Lecture 10: Nucleus Flashcards
How big is the nucleus of most cells?
5-10 µm diameter
What structure surrounds the nucleus?
The nuclear envelope
How many membranes is the nuclear envelope composed of?
Two phospholipid bi layers.
What is the role of a nuclear pore?
Control the movement of material in and out of the cell.
What can leave the nuclear pore?
mRNA, tRNA and ribosomal units.
What can enter through the nuclear pore?
Control signals (msg from cell telling nucleus what to do e.g. turn a specific gene on). Because mRNA is let out of the cell it also makes sense that the nucleus must let in building materials needed to build these structures. If we think back to earlier lectures the RNA is a polymer of a smaller structure called a nucleotide. Finally, because things are being built the nucleus also needs to let in some molecules which can be used for energy, such as GTP (guanosine triphosphate). ‘Energy for chemical synthesis.’
Nuclear Lamina
If we go inside of the nucleus and look at the phospholipid bilayer there is fibrous structure called nuclear lamina. The nuclear lamina is made of intermediate filaments which is a type of cytoskeletal component which is harder for the cell to break down. Intermediate filaments are important for structure therefore the nuclear lamina maintains the shape of the nucleus holding it in a spherical shape. Nuclear lamina is made of a protein called lamin, without this the cell struggles to divide. Deficiencies of the protein lamin can lead to medical issues as if the cells cant divide the aging process increases - thus shape of nucleus is important.
Describe the process of DNA packing
Double helix 2nm in diameter > Double helix interacts with proteins called histones (H2 to H4) beads formed are called nucleosomes (length of DNA wrapping around a histone) > Forms a 10nm diameter fibre > DNA interacts with H1 > Causes 10 nm fibre to coil to form a 30 nm fibre > 30nm fibre loops to form 300nm fibre > during cell division 300nm fibres coil to form metaphase chromosomes which can only be seen during metaphase. DNA is usually a 300nm fibre.
Euchromatin vs Heterochromatin
Euchromatin is less dense, contains genes not being used by the cell. Cell needs the genes therefore is less densely packed.
Heterochromatin on the other hand is much more dense as it contains genes not being used by the cell. Because the cell does not need accessibility to these genes they can be densely packed. However heterochromatin can be unpacked if it is needed. Such as when cell division occurs the genes needed will be unpacked, and form euchromatin.
