Chapter 3: The Nucleus
What is the nucleus?
The command center of the cell, containing the code for all of a cell’s enzymes and other proteins (i.e. DNA).
What is the nuclear envelope?
A selectively permeable barrier between the nuclear and cytoplasmic compartments.
What is the perinuclear space?
A narrow space that separates the two concentric membranes of the nuclear envelope.
What is the outer nuclear membrane continuous with?
The extensive cytoplasmic network of the rough endoplasmic reticulum (RER).
What is the nuclear lamina?
A highly organized meshwork of proteins which stabilizes the nuclear envelope.
What are lamins?
Intermediate filament proteins in the nuclear lamina that bind to membrane proteins and associate with chromatin in nondividing cells.
What are nuclear pore complexes?
Protein complexes that bridge the inner and outer nuclear membranes.
What are nucleoporins?
Proteins of the nuclear pore complex that regulate movement of macromolecules into the nucleus.
What are importins?
Proteins that facilitatie the the transfer of other proteins across the nuclear envelope.
What is chromatin?
The combination of DNA and its associated proteins, divided among 46 chromosomes.
What are chromatids?
The two identical chromatin units that comprise each chromosome after DNA replication but before cell division, held together by cohesin proteins.
What are histones?
Small basic proteins associated with the organization and packaging of DNA.
What is a nucleosome?
The structural unit of DNA and histones, consisting of a core of eight histones (two each of H2A, H2B, H3, and H4), around which is wrapped about 150 bp of DNA, which is stabilized by a larger histone (H1).
Describe the organization of DNA above the level of the nucleosome.
Nucleosomes interact to form a compact 30-nm fiber, which forms various loops that are tethered to and stabilized by interactions with protein scaffolds.
What is euchromatin?
Light-staining, finely dispersed, transcriptionally active chromatin.
What is heterochromatin?
Dark-staining, highly condensed., transcriptionally inactive chromatin.
What is constitutive heterochromatin?
Heterochromatin containing repetitive, gene-poor DNA sequences (e.g. centromeres and telomeres)
What is facultative heterochromatin?
Heterochromatin that can undergo reversible transitions from compact, transcriptionally silent states to more open, transcriptionally active conformations.
What is a Barr body?
Facultative heterochromatin occurring as one of the two X chromosomes in females, remaining tightly coiled, and transcriptionally inactive.
What are chromosomal territories?
The discrete areas within the nucleus occupied by each individual chromosome.
What does it mean that the members of each chromosomal pair are "homologous"?
Although derived from different parents, they contain alleles of the same genes.
What does it mean if a cell is diploid?
It contains 2n chromosomes, where n is the number of unique chromosomes in a species (23 in humans).
What does it mean if a cell is haploid?
It contains half the diploid number of chromosomes, each pair having been separated during meiosis.
What is a karyotype?
A digitally arranged microscopic photograph of stained chromosomes that allows them to be analyzed.
What is the nucleolus?
A generally spherical, highly basophilic subdomain of nuclei in cells actively engaged in protein synthesis.
What is neoplastic proliferation?
Cells that become transformed to grow at a higher rate and in an uncoordinated manner, which can be either benign or malignant (i.e. cancer).
What is the cell cycle?
The regular sequence of events that produce new cells.
What are the four phases of the cell cycle?
- G1 (RNA and protein synthesis)
- S (DNA replication)
- G2 (preparation for mitosis)
What is the G0 phase?
A temporary or permanent suspension of cell cycle activities in new postmitotic cells.
How is cell cycling is activated in postmitotic G0 cells?
By protein signals from the extracellular environment called mitogens or growth factors.
What are cyclins?
A family of cytoplasmic proteins that regulate the cell cycle through activation of cyclin-dependent kinases.
What are cyclin-dependent kinases (CDKs)?
Enzymes that phosphorylate specific proteins to trigger the activities of the next phase of the cell cycle.
What occurs if problems encountered at checkpoints in the cell cycle are not corrected quickly?
Tumor suppressor genes are activated and cell’s activity is redirected toward apoptosis.
What are proto-oncogenes?
Genes coding for proteins important in the control of cell proliferation and differentiation.
What are oncogenes?
Mutations of proto-oncogenes that cause uncontrolled cell growth and a potential for cancer.
What is mitosis?
The period of cell division, consisting of four stages: prophase, anaphase, metaphase, and telophase.
What is interphase?
The period between mitoses (the G1, S, and G2 phases).
What events occur during prophase?
- the nucleolus disappears and the replicated chromatin condenses into chromosomes
- the centrioles migrate to opposite poles of the cell and organize the mitotic spindle
- the nuclear lamina and nuclear pore complexes disassemble and disperse in vesicles
What events occur during metaphase?
- kinetochores at the centromere of each chromosome attach to the mitotic spindle
- microtubules move chromosomes into alignment at the equatorial plate
What events occur during anaphase?
The sister chromatids (now called chromosomes themselves) separate and move toward opposite spindle poles by microtubule motor proteins and changes in the lengths of the microtubules.
What events occur during telophase?
- The two sets of chromosomes are at the spindle poles reverting to their uncondensed state
- the nuclear envelope begins to reassemble around each set of daughter chromosomes
- a belt-like contractile ring of actin and myosins develops at the cell’s equator
What is cytokinesis?
Constriction of the contractile ring formed in telophase, producing a cleavage furrow that progresses until the cytoplasm and its organelles are divided into two daughter cells, each with one nucleus.
What are stem cells?
Undifferentiated, self-renewing cells whose cycling serves to renew the differentiated cells of tissues.
What are progenitor cells?
Rapidly dividing progeny of stem cells, often called transit amplifying cells because they are along the path from the stem cell niche to a differentiated state.
What is meiosis?
A specialized process involving two associated cell divisions to produce haploid cells.
What is synapsis?
A process that occurs during prophase ,I in which homologous chromosomes form pairs called tetrads, and exchange DNA is by forming crossovers.
What are three key differences between meiosis and mitosis?
- mitosis involves one cell division, meiosis involves two cell divisions
- mitosis yields diploid (2n) cells, meiosis yeilds haploid (n) cells
- mitosis yeilds genetically identical cells, meiosis yeilds genetically unique cells (crossovers)
What is trisomy 21?
A genetic disease resulting from nondisjunction in the first meiotic division, producing a viable zygote containing three copies of chromosome 21 which, if fertilized, results in Down syndrome.
What is apoptosis?
A rapid, highly regulated cellular activity that shrinks and eliminates defective and unneeded cells.
What is the key difference between apoptosis and necrosis?
Apoptotic cells do not rupture and release their contents, thus it does not stimulate the inflammatory response like necrosis does.
What are Bcl-2 proteins?
A family of proteins that regulate the release of death-promoting factors from mitochondria in apoptosis.
What occurs in the mitochondira during apoptosis?
Bcl-2 proteins trigger it to release cytochrome c, which activates caspases, proteolytic enzymes that cause protein degradation throughout the cell.
What occurs to DNA during apoptosis?
It is cleaved into small fragments by endonucleases.
What occurs to the nuclear and cell volumes during apoptosis?
Destruction of the cytoskeleton and chromatin causes the cell to shrink quickly, producing small structures with dense, darkly stained pyknotic nuclei.
What occurs to the cell membrane during apoptosis?
It undergoes shape changes, such as “blebbing,” as proteins are degraded and lipid mobility increases.
What are apoptotic bodies?
Membrane-bound remnants of cytoplasm and nucleus that separate from an apoptotic cell, which quickly undergo phagocytosis.