BIOLOGY Volume 3 - A Guide to General Biology - 2004

23. THE CONTINUITY OF LIFE

As noted at the beginning of Chapter 5, one of the most fundamental principles of biology is the Cell Theory, which states that The Cell is the basic Structural and functional unit of All living organisms.

The cell theory was first formulated by Schleiden in 1838 and Schwann in 1839. Rudolf Virchow expanded it in 1855 by proclaiming that new Cells arise only from pre-existing cells through Cell Division. The recognition of the continuity of life prompted other scientists in the latter half of the 19th century to investigate Cell Structure and the MECHANISMS OF CELL division. The refinement of histological techniques and The Development of microscopes with higher resolving power revealed the vital role of The Nucleus—and particularly the Chromosomes contained within it—as the structures ensuring continuity between successive generations of cells. In 1879, Boveri and Flemming described the events occurring in the nucleus that result in The formation of two identical cells, and in 1887, Weismann suggested that Gametes are formed through a specialized type of division. These Two Types of division are known as Mitosis and Meiosis, respectively. Before studying them in detail, it is helpful to take a closer look at chromosomes.

23.1. Chromosomes

Class="center">23.1.1. Chromosomes and Karyotype

Chromosomes play the most crucial role in cell division, ensuring the transmission of Genetic information from one generation to the next. Such transmission is made possible by the DNA molecules contained within the chromosomes. In the period between nuclear divisions, each chromosome contains a single DNA molecule. Prior to division, an exact copy of this DNA molecule is synthesized next to it, transforming the chromosome into a paired structure consisting of two identical DNA molecules. These two PARTS OF THE chromosome are called chromatids. Each chromatid contains one of the two identical DNA molecules.

During the period between divisions, known as interphase, chromosomes are virtually indistinguishable as discrete structures under a Light Microscope, although the material composing them is stained by certain basic Dyes and is therefore termed Chromatin. At this stage, chromosomes appear as a tangled mass of long, thin threads. Immediately before nuclear division, they coil into much more compact and intensely staining structures that are shorter, thicker, and more distinct from one another. Fig. 23.1 shows a photograph of Human chromosomes in a cell at metaphase. It can be seen that each chromosome consists of two chromatids. The chromatids are joined together at a point called the centromere, which may be located anywhere along the length of the chromosome (Fig. 23.2).

Fig. 23.1. Photomicrograph of a human chromosome Complement (male). Each chromosome consists of two chromatids joined at a point called the centromere. The complement consists of 46 chromosomes. Note their varying shapes and different centromere positions.

Each cell of an Organism contains a specific number of chromosomes characteristic of that species. In humans, as seen in Fig. 23.1, this number is 46. The chromosome number varies widely among different species. For example, the fruit fly has only 8 chromosomes, whereas the small Spanish butterfly Lysandra has up to 380. Cats have 38 chromosomes and dogs have 78. In most species, each cell contains between 12 and 50 chromosomes. The units of heredity—genes—are arranged linearly along the chromosomes (Fig. 23.2). In humans, the number of distinct genes reaches nearly 100,000 (though recent data suggest the actual number is significantly lower).

Fig. 23.2. Simplified diagram of Chromosome structure. In reality, the number of genes in a chromosome ranges from several hundred to several thousand. Gene sizes also vary.

If chromosomes are cut out from a photograph like the one shown in Fig. 23.1 and arranged according to their size and shape, it becomes apparent that the cell actually contains pairs of chromosomes; these are called homologous chromosomes because they share the same structure. In Fig. 23.3, the chromosomes are arranged in homologous pairs. Such a photograph is called a karyogram, and the complete set of chromosomes is termed the karyotype. Fig. 23.3 depicts 23 pairs of chromosomes. The presence of paired chromosomes is due to the fact that an individual receives one set of chromosomes from the mother via the egg cell and the other from the father via the sperm cell. Upon the fusion of the sperm and egg during Fertilization, a zygote is formed containing two sets of chromosomes.

Among the chromosomes shown in Fig. 23.3, one pair can be seen consisting of unequal chromosomes designated by the letters X and Y. These are the sex chromosomes. The male, or Y chromosome, is shorter than the female, or X chromosome, and lacks certain genes present on the female chromosome (Section 24.6). Normally, homologous chromosomes carry genes that determine the same traits. The individual whose karyotype is shown in Fig. 23.3 is a male (XY). A female karyotype would feature two X chromosomes (XX). Chromosomal Mutations can sometimes be observed in karyograms (Chapter 24).

Fig. 23.3. Human karyogram (male) compiled from the photograph in Fig. 23.1. Autosomes (non-sex chromosomes) are grouped (A to G) according to size. The sex chromosomes are placed in a separate group: X is female, Y is male. Human cells contain 22 pairs of autosomes and one pair of sex chromosomes. Genes located on autosomes are called autosomal genes, and those located on sex chromosomes are called sex-linked.

23.1.2. Haploid and Diploid Cells

Species whose cells contain two sets of chromosomes are called diploid and are designated by the symbol 2n. The vast majority of animals and approximately half of plants are diploid, meaning they possess two sets of chromosomes. The cells of some primitive organisms contain only a single set of chromosomes and are termed haploid (symbol n; see, for example, the section on Morphology/12.html">ALTERNATION OF GENERATIONS in Chapter 2). Furthermore, all gametes are haploid. Some organisms, including many plants, contain three or more sets of chromosomes; such organisms are called polyploid, but they will not be discussed in this chapter.

Having two sets of chromosomes offers two main advantages:

1) genetic variation increases, since each individual inherits traits from both parents;

2) if a gene is defective in one of the two chromosomes, the corresponding gene on the homologous chromosome can compensate for this defect.

23.1.3. Why Are There Two Types of Nuclear Division?

By the end of the 19th century, it was discovered that nuclear division occurs via two different mechanisms. This is absolutely essential whenever an organism's life cycle includes a stage of sexual reproduction (Fig. 23.4).

Fig. 23.4. Diagram of an animal life cycle.

The development of all Multicellular Organisms begins with a single cell. Each cell division is preceded by nuclear division. If the chromosome number in the nucleus were halved with each division, the number of chromosomes in every cell would very soon be reduced to almost none. Meanwhile, daughter cells contain the same number of chromosomes as the parent cells, so that a constant chromosome number is maintained across all cells of a given organism. This is achieved through a type of division known as mitosis.

However, as shown in Fig. 23.4, in a life cycle involving sexual reproduction, a zygote is formed by the fusion of two cells: a male and a female gamete. If these cells each contained two sets of chromosomes, the zygote and all subsequent cells would contain four sets, and this number would double with every generation. Therefore, at some stage in The life cycle, the nucleus must undergo a different type of division to reduce the chromosome number. Subsequently, upon zygote formation, the diploid state is restored. Nuclear division that reduces the number of chromosome sets in daughter cells from two to one is called meiosis or reduction division.

23.1.4. Summary

Mitosis is a type of nuclear division that produces two daughter nuclei containing chromosome sets identical to those of the parent cell. Usually, nuclear division is immediately followed by the division of the whole cell, forming two daughter cells. Mitosis, followed by cell division, leads to an increase in cell number, thereby driving growth, tissue repair, and cell replacement in eukaryotes. In Unicellular Eukaryotes, mitosis serves as a mechanism of asexual reproduction, leading to an increase in population size.

Meiosis is a process of nuclear division resulting in daughter nuclei, each containing half the number of chromosomes of the parent nucleus. Meiosis is also referred to as reduction division because it reduces the chromosome number in a cell from diploid (2n) to haploid (n). The Biological Significance of meiosis is that in sexually reproducing species, it ensures a constant chromosome number across generations. Meiosis occurs during the formation of gametes in animals and spores in plants. The subsequent fusion of haploid gametes during fertilization restores the diploid chromosome number.