nucleus (histo)

nucleus

• largest organelle of the cell

• contains most of the DNA and mechanisms for RNA

• houses the nucleolus

nucleolus

• location for assembly of ribosomal subunits

• center for ribosomal RNA synthesis (rRNA)

nucleus components

• chromatin

• nucleolus

• nucleoplasm

chromatin

genetic material of the cell

nucleoplasm

containing macromolecules and nuclear particles involved in maintenance of the cell

nuclear envelop

• composed of inner and outer nuclear membrane

• the space btw them is called the perinuclear cisterna

inner nuclear membrane

• has proteins that associate with lamin (a form of intermediate filament)

• lamin forms nuclear ports with organize perinuclear chromatin

• docking sites for RNA and helps reassemble the nucleus after mitosis

outer nuclear membrane

• continuous with RER

• studded with ribosomes for nuclear membrane protein synthesis

nuclear pore complex

• spans both nuclear membranes

• composed of 3 stacked ring like arrays of proteins

• has cytoplasmic fibers (transporters) and a nuclear basket

• importins and exportins

exportins

• transport macromolecules from the nucleus to the cytoplasm

• regulated by family of GTP binding proteins called Ran

importins

• transport cargo from the cytoplasm to the nucleus

• regulated by family of GTP binding proteins called Ran

chromatin

• cells DNA and associated proteins in the “relaxed state”

heterochromatin

• condensed form of DNA located primarily at peripheral portion of the nucleus (not transcriptionally active)

• can be seen with light microscopy

euchromatin

• unwound DNA, transcriptionally active

• “beads on a string” stage

• cant be seen with light microscopy

nucleosomes

• an octamer composed of 2 subunits of H2a, H2b, H3, and H4

• H1 helps compact chromatin by binding coiled nucleosomes

histone tails

• subject to enzymatic modification to condense or open chromatin

• methylation usually condenses

• acetylation opens

how many chromosomes do humans have

46

how many pairs of chromosomes do humans have

23

karyotype

visualization of chromosomes in a cell

mRNA

coding RNA that function as the template for protein synthesis

rRNA

combines with proteins to form the two ribosomal subunits to function in protein synthesis

tRNA

binds to amino acids to carry them to the correct sites on the mRNA in protein synthesis

RNA

single stranded and turn over rapidly in the cell

micro RNA

• non-coding RNAs transcribed by RNA polymerase I and II

• post transcriptionally regulate gene expression by binding to the 3’ UTR of mRNA

• causes degradation or prevents translation

silencing RNA

• similar to micro RNA

• reduces level of expression of a gene

nucleoplasm

contains chromatin, ribonuclear proteins, interchromatin, and perichromatin granules

perinuclear granules

• located at periphery of nucleus

• main site of RNA export from the nucleus

interchromatin granules

located throughout the nucleus

cell cycle phases

mitosis and interphase

mitosis

when the cell divides all of it contents

interphase

• when cell replicates DNA and organelles

• 3 phases: G1, S, G2

G1 phase

• when the synthesis of macromolecules (RNA and protein) for DNA duplication begin

S phase

• when DNA is duplicated

• somatic cell ploidy goes from 2n —> 4n

G2 phase

• when the cell undergoes preparations for mitosis

• proofreading of DNA, protein synthesis for cell division

prophase

• chromosome condensation and joining at centromeres

• centrosome divides and forms 2 regions of 2 centrioles on opposite sides of cell

• many microtubules

prometaphase

• phosphorylation of nuclear lamins and disassembly of nucleus

• kinetochore microtubules attach to kinetochore of chromosomes

• proteins cohesins and condensins hold chromosomes together

metaphase

• duplicated chromosomes align on the mitotic spindles

• forms metaphase plate

anaphase

sister chromatids are pulled to opposite poles and appearance of cleavage furrow

telophase

cytokinesis, dephosphorylation and restoration of the nucleus

necrosis

• cell death due to mechanical damage

• causes cell to rupture and spill its contents into extracellular space

• unregulated and harmful to undamaged cells bc of induced inflammation

apoptosis

• programmed cell death

• highly regulated process

• cell fragments (without leakage) are engulfed by macrophages

• 2 types: extrinsic and intrinsic

extrinsic

• cytokines bind their extracellular receptor on cells and trigger a signaling cascade that activates the caspases

• leads to nuclear and cytoskeletal degradation

• phagocytosis by macrophages

intrinsic

• molecular (ex: DNA) damage or stress triggers a signaling cascade that assembles the death inducing signal complex

• causes cytochrome c to leak from mitochondria and activate apoptosis cascade