The Cell Cycle

Topics: Mitosis, Cell cycle, Chromosome Pages: 5 (649 words) Published: November 8, 2014
The Cell Cycle
Mitosis: the process by which cells reproduce themselves, resulting in daughter cells that contain the same amount of genetic material as the parent cell.


Cell Division Occurs in a series of stages of phases

occurs before mitosis begins
Chromosomes are copied (# doubles)
Chromosomes appear as threadlike coils (chromatin) at the start, but each chromosome and its copy(sister chromosome) change to sister chromatids at end of this phase

1st step in Mitosis
Mitosis begins (cell begins to divide)
Centrioles (or poles) appear and begin to move to opposite end of the cell. Spindle fibers form between the poles.

2nd step in Mitosis
Chromatids (or pairs of chromosomes) attach to the spindle fibers.

3rd step in Mitosis
Chromatids (or pairs of chromosomes) separate and begin to move to opposite ends of the cell.

4th step in Mitosis
Two new nuclei form.
Chromosomes appear as chromatin (threads rather than rods).
Mitosis ends.
occurs after mitosis
Cell membrane moves inward to create two daughter cells – each with its own nucleus with identical chromosomes.

Cell Cycle

DNA associates with special proteins to form more stable structure called chromosomes (different proteins in prokaryotes and eukaryotes, so chromosomes built different) Chromosomes are found inside nucleus in eukaryotes

Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1 set of 23 from sperm) Each chromosome contains many genes
Gene is a segment of DNA that is responsible for controlling a trait (e.g., coding for a specific protein)

The cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)
Interphase (cell growth and copying of chromosomes in preparation for cell division)

Interphase (about 90% of the cell cycle) can be divided into subphases: G1 phase (“first gap”)
S phase (“synthesis”)
G2 phase (“second gap”)

Distribution of Chromosomes during Cell Division

In preparation for cell division, DNA is replicated and the chromosomes condense Each duplicated chromosome has two sister chromatids, which separate during cell division The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

Mitosis is conventionally divided into five phases:
Cytokinesis is well underway by late telophase
1. Prophase
Chromatin condenses; this causes the chromosomes to begin to become visible. Centrosomes separate, moving to opposite ends of the nucleus. The centrosomes start to form a framework used to separate the two sister chromatids called the mitotic spindle that is made of microtubules. Nucleolus disappears.

Prophase. The chromatin is condensing.The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is starting to form.

2. Prometaphase
Nuclear envelope fragments.
Chromosomes become more condensed.
A kinetochore is formed at the centromere, the point where the sister chromatids are attached. Microtubules attach at the kinetochores.

Prometaphase we now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment.

3. Metaphase
Chromosomes align on an axis called the metaphase plate.
Note: the spindle consists of microtubules, one attached to each chromosome.

Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate.

4. Anaphase
Each centromere splits making two chromatids free.
Each chromatid moves toward a pole.
Cell begins to elongate, caused by microtubules not associated with the kinetochore.

Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to...
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