Meiosis can be defined as the process in which a cell is divided into germline cells to form gametes. Meiosis is an important step in reproduction because each gamete that is formed has half the number of chromosomes that were in the original normal cells in the two gametes from each parent cell combined and thus resulting in the same number of chromosomes in the resulting zygote organism. The sketches below illustrate the phases of meiosis (Crick 368).
Thus, some features that take place in each of the phases include (Mayr & Provine 324):
- Interphase (parent cell) – it is the phase where the cell replicates its chromosomes and each chromosome usually has two daughter chromatids that are held by the help of centromere
- Prophase 1 – the chromosomes in the cell coil up resulting in the formation of the spindle, and the homologous chromosomes come together through matching gene-by-gene resulting in the formation of a tetrad. At this stage crossing over may occur resulting in the exchange of genetic material and usually, such process occurs two or three times per homologous chromosome pair. This crossing over results in combinations of alleles on a chromosome.
- Metaphase 1 – spindle fiber is attached by each chromosome centromere, and then the tetrads are pulled by the spindle fibers to the equator of the spindle. The homologous chromosomes are then lined up side by side as tetrads.
- Anaphase 1 – the homologous chromosomes then separate and move towards the end of the cell but the centromeres do not split.
- Telophase 1 – At this phase, the spindle breaks and the chromosomes uncoil while the cytoplasm divides to yield two new cells in which each cell comes with genetic material from each parent cell.
- Prophase II – the spindle forms in the new cells and the fibers are then attached to the chromosomes.
- Metaphase II – the chromosomes come to the center of the cell and they are randomly lined up at the equator.
- Anaphase II – in each of the chromosomes, the centromere splits, and the sister chromatids separate and move to opposite poles of the cell.
- Telophase II – the nuclei forms again, the spindles then break down, and the cytoplasm divide.
At the end of the meiosis process, four daughter cells from the original cell are formed in which each pair of cells contains one chromosome from each parent and these chromosomes through gametes transmit genes to the offspring (Strachan & Read 435). Thus, combining different cells from the parents comes with different characteristics that determine the genetic makeup of the offspring, and thus meiosis process is commonly associated with variation in genetic makeup (Fox & Wolf 60).
Structure of Ribosomes and Their Role in Organelles in Translation
The genetic information of most living organisms is stored in the genome sequences of their deoxyribonucleic acid. A major extent of this genome sequence carries the most functional tasks in all extant organisms. The information contained in the DNA is only made possible by the transcription of genes to the ribonucleic acids that are later translated into various amino acids (Crick 378).
The ribosome is an important component that helps in making proteins. Generally, the ribosome is protein builders or can be referred to as protein synthesizers of the cell. Ribosome connects amino acids forming a chain (Meyer 98). Ribosomes can be found in many places within a cell. Ribosomes can be found floating in the cytoplasm while others can be found on the endoplasmic reticulum; these ribosomes are commonly referred to as rough. Those ribosomes that are found in the cytoplasm make proteins. Ribosome brings together two subunits or pieces (Fox & Wolf 56). One piece is called large while the other is small. An important fact is that DNA makes RNA, which then makes protein. The sequence of the genes is copied to the messenger RNA (mRNA), and the information is received by the ribosome and used to make proteins. This process of protein formation can be understood through an analysis of translation (Purves & Sadava 256).
The translation is the first phase of the biosynthesis of protein. The translation process is the process of producing proteins through the decoding of the mRNA that was produced during transcription. It takes place in the cytoplasm at the ribosome. Small and large subunits make up the ribosome in which it surrounds mRNA. According to the rules possessed by genetic code, the mRNA is decoded resulting in a specific polypeptide (Greenwood, Bartusiak & Burke 85). It utilizes a sequence of mRNA as a template in which it helps in synthesizing the chain of the amino acids forming the protein. However, not all types of RNA are translated into sequences of amino acids. The translation process usually follows four stages, which are the activation, initiation, elongation, and termination stages. In the activation stage, the correct sequence of amino acids is bonded with the appropriate tRNA (Solms, Turnbull & Sacks 34). The initiation stage enables the small subunit of the given ribosome to join the 5’-end and it is supported by the initiation factors (IF) (Mayr & Provine 67). In the termination phase, no tRNA is recognized since the A sites of the ribosome face a stop codon. Hence, the end side of 5’ gives rise to N-terminus proteins, and the direction of the given translation will result in N → C (Olby 67).
- Crick, F. ‘The Origin of the Genetic Code’. Journal of Molecular Biology, vol. 38 (1968). P. 367 – 379.
- Fox, C. & Wolf, J. Evolutionary Genetics. New York: Oxford University Press, 2006.
- Greenwood, A., Bartusiak, M. & Burke, B. Science at the Frontier. New York: National Academies Press, 1992.
- Mayr, E. & Provine, W. The Evolutionary Synthesis. Chicago: Harvard University Press, 1998.
- Meyer, A. Hunting the Double Helix. London: Allen & Unwin Publishers, 2005.
- Olby, R. The Path to the Double Helix. New York: Courier Dover Publications, 1994.
- Purves, W. & Sadava, D. Life, the Science of Biology, 7th Ed. London: Sinauer Associates.
- Strachan, T. & Read, A. Human Molecular Genetics 3. London: Graland Sciences, 2004.
- Solms, M., Turnbull, O & Sacks, O. The Brain and the Inner World. New York: Other Press, 2002.
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