Cell Replication
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 mitosis:
- 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.
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.
Interphase
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 Replication
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 helix shape.
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.
Mitosis
During mitosis, the cell divides into two daughter cells. Mitosis can be divided into four subphases: prophase, metaphase, anaphase, and telophase.
Prophase
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 genetic material.
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.
Metaphase
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.
Anaphase
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
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.
Cytokinesis
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.
