The Nucleus

Question 1

What are the functions of the nucleus?

Answer 1

Act as the ‘head office’ of the activities of the cell.

Gene transcription takes place operated by RNA polymerase.

RNA transcripts are cut (splicing) and modified before being shipped to the cytoplasm.

Synthesis and replication of DNA occurs before cell division.

Question 2

What is the shape of the nucleus?

Answer 2

The shape of the nucleus is not always round, they can vary depending on the type of cell.

Question 3

Describe the staining of the nucleus.

Answer 3

The nucleus is stained with basic dyes, often forming blue tones.

This is due to a material within the nucleus called chromatin.

Question 4

What regions are heterchromatin vs euchromatin?

Answer 4

Regions of dense chromatin are called heterochromatin.

While the more filamentous/granular regions are called euchromatin.

Question 5

Describe the appearance of heterochromatin under TEM.

Answer 5

The darker areas are associated with the heterochromatin, which can be found:

• Just below the membrane (marginal heterochromatin).
• Localized in small aggregates below the nuclear membrane (karyosome).
• Around the nucleolus (nucleolar associated heterochromatin).

Question 6

Describe the appearance of euchromatin under TEM.

Answer 6

The lighter grey areas are the regions of euchromatin.

Question 7

Describe the appearance of the nucleus during interphase.

Answer 7

The nucleus is obviously observes during interphase, with a LM both in vivo and after fixation and staining.

Regions of neucleoli (or neucleolus) are visible.

46 filaments of curled chromatin within the nucleus.

Question 8

Describe the appearance of the nucleus during mitosis/meiosis.

Answer 8

When a cell divides by mitosis/meiosis, strands of chromatin are displayed in the form of chromosomes.

Neucleoli (neucleolus) are not visible as they dissolve.

46 chromosomes can be observed (23 pairs).

Question 9

Describe the structure of a single strand of chromatin.

Answer 9

The chromatin is formed by a filament of DNA with regular intervals with globular particles called nucleosomes.

A DNA linker between 1 nucleosome and another.

Question 10

Describe the consequent folding of chromatin.

Answer 10

A type of histone called H1 (histone 1) will act to close the histones together, so it will be between the DNA linker and the histone molecules to form a solenoid structure.

This solenoid filament is further folded into loops

The maximum number of folding of solenoid filaments is achieved through the formation microconvuli. These are compact structures formed from loops that are folded and spiralized.

Question 11

At what degree of folding is DNA considered euchromatin?

Answer 11

All degrees of folding up till the loops of solenoid are considered euchromatin.

Question 12

At what degree of folding is DNA considered heterochromatin?

Answer 12

After the loops of the solenoid are grouped together, this is then considered to be heterochromatin.

Question 13

When do chromosomes appear?

Answer 13

Chromosomes appear in the prophase of mitosis.

Question 14

What are the classifications of chromosomes according to its centromere position?

Answer 14

Metacentric - centromere is in the middle.
Sub-metacentric - centromere is slightly off center.
Acrocentric - Centromere is completely off centre.

Question 15

Describe the composition of the human karyotype.

Answer 15

22 pairs of autosomal chromosomes.
1 pair of sexual chromosome.
23 pairs of chromosomes overall.

Question 16

Describe the main features of the nucleolus.

Answer 16

Variable size and quantity.

3 nuclear components:
Fibrillar centre
Dense fibrillar component
Granular component which is everything else.

The ribosome factory

Question 17

What is the function of the nucleolus?

Answer 17

Protein synthesis and production of ribosomes (assembles ribosomal RNA).

Question 18

What is the nuclear matrix?

Answer 18

After digesting the nucleus with RNAse and DNAse, the filaments of DNA and RNA is removed.

A proteic material called the nuclear matrix is left.

Question 19

What makes up the nuclear matrix?

Answer 19

The nuclear matrix constitutes the structure of the nucleoplasm, formed by a fibrous reticulum whose protein structure has not yet been defined.

Question 20

What are domains?

Answer 20

It’s been shown that there are specific areas occupied by chromosomes.

These areas are called domains and seem to be occupied by specific chromatin material.

Question 21

Describe the proteins that associate with filaments inside and outside the nucleus.

Answer 21

SUN, Samp and Emerin proteins make contact with filaments of chromatin inside the nucleus, and determine nuclear space organisation.

They also appear to be implicated in DNA replication and transcription.

These are also connected with filaments of the cytoskeleton, contributing to a complex mechanism of cell action that implies that everything is connected.

Question 22

What is the function of the nuclear envelope?

Answer 22

The nuclear envelope encloses the nucleoplasm and separates it from the cytoplasm.

Question 23

What is the structure of the nuclear envelope?

Answer 23

The nucluear envelope is made up of a double membrane with pores.

Question 24

How can we observe the nuclear envelope?

Answer 24

Although the nucleus is identifiable with a LM, the nuclear envelope is not visible.

It is visible with the TEM.

Question 25

Describe the structures associated with the nuclear envelope.

Answer 25

Protein filaments called nuclear lamina are coated on the inner membrane.

The outer membrane is continuous with the endoplasmic reticulum.

There are also ribosomes on the outer surface of the membrane.

On the inner membrane there are the nuclear pore complexes, which will permit the movement of material in and out of the nucleus.

Question 26

What is the nuclear pore?

Answer 26

The nuclear pore will cover both membranes and essentially create a hole to allow communication between cytoplasm and nucleus.

Question 27

Describe the structure of the nuclear pore.

Answer 27

There are filaments on the cytoplasmic side that form antenna-like structures to help materials enter.

A core that is the same on both sides composed of many proteins.

In the nuclear side there is a nuclear basket.

Question 28

What molecules can pass through the nuclear pore?

Answer 28

Small molecules (<50kD) can pass freely through the nuclear envelope/pores.

Larger molecules can pass with active transport.

Question 29

How can proteins pass through the nuclear pore?

Answer 29

Proteins have a sequence called Nuclear Localisation Sequence (NLS) that is recognized by importin in the nuclear envelope.

To exit, a Nuclear Exporting Sequence (NES) is recognized by the exportin.

Question 30

What is the structure and function of the nuclear lamina?

Answer 30

They are formed by bundles of intermediate filaments.

This both maintain the structure of the nuclear envelope, as well as contributing to the formation of those domains for where chromatin attaches.

Question 31

Describe a pathology associated with mutations in the nuclear lamina.

Answer 31

Hutchinson-Gilford progeria syndrome

Characterized by a dramatic, rapid aging in childhood. In more than 80% of cases, the gene defect is a single spontaneous mutation in codon 608 of the LMNA gene, which codes for lamin A and C.

A form of a mutant of lamin A is progerin.