Biology Today — May 2017

Nucleus

Nuclear envelope

•    Separates nucleus from cytoplasm. It is made up of two lipoprotein and

trilaminar membranes, each of 60-90A° thickness.

•    Inner membrane is smooth while outer may be smooth or have ribosomes

like RER.

•    Two membranes are separated by 100-500 A° wide space called

perinuclear space.

•    Contains large number of pores that regulate transport of substances

inside and outside of the nucleus, e.g., ribosomes, RNAs, proteins, etc.

Nucleoplasm

•    Transparent, semifluid, colloidal substance that fills

inside the nucleus.

•    Contains nucleosides and enzymes for DNA and RNA

synthesis and functioning of nucleoproteins, DNA, RNA,

etc. Some proteins of nucleoplasm help in spindle

formation.

Nuclear matrix

•    Comprises of two types of intermediate filaments, lamin

A and lamin B.

•    Forms a dense layer below the nuclear envelope called

nuclear lamina in which ends of chromatin fibres are

embedded.

•    Nuclear matrix and nuclear lamina form scaffold

for chromatin. Attachment sites to telomeric parts,

components of nuclear pore complex and impart

mechanical strength to nuclear envelope.

Chromatin

•    Hereditary, DNA-protein complex, named by Flemming in 1879.

•    Occurs in the form of fine overlapping coiled fibres which form a network

called chromatin reticulum.

•    Are distributed throughout the nucleoplasm.

•    Differentiated into two regions-euchromatin and heterochromatin.

•    Based on its size, heterochromatin is known as chromocentre,

karyosome or false nucleolus.

•    The whole of chromatin is not functional, generally, only a portion of

euchromatin takes part in transcription.

•    During prophase, chromatin condenses to form chromosomes.

Nucleolus

•    Discovered by Fontana (1781), described by Wagner (1840) and named by Bowman

(1840).

•    Naked, round or irregular structure, attached to chromatin at specific regions called

nucleolar organiser region (NOR).

•    Generally 1-4 nucleoli are found in a nucleus.

•    It is the principal site for the development of ribosomal RNAs, centre for the formation of

ribosome components, stores nucleoproteins and is essential for spindle formation during

nuclear division.

Perinucleolar chromatin

Intranucleolar chromatin

Matrix (Pars amorpha)

Fibrillar portion (RNA fibrils)

Fig.: Detailed structure of nucleolus

Granular portion (Ribosomal

precursor)

Table: Differences between euchromatin and heterochromatin

Euchromatin Heterochromatin

(i) It is narrower, 10-30 nm in diameter. It is thicker, 100 nm in diameter.

(ii) It is somewhat diffused and lightly stained. It is condensed and darkly stained.

(iii) It forms the bulk of chromatin. It is present at certain places in the chromatin.

(iv) It contains active genes and takes part in transcription. Heterochromatin does not possess active genes and hence transcription

is absent.

(v) Euchromatin is affected by a number of factors like

pH, temperature and hormones.

Heterochromatin is not influenced by these factors.

(vi) Crossing over is quite common. Hetertochromatin inhibits crossing over.

(vii) It replicates early. It replicates late in the S-phase.

(viii) Nucleosome strand has minimum coiling. Nucleosome strand has solenoid coiling.

Functions

• It stores genetic information and transmit it to the next generation.

• It controls cellular activities like growth, development, reproduction and metabolism.