What makes nuclei exact copies of the original

The cleavage furrow forms because of the action of a contractile ring of overlapping actin and myosin filaments. As the actin and myosin filaments move past each other, the contractile ring becomes smaller, akin to pulling a drawstring at the top of a purse.

When the ring reaches its smallest point, the cleavage furrow completely bisects the cell at its center, resulting in two separate daughter cells of equal size Figure 3. Figure 3: Mitosis: Overview of major phases The major stages of mitosis are prophase top row , metaphase and anaphase middle row , and telophase bottom row.

This page appears in the following eBook. Aa Aa Aa. What Are the Phases of Mitosis? Figure 1: Drawing of chromosomes during mitosis by Walther Flemming, circa What Happens during Prophase? What Happens during Prometaphase? Each microtubule is highly dynamic, growing outward from the centrosome and collapsing backward as it tries to locate a chromosome.

Eventually, the microtubules find their targets and connect to each chromosome at its kinetochore , a complex of proteins positioned at the centromere. The actual number of microtubules that attach to a kinetochore varies between species, but at least one microtubule from each pole attaches to the kinetochore of each chromosome. A tug-of-war then ensues as the chromosomes move back and forth toward the two poles.

What Happens during Metaphase and Anaphase? Figure 2: Types of microtubules involved in mitosis. During mitosis, several types of microtubules are active.

What Happens during Telophase? During telophase , the chromosomes arrive at the cell poles, the mitotic spindle disassembles, and the vesicles that contain fragments of the original nuclear membrane assemble around the two sets of chromosomes. Phosphatases then dephosphorylate the lamins at each end of the cell.

This dephosphorylation results in the formation of a new nuclear membrane around each group of chromosomes. When Do Cells Actually Divide? Figure 3: Mitosis: Overview of major phases. The major stages of mitosis are prophase top row , metaphase and anaphase middle row , and telophase bottom row. Cells spend most of their life in this non-dividing phase. See the graphic below. Prophase : The chromosomes coil and shorten, and become visible. It becomes apparent that the chromosomes have duplicated.

The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide.

Nucleotides are arranged in two long strands that form a spiral called a double helix. An important property of DNA is that it can replicate, or make copies of itself. The particular sequence of bases along the DNA molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could infer the order of the bases in one strand from the bases in the other, complementary strand.

After a great deal of debate and experimentation, the general method of DNA replication was deduced in by two scientists in California, Matthew Meselson and Franklin Stahl. This method is illustrated in Figure 3. Stage 1: Initiation. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including helicase , untwist and separate the two strands of DNA. Stage 2: Elongation. Each strand becomes a template along which a new complementary strand is built.

DNA polymerase brings in the correct bases to complement the template strand, synthesizing a new strand base by base. This growing strand continues to be built until it has fully complemented the template strand. Stage 3: Termination. Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete. Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand.

As you might imagine, it is very important that DNA replication take place precisely so that new cells in the body contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless.

Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and corrects them. Once the process of DNA replication is complete, the cell is ready to divide. You will explore the process of cell division later in the chapter. Watch this video to learn about DNA replication.