Nucleus

Nucleus

control center of eukaryotic cell that stores genetic material (DNA) and directs cellular activities through gene expression

Nuclear Envelope

double membrane separating nucleus from cytoplasm

nuclear pores

openings in the envelope allowing controlled movement of materials

nucleoplasm

gel-like fluid inside the nucleus containing chromatin, nucleolus, and enzymes

chromatin

threadlike DNA-protein complex that condenses into chromosomes during cell

Nucleolus

dense region where the ribosomal RNA is made and ribosomes subunits are assembled

Function of the Nucleus

Genetic Material Storage

cell control

central dogma

Nuclear Import

1. Cytosolic protein with a nuclear localization signal (NLS) binds to importin.

2. Complex moves through nuclear pore complex (NPC) into the nucleus.

3. Inside, importin binds Ran-GTP, releasing the cargo protein.

4. Ran-GTP-importin complex exits to the cytoplasm.

5. GTP hydrolysis converts Ran-GTP to Ran-GDP, releasing importin for reuse.

Nuclear Export

most of it is Ran-dependent

Ran

small GTP-binding protein providing energy and direction

Nuclear Export (Ran-Dependent Pathway)

1. Cargo protein with nuclear export signal (NES) binds exportin.

2. Exportin + Ran-GTP + cargo → forms export complex.

3. Complex moves through NPC.

4. RanGAP hydrolyzes GTP → complex dissociates; cargo released.

5. RanGDP returns to nucleus and is recharged by RanGEF.

Nuclear Export (Ran-Independent Pathway)

1. mRNA must be processed – capped, spliced, polyadenylated.

2. Adaptor proteins (TREX complex, NXF1/NXT1) bind to mRNA.

3. Complex moves through NPC via nucleoporins.

4. RNA helicases use ATP to release mRNA into cytoplasm.

Genes

fundamental units of heredity made up of DNA

RNA Viruses

some virus use RNA as genetic material

Base pairing rule

A=T, G=C

Purine-Pyrimidine Balance

(A+G) = (T+C)

Species Specifity

DNA base ratios are constant within a species but vary between species

DNA packaging in bacteria

DNA stored in bacterial chromosomes and plasmids

DNA packaging in Eukaryotes

DNA stored as chromatin and chromosomes

Histone tails

undergo chemical modification that affects packing

beads on a string 

nucleosomes connected by a linker DNA

30-nm fiber

condensed nucleosomes

looped domains

30-nm fibers form loops

condensed chromatin

highly packed during mitosis

chromosomes

fully compacted DNA during cell division

Euchromatin

lightly packed, active in transcription

heterochromatin

densely packed, transcriptionally inactive

constitutive heterochromatin

centromeres and telomeres

karyotype

distinct chromosome banding patterns

Fluorescent In Situ Hybridization (FISH)

detects specific DNA or RNA sequences in intact cells