Cell: The Cell Reproduction
The Cell Cycle
Cells go through cell cycle as they grow and divide. Before the cell divides, it spends most of its life in a period of growth and development called interphase. Some cells, such as neurons and muscle cells are always in the interphase stage. These are the cells that no longer divide. Interphase can be divided into three substages: Gap 1 (G¹), Synthesis (S), and Gap 2 (G²). The entire cell division process follows interphase and is called mitosis stage (M).
Interphase: Period of Cell Growth
1. Gap 1 – During this stage, the cell produces proteins, carbohydrates, and lipids. As a result, cellular components, such as cell organelles increase in number and the cell grows in size.
2. Synthesis – This stage is characterized by DNA replication. DNA molecules increase in number prior to cell division.
3. Gap 2 – Similar to G1, G2 is a period of synthesis and preparation. Mitochondria and other cell organelles further divide and increase in number. More proteins and carbohydrates are produced. The cell now gets ready for cell division.
Chromosomes and Cell Division
Each species has a characteristic number of chromosomes. Chromosomes are condensed threadlike bodies consisting of DNA and special proteins. The complete set of chromosomes is called diploid number, designated by the symbol 2n. For instance, the diploid number in humans is 46, 8 in Drosophila melanogaster (fruitfly), 4 in Ascaris (giant roundworm) and 20 in corn. Some cells contain only half of the diploid number of chromosomes. These cells have a haploid chromosomes number (n). since the diploid number in humans is 46 and the haploid number is 23, we can represent these chromosome numbers as 2n=46 and n=23.
Body cell divides by means of mitosis. Mitosis is a type of cell division that maintains a constant chromosome number. In mitosis, a diploid (2n) parent cell gives rise to two daughter cells, each with a diploid (2n) chromosome number. The daughter cells are identical to each other and to the parent cell in terms of chromosome number. The skin cells have 46 chromosomes. Similarly, all new skin cells produced through mitosis have 46 chromosomes. Meiosis is a special type of cell division, which leads in the formation of haploid (n) gametes from a diploid (2n) parent cell. At the onset of puberty, a person starts producing gametes (egg cell for females and sperm cell for males) from meiotic divisions of diploid cells within the ovaries and testes.
Mitosis
Mitosis involves division of the nucleus (karyokinesis), followed by division of the cytoplasm (cytokinesis). Mitosis is a continuous process. With the help of sophisticated techniques in cell biology, the sequence of events in mitosis can now be observed.
Karyokinesis: Nuclear Division
1. Prophase. Prior to cell division, the DNA replicates and is packed with special proteins called histones. When a cell is not dividing, the DNA (associated with histones) exists as thin threads called chromatin. During prophase, the chromatin becomes highly condensed and appears as colored bodies known as chromosomes. Each chromosome is composed of two strands called chromatids, joined together at the center by a centromere. As the chromosomes become more condensed, the nucleolus and the nuclear membrane disintegrate. In animal cells, a pair of centrioles moves to opposite poles of the cell and begins to form threadlike spindle fibers. Short microtubules radiate from each centriole, forming aster.
2. Metaphase. The double-stranded chromosomes become attached to each spindle fiber by their centromere. The spindle fibers pull the chromosomes toward the center of the cell (equator) where they line up. By the end of metaphase, the centromeres divide.
3. Anaphase. The centromeres break apart and the chromatids are pulled apart by the spindle fibers. The chromatids move away from each other and migrate toward the opposite poles.
4. Telophase. During this last stage of cell division, the centrioles and spindle fibers start to disappear, chromosomes unwind and return to threads of chromatin and a new nuclear membrane forms around each mass of nuclear material.
Cytokinesis: Nuclear Division
Following karyokinesis, the cytoplasm divides and two cells are formed. In animal cells, cytokinesis is accomplished by the pinching in of the plasma membrane and the cytoplasm. This process of pinching in is so called formation of cytoplasmic cleavage. Plant cells cannot perform cytoplasmic cleavage because of their rigid cell walls. Instead, a cell plate forms at the middle, between the two daughter nuclei. The cell plate marks the boundary between the two daughter cells. New plasma membrane and cell wall later form from the cell plate.
Types of Reproduction
Reproduction is the process by which an organism produces its own kind. There are two types of reproduction among living things, (1) asexual reproduction and (2) sexual reproduction.
1. Asexual Reproduction – new organisms are produced from one parent. Offspring produced by asexual reproduction have identical genetic material (DNA) with that of the parent. Mitosis is an asexual mode of reproduction. All new leaves, stem and roots that grow out from a newly planted horse radish stem have been produced by mitosis. New potato plants can be grown from potato tubers and new strawberry plants, from runners. Bacterial cells split into two cells in asexual process called binary fission. Yeasts, hydra, and other coelenterates reproduce by budding. Budding is a type of asexual reproduction by which new organisms grow from the body of the parent plant and later break away from it.
2. Sexual Reproduction – involves the fusion of gametes or sex cells from two parents. The gamete from the male parent is called the sperm and the gamete from the female parent is called the egg. The offspring contains different genetic material from that of either parent.
Mitosis
Prophase: chromosomes and centrioles )already in their duplicated form) become distinct. Nuclear membrane and nucleolus disintegrate, and centrioles move toward opposite poles.
Metaphase: chromosomes align around the center of the cell.
Anaphase: sister chromatids separate and move toward opposite poles.
Telophase: nucleolus reappears and nuclear membrane forms around each mass of chromosomes. Division of cytoplasm (cytokinesis) takes place.
Cytokinesis: two diploid daughter cells are formed.
Meiosis … 
Category: Biology
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