Interphase nucleus

Question 1

Functions of a nucleus

Answer 1

Segregates the genome
separates transcriptional and translational material
Usually only one per cell
There is a matrix to organize and regulate the functionality of genes

Question 2

Nuclear matrix

Answer 2

2% of the nucleus, its essentially the the loops ofs of the chromatin with the rest of the chromatin removed, it confers a non-random order of chromatin, to enable transcription and translation to be orderly

Question 3

Components of the nuclear matrix

Answer 3

1. Nuclear envelope/ pore complex/ lamina- this is the border of the nucleus
2. nucleolus
3. internal matrix- this is 99% of the nuclear matrix

Question 4

Nuclear Envelope

Answer 4

Nuclear envelope is composed of a double membrane

Outer membrane: harbors ribosomes and is continuous with the RER

Inner membrane: holds the nuclear lamina and marginal heterochromatin

The nuclear envelope also hosts the nuclear pore complexes

Question 5

Nuclear pore complex

Import complexes

Answer 5

Nuclear pore complex: thousands/nucleus and they occur between segments of nuclear envelope

has members of the 30-member nucleoporin family, stacked in 3 octomeric rings

They bind to and transport stuff transported between the cytoplasm and the nucleus

Import complex: has the nuclear protein cargo bound to an import receptor

Nuclear pores are multifunctional:
Some proteins have both NLS and NES (to get in and out of the nucleus)
any type of RNA can be exported
Import/export can be done through all pores

Question 6

How the pore imports proteins

Answer 6

Proteins are the only thing imported in the nucleus, over 45 k need to be imported, under they can just diffuse in

Import complex: have NLS (Nuclear localization sequences)

Import complex (shipment contents): has the nuclear protein (cargo) bound to an import receptor

1. The Cargo proteins have an NLS (nuclear localization sequence)- the NLS is a sequence rich in K+ and R+ AAs
2. The import receptor: its a protein that binds to the NLS-bering cargo and escorts it to the nuclear pore

The receptor is composed of importin alpha/Beta alpha binds to the NLS on the cargo

Question 7

Nuclear import Steps

Answer 7

In the cytoplasm, NLS binds to Cargo, and importin binds to the NLS (alpha subunit)

The complex migrates to a nuclear pore

The Beta subunit docks to a nucleoporin protein

The import complex translocates thru the pore, then at the end of the tunnel Ran-GTP (energy) binds and releases the cargo

Question 8

How the nuclear pore exports

Answer 8

Proteins and RNAs can be exported, Export cargo have NES signals (Nuclear export sequence high in Leu)

They also have an export receptor bind to them (exportin)

And need Ran- GTP to make the whole complex and bind to the pore

Question 9

Energy for Nuclear Import/Export

Answer 9

its done via the gradient of Ran GTP

Ran GTP concentration is highest inside the nucleus
In the nucleus Ran-GTP binds to the import receptor causing ejection of cargo
In the nucleus Ran-GTP binds to the export receptor to assemble the complex

Both import and export receptors then go to the cyto with the Ran-GTP still attached where Ran-GTP is hydrolyzed to RanGDP

Question 10

Nuclear Lamina

Answer 10

sits between the inner nuclear envelope and marginal heterochromatin, contains 3 lamins (A, B and C)

Lamin B binds to the inner membrane via binding to the LBR (lamin B receptor)

Lamins A and C bind to the marginal heterochromatin

Question 11

Functions of the Nuclear lamina

Answer 11

During interphase, it maintains the nucleus as a sphere
During mitosis, nucleus becomes disrupted due to the hyper phosphorylation of lamins A and C, but Lamin B stays bound to LBR to mediate reformation of the nuclear envelope at telophase
Lamin mutations mess up the interphase nucleus

Question 12

Lamin genes

Answer 12

LMNB-> encodes the lamin B protein
LMNA-> encodes the lamin A and C proteins (splice variants)

Mutations of the LMNA gene cause laminopathies-> muscular dystrophy cardiomyopathy and premature aging (progeria)

Question 13

Nucleolus

Answer 13

its a Ribosome factory, rDNA genes are set up in a part of the nucleolus called the nucleuolus organizer where 10 chromosomes (5 pairs) compromise each nucleuolus

Nucleolus is only shown in interphase NOT in mitosis

They are not membrane bound

Question 14

Synthesis and assembly of Ribosomes

Answer 14

RNA pol- 1 synthesizes rRNA transcripts in the nucleolus and processed into 18s, 5.8s and 28s (another 5s is transcribed by Pol 3)

Ribonucleoproteins (RNPs) bind to the rRNA transcripts inside the nucleolus

A small 40s (18s) and a large 60s (5.8, 28 and 5) subunti is formed in the nucleus, then migrate thru the pores and assemble int a large 80 s ribosome in cyto

Question 15

Internal Nuclear Matrix

Answer 15

98% of the total nuclear matrix
Not much is known about it, but it confers a non random order to where the chromatin hangs out, it organizes orderly replication and transcription of DNA

contains cell specific matrix proteins (NMPs)

DNA replication and transcription occurs in nuclear matrix (where mRNA is transcribed)