Mitosis in Animal and Plant Cells

The Bristlecone Pine

There is a tree in the White Mountains of California that scientists believe is more than 5,000 years old. This tree, a bristlecone pine, is the oldest known individual tree in the world.

Scientists who are interested in the long lifespan of the bristlecone pine have compared seeds and pollen from bristlecone pines of different ages. They found that the cells of the old trees appeared just as young as the cells of the younger trees.

No one really understands how the bristlecone pine can live so long. This is something scientists are continuing to study. But scientists know it has something to do with the life cycle of the tree’s cells. A life cycle is the series of developmental stages an organism passes through on its way from birth to death. The lifespan of a cell ranges from 1 day to more than 100 years.

All cells follow a similar pattern of development. All cells first grow, and then they divide to produce new daughter cells, which are cells formed by the division of a parent cell. Those daughter cells contain DNA from the parent cell. This ordered series of events, which moves from growth to division, is called the cell cycle.

The Growing Phase

A typical animal cell cycle lasts roughly 24 hours, although in some animal cells it lasts less than 8 hours, while in other kinds of cells it takes more than a year. The majority of the cell cycle is spent in interphase—the period of the time when a cell performs its normal functions. For example, a human muscle cell contracts and relaxes during interphase, enabling you to move.

New nucleotides are added to each unzipped strand of the DNA molecule: adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C). The two new strands are both exact replicas of the original DNA molecule. The two identical chromosomes that result from DNA replication are referred to as sister chromatids. They are linked together by the centromere. The chromosomes are not visible during interphase.

Interphase is also when the cell grows, nearly doubling its size. Finally, DNA is replicated during interphase. To replicate means to make a copy of. DNA’s structure allows it to replicate because each strand of the double helix runs in opposite directions. Before a cell divides, the twisted, tightly packed double helix unwinds and separates its two strands, unzipping down the the cell cycle middle. Each strand serves as a template for a new strand of DNA.

The Dividing Phase

Once a cell has replicated its DNA, it gets ready to divide so it can pass along its genetic information to its offspring. Cell division refers to the splitting of a single cell into daughter cells, each with DNA from the parent cell. Cell division is a natural part of a cell’s life cycle, so all cells have proteins that tell them when to divide.

Once DNA replication happens in prokaryotic cells, a new membrane forms that will eventually divide the cell, including the duplicated DNA, in two. This process happens quickly—for example, in ideal conditions, the life cycle of bacteria occurs every 30 minutes.

The eukaryotic life cycle is more complex than the prokaryotic life cycle because eukaryotes have multiple chromosomes within a nucleus. Most plant and animal cells take between 10 and 20 hours to double in number, while some duplicate much more slowly. For example, human skin cells divide almost constantly, while nerve cells in adult animals almost never divide. .

There are several reasons why cells divide. One important reason is growth. The more cells an organism has, the larger that organism is. For example, humans start off as a single cell. By the time they are adults, they have trillions of cells. Almost all of those cells, with the exception of eggs, sperm, and red blood cells, contain the same copy of DNA because of DNA replication and cell division.

Cell division also allows cells to repair damaged cells or replace dead cells. For example, human skin cells constantly divide so they can replace damaged or dead skin cells. Muscle cells also divide frequently to replace cells damaged by exercise or injury.