Chromosomes, Cell division, and DNA
Chromosomes are thread-like structures found in the nucleus of all living cells. Through various researches, it has been identified that chromosomes carry genetic information and are responsible for cell division (Madigan et al., 2008). Chromosomes are composed of strands of Deoxyribonucleic Acid (DNA) wrapped around histones which make it possible to fit in the cell nucleus. Different species of animals contain different numbers of chromosomes in their cell nucleus. On the chromosome, there are a series of genes that control different traits in that organism (Goodenough et al., 2007). A chromosome is made of two vertical strands known as chromatids attached at the centre by a structure known as the centromere. Two sister chromatids form a chromosome which is attached by a centromere.
Additionally, chromosomes occur in pairs known as homologous chromosomes. Therefore centromere functions to hold the sister chromatids together, during cell division it splits; hence the chromosomes are duplicated. During different stages of cell divisions, chromosomes appear differently when viewed under a powerful microscope.
Figure 1: Chromosome structure
Chromosomes have a complex structure with DNA making the base of their structures. Chromosomes occur in different types, such as metacentric, submetacentric, acrocentric, and telocentric. Metacentric chromosomes are a form of chromosomes that have centromere located exactly in the centre, making the two sections of these chromosomes to be equal. In submetacentric chromosomes, the centromere is slightly off the centre, making the sections to be unequal in length. It has been noted that acrocentric chromosomes have two sections which are unequal since the centromere is not located at the centre making the chromosome to have one long section and another short one (Goodenough et al., 2007). The last form of chromosomes is telocentric chromosomes in which the centromere is located at the very end of the chromosome. Human beings lack telocentric chromosomes but are found in species such as mice.
Chromosomes have many functions in living organisms. In 1902, Bover and Sutton pointed out the primary purpose of the chromosome is heredity for the first time (Goodenough et al., 2007). Since then, it has been established that chromosomes have different roles in living organisms. The basic function is to carry genetic codes in the form of DNA molecules. DNA provides genetic code for various cellular activities such as growth, reproduction and survival of the organisms. Chromosomes have histones and proteins which protect DNA from enzymes and physical destruction during cell division. Chromosomes play a crucial role in cell division as they divide giving rise to daughter cells. The spindle fibre contracts the centromere splitting the chromosomes to ensure proper DNA distribution to daughter cells
Cell division comprises of all events that take place for the parent cell to divide, giving rise to two or more daughter cells, forming a cell cycle. Chromosomes play a vital role in cell division as they are responsible and take part in the division. Cell division in human beings is of two types: mitosis and meiosis. During mitosis, the parent cell divides to give rise to two daughter cells, each with the same number of chromosomes as the parent cell. In mitosis, the cell cycle comprises of interphase and cell division. Mitosis is responsible or new cells in human beings, growth of organisms and replacement of dead cells in all living organisms (Goodenough et al., 2007). During this type of division, chromosomes divide followed by separation of sister chromatids. Spindle fibres are essential for chromosome division. During interphase, the cell prepares itself to divide by synthesizing DNA and synthesizing energy that will be used during the process of cell division. Cell organelles such as mitochondria and ribosomes are doubled. Additionally, the cell makes proteins that are essential for cell division such as microtubules.
Figure 2: The cell cycle
Mitotic phases are divided into four that is, prophase, metaphase, anaphase and telophase. Although the phases are shown as separate, they are continuous from one phase to another. The first phase is prophase in which chromosomes become visible under a powerful microscope. Spindle fibres start to form between chromosomes. The nuclear membrane begins to disintegrate and chromosomes coil and condense. The chromosomes at this point are visible; the spindle fibres attach the chromosomes at their centromeres (Goodenough et al., 2007). The second stage in mitosis is metaphase in which the chromosomes occupy the equator of the cell, and the spindles are fully formed. During anaphase of mitotic division, centromeres holding chromosomes divide thus separating sister chromatids. The sister chromatids now become the new chromosomes and move towards opposite poles. This ensures that each cell gets a copy of each chromosome type. In anaphase movement of chromosomes is aided by the function of spindle fibres which shorten and lengthen, resulting in chromosomal movement. In telophase stage, the chromosomes finally reach the poles. The spindles start to disintegrate, and daughter nuclei form. Cytokinesis then follows with the division of organelles and the cytoplasm. This division results into two daughter cells, each with the same number of chromosomes as the parent cell.
The second type of cell division is meiosis. Meiosis occurs in reproductive cells and is a reduction division which results in four daughter cells, each with a half the number of chromosomes as the parent cell. Meiosis occurs in two phases: meiosis one and meiosis 2.
Figure 3: Meiotic division
During the first meiotic division, homologous chromosomes come together and lining them side by side result in the formation of synapsis. This is important as it leads to a decline in the number of chromosomes. Chromosomes in meiosis one are aligned at the equator, pair and separate into daughter cells. Meiosis 1 has various stages that are prophase 1, metaphase 1, and anaphase 1. In prophase one synapsis take place and the nuclear membrane disintegrates. Homologous chromosomes come together, resulting in crossing-over which leads to the exchange of genetic materials. In metaphase 1, homologous chromosomes are arranged independently along the equator of the cell (Goodenough et al., 2007). This arrangement ensures that there will be no same combination of genes and chromosomes. In anaphase 1, homologous chromosomes break and migrate towards the poles. This results in two daughter cells with the same number of chromosomes as the parent cell. After meiosis 1, the second phase of cell division follows which the second meiotic division is resulting in four daughter cells. Each daughter cell so formed has a half the number of chromosomes as their parent cells.
Figure 4: First and second meiotic division
Meiosis is essential in maintaining a constant number of chromosomes from one generation to another.
Chromosomes carry genetic information in the form of deoxyribonucleic acid (DNA). On portions of DNA molecules, there are series of gene codes that regulate certain traits in living organisms. The DNA double-helix model was first proposed by Watson and Crick (Goodenough et al., 2007). According to the model, a DNA molecule is made up of polymer of nucleotides. A DNA strand is made up of three components: a phosphate, nitrogenous base and a pentose sugar. In eukaryotes, DNA is double-stranded. It is composed of two DNA strands that twist on one another. The sugar molecule and the phosphate form the backbone of the DNA structure with the nitrogenous base arising from the sides of the backbone. The two strands are held together by the pairing of the nitrogenous bases which have a hydrogen bond. Generally, there are four nitrogenous bases, namely: Adenine, cytosine, thymine and guanine. During the pairing of the bases, Guanine pairs with Cytosine while Thymine pairs with adenine forming hydrogen bonds (Madigan et al., 2008). Paired bases are essential for the effective functioning of a DNA molecule. It is important to note that guanine and adenine comprise the purine structure, which has two rings while cytosine and thymine form pyrimidine structure with one ring.
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Figure 5: DNA double helix, DNA ladder structure and one pair of base
DNA molecule in eukaryotes is wrapped on protein molecule forming nucleosomes. The coils are further coiled during meiosis and mitosis to fix on chromosomes, thus facilitating their movements. DNA plays various functions in all living organisms. The primary role of DNA is inheritance and control of characteristics of all living organisms (Goodenough et al., 2007). DNA molecules are inherited from parent to offspring through gene replication. In gene replication, a new exact copy is synthesized from the original copy. It is has been found clearly that DNA is a self-replicating material; thus, it can form a new strand from the original one. During replication, three main steps take place that is initiation, elongation and termination. Before the start of replication, the DNA double helix must unzip with the help of enzyme helicase, which breaks the hydrogen bonds holding the complementary DNA strands together. This is followed by DNA elongation and termination, resulting in a new DNA strand with exact genetic information as the parent strand.
DNA is responsible for the storage of genetic material in coded form. Additionally, DNA is responsible for the formation of amino acids through translation. In the process of translation, the DNA sequence determines the sequence of amino acids in a protein molecule. RNA is responsible for the synthesis of proteins. The parent cell first transcribes genes onto RNA segments using base-pairing logic (Madigan et al., 2008). Finally, DNA is responsible for the transcription of RNA. Through the process, DNA is converted to RNA in the same version as DNA replication. Although the process is similar to replication, base Uracil replaces Thymine.
Citations and References
Goodenough, J., McGuire, B., & Wallace, R. A. (2007). Biology of humans. Prentice-Hall bei Pearson.
Madigan, M. T., Martinko, J. M., Dunlap, P. V., & Clark, D. P. (2008). Brock biology of microorganisms 12th edn. Int. Microbiol, 11, 65-73.