Interphase Nucleus

Question 1

Nuclear Matrix

Answer 1

Confers non-random anatomical order

Organizes chromatin to enable orderly replication & transcription

Question 2

Components of the nuclear matrix

Answer 2

1. Nuclear envelope/poor complex/lamina
2. Nucleolus
3. Internal Matrix (90%)

Question 3

Nuclear Envelope

Answer 3

Double membrane
Outer Membrane - ribosomes/ER
Inner Membrane - Nuclear Lamina and marginal heterochromatin

Question 4

Nuclear Pore Complex

Answer 4

Thousands/nucleus
Occurs between segments of nuclear envelope
proteins <40kD can diffuse (passively) thru, others must be regulated by rapid active transport systems

Question 5

Nucleoporin

Answer 5

Proteins found in pore complex
30 member family, stacked in 3 octameric rings
Bind and transport complexes from cytoplasm, consisting of a nuclear (cargo) protein plus an nuclear localization sequence (NLS) receptor protein
Lysine and Arginine residues (basic) confer that

Question 6

Nuclear (cargo) proteins

Answer 6

have a NLS

NLS = zipcode of basic amino acids (K+,R+)

Question 7

NLS-Receptor Proteins

Answer 7

Bind to NLS-bearing nuclear cargo proteins and escort them to the nuclear pore

Question 8

Nuclear Import

Answer 8

1. Nuclear cargo proteins (with NLS) binds NLS-receptor
2. Receptor docks to a nucleoporin in the pore
3. Translocaiton thru Pore (active transport using Ran-GTP)
4. Release to nuclear interior

Question 9

Phosphorylation of NLSs

Answer 9

Regulates entry of proteins into the nucleus

Can’t be imported

Question 10

Nuclear Export

Answer 10

tRNAs, mRNAs, ribosome subunits, proteins

Question 11

Exportins

Answer 11

Nuclear Export Sequences (NESs) = zip codes for export

Typical NES is LPPLERTL (enriched in leucine residues)

Question 12

Selectivity of Nuclear Pores

Answer 12

Nuclear Pores are multi-functional!

Every single pore has the same activity, can import/export all products

Question 13

Energy for Nuclear Import/Export

Answer 13

Ran-GTP gradient

- High concentrations of Ran-GTP in the nucleus bind to empty import receptors and loaded export receptors 
   - These move along the grandient to the cytoplasm where Ran-GTP is hydrolyzed

Question 14

Nuclear Lamina

Answer 14

Resides between the inner nuclear envelope and marginal heterchromatin

Question 15

Lamina Proteins

Answer 15

3 Proteins: lamins A,B,C
Lamin B attaches to nuclear envelope via the Lamin B Receptor.
- Lamins A and C bind to Lamin B
Lamins A & C interact with the marginal heterochromatin

Question 16

Functions of Nuclear Lamina

Answer 16

Interphase: maintains nucleus as sphere
Mitosis: hyper-phosphorylation of A+C –> soluble
Lamin B stays bound to the LBR which mediates reformation of nuclear envelope at telophase

Question 17

Mutations of Lamin Gene

Answer 17

LMNA gene encodes lamins A & C

mutations of LMNA cause muscular dystrophy and cardiomyopathy, and premature aging

Question 18

Nucleolus

Answer 18

Only function is to make ribosomes

400 genes that make up the ribosomal RNA, 40 of them on each of 10 chromosomes

Question 19

Internal Nuclear Matrix

Answer 19

Comprises 98% of the total nuclear matrix

Confers non-random order of chromatin