Lesson: Chapter - 6

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 itosism 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:

Video Lesson - Cell Replication

1. DNA packaged into chromosomes must replicate.
2. Copies of the chromosomes and organelles must migrate to opposite ends of the cell.
3. 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.

Video Lesson - Interphase

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.

Video Lesson - DNA Replication

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.

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