NUCLEUS

• Nucleus was first discovered by Robert Brown (1831) in orchid cells. 
• It is the most important part of the cell which directs and controls all the cellular functions. 
• That’s why the nucleus is very often regarded as ‘director of the cell’. 
• Presence of a true nucleus with nuclear membrane and linear chromosomes is the characteristic of all the eukaryotic cells. 
• However, there are some exceptions viz., mature mammalian RBCs, sieve tubes of phloem, tracheids and vessels of xylem.

• As far as the number of nuclei in a cell is concerned, most eukaryotic cells have a single nucleus within them. 
• However, the number may vary in some cells. 

• Depending on the number of nuclei cells may be of following types :
• Anucleate (without nucleus) : Mammalian RBCs.
• Uninucleate : Most Eukaryotic Cells.
• Binucleate : Basidiomycetes, Paramoecium
• Multinucleate : Phycomycetes like Mucor, Rhizopus etc.Red Algae.

• The true Nucleus may be defined as: ‘The cellular structure limited externally by a nuclear membrane surrounded by cytoplasm which contains linear nucleoproteinous chromosomes and carry genetic information from generation to generation’. 
• The carrier of genetic information about the nature of nuclei was established by Hammerling (1953) who worked on the macroscopic unicellular alga, Acetabularia and concluded that the morphology of the plant is solely determined by the type of nucleus contained in the plant body.

STRUCTURE OF NUCLEUS

• Study of the cell cycle has revealed that each cell has two phases in its cycle:

1. Interphase and 
2. Phase of cell division.

• In fact, interphase is the phase between two cell divisions. 
• This is much longer than the phase of cell division; the structure of the nucleus is studied in this interphase only.
• The electron microscopic studies of interphase nucleus have revealed that the nucleus may consists of following four parts:

1.  Nuclear Membrane: 

• It limits the nucleus externally and is also known as karyotheca. 
• It is bilayered, lipoproteins and trilaminar in nature. 
• Outer membrane is called ecto-karyotheca and the inner is endo-karyotheca. 
• The outer membrane is studded with ribosomes while the inner is free of that. 
• The two membranes have a thickness of 75-90 Å  And are apart from each other by a distance of 100-300Å. 
• This space is called perinuclear space.

• The nuclear membrane has many pores. 
• Its number may vary from 1000-10000 in a nucleus. 
• Each pore is about 400-1000 Å in diameter. 
• The number and size of pores may depend on the needs of the cell. 
• Each nuclear pore is fitted with a cylindrical structure called annulus. 
• The pore and the annulus both collectively form the pore complex or pore basket.

2. Nucleoplasm: 

• It is transparent semi fluid, homogenous, colloidal ground substance inside the nuclear membrane. 
• It is also called nuclear sap, karyolymph or karyoplasm. 
• Nuclear chromatin and nucleolus are embedded within nucleoplasm, chemically, it is formed of water, sugars, minerals (Mn2+, Mg2+, etc.), 
• Nucleotides, ribosomes, enzymes, DNA and RNA polymerases, mRNA, tRNA molecules etc. It is alkaline in nature (pH = 7.4  ).

Functions :

• Nucleoplasm forms the skeleton of nuclei and helps in maintaining their shape.
• The process of transcription takes place in the nucleoplasm in which different molecules of RNA are formed.
• It supports nuclear chromatin and the nucleolus.
• Ribosomal subunits are synthesized in the nucleoplasm.

3. Chromatin Net or thread : 

• Electron microscopic studies of well stained eukaryotic nuclei have revealed that presence of darkly stained network of long, fine and interwoven threads which is called chromatin net or thread. 
• It is also known as nuclear reticulatum. 
• It was first reported by Fleming in 1882. During the phase of cell division, the chromatin net is transformed into chromosomes due to high condensation of DNA molecules. 
• These chromosomes are rod like and have definite shape and size chracteristic of an organism.
• The chromatin is chemically nucleoprotein and formed of nucleic acid (DNA) and base proteins i.e., histones . 
• It may be classified in to two categories:

Heterochromatin : 

• It is made of comparatively thick regions which are darkly stained. 
• DNA strands in this chromatin are more condensed. 
• Transcriptionally, it is inactive and late replicative. 
• It does not contain active genes.

Euchromatin : 

• It is true chromatin and is formed of thick and less darkly stained areas. 
• It has loose, less condensed DNA which is transcriptionally, inactive and early replicating.

4.   Nucleolus : 

• Within each nucleus, there is a darkly stained, granular, naked and large organelle without limiting membrane. It was discovered by Fontana in 1781. 
• The term nucleolus was coined by Bowman (1840). 
• The size of nucleolus is comparatively larger in those cells which have rapid rates of protein biosynthesis.

• The position of the nucleolus is generally definite within the nucleus. 
• It is associated with the nucleolar organizer region (NOR) of the nuclear chromosome.
•  It is absent in muscle fibers , RBC, Yeast, sperm and prokaryotes. 
• In general, each nucleus has one or two nucleoli. 
• Its number depends on the number of chromosomes in the species. 
• For each haploid set of chromosomes in the nucleus, there is a single nucleolus. 
• However, a pair of nucleoli may be found in haploid nuclei. 
• In human beings, two pairs of nucleoli are found in each diploid nuclei. 
• In human beings, two pairs of nucleoli are found in each diploid nucleus Xenopus oocytes may contain upto 1000 nucleoli in the nucleoli in the nucleus.

Ultrastructure :  

• The ultrastructure of nucleolus was studied by Borysko and Bang in 1951 and again by Berhard in 1952. 
• On the basis of electron microscopic studies of the structure of nucleolus, de Robertis et al., (1971) described it to be made up of four parts:

I. Fibrillar regions: 

• This part is made up of ribonucleoprotein fibers. 
• It is also called nucleolemma. 
• Each fiber has a length of around 50-80 Å. 

Ii. Granular regions : 

• This part has many granules each having the diameter of 150-200 Å. 
• These are derived from nucleolar fibers, chemically, these granules are also ribonucleoproteins.

Iii. Protein region: 

• This proteinous part is also called pars amorpha. 
• This is the fluid part of the nucleolus in which other parts are found.

iv.Chromatin part:  

• It is made up of chromatin fibers containing DNA. 
• These DNA molecules function as templates for RNA synthesis. 
• The chromatin part may be differentiated into two parts 

a) Perinucleolar Chromatin: 

• It forms a covering or envelope around the nucleolus. 
• It may have ingrowths at certain places inside the nucleolus, which are called trabeculae.

b) Intra-nucleolar chromatin: 

• These chromatin fibers are found in the internal protein region. 
• These form many septa-like structures. 

• Each nucleolus has a dense fibrillar region due to the presence of which it is associated with the nuclear organizer region of chromosomes. 
• These regions have been reported to contain many copies of  DNA responsible for synthesis of ribosomal RNA. 
• These rRNA molecules are rapidly synthesized in this region. 
• The proteins of ribosomes are synthesized in the cytoplasm which is transported to the nucleus and finally to the nucleolus. 
• The rRNA and protein molecules combine to form complete ribosome molecules. 
• These newly synthesized ribosomes are associated with thin fibrils of RNA and look like beaded strings. 
• This structure is called nucleo-nema. 
• On the basis of the presence and structure of nucleo-nema, following three types of nucleoli may be recognized: 

• Nucleolus with nucleo-nema which is more common is all types of cells.
• Nucleolus without nucleo-nema which is commonly found in salivary gland cells.
• Ring shaped nucleolus containing ribonucleoprotein granules and RNA fibrils.
• This is common in endothelial cells and muscle cells.

Functions of Nucleolus

• Nucleoli are the site of rRNA biosynthesis.
• It stores rRNA.
• It helps in the biogenesis of ribosomes.
• It helps in the formation of spindle fibers.
• It plays an important role in mitosis.

Functions of Nucleus

• It controls all the cellular functions. 
• It controls the synthesis of all the structural and enzymatic proteins.
• Synthesis of all the 3 types of RNA (mRNA, tRNA and rRNA) takes place in the nucleus.
• It plays important role in cell division.
• Cell growth is controlled by nucleus
• Nucleus controls cellular differentiation by regulating differential gene expression
• It induces genetic variation and thus helps in organic evolution.
• Sexual reproduction happens due to fusion of two nuclei gametes of opposite sex.
• Due to presence of all these organelles and other structures, a cell functions as self-regulatory systems and provides a definite set of characteristics to different organisms.

NUCLEOLUS cycle & NUCLEOLOGENESIS

• Nucleolus can be seen as a very conspicuous structure in the interphase nucleus. It disappears during mitosis and reappears at the next interphase. 
• The process by which the nucleolus is formed, is described as nucleologenesis. 
• During prometaphase to early telophase, when the nucleolus remains disappeared, a number of non-ribosomal nucleolar proteins as well as U3 s- RNA are found in 
• (i) the peripheral regions of chromosomes and in the 
• (ii) nucleolus derived foci (NDF) found as cytoplasmic particles 1-2 in diameter;

CHROMOSOMES

• Chromosomes are rod-like or filamentous bodies, which are typically present in the nucleus and become visible during the stage of cell division. 
• Presence of true chromosomes is the characteristic of eukaryotic cells.
• Literally, the term chromosomes have been derived from two Greek words; Chroma and soma meaning ‘colored body’. 
• This is named because they appear as darkly stained bodies during cell division when stained with a suitable dye and viewed under compound microscope.
• Chromosomes can well be defined in following way “Chromosomes are individual protoplasmic entities found in the nuclei of almost all eukaryotic cells which multiply themselves through sequential cell divisions and provide physiological and morphological stability to protoplasm and so to a particular individual”
• Chromosomes were first observed by Straburger (1875) in mitotically dividing cells and the name ‘chromosome’ was proposed by Waldeyer in 1888.
• Each species has a definite and constant number of chromosomes in their cells. 
• The chromosome number found in somatic cells of the species is called somatic chromosome number and is usually represented by ‘2n’. This is because, ordinarily, somatic cells contain two copies of each chromosome which are morphologically identical and also have the same gene content and gene location. 
• They are known as homologous chromosomes.
• Chromosomes can darkly be stained by treating the dividing cells by acetocarmine, aceto-Orcein, fuelgen and some other basic dyes.
• In the plant kingdom, the lowest number of chromosomes is found in Haplopappus gracilis and highest in Ophioglossum reticulatum. 
• In the animal kingdom, Ascaris mega-locafella has been found to have the lowest number of chromosomes.
• Chromosomes are chemically nucleio protein consisting of DNA and proteins. 
• The bear genes therefore, regarded as ‘bearer of heredity.’
• Each chromosome is made up of two longitudinally held chromatids which are visible during mitotic metaphase.
• The two chromatids of a chromosome are joined at centromere, the main function of which is the formation of spindle fibers during cell division.
• Nucleolus within nucleus is associated with secondary constriction of chromosomes. Therefore, the latter is called ‘nucleolar organizer.

1. Who discovered the nucleus?
(A) Robert Brown
(B) Robert Hooke
(C) Schleiden
(D) Schwann
Answer: (A) Robert Brown

2. Why is the nucleus called the “director of the cell”?
(A) It controls cell division
(B) It stores genetic information
(C) It directs all cellular functions
(D) All of the above
Answer: (D) All of the above

3. In which type of cells is a true nucleus found?
(A) Prokaryotic cells
(B) Eukaryotic cells
(C) Bacteria
(D) Viruses
Answer: (B) Eukaryotic cells

4. Which type of cells lack a nucleus?
(A) Mammalian red blood cells (RBCs)
(B) Nerve cells
(C) Muscle cells
(D) Skin cells
Answer: (A) Mammalian red blood cells (RBCs)

5. What is the fluid inside the nucleus called?
(A) Cytoplasm
(B) Nucleoplasm
(C) Mitochondria
(D) Golgi body
Answer: (B) Nucleoplasm

6. How many layers does the nuclear membrane have?
(A) Single
(B) Double
(C) Triple
(D) None
Answer: (B) Double

7. How many pores can be found in the nuclear membrane?
(A) 10-100
(B) 100-1000
(C) 1000-10000
(D) 50000-100000
Answer: (C) 1000-10000

8. Which major molecule is present inside the nucleus?
(A) DNA
(B) RNA
(C) Protein
(D) Lipid
Answer: (A) DNA

9. Who demonstrated that the nucleus carries genetic information?
(A) Hammerling
(B) Schleiden
(C) Schwann
(D) Mendel
Answer: (A) Hammerling

10. What is the primary function of the nucleolus?
(A) DNA synthesis
(B) RNA synthesis
(C) ATP production
(D) Lipid metabolism
Answer: (B) RNA synthesis

11. What is the main component of the nucleoplasm?
(A) Water
(B) Proteins
(C) Sugars
(D) All of the above
Answer: (D) All of the above

12. The nuclear membrane is also known as:
(A) Karyotheca
(B) Cytoplasm
(C) Plasmalemma
(D) Chromatin
Answer: (A) Karyotheca

13. The outer membrane of the nuclear envelope is continuous with:
(A) Mitochondria
(B) Endoplasmic reticulum
(C) Golgi apparatus
(D) Ribosomes
Answer: (B) Endoplasmic reticulum

14. Which type of chromatin is transcriptionally active?
(A) Heterochromatin
(B) Euchromatin
(C) Perichromatin
(D) None of the above
Answer: (B) Euchromatin

15. The nucleolus is primarily involved in the synthesis of:
(A) DNA
(B) rRNA
(C) tRNA
(D) mRNA
Answer: (B) rRNA

16. The chromatin network was first observed by:
(A) Waldeyer
(B) Flemming
(C) Brown
(D) Schwann
Answer: (B) Flemming

17. Which of the following is absent in the nucleolus?
(A) DNA
(B) rRNA
(C) Histones
(D) Lipids
Answer: (D) Lipids

18. What is the function of nuclear pores?
(A) ATP production
(B) Transport of molecules
(C) DNA replication
(D) None of the above
Answer: (B) Transport of molecules

19. Which phase of the cell cycle is the structure of the nucleus best studied?
(A) Interphase
(B) Prophase
(C) Metaphase
(D) Telophase
Answer: (A) Interphase

20. The pH of the nucleoplasm is approximately:
(A) 6.5
(B) 7.0
(C) 7.4
(D) 8.0
Answer: (C) 7.4

21. The term “chromosome” was coined by:
(A) Waldeyer
(B) Flemming
(C) Mendel
(D) Morgan
Answer: (A) Waldeyer

22. Chromosomes are composed of:
(A) DNA and proteins
(B) RNA and lipids
(C) Carbohydrates and proteins
(D) None of the above
Answer: (A) DNA and proteins

23. The structure that holds two chromatids together is called:
(A) Centriole
(B) Centromere
(C) Kinetochore
(D) Nucleosome
Answer: (B) Centromere

24. Which of the following organisms has the lowest chromosome number?
(A) Human
(B) Fruit fly
(C) Haplopappus gracilis
(D) Ophioglossum reticulatum
Answer: (C) Haplopappus gracilis

25. What is the chromosome number in human somatic cells?
(A) 23
(B) 46
(C) 22
(D) 44
Answer: (B) 46

26. A haploid cell contains how many sets of chromosomes?
(A) One
(B) Two
(C) Three
(D) Four
Answer: (A) One

27. What is the function of the centromere?
(A) DNA replication
(B) Spindle fiber attachment
(C) Protein synthesis
(D) Lipid synthesis
Answer: (B) Spindle fiber attachment

28. What are homologous chromosomes?
(A) Chromosomes that are identical
(B) Chromosomes with the same genes but different alleles
(C) Chromosomes without a centromere
(D) None of the above
Answer: (B) Chromosomes with the same genes but different alleles

29. The secondary constriction of a chromosome is known as:
(A) Nucleolar organizer region
(B) Kinetochore
(C) Telomere
(D) None of the above
Answer: (A) Nucleolar organizer region

30. DNA in chromosomes is packed with proteins called:
(A) Actin
(B) Tubulin
(C) Histones
(D) Myosin
Answer: (C) Histones

31. What are telomeres?
(A) The middle part of the chromosome
(B) Protective ends of chromosomes
(C) Regions where spindle fibers attach
(D) None of the above
Answer: (B) Protective ends of chromosomes

32. What is the function of the nucleolus?
(A) DNA replication
(B) rRNA synthesis
(C) Protein transport
(D) Cell membrane formation
Answer: (B) rRNA synthesis

33. Chromosomes that determine sex are called:
(A) Autosomes
(B) Allosomes
(C) Metacentric chromosomes
(D) Telocentric chromosomes
Answer: (B) Allosomes

34. Which type of chromosome has the centromere near the center?
(A) Acrocentric
(B) Telocentric
(C) Metacentric
(D) Submetacentric
Answer: (C) Metacentric

35. The process of forming a new nucleolus after mitosis is called:
(A) Nucleologenesis
(B) Nucleocytosis
(C) Nucleolysis
(D) None of the above
Answer: (A) Nucleologenesis

36. What is a karyotype?
(A) A chart showing all chromosomes of an organism
(B) The function of ribosomes
(C) A type of genetic mutation
(D) None of the above
Answer: (A) A chart showing all chromosomes of an organism

37. What is the function of spindle fibers in cell division?
(A) To replicate DNA
(B) To separate chromosomes
(C) To synthesize proteins
(D) To digest waste
Answer: (B) To separate chromosomes

38. How many nucleoli are found in a human diploid nucleus?
(A) One
(B) Two
(C) Four
(D) Six
Answer: (C) Four

39. Which phase of mitosis does the nuclear membrane break down?
(A) Prophase
(B) Metaphase
(C) Anaphase
(D) Telophase
Answer: (A) Prophase

40. The largest human chromosome is:
(A) Chromosome 1
(B) Chromosome 21
(C) Chromosome X
(D) Chromosome Y
Answer: (A) Chromosome 1