Nucleus structure Flashcards
What is heterochromatin and euchromatin
Heterochromatin and euchromatin are two major categories of chromatin higher order structure. Heterochromatin has condensed chromatin structure and is inactive for transcription, while euchromatin has loose chromatin structure and active for transcription.
Inside of the nucleus structure
Inside of the nucleus is not uniform and there is a system; a geography or a pattern within a nucleus
The size of the nucleus can be anywhere between 5microns to 20 microns (a micron being 10^-6)
What is a nucleolus
A sub region of a nucleus and has its own structure and its own definite regions (nucleolar organising center; pars fibrosis)
A nucleus may have one or more nucleoli.
What is a hetero chromatin
Regions of the chromosomes that has become extremely compacted- so dark that it appears dark in an electron microscope. Because they are so dense, electrons cannot get through them. These dense compacted regions of chromosomes normally indicate that these regions are inactive- they are not being actively used in transcription.
The nuclear pores allows the interior of the nucleus to communicate with the rest of the cell; because it carries out its own functions, it needs ingredients to do so. For this, the nuclear pores are important as this allows the flow of materials in and out.
Overview of the nucleus
The nucleus takes up most of the space within the eukaryotic cell
Nucleus needs to replicate too so goes through nuclear division (through mitosis) followed by a separation of the cells (cytokinesis)
The interior of the nucleus with all the material inside is called the nucleoplasm
The nucleoplasm communicates with the cytosol (rest of the cell) through nuclear pores (that lies within double layer nuclear membrane (nuclear envelope))
The space between the double layered nuclear membrane is called the peri nuclear space. This whole nuclear envelope is supported by a scaffolding/framework that lies within the nucleus, underneath the nuclear envelope. This is called the nuclear lamina.
When we look within the structure of a nucleus, we will find heterochromatin, euchromatin and nucleolus.
What drives the nucleus
One of the main driving forces of the nucleus is make RNA; transcribing DNA into RNA. To do that, the nucleus has to import proteins (enzymes that enable this to happen). The nucleotides has to also be imported from which the nucleus is going to build the RNA. And also once it has made the RNA, it has to send the RNA back out to the cytoplasm for it to be translated.
How does transcription and translation occur in eukaryotic cells and also in prokaryotic cells
These processes are decoupled. DNA is transcribed into RNA within the nucleus and the RNA is translated in the cytoplasm- there is a geographical difference between them.
Unlike prokaryotes, where these processes are coupled.
What happens to the nuclear envelope and the RER?
It branches off to the rough endoplasmic reticulum. The peri nuclear space is contiguous to the RER; meaning that they share the same borders and the interiors are connected.
What do you call the inside of a membrane sac or a layered membrane?
The lumen
So, another name for the lumen of the nuclear envelope is the peri nuclear space.
It is about 20-40nm across
(Nm being 10^-9)
What is a nuclear pore
Not completely open; is gated
Nuclear lamina- what are lamins
Nuclear lamina is a mesh of fibres that support the nuclear envelope; this allows the nucleus to have a shape.
Similar function to cytoskeleton.
Nuclear lamina is composed by a type of fibre proteins called lamins. These belong to the class of fibres known as the intermediate filaments and are formed by polymerised sub-units.
Some of the intermediate filaments form part of the cytoskeleton.
Lamins are a special type of intermediate filament that form a scaffold under the nuclear envelope.
DNA can attach themselves to the lamins as well as the envelope. This provides a scaffold to which the nuclear envelope can attach and give shape to the nucleus and probably is a place where we can attach all our chromosomes in the inside.
Nuclear lamina- monomeric units
Lamins are monomeric units and they can hybridise/ polymerise with each other to make long fibres (long, stretchy fibrous protein). So, we can polymerise them to form the mesh of fibres to provide the scaffolding but we can also depolymerise them to the monomeric units (which breaks down the scaffolding). This results in the nuclear envelope breaking down.
Lamins and mitosis
In mitosis, we need to depolymerise the lamins to allow the nuclear envelope to fall apart and then reform at the end of the mitosis.
This polymerisation and depolymerisation of the lamins (of these fibres) is controlled by a process called phosphorylation (a type of post-translational modification).
This is special to eukaryotic cells- they can modify their proteins after the proteins have been made.
What are types of modifications that can be made to proteins once they are made
And what are the enzymes that help this are called
One of them is phosphorylation- this is where a phosphate group is added to the protein. This phosphate group can also be taken off- can be dephosphorylated. There are enzymes that add phosphates called kinases and the enzymes that remove the phosphate groups can called phosphotases.
Quite often the act of phosphorylation acts as a switch on the protein- switches it between two stages; the active or deactive stage.
This is very common in eukaryotic cells rather than prokaryotic cells
How to polymerise and depolymerise lamins.
The polymerisation of these lamins to form these big fibres is controlled by this act of phosphorylation. When you phosphorylate lamins, the depolymerise. So, when you add a phosphate group to the lamins, the whole fibres fall apart. When you remove the phosphate, they reassemble.
