端木·宇 2008-6-19 22:22
Cell Replication
Eukaryotic cell replication is a process by which cells duplicate theirgenetic material and then divide to yield two daughter cells. In thissection, we will discuss one type of cell reproduction called [b]mitosis[/b]that produces an exact copy of the original cell, including an exactreplication of DNA. In the next chapter, we will move on to discussmeiosis, a different form of cell replication that leads to thecreation of sex cells. Millions of rounds of mitosis take place duringthe development of large multicellular organisms. Three separate tasksmust be completed for a successful round of mitosis: [list=1][*]DNA packaged into chromosomes must replicate.[*]Copies of the chromosomes and organelles must migrate to opposite ends of the cell.[*]The cell must physically split into two separate cells.[/list]
The [b]cell cycle[/b] is the recurringsequence of events that includes the duplication of a cell’s contentsand its subsequent division. The cell cycle is divided into two phases:interphase and mitosis proper. During interphase, the cell copies itsDNA and prepares for division. The cell splits into two daughters inthe stages of mitosis.
[b] Interphase[/b]
During interphase, the cell prepares for thedivision it will undergo during mitosis. Such preparation involvesmaintaining its normal activities, growing to a size that can supportcell division, and replicating its DNA.
[b] DNA Replication[/b]
DNA replicates so that from one helix of DNAemerge two “daughter” helices. These daughter helices are exact copiesof the parental helix. DNA creates daughter helices by using theparental strands of DNA as a template.
The first step in DNA replication is theseparation of the two DNA strands that make up the helix that is to becopied. An enzyme called DNA helicase untwists the helix to form a Yshape called a replication fork. The replication fork moves down theDNA strand, splitting it into two single strands. Next, an enzymecalled DNA polymerase helps new nucleotides line up next to the twoseparated strands, according to the rules of base pairing: adenine andthymine pair with each other, and guanine and cytosine pair with eachother.
As new nucleotides line up at theappropriate spots along the original strand, they form the “rungs” onthe new DNA molecule. Ultimately replication produces two new DNAmolecules that are identical to the original molecule. Replication iscomplete when both of the new strands have formed and rewound intotheir characteristic double helix shape.
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[b] The Products of Replication[/b]
During interphase, every chromosome isreplicated. In a human cell, for example, all 46 chromosomes arereplicated. But that doesn’t double those 46 chromosomes into 92chromosomes like you might think. Instead, after replication, each ofthe two new chromosomes are joined together at their middle by a regioncalled a centromere. The result is an X-shaped structure.
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The two halves of the structure are called [b]chromatids[/b].The entire structure, even though it has doubled in size, is stillcalled a chromosome. Since we call each double-chromatid structure achromosome, a cell that has replicated all of its DNA to prepare fordivision is still said to contain the diploid number of chromosomes,which is 46 in humans.
[b] Mitosis[/b]
During mitosis, the cell divides into twodaughter cells. Mitosis can be divided into four subphases: prophase,metaphase, anaphase, and telophase.
[i] Prophase[/i]
Prophase begins when the double-chromatidchromosomes are fully formed and can be seen clearly under amicroscope. After the chromosomes have formed, microtubule structurescalled [b]centrioles[/b] move to opposite ends of the cell. As the centrioles separate, a fanlike array called the [b]mitotic spindle[/b]forms between them. In later phases of mitosis, the spindle willfunction as a guide to help the replicated chromosomes divide neatlyinto two groups of complete genetic material.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/6e566e2f5c25ebf8e73ac5ebd8b5cfa9.gif[/img][[/align] In prophase, the nuclear membrane dissolvesand the chromosomes attach to the spindle at their centromere. Withchromosomes secured on the spindle, the cell is ready to enter the nextphase of mitosis, metaphase.
[i] Metaphase[/i]
Metaphase begins when the spindle iscompletely formed. The phase is marked by the alignment of chromosomesat the middle of the cell, halfway between each of the mitotic spindlepoles along a plane called the metaphase plate.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/92696fbef4361981cc77da586847f5e8.gif[/img][/align] Once the chromosomes are aligned correctly, the cell enters anaphase, the third stage of mitosis.
[i] Anaphase[/i]
During anaphase, the pairs of chromosomes atthe center of the cell separate into individual chromosomes, which moveto opposite sides of the cell. The microtubule and spindle fibersfacilitate this motion. The cell also begins to elongate in preparationfor splitting.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/339a8c8fb2ce55339abdf1615170a610.gif[/img][/align] When the chromosomes reach their destinationat the opposite poles of the cell, anaphase gives way to telophase, thefourth and final stage of mitosis.
[i] Telophase[/i]
Telophase begins when the chromosomes reachopposite poles. Small pieces of nuclear membrane in the cell begin tore-form around the group of chromosomes at each end, creating twonuclei in one cell. When the chromosomes are once again surrounded by aprotective envelope, they relax and resume their interphase appearanceas a stringy tangle. No longer needed, the spindles fall apart duringthis stage, and a nucleolus re-forms inside each nucleus.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/eef5da83305fe91822cb68d301386acc.gif[/img][/align][b] Cytokinesis[/b]
Although mitosis officially ends with telophase,at this point, the cell is not yet actually split into two new cells.The final cleavage is not exactly its own stage, but it does have itsown name: cytokinesis, literally “cell division.”
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When the two nuclei reach opposite poles of the cell, the cell pinches in the middle, ultimately leading to cleavage.
