CELLULAR REPRODUCTION

The Basics of Cellular Reproduction

We humans, like other multicellular organisms, begin life as a single cell. In nine short months, however, we become trillions of cells because cellular reproduction has occurred over and over again. Even after we are born, cellular reproduction doesn’t stop –it continues as we grow, and when we are adults, it replaces worn-out or damage tissues. Right now, your body is producing thousands of new red blood cells, skin cells, and cells that line your respiratory and digestive tracts. If you suffer a cut, cellular reproduction helps repair the injury.

                Cellular reproduction is also necessary for the reproduction of certain organisms. When an amoeba splits, two new individual amoebas are produced. The process is called asexual reproduction because it doesn’t require a sperm and an egg. The next chapter concerns the production of egg and sperm, which are needed for sexual reproduction.

                One way to emphasize the importance of cellular reproduction is to say that “all cells come from cells.” You can’t have a new cell without a pre-existing cell. And you can’t have a new organism without a pre-existing organism. Cellular reproduction is necessary for the production of both new cells and a new organism.

Chromosomes

Cellular reproduction always involves two important processes: growth and cell division. During growth, a cell duplicates the contents of its cytoplasm and it DNA. Then, during division, the cytoplasm and the DNA of the parent cell are distributed to the so-called daughter cells. (these terms have nothing to do with gender; they are simply a way to designate the beginning cell and the resulting cells.)

                The passage of DNA to be daughter cells is critical because cells cannot continue to live without a copy of the genetic materials. Especially in eukaryotic cells, passage of DNA to the daughter cells present a problem because of the large quantity of DNA in the nucleus. For example, a human cell contains about 2 meters of DNA and a nucleus is only 5 to 8 micrometers (µm) in diameter. During cellular reproduction, DNA is packaged into chromosomes, which allow DNA to be distributed to the daughter cells. A chromosome contains DNA, and it also contains proteins that help package the DNA and possibly function in utilizing the DNA as well.

Chromatin into chromosomes

When a eukaryotic cell is not undergoing cell division, the DNA and associated proteins have the appearance of thin threads called chromatin. Closer examination reveals that chromatin is periodically wound around a core of eight protein molecules so that it looks like beads on a string. The protein molecules are histones, and each bead is called nucleosome.

                Just before cell division occurs, the chromatin coils tightly into a fiber that has several nucleosomes to a turn. Then the fiber coils again before it loops back and forth and condenses to produce highly compacted chromosomes. Each species has a characteristic number of chromosomes; a human cell has 46 chromosomes. We can easily chromosomes with a light microscope because just before division occurs a chromosome is 10,000 times more compact than is chromatin.

                Another important event, that occurs in preparation for partition of chromosomes is DNA replication, when a DNA make a copy of itself. By the time we can clearly see the chromosomes, they are duplicated.  A duplicated chromosome is composed of two identical halves called sister chromatids held together  at a constricted region called a centromere. Each sister chromatid contains an identical DNA double helix.

The Cell Cycle

We have already indicated that cellular reproduction involves duplication of cell contents followed by cell division. For cellular reproduction to be orderly, you would expect the first event to occur before the second event, and that’s just happens during the so-called cell cycle. The cell cycle is an orderly sequence of stages that takes place between the time a new cell has arisen from division of a parent cell to the point when it has given rise to two daughter cells. Duplication of cells contents occurs during the stage called interphase.

Interphase

Most of the cell cycle is spent in interphase. This is the time when a cell performs its usual functions, depending on its location in the body. The amount of time the cell takes for interphase varies widely. Some cells, such as nerve and muscle cells, typically remain in interphase and cell division is permanently arrested. These cells are said to have entered a G₀ stage. Embryonic cells complete the entire cell cycle in just a few hours. In contrast, interphase alone in a rapidly dividing mammalian cell, such as an adult stem cell, may last for about 20 hours, which is 90% of the cell cycle.

                DNA replication occurs in the middle of interphase and serves as a way to divide interphase into three stages: G₁, S, and G₂. G₁ is the stage before DNA replication, and G₂ is the stage following DNA synthesis. Originally, G stood for “gap,” but now we know how metabolically active the cell is, it is better to think of G as standing for “growth.” Protein synthesis is very much a part of these growth stages.

                During G₁, a cell doubles its organelles (such as mitochondria and ribosomes) and accumulates materials that will be used for DNA replication. Following G₁, the cell enters the S stage. The S stand for synthesis, and certainly DNA synthesis is required for DNA replication. At the beginning f the S stage, each chromosomes has one DNA double helix. At the end of this stage, each chromosomes is composed of two sister chromatids, each one have a double helix. Another way of expressing these events is to say that DNA replication result in duplicated chromosomes.

                Following the S stage, G₂ is the stage that extends from the completion of DNA replication to the onset of mitosis. During this stage, the cell synthesize proteins that will be needed for cell division, such as the protein found in microtubules. The role of microtubules in cell division is described in a later section.

M (Mitotic) Stage

Cell division occurs during the M stage, which encompasses both division of the nucleus and division of cytoplasm. The type of nuclear division associated with the cell cycle is called mitosis, which accounts for why this stage is called the M stage. As a result of mitosis, the daughter nuclei are identical to the parent cell and to each other-they all have the same number and kinds of chromosomes. Division of the cytoplasm, which starts even before mitosis is finished, is called cytokinesis.

Mitosis and Cytokinesis

Each sister chromatid of a duplicated chromosome carries the same genetic information because its DNA double helix has the same sequence of base pairs as did  the original chromosome. Thus, it is proper, once the chromatids have separated, to call them daughter chromosomes. Because each original chromosome goes through the same process of DNA replication followed by separation of the chromatids to form daughter chromosomes, the daughter nuclei produced by mitosis are genitically identical to each other and to the parent nucleus. In the simplest of terms, if the parent nucleus has 4 chromosomes, each daughter nucleus also has 4 chromosomes of exactly the same type. One way to keep track of the number of chromosomes in drawings is to count the number of centromeres. Because every chromosome has a centromere.

                Every animal has an even number of chromosomes because each parent contributed half of the chromosomes to the new individual. In drawing of mitosis, some chromosomes are colored red and some are colored blue to represent that half of the chromosomes are derived from those contributed by one parent and the other half are derived from chromosomes from the other parent.

The Spindle

While it may seem easy to separate the chromatids of only for 4 duplicated chromosomes, imagine the task when there are 46 chromosomes, as in humans, or 78, as in dogs. Certainly it is helpful that chromosomes be highly condensed before the task begins, but clearly some mechanism is needed to complete separation in an organized manner. Most eukaryotic cells rely on a spindle, a cytoskeletal structure, to pull the chromatids apart. A spindle has spindle fiber made of microtubules that are able to assemble and disassemble. First, the microtubules assemble to form the spindle that takes over the center of the cell and separates the chromatids. Later, they disassemble.

                A centrosome is the primary microtubule organizing center of a cell. Centrosome duplication occurs at the start of the S phase of the cell cycle and is completed by G₂.during the first part of the M stage, the centrosome separate and move to opposite side of the nucleus, where they form the poles of the spindle. As the nuclear envelope breaks down, spindle fibers take over the center of the cell. Certain ones overlap at the spindle equator, which is midway between the poles. Others attach to duplicated chromosomes in a way that ensures the separation of the sister chromatids and their proper distribution to the daughter cells. Whereas the chromosomes will be inside the newly formed daughter nuclei, a centrosome will be just outside.

                Traditionally, mitosis is divided into sequence of event, even though it is a continuous process. We will describe mitosis as having four phase: prophase, metaphase, anaphase, and telophase. These phases for a dividing plant nucleus. Plant cell have centrosomes but they are not clearly visible especially because they lack centriols. In animal centriols.