Aging refers to the accumulation of various deleterious changes that occur in tissues and cells as one's age advances, and this is responsible for the growing risks of diseases as well as death. It is a universal phenomenon occurring among every known living organism. Species get subjected to the biological aging process after they reach sexual maturity; the organism gradually starts to lose its biological and cellular functions (Carlson, & Riley, 1998). The organism's body organs become impaired, fertility decreases, cancerous cells grow, and the species inevitably starts becoming very prone to all the aging-related diseases. The decreasing biological attributes of aging reach a specific climate end as the species experiences its ultimate consequences and effects of aging, and that is death (Dong, Milholland, & Vijg, 2016). Aging is fatal and costly, especially to humans, since elderly individuals go through various treatments that start at the onset of aging. However, aging can be controlled by the biological processes and genetic factors that are inherent to human beings (Harman, 1998). The causes of aging, as well as the aging process, can be explained using various biological and biochemical mechanisms and some theories like the damage based and programmed aging theories, as discussed below.
Random Damage Accumulation (Stochastic Aging)
The stochastic theory of aging proposes that aging is; as a result, some small inevitable random changes which accumulate in the organisms' body with time (Sharpless & DePinho, 2007). This theory reflects that the aging effects are majorly due to the environment, inborn aging process, development, and genetic defects. The evidence for the stochastic aging are:
Mitochondria refer to the organelles found in the cells. They are responsible for maintaining cellular respirations as well as ATP creation, which is the energy source necessary for all mechanisms of a person's body( Guarente & Kenyon, 2000). Mitochondria are the power source for every living organism since their functions are very important, and the dysfunctioning of the mitochondria causes the death of the cells. They are the only organelles that possess their DNA, known as the mtDNA (Finkel & Holbrook, 2000). Dysfunctioning of the mitochondria is a good indicator of aging. It happens even when there is no significant change in the mtDNA, and this means that the age-related mitochondria dysfunctions are not necessarily triggered by the mutation of the mtDNA but by other factors. One of the potential causes of the malfunction of mitochondria in oxidative stress (Serrano & Blasco, 2007).Oxidative stress results due to oxygen-related electron loss.
The molecules which are responsible for the oxidation process are the free radicals, which are as a result of the oxygen in the air humans breathe. The free radicals or the oxidative species induce some wear, which causes aging (Finkel & Holbrook, 2000). Increased antioxidants have zero effects on an individual's life expectancy. However, the increased level of free radicals causes a longer life expectancy. Additionally, another cause of mitochondria dysfunction is the cell mitochondria communication vital for the replication of the mitochondria, and this makes them very important to the cells. Their functions degrade as an individual age with the needed protein regulation also degrading, and this causes problems with communication between the mitochondria and the cells (Russell & Kahn, 2007). Possible causes of deterioration of the mitochondria lead to the aging of the tissues since the mitochondria carry out significant roles in the body cells like ATP production and cellular respiration.
Stem Cell Exhaustion
The stem cell is the undifferentiated cell belonging to no particular organ and usually generates some specialized cells during cellular differentiation. This mechanism enables the stem cell to have some specific characteristics to specific types of cells. Adults' stem cells do not end up with just any kind of cells ( Oberdoerffer & Sinclair, 2007). For instance, the T and B lymphocytes and the red blood cells come from a single undifferentiated cell referred to as the hematopoietic stem cell. Stem cells also enable the renewing of organ cells, and they are usually stored in an individual's body to be used when the need arises. Some of these cells generally age and end up dying regularly hence the need to replace them.
As aging occurs, tissues are not able to regenerate well since there is a slow division of cells, and also there is no replacement of the stem cells. This can best be explained by the overexpression of proteins, which blocks the cell cycle as well as the DNA damage accumulation on these stem cells (Monaghan, 2010). This leads to an increased number of senescent cells found in the tissues and can cause various problems that depend on the affected organ. In cases of the hematopoietic stem cells, when the stem cells get exhausted, an immunodeficiency can occur. The exact same thing happens with an individual's brain, muscles, and bones (Tosato, Zamboni, Ferrini, & Cesari, 2007). The exhaustion of these stem cells is the primary cause of aging because it inhibits the renewal of the cells, and this causes the aging of the body organs.
Unfolded Protein Response
Proteins play a vital role in most of the cellular mechanisms, and their shape enables them to perform their duties and also get identified. The folding of the proteins enables them to have this effective shape. It is a physical process in which proteins tend to be functional through having the proper shape (Kirkwood & Austad, 2000). Every protein has particular unfolded shapes in which they are not functional. All the chemical properties present in every amino acid shape the proteins into specific shapes allowing them to carry their functions in the cells. Most of the diseases brought by age and, most importantly the neurodegenerative diseases like Parkinson's and Alzheimer's disease are a result of the development of the unfolded proteins in the cells, which lead to malfunctioning (Kirkwood, 1997). Dysfunctioning of the proteins is the major cause of aging since their major role in a person's body involves antibodies, communications, and hormones. The nucleus mitochondria and chaperone proteins communication tends to rule the behavior of the proteins; hence the deterioration.
Genome refers to the sum of all the genetic information in an individual or a species. Genome acts as the map building the entire organisms, and the information is often found in the nucleus cells. This information allows a person's cells to maintain and also build our bodies, and it is stored as DNA. Every specific DNA faction acts as a representation of a single characteristic (gene) of a person. An individual's genome has about 25, 000 to 30, 000 genes with nucleus containing the DNA having proteins known as histones ( Freitas & De Magalhaes, 2011).DNA and histones combination make up the chromosomes, and the chromosomes have all the genetic information replicated with specific cell divisions to carry the information to any newly established daughter cell. Throughout individual life, cells are usually divided several times, and in the older individuals, these genetic anomalies can be seen. Mutations occur during the DNA mutation process affecting essential genes and end up causing some genetic issues (Freitas & De Magalhaes, 2011). The mutation leads to malfunctioning affecting the remaining tissues when the cells fail to get eliminated through senescence and apoptosis. The DNA repair system plays a significant role during the aging process. Extra DNA repairing proteins are usually found in the aged individuals, and excess activation can cause the NAD+ to expire, hence inducing the death of the cells or apoptosis.
Connective Tissue Theory
This approach proposes that as time goes on, the biochemical processes develop connections between unconnected structures. Various cross-linkages take place rapidly, especially between 30 and 50 years of age (Kirkwood, 1977). There is no particular thing that has been identified as useful in stopping the occurrence of the cross-linkages. With age, elastin dries up and also cracks, and this makes the skin to have lesser elastin just like in the old adults; the skin becomes drier even wrinkled. With time, the decrease of the extracellular fluid, various deposits of chloride, calcium, and sodium build up in an individual's cardiovascular system.
Programmed theories of aging propose that aging can be predetermined through the changes of the programmed cells or even through changes in the immunologic or neuroendocrine systems. The theory's main argument is that aging is related to an individual's biological clock or genetic factors (Kenyon, 1996). Aging is often likely an evolved benefiting characteristic which results from some structured processes which seem complicated and some series of random events. Some of the evidence for support of the programmed aging include:
During the steps of the cell cycle, the cell usually divides and also replicates the DNA to carry the genetic information to the new cells. As the replication process occurs, an automatic modification occurs, the non-replication of specific end of a particular chromosome or telomere. This leads to shortening of all the telomeres with every cell division (DePinho & Wone, 2003). The telomere enzymes enable the telomeres to replicate fully, and it is usually found with the embryonic cells, cancer cells, and stem cells. Additionally, these telomeres have some protecting effects on the persons DNA since them chromosome portions with no genetic information and seem to erode throughout individual's life with every replication until it disappears and leave information bare of the DNA (Eisenberg, 2011). There is no more protection of the DNA as the replication occurs with the vital information getting chewed hence leading to cell apoptosis or death as well as the changes of the cellular cycles through the creation of the cancer cells ( Freitas & De Magalhaes, 2011). Telomere shortening is considered a biological clock is activating the cell senescence immediately; its time is up; this mechanism tends to limit the life expectancy of individual cells hence causing aging.
Epigenetic of Aging
Epigenetic refers to the study of all the mechanisms which manage gene expression pool. Genetics is majorly about genes, while epigenetic is mainly about the usage of these genes. Gene expression tends to vary whether it is activated or not depending on the environment it is in( Guarente & Kenyon, 2000). Most of the organs exhibit this variation since every cell has the same genetic information although the functions tend to differ and this shows the differences in the gene expressions based on the type of environment. Studying genetic alterations helps to correctly explain the aging process since all the changes undergone in the cell activity every specific time are shown. Gene modification and gene methylation of the histone proteins tend to be responsible for the various variations in the genome expressions as well as chromatin remodeling, which affects genome stability (Ken Haussmann & Mauck, 2008). These alterations are usually linked to diseases related to age like dementia, atherosclerosis, and cancer.
Biological/ Gene Clock Theory
The cells, as well as the whole organism, have a specific aging code that is genetically programmed stored in the DNA of the organism. It is a compromising genetic influence that predicts the occurrence of the diseases, physical conditions, as well as the ca...
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