The nucleus of each eucaryotic cell contains chromosomes
that control heredity. These chromosomes replicate during a
cell cycle that includes mitosis, cytokinesis, and interphase.
Each eucaryotic cell nucleus includes:
- Nuclear envelope: a double-layered
membrane around the nucleus, penetrated by occasional pores
- Nucleolus: a dense-staining, RNA-rich region within the nucleus,
responsible for the production of ribosomes
- Chromatin: long, thin threads of DNA, hard to see except
when they condense into chromosomes during cell division
Cell cycle: The cell cycle results in two identical cells.
Cell cycle
It consists of nuclear division (mitosis), cytoplasmic division (cytokinesis),
and interphase.
- Mitosis is division of the nucleus in somatic cells (all cells
except sex cells).
It preserves chromosome number without change.
Mitosis
- Prophase:
- Centrioles divide and separate
- Spindle forms between them, then enlarges
- Nuclear envelope and nucleolus fade from visibility
- Chromosomes thicken & coil more tightly, becoming visible as two
chromatids attached to a central centromere.
- Metaphase: Chromosomes line up on the equator of the cell to
form a metaphase plate.
- Anaphase:
- Centromeres of each chromosome divide in half, briefly doubling the
chromosome number (since we count centromeres).
- Sister chromatids (those derived from the same chromosome)
now separate and are pulled toward opposite poles.
- Telophase:
- Chromosomes reach opposite poles and grow too thin to see.
Mitosis
- Finally, nuclear envelope and nucleolus reappear.
- Cytokinesis (division of the cytoplasm, much simpler than mitosis)
- In animal cells (and other cells without rigid cell walls), cytoplasm
divides by constricting in the middle, forming a dumbbell shape.
- In cells with rigid cell walls (like plant cells), a series of
bubble-like vesicles fuse to form a cell plate alolng the
cell's equator.
- Interphase, the longest part of the cell cycle, during which nearly
all growth and metabolism occurs. Consists of:
Cell cycle
- G1 (first gap phase): time of most rapid growth;
many new organelles synthesized
- S (synthesis phase): DNA synthesis occurs; growth continues
- G2 (second gap phase): growth continues;
chromosomes become double-stranded; cell prepares to divide
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