STD-11 UNIT-3 CHA-8
CELL : STRUCTURE AND FUNCTIONS
Nucleus Structure
- A typical interphase nucleus is 5-25 um in diameter.
- It is differentiated into five parts - nuclear envelope , nucleoplasm , nuclear matrix, chromatin and nucleolus.
- It bounds the nucleus on the outside.
- The nuclear envelope separates the nucleus from the cytoplasm.
- It is made up of two lipoprotein and trilaminar membranes, each of which is 60–90Ă thick.
- The inner membrane is smooth.
- The outer membrane may be smooth or its cytoplasmic surface may bear ribosomes like the rough endoplasmic reticulum.
- The two membranes of the nuclear envelope are separated by an electron transparent perinuclear space.
- The space is 100–500 Å in width.
- The outer membrane is often connected to endoplasmic reticulum. Nuclear envelope contains a large number of pores or perforations.
- In some cases 10 % of the envelope is occupied by pores.
- The two membranes of the envelope become continuous in the region of pores.
- Nuclear pores have complex structure.
- They may have diaphragm, septum, plug of electron dense material or nucleoplasmin, blebs or annuli. Annuli are circular structures around the pores.
- The pores and their annuli form a pore complex called annulated pore.
- An annulated nuclear pore may possess 9 cylinders, one central and eight peripheral.
- Instead , there may be a network of granules and filaments.
- The nuclear pores control the passage of substances to the inside or outside of the nucleus, e.g. , RNAs, ribosomes, proteins.
- It is a transparent,semifluid and colloidal substance which fills the nucleus.
- It contains nucleosides and a number of enzymes (e.g. , DNA polymerase, RNA polymerase, nucleoside phosphorylase) which are required for the synthesis and functioning of DNA, RNA, nucleoproteins, etc.
- Some of the proteins present in nucleoplasm are essential for spindle formation.
- It is a network of fine fibrils of acid proteins that function as scaffold for chromatin.
- On the periphery, below the nuclear envelope, nuclear matrix forms a dense fibrous layer called nuclear lamina.
- Terminal ends of chromatin fibres or telomeres are embedded in nuclear or fibrous lamina.
- Nuclear matrix consists of two types of inter mediate filaments, lamin A and lamin B.
- Nuclear matrix and nuclear lamina form
- Scaffold for chromatin
- Attachment sites to telomeric parts.
- Mechanical strength to nuclear envelope.
- Components of nuclear pore complex
Chromatin
- It is hereditary DNA - protein fibrillar complex which is named so be cause of its ability to get stained with certain basic dyes ( Gk . chroma- colour ; Flemming , 1879 ).
- Chromatin occurs in the form of fine overlapping and coiled fibres which appear to produce a network called chromatin reticulum.
- Chromatin fibres are distributed throughout the nucleoplasm.
- They are differentiated into two regions- euchromatin and heterochro matin, Heitz ( 1928 ).
- Euchromatin is narrow (10–30nm thick) lightly stained and diffused fibrous part which forms the bulk of chromatin.
- Heterochromatin is wider (100 nm thick), darkly stained and condensed granular part which is attached here and there on the euchromatin.
- Depending upon the size of granules formed by heterochromatin they are called chromocentres, karyosomes or false nucleoli.
- The whole of chromatin is not functional.
- Generally only a portion of euchromatin which is associated with acid proteins takes part in transcription or formation of RNAs.
- During prophase of nuclear division, the chromatin fibres condense to form a definite number of thread - like structures called chromosomes.
- It was first discovered by Fontana in 1781, described by Wagner in 1840 and provided with its present name by Bowman in 1840.
- Nucleolus is a naked, round sightly irregular structure which is attached to the chromatin at a specific region called nucleolar organiser region (NOR).
- Commonly 1-4 nucleoli are found in a nucleus.
- Upto 1600 nucleoli are reported in the oocytes of Xenopus.
- A covering membrane is absent around nucleolus.
- Because of this the contents of nucleolus are in direct contact with the nucleo plasm.
- However, Calcium seems to help in maintaining its configuration. Nucleolus has four components - amorphous matrix, granular part, fibrillar portion and chromatin.
- Amorphous Matrix
- It is the homoge neous ground substance of the nucleolus.
- Matrix is formed of protein.
- Granular Portion
- It consists of gran ules of the size of 150-200 A which lie scattered in the amorphous matrix.
- The granules are formed of protein and RNA in the ratio of 2 : 1.
- They are believed to be precursors of ribosomes.
- Fibrillar Portion ( Nucleolonema )
- It is formed of a large number of small fibrils that are 50–80 A long.
- The fibrils are made up of both protein and RNA and are believed to be precursors of granules.
- Chromatin Portion
- It is that part of chromatin which is associated with nucleolus.
- Depending upon its position nucleolar chromatin is of two types- perinucleolar and intranucleolar.
- The perinucleolar chromatin lies around the periphery of the nucleolus . It gives rise to ingrowths or trabeculae which produce the intranucleolar chromatin.
- Nucleolus is the principal site for the development of ribosomal RNAs.
- It is the centre for the formation of ribosome components.
- Nucleolus stores nucleoproteins.The same are synthesised in the cytoplasm (over the ribosomes) and transferred to nucleolus.
- It is essential for spindle formation during nuclear division.
- Nucleus is an essential and integral part of the eucaryotic cell.
- It stores genetic information in its DNA molecules which can be passed on to daughter cells.
- It also controls cellular activities.
- Chromatin
- Nucleus contains hereditary material called chromatin.
- Chromatin is DNA protein complex.
- It is made of a number of fine fibres that condense to form chromosomes.
- Number of chromosomes is fixed for a species.
- They bear genes.
- Genetic Information
- Chromatin part of nucleus possesses all the genetic information that is required for growth and development of the organism , its reproduction, metabolism and behaviour (Hammerling, 1953 ).
- Cellular Activities
- Nucleus controls cell metabolism and other activities through the formation of RNAs (mRNA, rRNA, RNA ) which control synthesis of particular type of enzymes.
- Ribosomes
- Ribosomes are formed in nucleolus part of the nucleus.
- Variations
- All variations are caused by changes in genetic material present in the nucleus.
- Cell Growth and Maintenance
- With the help of RNAs, nucleus directs the synthesis of some structural proteins and chemicals required for cell growth and maintenance.
- Cell Differentiation.
- It directs cell differentiation by allowing certain particular sets of genes to operate.
- Cell Replication
- Replication of nucleus is essential for cell replication.
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