1.8 - the nucleus – Bioch / OB Flashcards
size of the nucleus
5-10µm in diameter
nuclear envelope
double membrane
outer membrane is continuous with the endoplasmic reticulum
inner and outer membranes are separated by a space = perinuclear space = around 20-40 nm
has nuclear pores
nuclear pores
holes in the nuclear envelope that are made up of proteins forming a nuclear pore complex
Cylinder in hole spanning the two nuclear membranes
nucleoplasm
interior of the nucleus is filled with a semi-fluid substance called the nucleoplasm = similar to cytoplasm
contains nucleotides, enzymes etc
nucleolus
dense, spherical region within the nucleus that is not membrane-bound
site of ribosomal RNA synthesis and the assembly of ribosomal subunits.
nuclear lamina
dense, fibrous network located just beneath the inner membrane of the nuclear envelope
composed of intermediate filament proteins called lamins
functions of the nuclear lamina
Provides mechanical support to the nuclear envelope, helping to maintain the shape and stability of the nucleus.
Involved in the anchoring of chromatin to the nuclear periphery, which helps in organizing the DNA within the nucleus.
Plays a role in the disassembly and reassembly of the nuclear envelope during cell division (mitosis).
nuclear functions
gene replication + repair
genetic transcription
ribosome production
DNA in interphase
genetic material found as chromatin – largely invisible + thin strands of genetic material
more accessible for transcription factors and important enzymes such as RNA polymerase II.
type of DNA in interphase
euchromatin or heterochromatin
heterochromatin
more densely stored + wrapped tightly around deacetylated histones so that it is inaccessible for transcription
deacetylated
positive charge on histones is retained = strengthens bonding
histones
proteins around which DNA wraps to form nucleosomes – basic unit of chromatin
euchromatin
loosely packed and less condensed than heterochromatin
structure allows for easier access to DNA by the transcription machinery
associated with active genes
regulation of euchromatin
-regulated through acetylation, methylation, and other similar methods so that it can be appropriately expressed by the cell
Acetylation
neutralises the positive charge on histones, loosening their interaction
2 classes of heterochromatin
constitutive
facultative
constitutive
poorly expressed
contains genes that are permanently silenced
genes transcriptionally inactive
often found in centromeres + telomeres
function of constitutive
Aids in the physical separation of chromosomes during mitosis and meiosis, preventing chromosomal entanglement
Helps maintain chromosomal integrity and stability during cell division.
Facultative
can switch between an active (euchromatic) state and an inactive (heterochromatic) state
reversible = can be decondensed into euchromatin when specific genes need to be expressed
well known example of facultative heterochromatin
X chromosome inactivation in females
descrive x chromosome inactivation
in early embryonic development – cells randomly choose one of the 2 X chromosomes to inactivate – some will inactive mother + some father = leads to mosaicism – different cells express different X-linked genes
inactivated X chromosome condenses into a dense structure called the Barr body
reversible process – that’s why its facultative
why do we have x chromosome inactivation
ensures that females do not have a double dose of X-linked gene expression compared to males, who only have one X chromosome
what is argued to define eukaryotes
nuclear envelope
