Eukaryotic cell replication is a process by which cells
duplicate their genetic material and then divide to yield two daughter
cells. In this section, we will discuss one type of cell reproduction
called mitosis that produces an exact copy of the original
cell, including an exact replication of DNA. In the next chapter,
we will move on to discuss meiosis, a different form of cell replication
that leads to the creation of sex cells. Millions of rounds of mitosis
take place during the development of large multicellular organisms.
Three separate tasks must be completed for a successful round of
DNA packaged into chromosomes must replicate.
of the chromosomes and organelles must migrate to opposite ends
of the cell.
cell must physically split into two separate cells.
The cell cycle is the recurring sequence
of events that includes the duplication of a cell’s contents and
its subsequent division. The cell cycle is divided into two phases:
interphase and mitosis proper. During interphase, the cell copies
its DNA and prepares for division. The cell splits into two daughters
in the stages of mitosis.
During interphase, the cell prepares for the division
it will undergo during mitosis. Such preparation involves maintaining
its normal activities, growing to a size that can support cell division,
and replicating its DNA.
DNA replicates so that from one helix of DNA emerge two
“daughter” helices. These daughter helices are exact copies of the
parental helix. DNA creates daughter helices by using the parental
strands of DNA as a template.
The first step in DNA replication is the separation of
the two DNA strands that make up the helix that is to be copied.
An enzyme called DNA helicase untwists the helix to form a Y shape
called a replication fork. The replication fork moves down the DNA
strand, splitting it into two single strands. Next, an enzyme called
DNA polymerase helps new nucleotides line up next to the two separated
strands, according to the rules of base pairing: adenine and thymine
pair with each other, and guanine and cytosine pair with each other.
As new nucleotides line up at the appropriate spots along
the original strand, they form the “rungs” on the new DNA molecule.
Ultimately replication produces two new DNA molecules that are identical
to the original molecule. Replication is complete when both of the
new strands have formed and rewound into their characteristic double
The Products of Replication
During interphase, every chromosome is replicated. In
a human cell, for example, all 46 chromosomes are replicated. But
that doesn’t double those 46 chromosomes into 92 chromosomes like
you might think. Instead, after replication, each of the two new
chromosomes are joined together at their middle by a region called
a centromere. The result is an X-shaped structure.
The two halves of the structure are called chromatids.
The entire structure, even though it has doubled in size,
is still called a chromosome. Since we call each double-chromatid
structure a chromosome, a cell that has replicated all of its DNA
to prepare for division is still said to contain the diploid number
of chromosomes, which is 46 in humans.
During mitosis, the cell divides into two daughter cells.
Mitosis can be divided into four subphases: prophase, metaphase,
anaphase, and telophase.
Prophase begins when the double-chromatid chromosomes
are fully formed and can be seen clearly under a microscope. After
the chromosomes have formed, microtubule structures called centrioles move
to opposite ends of the cell. As the centrioles separate, a fanlike array
called the mitotic spindle forms between them. In later
phases of mitosis, the spindle will function as a guide to help
the replicated chromosomes divide neatly into two groups of complete
In prophase, the nuclear membrane dissolves and the chromosomes
attach to the spindle at their centromere. With chromosomes secured
on the spindle, the cell is ready to enter the next phase of mitosis,
Metaphase begins when the spindle is completely formed.
The phase is marked by the alignment of chromosomes at the middle
of the cell, halfway between each of the mitotic spindle poles along
a plane called the metaphase plate.
Once the chromosomes are aligned correctly, the cell enters
anaphase, the third stage of mitosis.
During anaphase, the pairs of chromosomes at
the center of the cell separate into individual chromosomes, which
move to opposite sides of the cell. The microtubule and spindle
fibers facilitate this motion. The cell also begins to elongate
in preparation for splitting.
When the chromosomes reach their destination at the opposite
poles of the cell, anaphase gives way to telophase, the fourth and
final stage of mitosis.
Telophase begins when the chromosomes reach opposite poles.
Small pieces of nuclear membrane in the cell begin to re-form around
the group of chromosomes at each end, creating two nuclei in one
cell. When the chromosomes are once again surrounded by a protective
envelope, they relax and resume their interphase appearance as a
stringy tangle. No longer needed, the spindles fall apart during
this stage, and a nucleolus re-forms inside each nucleus.
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 its own
name: cytokinesis, literally “cell division.”
When the two nuclei reach opposite poles of the cell,
the cell pinches in the middle, ultimately leading to cleavage.