Nucleus Flashcards
Components of nucleus
nuclear envelope(membrane), nucleoplasm, chromatin, nucleolus
Chromatin of nucleus contains
DNA encoding genetic apparatus of chromosomes
nuclear pores are formed by the fusion of
the inner and outer nuclear membranes and associated with nuclear pore complex
Nuclear pore complex contains how many different proteins
> 450 different proteins
What is a nuclear pore complex?
proteins arranged around each nuclear pore in an octagonal symmetry (cylindrical aqueous channel opening)
What can diffuse across NPC?
Ions, small molecules and proteins
Transport of larger molecules in across NPC requires
nuclear pore receptor proteins (fibril associated)
All proteins in the nucleus are imported from
cytoplasm
Nuclear pore receptor proteins recognize what on the protein?
nuclear localization signals
When larger molecules are recognized they are transported via
active transport
Protoplasm surrounded by nuclear envelope
nucleoplasm
two forms of chromatin that reside with the nucleus
heterochromatin and euchromatin
Where is heterochromatin located in the nucleus
concentrated at nucleus periphery, around the nucleolus and scattered throughout the nucleoplasm
Heterochromatin is transcriptionally
inactive
Euchromatin is transcriptionally
active
Chromatin
Double stranded DNA in chromatin complexed with histones and non-histone acidic proteins.
Histones have
+charged amino acids that bind to DNA
Nucleosome
fundamental packing unit of chromatin
How many copies of each histone does nucleosome contain?
2 copies of each H2A, H2B, H3, and H4, which form the nucleosome core around which the DNA double helix is wrapped two full turns
Describe what composes the “beads of a string” analogy for chromatin
beads= nucleosomes; string=linker DNA
condensed chromatin contains
an additional histone, H1, which wraps around groups of nucleosomes forming 30nm diameter fibers, the structural unit of the chromosome
When do chromosomes become visible?
Chromosomes become visible during mitosis and meiosis when their chromatin is condensed
Barr body
corresponds to one of the two X chromosomes; present in nearly all somatic cells of female mammals
What phase is the barr body (inactive X chromosome) visible as a dark staining evagination protruding from the nucleus
Interphase
Two major periods of cell cycle
Interphase and Mitosis
Cells in G0 state
Temporarily suspended in nondividing resting cells. Such cells may reenter cycle and divide again
Which is longer? Interphase or Mitosis
Interphase is much longer
Period during which cell doubles in size and DNA content
Interphase
Three phases of interphase
G1, S, and G2
Cells that fail to reach restriction point become
resting cells and enter the G0 state
How long does the G1 phase last
lasts from a few hours to several days
In the cell cycle, G1 phase(gap phase) occurs when
Just after mitosis-cell growth and protein synthesis occur-restoring daughter cells to normal volume and size
The synthesis of regulatory proteins in G1 phase enables the cell to
reach a threshold(restriction point) and proceed to S phase
What occurs in the S phase (synthetic phase)
DNA replication and protein synthesis occur. Chromosomes duplicated
Centrioles in S phase
self-duplicated
Cells in S phase will uptake
tritiated thymidine- counted in scintillation counter or by autoradiography
When does G2 phase(gap phase) occur in the cell cycle?
Follows the S phase and extends to mitosis
What occurs in G2 phase (the gap phase)
Cell resting and preparing to divide: centrioles grow to maturity; energy for mitosis is stored; RNA and proteins necessary for mitosis are synthesized
Cell cycle control factors
cyclins; cyclin-dependent kinases
CDK
cyclin dependent kinases
Cyclins and cyclin dependent kinases basic function
regulate the cell cycle and move it in one direction
How are cyclins and cyclin dependent kinases controlled
controlled by regulatory proteins – inhibit activity of kinases (cyclin kinase inhibitors) and degrade cyclins (polyubiquinated and degraded by proteosome
Start Kinase is used to
initiate S phase
M-phase promoting factor
initiate mitosis
Two primary ways to control cell cycle
1) Regulation of genes that suppress cell proliferation 2) Regulation of growth factors that stimulate cell growth
Tumor suppressor gene-Normal cell has
2 copies of retinoblastoma gene.(Normal Rb protein prevents mitosis (entry into S phase))
If one copy of Rb mutated
other copy of gene still suppresses growth
Mutation in both tumor suppressor genes
results in abnormal Rb gene product and cancer
p53
DNA repair gene which exhibits frequent mutations; p53 acts prior to DNA replication by detecting DNA damage and delaying entry into S phase until damage repaired
mutation in how many of four subunits may result in an abnormal p53 protein
mutation in 1 of 4 p53 subunits may result in abnormal p53 protein
When does mitosis occur
follows the G2 phase and completes the cell cycle
Karyokinesis
Division of the nucleus
Division of the cytoplasm
cytokinesis
Mitosis results in
two identical daughter cells
Centrosome
Pericentriolar material plus associated 2 centrioles oriented orthogonally to each other. Centrosomes function as microtubule organizing centers and function as the mitotic spindle pole during mitosis
Astral microtubules
radiate from centrosomes; function to separate the spindle poles and position them in the cell
Polar microtubules
extend from each pole and overlap in middle of cell- responsible for pushing poles of cells apart
Kinetochore microtubules
attach to kinetochore that forms at the centromere of duplicated chromosomes- moves separated daughter chromatids to each pole
Centromere
constriction in the middle of the attached sister chromatids where a protein complex (kinetochore) is assembled. Kinetochore microtubules
will attach to the kinetochore and pull the sister chromatids towards opposite poles of the cell
Normal process of programmed cell death. May be triggered by multiple pathways and stimuli (cytokines, toxins, hormones, and other cell stressors – radiation, viruses, oxygen or nutrient deprivation)
Apoptosis
When are caspases activated
Caspases are activated in apoptosis and drive apoptotic cascade
Function of caspases
cleave cell proteins and activate other enzymes like DNAse
