UNIT 5.4. MEMBRANE-BOUND ORGANELLES: NUCLEUS

Nucleus

It is the controlling center of the cell. This is where the genetic information will be stored and regulated. Furthermore, is very crucial in central dogma mechanism and cellular processes

nucleus

The presence of a ________ distinguishes eukaryotic cells from prokaryotic cells

Ernest Rutherford (1871–1937)

• Nucleus Theory (1910)

• Gold foil experiment with alpha particles

• Key Observations:

  • Most alpha particles passed straight through → atom is mostly empty space

  • Some alpha particles deflected at large angles → presence of a dense central nucleus

Mosely

Proved that the nucleus have a positive charge

• Drosophila melanogaster in embryonic stages

• myocytes

examples of multinucleate cells

• mammalian RBCs

• cells of lens of vertebrae eye

examples of anucleate cells

1. Unicellular

2. Multicellular

2 Main types of Eukaryotic cells:

Unicellular

2 Main types of Eukaryotic cells:

• made up of single cell that carries out all life processes

  • Fungi

  • Protista

Multicellular

2 Main types of Eukaryotic cells:

• made up of numerous cells;

• numerous cells are specialized to performs a specific function

  • Plants

  • Animals

1. Nuclear membrane

2. Nucleoplasm

3. Nucleolus

4. Chromatin

4 Components of Nucleus

Nuclear envelope

4 Components of Nucleus:

• pore-riddled

• also known as nucleolemma

• Separates the nuclear material from cytoplasm

• Consists of two lipid bilayers

  • Outer membrane

  • Inner membrane

• The nuclear envelope is a double-layered membrane perforated with pores, which control the flow of material going in and out of the nucleus.

Nuclear membrane

4 Components of Nucleus:

1) Shape and stability: helps the nucleus from collapsing

2) Compartmentalizing: separates the nucrear material from cellular material

3) Regulation of substances: allow the exchange of materials

4) Communication: develops a chemical connection between nucleus and cell

1. Outer nuclear membrane

2. Perinuclear space

3. inner nuclear membrane

Parts of nuclear membrane

Outer nuclear membrane

Parts of nuclear membrane:

• continuous w the ER

• functionally homologous to ER membrane

• 7-8 nm

ribosomes ;

membrane proteins (for cytoskeleton binding)

The cytoplasmic surface of the outer nuclear membrane has ________ that are different in composition of protein and t_ese ribosomes are enriched in ______________________________________.

Perinuclear space

Parts of nuclear membrane:

• “lumen of envelope”

• Space is present between ONM and INM

• 20-40 nm thick.

• same content as with the ER lumen

Inner Nuclear Membrane

4 Components of Nucleus:

• contains proteins that are specific to nucleus are present, involved in nuclear structure, organization, and gene regulation

• Help maintain nuclear shape, mechanical stability, and chromatin organization

• Lamin B receptor (LBR)

• Lamina-associated polypeptides (LAP, LAP2)

• Emerin

• MAN1

• Nurim

INM Proteins

muscular dystrophies & lipodystrophy

Mutations in emerin and nuclear lamins are linked to

RER

INM integral proteins are synthesized on the ___________

lateral diffusion

Integral proteins reach the inner nuclear membrane by _________________ through the continuous ER–nuclear envelope membrane system

phosphorylation ;

cylin-dependant protein kinase

The nuclear membrane disassembles and the nuclear lamina depolymerizes due to ________________ of nuclear lamins by___________________________

Nuclear Pore Complexes (NPC)

The nuclear envelope membrane proteins also disassemble by phosphorylation leading to disassembling of _______________________

nuclear import receptors

NPC proteins become bound to ________________, which play an important part in the reassembly of NPCs at the end of mitosis.

chromatin

Offset:

The nuclear envelope forms around ________.

Ran-GTP

Offset:

Chromatin is surrounded by a shroud of _________

nuclear import receptors

Offset:

The assembly of NPCs is started by _____________ from NPC proteins displaced by the Ran-GTP cloud.

ER

Offset:

Membranes from _______ fuse over the chromatin until a complete nuclear envelope is formed.

Onset (Pre-mitosis)

DURING MITOSIS THE NUCLEAR ENVELOPE DISASSEMBLES:

★ The nuclear membrane disassembles and the nuclear lamina depolymerizes due to phosphorylation of nuclear lamins by cylin-dependant protein kinase.

★ The nuclear envelope membrane proteins also disassemble by phosphorylation leading to disassembling of NPCs.

★ NPC proteins become bound to nuclear import receptors, which play an important part in the reassembly of NPCs at the end of mitosis.

★ Motor proteins in microtubules also help in tearing down the nuclear membrane.

Offset (Post-mitosis)

DURING MITOSIS THE NUCLEAR ENVELOPE DISASSEMBLES:

★ The nuclear envelope forms around chromatin. Chromatin is surrounded by a shroud of Ran-GTP.

★ The assembly of NPCs is started by nuclear import receptors from NPC proteins displaced by the Ran-GTP cloud.

★ Alongside the nuclear envelope membrane proteins with dephosphorylated lamins start attaching to chromatin again.

★ Membranes from ER fuse over the chromatin until a complete nuclear envelope is formed.

nuclear pore

• Most distinctive feature of the nuclear envelope

• Composed of small cylindrical channels

• Provides direct communication between cytosol and nucleoplasm

• Formed where inner and outer nuclear membranes fuse

• Highly structurally complex

• Regulates and controls selective transport of proteins, RNAs, and other macromolecules

freeze-fracture microscopy

Nuclear pore is readily visible using __

Small molecules;

passive diffusion

___ are able to pass rapidly through open channels in nuclear pore complex by _____.

macromolecules;

selective, energy-dependent mechanism

In contrast, ____ are transported by a ____ that acts predominantly to import proteins to the nucleus and export RNAs to the cytoplasm

Nuclear Localization Signal

bipartite, consisting of a Lys-Arg sequence, followed by a Lys-Lys-Lys-Lys sequence located ten amino acids farther downstream.

Nuclear Localization Signals (NLS)

• Short specific amino acid sequences that target proteins to the nucleus

• Recognized by nuclear transport receptors (importins)

• Guide proteins through the nuclear pore complex (NPC)

• Commonly rich in basic amino acids (lysine and arginine)

• Amino acids involved are often clustered together but not always directly adjacent

Nucleoplasm/ Karyoplasm

4 Components of Nucleus:

• A jelly-like (made mostly of water) matrix within the nucleus

• All the other materials “float” inside

• Helps the nucleus keep its shape and serves as the median for the transportation of important molecules within the nucleus

• It is completely enclosed within the nuclear membrane or nuclear envelope.

Nucleolus

4 Components of Nucleus:

• Ribosome factory

• Large, prominent structures

• Doesn’t have membrane

• most cells have 2 or more

• takes up around 25% of the volume of the nucleus

• This structure is made up of proteins and ribonucleic acids (RNA)

Nucleolus

4 Components of Nucleus:

● Site of transcription

● Assemblage of ribosomes

● Synthesis of ribosomes and RNA

5S

5.8S

18S

28S

Ribosomes of higher eukaryotes contain four rRNAs:

RNA polymerase I ;

45S pre-rRNA.

The 5.8S, 18S, and 28S rRNAs are transcribed as a single unit in the nucleolus by _________________ —>

5S rRNA ;

RNA polymerase III

The _________ is transcribed outside the nucleolus by ____________and later imported into the nucleolus for ribosome assembly.

rRNAs

plays key roles in decoding genetic information and catalyzing peptide bond formation

200 copies

2, 000 copies

The human genome, for example, contains about _________ of the gene that encodes the 5.8S, 18S, and 28S rRNAs and approximately _________ of the gene that encodes 5S RNA.

tandem arrays ;

5

• chromosome 13

• chromosome 14

• chromosome 15

• chromosome 21

• chromosome 22

The genes for 5.8S, 18S, and 28S rRNAs are clustered in___________ on ___ different human chromosomes

single tandem array ;

chromosome 1

The 5S rRNA genes are present in a_______________ on ________.

oocytes

The importance of ribosome production is particularly evident in _______ in which the rRNA genes are amplified to support the synthesis of the large numbers of ribosomes required for early embryonic development.

Xenopus oocytes

In ______________, the rRNA genes are amplified approximately two-thousand-fold, resulting in about one million copies per cell.

nucleolar organizing regions (NORs)

rRNA genes are located in _______

tandemly repeated rRNA genes ;

non-transcribed spacer DNA

Each NOR contains_______________________, separated by________________________

organization of rRNA genes —> initiation —> elongation —> christmas tree apprearance —> high polymerase density —> 45S pre-rRNA —> cleavage, trimming, chemical moifications —> mature 5.8S, 18S, 28S —> combine with ribosomal proteins —> 40S and 60S ribosomal subunits —> export to the cytoplasm

rRNA Transcription and Processing steps

Chromatin

4 Components of Nucleus:

• DNA in a condensed form attached to a histone protein

1. Nucleic acids

2. Proteins

2 components of chromatin:

NA

2 components of chromatin:

• DNA (primary nucleic acid) + small amount of RNA 9transit to the cytoplasm)

Proteins

2 components of chromatin:

• a. Histones (basic pH) – core histones (H2A, H2B, H3 & H4),
  Linker histone (H1)

• b. Non-histone proteins

nucleosomes ;

linker DNA

• Electron microscopy of interphase chromatin shows ellipsoidal beads

• These beads are called ________________, connected by__________

Euchromatin = lighlty staining

Heterochromatin = darkly staining

2 regions of chromatin

Euchromatin

Heterochromatin

Chromosome

● It controls the activities of cell

● Information of the form of genes is located in chromosomes

● Control inheritance and metabolism

DNA

contains the information needed for the creation of proteins (which include enzymes and hormones) and is stored in the nucleus, as already said, in the form of chromatin or chromosomes.