Abstract
Cells divide for three main reasons: growth and development, replace worn-out or injured cells, and reproduction of offspring. Cell division is part of the cell cycle divided into five distinct phases. The diploid state of the cell is the normal chromosomal number in species. During sexual reproduction, the cell's chromosome number is reduced to a haploid state to ensure constancy in chromosome number and thus continuation of the species. The process of cell division is controlled by regulatory proteins. Mitosis occurs in all body cells and is divided into four phases. Meiosis, which occurs in only the germ cells involved in reproduction, divides the chromosomes in two rounds termed meiosis I and meiosis II (reduction division). The human lifecycle starts with gametogenesis, the process that forms gametes which then combine to form a zygote. The zygote quickly becomes an embryo and develops rapidly into a foetus. This chapter explores cell division.
TopChapter Outline
4.1 Why Cells Divide
4.2 The Cell Cycle
4.3 Regulation of Cell Division
4.4 Mitosis
4.5 Meiosis and Sexual Reproduction
4.6 The Human Life Cycle
4.7 Mitosis and Meiosis Compared
Chapter Summary
TopLearning Outcomes
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Explain the reasons why cells divide and what is accomplished.
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Understand that actual cell division is part of a bigger picture – the cell cycle.
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Describe the mechanics of the cell cycle.
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Comprehend that cell division is highly regulated by cellular molecular signals.
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Understand that some cells remain in a dividing state forever while others remain arrested forever.
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Explain the two different types of cell division (mitosis and meiosis) and various phases of each.
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Understand the role of mitosis in living organisms.
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Recognize that meiosis involves reduction division, genetic reshuffling, and sexual reproduction.
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Understand the human life cycle and how mitosis and meiosis fit in.
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Be able to compare and contrast the two types of cell division.
Top4.1 Why Cells Divide
Living organisms are composed of basic units of organization called cells. Some organisms remain as single cells (for example, bacteria and some protozoans like the malaria causing Plasmodium falciparum); while others differentiate into complex multicellular organisms (for example, worms, spiders, dogs, plants, humans) that are made up of billions of cells that function together as an organism. Why do cells need to divide? There are three main reasons why cells divide: (1) for growth and development; (2) to replace old, worn-out or injured cells (both of these occur in a type of cell division known as mitosis), and (3) for sexual reproduction (in a type of cell division known as meiosis).
1. Growth and Development
Many sexually reproducing animals start life as fertilized eggs and must grow, develop, and mature into adulthood. This growth and development means that cells must be able to divide and increase in number with subsequent specialization into functional cells, tissues, systems, and eventually individual organisms. Growth and development of the fertilized egg continues in stages until the young mature enough to be born. To illustrate, the human fertilized egg or zygote divides repeatedly within the first 24 hours to form a ‘ball of cells’ usually referred to as a morula which transforms into a blastula (or blastocyst) by end of first week (Figure 1).
Figure 1. Animal zygote development. Note the various stages fertilized egg, 2-cell stage, 4-cell stage, and blastula (blastocyst).
In humans, this ‘ball’ continues to divide and develop in the next few weeks to a well differentiated and recognizable human foetus by four to five months (Figure 2).
Figure 2. Human foetus development. Note the various stages of development from fertilized egg to nine months.
Continued growth and development in the next several months fully develops the foetus until it is ready for birth as a neonate (Figure 3). After birth, the neonate, now an infant, continues to grow and develop until maturity. In adulthood, cell growth continues (albeit relatively more slowly) until death occurs. This continued growth and development is a defining aspect of life as new blood cells are formed, bones are dissolved and rebuilt, hair grows, the organism adds or loses weight, and many other metabolic functions that characterize living organisms occur.
Figure 3. Source: Image used under license from Shutterstock.com Key Terms in this Chapter
Cell Theory: All living things are made up of one or more cells.
Growth Factors: Molecular signals that prioritize and promote rapid cell division.
Crossing Over: The exchanging of genes between two non-sister chromatids that occurs during prophase I.
Gametogenesis: The process of the gamete production.
Deoxyribonucleic Acid (DNA): A type of nucleic acid that contains an organism’s hereditary information; composed of adenine, guanine, thymine, and cytosine bases.
Synapsis: The process of homologous chromosome pairing during the initial phases of meiosis.
Germ Cells: Reproductive cells (ovaries in females and testes in males).
Meiosis: A type of cell division that results in germ cells for sexual reproduction.
Centromere: A structure that links two chromatids together to form a chromosome.
Chromatids: Half of an identical copy of a replicated chromosome.
Mitosis: A type of cell division that results in two identical copies of the original cell.
Reduction Division: The first cell division in which the number of chromosomes is reduced by half.
Evolution: The idea of change in the genetic composition over time.
Mitotic Promoting Factor (MPF): A checkpoint that regulates the transition of cells from G2 to M phase.
Chromosomes: Units that contain chromatin specific for each species (e.g., humans have 46 chromosomes).