The Argument-
Shenk says that while identical twins might seem to be exact
replicas of each other, there are many "hidden dissimilarities" (81).
He points out that while the Jim twins had many similarities, that "for
every tiny similarity between the Jim twins, there were thousands of tiny (but
unmentioned) dissimilarities" (81). However, given that the exact same
environment was not shared by the Jim twins, is it possible to have two
individuals be exactly the same? Can identical twins be more than just similar
to each other and be actually identical to each other? Consider organisms that
reproduce asexually and live in colonies. They share the exact same genetic
information as the parent organism and live in the same environment, so are
they identical or different in comparison to the parent? Consider the theme of continuity
and change and refer to Chapter 27 and 28 for the organisms that live in
colonies (bacteria and protists).
Matthew Yang (matt.y.yang2013@gmail.com)
Given the complexity of the human body and personality, it would be incredibly difficult to create two completely identical humans. Every person comes across different experiences and meets different people, all of which stimulate different gene expression and change personality that can make two individuals different regardless of how similar their environments seems. It is entirely unrealistic to keep humans in such similar environments that they grow up to be identical, so much so that it would require leaving the two twins in a completely blank room with no contact to the outside world, and even that might not be enough, as they’d emerge from the womb at different times, potentially be handled by different doctors, etc. Also, the children themselves could be factors in individualizing themselves. Shenk states that “child twins growing up together might often cling to their differences” (Shenk 82), and in doing so the twins would exacerbate their differences to have two separate personalities. Because every life experience has such an important impact on growth and personality, practically it is impossible to create two completely identical twins.
ReplyDeleteHowever, bacteria that reproduce asexually and live in colonies are much simpler organisms, and therefore a much different case from humans. They share the exact same genes (as do identical twins), but they also share a much smaller environmental, and also do not have complex personalities that humans have. Although at first it may seem that all bacteria would therefore become completely identical, that is not true. Albeit there may be a few bacteria that turn out identical to the parent cell due to identical genes and environment, bacterial populations are actually widely diverse. The rapid reproduction and mutation of bacteria means that even daughter cells will be genetically different from the parent cells in one way or another (Campbell 561). A single point mutation can cause frameshift that changes the entire translation of the genes in a prokaryote by changing the codon sequences that are translated (Campbell 346).
According to a study conducted by Peter L. Graumann of Universität Freiburg, bacteria that are genetically identical can actually switch between two different types of behavior, which occurs in colonies that are called bistable populations. Therefore, even bacteria can demonstrate unique gene expression despite having identical genes and living in identical environments. Identical environment does not necessarily translate to identical gene expression. Not only does environment affect gene expression, but mutations and internal cell environment can as well, causing seemingly identical organisms to behave differently.
This relates to the theme of continuity and change due to the fact that, despite the continuity of genes within asexually reproducing bacteria, gene expression still changes due to mutations and internal or external environment. The theme of continuity and change relates to the way that some aspects of life remain continuous, while others vary, evolve, and change over time. Although genes and environment can remain continuous from generation to generation, some things such as gene expression must eventually change. This is also how the mechanism of evolution works, because even though in many ways daughter cells must stay the same as the parent, in other ways it must change, whether through mutations or internal changes, because no cell can stay exactly the same as the parent. In this way populations and whole species adapt to changing environments. As demonstrated by both humans and bacteria, gene expression always allows a way for an organism to change from one generation to the next.
Aaron Zalewski (bitquest@yahoo.com)
The Argument: By the very nature of the GxE model, it is essentially impossible to create absolutely identical organisms, especially in the case of organisms as large and complex as humans. Even identical twins, which share the exact same genome, can vary quite drastically phenotypically due to variation in the external stimuli that they encounter. It is the culmination of these environmental differences that result in the variation in gene expression between identical twins, making them anything but identical. As Shenk notes, “we inherit [our genome]- and we also become [via disparities in external stimuli]” (83). In addition to the variation in external stimuli that organisms possessing the same genome encounter, variations in epigenetic inheritance can lead to different degrees of gene transcription into RNA, thus leading to significant distinctions in the production of proteins (Campbell 358). Processes such as histone acetylation, in which acetyl groups attach to lysines in histone tails and promote gene transcription, and DNA methylation, in which enzymes can methylate certain bases of DNA and cause long-term inactivation of genes, can vary between even identical twins due to distinctions in epigenetic inheritance. According to a study published by The Scientist, while methylation patterns of DNA in identical twins were proven to be more similar to methylation patterns in both fraternal twins and other siblings, epigenetic inheritance was not exactly the same in identical twins (http://classic.the-scientist.com/blog/display/55342/). Thus, while the genomic sequence of identical twins is the same, both their environmental stimuli and epigenetic inheritances can vary drastically, resulting in different degrees of gene expression.
ReplyDeleteIn the case of bacteria, which reproduce asexually via binary fission (thus sharing an identical genome) and occupy a much smaller environment (leading to less variation in external stimuli), the production of identical microorganisms seems much more likely. However, two factors can contribute to genetic diversity within colonies of bacteria: rapid reproduction/mutation and genetic recombination via transformation, transduction, or conjugation. The rapid reproduction of bacterial cells via binary fission increases “the probability of a spontaneous mutation occurring” (Campbell 561) via insertions, deletions, and/or base-pair substitutions. Genetic variation can also occur via genetic recombination in a bacterial colony. Transformation, in which a bacterial cell uptakes foreign DNA from its environment, alters the genotype of the cell and possibly its phenotype. Transduction, in which a bacteriophage transports bacterial genes from one host cell to another, also leads to variation in genotype within a colony of bacterial cells. Finally, conjugation can cause genetic variation within a colony, as two bacterial cells join via a sex pilus and one cell is the donator and the other is the recipient of DNA. Thus, while the possibility of identical bacterial cells is present (and common), rapid reproduction/mutation and genetic recombination result in variations in genome within a bacterial colony.
The biological concept of continuity and change is highly applicable to the reproduction of both humans and prokaryotes such as bacteria. As Aaron mentioned, while many aspects of life remain continuous over time, genetic variation via mutation and recombination result in change, even within the same species.
Nick Sotos (nsotos13@gmail.com)