Nucleus, nuclear import, Nucleolus

Question 1

The Nucleus function

Answer 1

1. Storage, replication, and repair of genetic material.
2. Expression of genetic material:

• Transcription: mRNA, tRNA, rRNA
• RNA splicing

3. Ribosome biosynthesis

Question 2

Nuclear enveloped contains

Answer 2

-Nuclear membrane
- Nuclear pores
- Nuclear lamina

Question 3

Nuclear content

Answer 3

-Chromatin
- Nucleoplasm
-Nucleolus

Question 4

Nuclear envelope

Answer 4

• 2 parallel phospholipid bilayers separated by 10-50nm space.

Question 5

Outer nuclear membrane (ONM)

Answer 5

Binds ribosomes and is
continuous with rough
endoplasmic reticulum

Question 6

Inner nuclear membrane (INM)

Answer 6

Has integral proteins and
connects to nuclear lamina

Question 7

Importance of the nuclear envelope

Answer 7

• Separates nuclear content from cytoplasm
• Spacecially separates transcription and translation processes
• Selective barrier that
  allows limited movement
  of molecules between
  nucleus and cytoplasm

Question 8

Nuclear Lamina

Answer 8

• Support the nuclear envelop
  -Thin meshwork of filamentous proteins
• Nuclear lamina is bound to inner membrane of the nuclear envelop (NE) by integral membrane proteins.
• Provides structural support for nuclear envelop.
• Attachment sites for chromatin

Question 9

Nuclear Pore

Answer 9

• Gateways between cytoplasm and nucleus
• 3000-4000 pores/nucleus
• Pores are found where inner and outer membranes fuses.
• Complex structure that
  involve the arrangement of
  different types of proteins

Question 10

Nuclear Pore complex

Answer 10

• Composed of nucleoporins (NUPs)(a large family of different proteins
• Octagonal symmetry / basket-like Projects into cytoplasm and nucleoplasm
• The nuclear pore complex is a supramolecular complex.

(In other
words, it is very big from a protein arrangement point-of-view.)
* 15-30 times the size of a ribosome

Question 11

Passive diffusion of molecules
that are 40 kDA or less:

Answer 11

• Rapid
• 100 molecules/minute/pore

Question 12

Regulated movement of larger
molecules:

Answer 12

• Slow
• 6 molecules/minute/pore

Question 13

Nuclear import

Answer 13

• Regulated movement of proteins into the nucleus requires a Nuclear Localization Signal (NLS).
• NLS = several positively charged amino acids within the protein sequence.

Question 14

Steps of nuclear import

Answer 14

1. Protein with NLS (cargo) interacts
   with Importin protein in cytoplasm
2. Cargo/Importin complex interacts
   with FG-NUPs (see Slide 15) at the
   NPC and enters the nucleoplasm
3. Ran-GTP interacts with Importin; cargo dissociates and stays in nucleoplasm
4. Ran-GTP/Importin complex exits
   nucleus through NPC
5. GTP hydrolysed to GDP. Importin
   released in cytoplasm to find new cargo

Question 15

Nucleocytoplasmic trafficking

Answer 15

Nuclear import and export are critical for cellular function:
- Nucleotides for transcription

• Structural proteins (e.g., lamins)
• DNA packaging proteins (e.g., histones)
• Proteins for DNA replication, repair, and transcription
• Proteins for RNA processing (splicing) and export
• Proteins for ribosomes synthesis and export

Question 16

Nucleolus

Answer 16

• Largest structure inside the
  nucleus of eukaryotic cells.

-Primary function of the nucleolus
is biosynthesis of ribosomes.

Question 17

Ribosomes

Answer 17

Ribosomes consist of two major
components:

• The small ribosomal subunits,
  which read the RNA, and the large subunits, which join amino acids to form a polypeptide chain.

-Each subunit consists of one or more ribosomal RNA (rRNA) molecules and a variety of ribosomal proteins.

Question 18

Ribosome Biosynthesis

Answer 18

• Synthesis of rRNAs
• rRNA processing
• Assembly of subunits— rRNA + ribosomal proteins

-40S and 60S subunits are exported to cytoplasm where they are assembled as 80S ribosomes