Nucleus ultra structure and function

1. Structure and function of nucleus
6-11-2020

2. Content
● Introduction
● Structure
● Function

3. • Nucleic acid was first discovered by Frindrich Miescher from the nuclei of the pus
cell (Leukocytes) from discarded surgical bandages and called it nuclein. Nuclein
was latter shown to be a mixture of a basic protein and phosphorous containing
organic acid, now called nucleic acid.
Introduction

4. Introduction

5. • The nucleus​ is a membrane-bound structure that contains the cell’s hereditary information and
controls the cell’s growth and reproduction.
 It is the command centre of a eukaryotic cell and is commonly the most prominent organelle in
a cell accounting for about 10 percent of the cell’s volume.
 In general, a eukaryotic cell has only one nucleus. However, some eukaryotic cells are
enucleated cells (without a nucleus), for example, red blood cells (RBCs); whereas, some are
multinucleate (consists of two or more nuclei), for example, slime molds.
Introduction

6.  The nucleus is separated from the rest of the cell or the cytoplasm by a nuclear
membrane.
 As the nucleus regulates the integrity of genes and gene expression, it is also
referred to as the control center of a cell.
Introduction

7. Introduction

8. Structure

9. Structure of Nucleus

10. Structure

11. • The structure of a nucleus encompasses the nuclear membrane, nucleoplasm, chromosomes,
and nucleolus.
 The nuclear membrane is a double-layered structure that encloses the contents of the nucleus.
The outer layer of the membrane is connected to the endoplasmic reticulum.
 Like the cell membrane, the nuclear envelope consists of phospholipids that form a lipid
bilayer.
 The envelope helps to maintain the shape of the nucleus and assists in regulating the flow of
molecules into and out of the nucleus through nuclear pores. The nucleus communicates with
the remaining of the cell or the cytoplasm through several openings called nuclear pores.
Nuclear Membrane

12.  Such nuclear pores are the sites for the exchange of large molecules (proteins and
RNA) between the nucleus and cytoplasm.
• A fluid-filled space or perinuclear space is present between the two layers of a
nuclear membrane
Nuclear Membrane

13. • The Nuclear Envelope and Traffic between the Nucleus and Cytoplasm
• The nuclear envelope separates the contents of the nucleus from the cytoplasm and provides
the structural framework of the nucleus.
• The nuclear membranes, acting as barriers that prevent the free passage of molecules between
the nucleus and the cytoplasm, maintain the nucleus as a distinct biochemical compartment.
The sole channels through the nuclear envelope are provided by the nuclear pore complexes,
which allow the regulated exchange of molecules between the nucleus and cytoplasm.
Traffic between the Nucleus and Cytoplasm

14. • The selective traffic of proteins and RNAs through the nuclear pore complexes not only
establishes the internal composition of the nucleus, but also plays a critical role in regulating
eukaryotic gene expression.
• The nuclear envelope has a complex structure, consisting of two nuclear membranes, an
underlying nuclear lamina, and nuclear pore complexes (Figure 8.1).
Traffic between the Nucleus and Cytoplasm

15. Traffic between the Nucleus and Cytoplasm

16. • An electron micrograph of a nucleus. The inner and outer nuclear membranes are
joined at nuclear pore complexes. An electron micrograph illustrating the
continuity of the outer nuclear membrane with the endoplasmic reticulum. Sche-
matic of the nuclear envelope. The inner nuclear membrane is lined by the nuclear
lamina, which serves as an attachment site for chromatin.
Traffic between the Nucleus and Cytoplasm

17. • The nucleus is surrounded by a system of two concentric membranes, called the inner and
outer nuclear membranes.
• The outer nuclear membrane is continuous with the endoplasmic reticulum, so the space
between the inner and outer nuclear membranes is directly connected with the lumen of the
endoplasmic reticulum.
• In addition, the outer nuclear membrane is functionally similar to the membranes of the
endoplasmic reticulum and has ribosomes bound to its cytoplasmic surface. In contrast, the
inner nuclear membrane carries unique proteins that are specific to the nucleus.
Structure of the Nuclear Envelope

18. • The critical function of the nuclear membranes is to act as a barrier that separates
the contents of the nucleus from the cytoplasm. Like other cell membranes, the
nuclear membranes are phospholipid bilayers, which are permeable only to small
nonpolar molecules
Structure of the Nuclear Envelope

19. • Permeability of phospholipid bilayers
• Small uncharged molecules can diffuse freely through a phospholipid bilayer.
However, the bilayer is impermeable to larger polar molecules (such as glucose
and amino acids) and to ions.
Structure of the Nuclear Envelope

20. • Other molecules are unable to diffuse through the phospholipid bilayer. The inner
and outer nuclear membranes are joined at nuclear pore complexes, the sole
channels through which small polar molecules and macromolecules are able to
travel through the nuclear envelope
Structure of the Nuclear Envelope

21. • The nuclear pore complexes are the only channels through which small polar molecules,
ions, and macromolecules (proteins and RNAs) are able to travel between the nucleus and the
cytoplasm. The nuclear pore complex is an extremely large structure with a diameter of about
120 nm and an estimated molecular mass of approximately 125 million daltons—about 30
times the size of a ribosome. In vertebrates, the nuclear pore complex is composed of 50 to
100 different proteins. By controlling the traffic of molecules between the nucleus and
cytoplasm, the nuclear pore complex plays a fundamental role in the physiology of
all eukaryotic cells.
The Nuclear Pore Complex

22. • RNAs that are synthesized in the nucleus must be efficiently exported to the cytoplasm, where
they function in protein synthesis. Conversely, proteins required for nuclear functions
(e.g., transcription factors) must be transported into the nucleus from their sites of synthesis in
the cytoplasm. In addition, many proteins shuttle continuously between the nucleus and the
cytoplasm. The regulated traffic of proteins and RNAs through the nuclear pore complex thus
determines the composition of the nucleus and plays a key role in gene expression.
The Nuclear Pore Complex

23. • Depending on their size, molecules can travel through the nuclear pore complex by one of two
different mechanisms (Figure 8.5).
• Small molecules and some proteins with molecular mass less than approximately 50 kd pass
freely across the nuclear envelope in either direction: cytoplasm to nucleus or nucleus to
cytoplasm.
• These molecules diffuse passively through open aqueous channels, estimated to have
diameters of approximately 9 nm, in the nuclear pore complex.
The Nuclear Pore Complex

24. • Most proteins and RNAs, however, are unable to pass through these open channels. Instead,
these macromolecules pass through the nuclear pore complex by an active process in which
appropriate proteins and RNAs are recognized and selectively transported in only one
direction (nucleus to cytoplasm or cytoplasm to nucleus).
• The traffic of these molecules occurs through regulated channels in the nuclear pore complex
that, in response to appropriate signals, can open to a diameter of more than 25 nm—a size
sufficient to accommodate large ribonucleoprotein complexes, such as ribosomal subunits.
The Nuclear Pore Complex

25. • It is through these regulated channels that nuclear proteins are
selectively imported from the cytoplasm to the nucleus while
RNAs are exported from the nucleus to the cytoplasm.
• Small molecules are able to pass rapidly through open channels
in the nuclear pore complex by passive diffusion. In contrast,
macromolecules are transported by a selective, energy-
dependent mechanism that acts predominantly to
import proteins to the nucleus and export RNAs to the
cytoplasm.
The Nuclear Pore Complex

26. The Nuclear Pore Complex

27. • Model of the nuclear pore complex
• The complex consists of an assembly of eight spokes attached to rings
on the cytoplasmic and nuclear sides of the nuclear envelope. The
spoke-ring assembly surrounds a central channel containing the central
transporter. Cytoplasmic filaments extend from the cytoplasmic ring,
and filaments forming the nuclear basket extend from the nuclear ring.
The Nuclear Pore Complex

28. • Nucleoplasm is the gelatinous substance within the nuclear envelope.
• Also called karyoplasm, this semi-aqueous material is similar to the cytoplasm and is
composed mainly of water with dissolved salts, enzymes, and organic molecules suspended
within.
• The nucleolus and chromosomes are surrounded by nucleoplasm, which functions to cushion
and protect the contents of the nucleus.
Nucleoplasm

29. • Nucleoplasm also supports the nucleus by helping to maintain its shape.
Additionally, nucleoplasm provides a medium by which materials, such as
enzymes and nucleotides (DNA and RNA subunits), can be transported
throughout the nucleus. Substances are exchanged between the cytoplasm and
nucleoplasm through nuclear pores.
Nucleoplasm

30.  Contained within the nucleus is a dense, membrane-less structure composed of RNA
and proteins called the nucleolus.
 Some of the eukaryotic organisms have a nucleus that contains up to four nucleoli.
 The nucleolus contains nucleolar organizers, which are parts of chromosomes with
the genes for ribosome synthesis on them. The nucleolus helps to
synthesize ribosomes by transcribing and assembling ribosomal RNA subunits. These subunits
join together to form a ribosome during protein synthesis.
 The nucleolus disappears when a cell undergoes division and is reformed after the completion
of cell division.
Nucleolus

31. • The nucleolus is the site of ribosome and ribosomal RNA production. On microscopy, it
appears as a large dense spot within the nucleus. After a cell divides, a nucleolus is formed
when chromosomes are brought together into nucleolar organizing regions. During cell
division, the nucleolus disappears.
Nucleolus

32.  The nucleus is the organelle that houses chromosomes.
 Chromosomes consist of DNA, which contains heredity information and instructions for cell
growth, development, and reproduction.
 Chromosomes are present in the form of strings of DNA and histones (protein molecules)
called chromatin.
 When a cell is “resting” i.e. not dividing, the chromosomes are organized into long entangled
structures called chromatin.
Chromosomes

33.  The chromatin is further classified into heterochromatin and euchromatin based on the
functions. The former type is a highly condensed, transcriptionally inactive form, mostly
present adjacent to the nuclear membrane. On the other hand, euchromatin is a delicate, less
condensed organization of chromatin, which is found abundantly in a transcribing cell.
• Besides the nucleolus, the nucleus contains a number of other non-membrane-delineated
bodies. These include Cajal bodies, Gemini of coiled bodies, polymorphic interphase
karyosome association (PIKA), promyelocytic leukemia (PML) bodies, paraspeckles, and
splicing speckles.
Chromosomes

34. • The nucleus is completely surrounded by the nuclear envelope. This consists of both
an inner and outer membrane which run parallel to each other. The envelope is perforated
by small gaps known as the nuclear pores. These pores are around 100nm wide in true
diameter, however due to the presence of central regulatory proteins the true size of the gap is
around 9nm.
• This small size controls the passage of molecules into and out of the nucleus. Larger
molecules such as larger proteins and nucleic acid are unable to pass through these pores, and
so the function of the nuclear envelope is to selectively separate the contents of the nucleus
from that of the cytoplasm.
Nuclear envelope

35. • Mechanical support for the nucleus is provided by the nuclear lamina.
• This is a protein mesh, which is more organised on the internal surface on the
nucleus than on the cytoplasmic surface.
Nuclear lamina

36. • Chromatin describes DNA that is complexed with proteins. The primary protein components
of chromatin are histones, which are highly basic proteins that associate readily with DNA.
Histones combined with DNA form nucleosomes, which are the subunit of chromatin.
Specifically, a nucleosome describes a segment of DNA associated with 8 histone proteins. By
associating with histones, DNA is more compact and able to fit into the nucleus.
• Chromatin can exist as either euchromatin or heterochromatin. Euchromatin is the form of
chromatin present during gene expression, and has a characteristic ‘beads on a
string’ appearance. It is activated by acetylation. In contrast, heterochromatin is the
‘inactive’ form, and is densely packed. On electron microscopy, euchromatin stains lighter
than heterochromatin which reflects their relative densities.
Chromatin

37. • The nucleus provides a site for genetic transcription that is segregated from the location
of translation in the cytoplasm, allowing levels of gene regulation that are not available
to prokaryotes. The main function of the cell nucleus is to control gene expression and
mediate the replication of DNA during the cell cycle.
 It controls the hereditary characteristics of an organism.
 The organelle is also responsible for protein synthesis, cell division, growth, and
differentiation.
Functions of Nucleus

38.  Storage of hereditary material, the genes in the form of long and thin DNA (deoxyribonucleic
acid) strands, referred to as chromatin.
 Storage of proteins and RNA (ribonucleic acid) in the nucleolus.
 The nucleus is a site for transcription in which messenger RNA (mRNA) are produced for
protein synthesis.
 During the cell division, chromatins are arranged into chromosomes in the nucleus.
 Production of ribosomes (protein factories) in the nucleolus.
• Selective transportation of regulatory factors and energy molecules through nuclear pores.
Functions of Nucleus

39. • The information above can be simplified into three key functions:
 Cell compartmentalisation: The presence of a selectively permeable nuclear envelope
separates the contents of the nucleus from that of the cytoplasm.
 Gene expression: Gene expression first requires transcription, which is the process by which
DNA is transcribed into mRNA. As the nucleus is the site of transcription, proteins within the
nucleus play a key role in regulating the process.
 Processing of pre-mRNA: Newly synthesised mRNA molecules are known as pre-mRNA.
Before they exit the nucleus, they undergo a process known as post-
transcriptional modification where molecules are added or removed from the structure.
Functions of Nucleus

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