus or the cell’s ribosome factory. Its function is the manufacture and assembly of ribosomes. Nucleolus was discovered by Fontana in 1781, described by Wagner in 1840 and provided with its present name by Bowman in 1840. These points are a part of the NEET 2022 Syllabus as well.
The nucleolus is attached to the chromatin at a specific region called the nucleolar organiser region (NOR). Commonly 1-4 nucleoli are found in a nucleus. However, upto 1600 nucleoli are reported in the oocytes of Xenopus. A covering membrane is absent around the nucleolus. Because of this, the contents of the nucleolus are in direct contact with the nucleoplasm. However, calcium seems to help in maintaining its configuration.
Nucleolus stores nucleoproteins, which are synthesised in the cytoplasm and transferred to the nucleolus. If you want to learn these concepts in a face to face class, Find Aakash Coaching Institute Near You.
Functions of the nucleus
- The nucleus is an indispensable part of the eukaryotic cells. It stores genetic information in the form of DNA, which can be passed from generation to generation. It also controls cellular activities.
- The nucleus contains hereditary material in the form of chromatin. Chromatin possesses all the genetic information required for the growth and development of the organism, its reproduction, metabolism and behaviour.
- The nucleus controls cell metabolism and other activities by forming RNAs (mRNA, rRNA, tRNA) that control the synthesis of a particular type of enzyme.
- The nucleolus part of the nucleus helps in the manufacture of ribosomes.
- The nucleus is very important for cell division and differentiation to occur inside the cell.
- The cell can only replicate if the nucleus replicates. So, the nucleus is important for cell replication.
Fun facts about the nucleus
- The nucleus was the first cell organelle to be discovered.
- The nucleus was first observed in orchid root cells by Robert Brown in 1831.
- Another name for the nucleus is karyon, so the study of the nucleus is called karyology.
- The nuclear membrane disintegrates during cell division but regenerates after the two new cells separate. Check NEET FAQs for more biology-related fun facts.
We now know that the nucleus is the most important organelle of the cell. The functions of all the organelles of the cell depend on it. If you understand this topic thoroughly, it will be helpful for your NEET Syllabus as well. All the necessary information regarding NEET 2022, like NEET 2022 notification, NEET 2022 registration, NEET 2022 Eligibility Criteria, NEET 2022 Application form, have been provided here for your reference if it is your goal to attend this competitive exam. Preparing for a competitive exam can be challenging; only the fittest survive. Work hard starting from now itself and set your dream score. Keep an eye on NEET 2022 Result, NEET 2022 Counselling and NEET 2022 Cut-off here to get an idea.
FAQs
1. Do all the cells have a single nucleus?
Although most cells are uninucleate, some cells may be binucleate (containing two nuclei) or multinucleate (containing multiple nuclei). An example of binucleate cells is paramecium caudatum, and examples of multinucleate cells are rhizopus and vaucheria. Even some cells lack a nucleus, e.g. mammalian RBCs and mature sieve cells of vascular plants. If you have more such questions, check Biology Concepts.
2. What happens if we remove the nucleus from the cell?
The nucleus is an integral part of the cell. It carries out all the cell's metabolic functions, tells the ribosomes when to start protein synthesis and decides when the cell must undergo division. Even autonomous cell organelles like mitochondria and chloroplast are dependent on the nucleus for their function. So, if the nucleus is removed, the cell won't be able to function properly. If you have more questions regarding the nucleus and how it functions, check NCERT Solutions for Class 12 Biology.
3. What question can be expected from this topic in NEET-UG 2022?
NEET UG contains the objective type of questions. They give you four options, from which you have to select one correct option. From this topic, you can expect questions that have the single best answer, like “all cells contain nucleus except mammalian RBCs”. Such kinds of questions where exceptions are asked are to be expected in the paper. Having said that, it's impossible to predict the questions that will appear in a competitive exam. However, you can go through the NEET Previous Year Question Paper to get acquainted with the pattern of questions asked. Check NEET Mock Tests and NEET 2022 Crash Course to get an idea about the pattern of questions asked in NEET. You can also attend NEET Mock Tests 2022 for practice. Also, checking out Important Concepts might help you in this regard.