On page 45, Shubin writes, “there is a deep similarity among every cell inside our bodies: all of them contain EXACTLY the same DNA”. If this is true, then how do muscle, tissue, and bone develop within our bodies? Explain using what we have learned throughout the year about DNA and gene expression.
Matt Micucci (coochqbk@sbcglobal.net)
Shubin explains the process by stating that “our bodies are a composition of individual genes turning on and off inside each cell during our development” (46). Basically, our bodies are made up of many different types of cells that originate from early in development through cell differentiation, the process in which cells “become specialized in structure and function” (Campbell 366). Cell differentiation itself is a direct result of gene regulation, as almost all the cells in the body (save for red blood cells, since they have no nucleus) have a copy of the same genome; the only thing that tells a cell to differentiate is what genes turn on or off during development. Thus, even though bone, muscle, fat and skin cells all carry the same DNA, certain genes are turned on or off, changing the cell’s characteristics.
ReplyDeleteDuring embryonic development, there are two things that tell a cell what genes to turn on or off: cytoplasmic determinants and inductive signals. The former are maternal substances in the egg that influence development by being distributed differently in splitting cells (Campbell 367-368); because they are originally somewhat unevenly distributed in the cytoplasm of the unfertilized egg, fertilization causes the determinants to move in different concentrations to the cells as they divide. Cytoplasmic determinants change cell differentiation by regulating gene expression, turning specific genes on or off (Campbell 368). Inductive signaling is more important as the embryo continues dividing due to the change in environment around the embryonic cells. When an embryonic cell comes into contact with another one or secretes growth factors, the target cell may experience a change through the process of induction, changing gene expression by sending specific signal molecules (Campbell 368).
The way that gene expression is actually changed is through either DNA methylation or histone modification, two methods the cell uses not to modify DNA, but rather to modify how the DNA is read (http://www.nature.com/scitable/topicpage/gene-expression-regulates-cell-differentiation-931). Histones, proteins in chromatin around which DNA winds, have tails which can bind to neighboring nucleosomes, making the chromatin more compact and thus preventing transcription; when the histone tails are acetylated, they loosen the bonds, thus turning the genes on in that particular region (Campbell 358). DNA methylation works in the opposite way, turning genes off rather than on, as methyl groups are attached to the nitrogenous bases of DNA by special enzymes, preventing transcription.
To see how this applies to the question at hand, the reason that muscle, bone, and other tissue cells develop specially in our bodies is due to stem cells. Embryonic stem cells are pluripotent, meaning they can differentiate into any kind of cell, and this is why we can develop so many kinds of cells from just one embryo. Other stem cells can be found throughout the body, though, including in the bone marrow and in the brain; the bone marrow contains hematopoietic stem cells, which form all the blood cells in the body, and mesenchymal stem cells, which form bone, cartilage, fat and connective tissue cells, while the brain contains special stem cells that create oligodendrocytes, astrocytes, and neurons (http://stemcells.nih.gov/info/basics/basics4.asp). After a cell has undergone the process of determination, the cell has differentiated completely and irreversibly (Campbell 368). Determination is evident when a cell begins producing tissue-specific proteins, signaling that the cell is now of one specific type, whether nerve, bone, muscle, skin, or any other kind. Examples are liver cells specializing in albumin creation, lens cells creating crystallin, and skeletal muscle cells creating special forms of myosin and actin (Campbell 368). This development of tissue-specific proteins is the final result of gene regulation differentiating cells into what we see as bone cells, muscle cells, or tissue cells.
Eugene Bulkin (doubleaw002@gmail.com)
Each and every cell nucleus in the human body contains identical DNA. In fact, it is this distinctive DNA that distinguishes us as individuals. Yet, even within a single individual, DNA further distinguishes between various organs and tissues. Several mechanisms allow the DNA in one cell to code for a heart cell, while identical DNA may code for a stomach cell in another.
ReplyDeleteLike Eugene said, cell differentiation is a direct result of regulation. Because the body is able to regulate which genes are turned on and off in a particular cell, cells are able to exhibit specific functions and in turn develop into organs and tissues. Most organs and tissues are developed during embryonic development using stem cells. Stem cells are unspecialized cells that are capable of two important things: dividing indefinitely and differentiating into a specialized cell to form a tissue or organ (Campbell 415). The stem cells ability to indefinitely divide ensures a future supply of stem cells for tissue replenishment and provides research scientists with an easily accessible pool of cells. A stem cells ability to differentiate allows tissues and organs to form. During development, and when replenishing damaged tissues, various proteins travel through stem cells, to their nucleus where they program the cell to read only certain DNA. This specifies the cell’s function (http://ts-si.org/biology/29261-cell-communities-self-organize-into-healthy-organ-tissue).
Eugene is also correct in that a cell may modify what genes are expressed rather than the genes themselves, and this is why all cells, regardless of their purpose, carry identical DNA. Many mechanisms are used to turn off and on a cell’s ability to read certain genes. One way gene expression can be turned off is by inhibiting RNA polymerase from attaching to the DNA strand. In operons, for example, active repressors change the shape of the operator and prevent RNA polymerase from binding. The structure, acetylation, and methylation of histones - the proteins that DNA is wrapped around in chromosomes - also affect which genes can be translated or read (Campbell 358). Acetylation loosens the binding of histones and exposes more DNA, giving transcription proteins easier access to this DNA. Methylation of DNA tends to have more long term effects and can begin in early cell differentiation that occurs in the embryo. Methylation generally turns of gene expression permanently. Transcription factors also affect gene expression by making it easier for RNA polymerase to attach to certain genes. Ultimately, by being able to regulate which genes are expressed, cells are able to specialize in function and eventually form tissues and organs.
But beyond just gene expression, other apparatuses are necessary to allow for organ and tissue development. Cells are also able to communicate with one another to help them organize more efficiently. By transporting molecules between one another and recognizing different molecules located on the membranes, cells can distinguish cells of different functions (Shubin 128). Some such molecular cues that are exchanged between cells are referred to as positional information cues. These molecular signals help indicate to a cell where it should be located relative to the body axes and other cells (Campbell 369). Also, the proteins present in cell membranes allow cells to recognize each other in addition to recognizing pathogens (http://webcache.googleusercontent.com/search?q=cache:K4ae8us
TOFAJ:202.114.65.51/fzjx/wsw/newindex/website/cellb/chapter2/protein.html+cell+recognition+communication&cd=2&hl=en&ct=clnk&gl=us&source=www.google.com). By recognizing cells, a cell can locate other cells with similar functions and ultimately situate itself in its proper location.
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Once numerous cells have been programmed and located properly, rivets can be used to bind cells similar in function (Shubin 127). The molecular “rivets” that allow this binding can be found in various forms. Some rivets have two molecules each bind to the outside of a cell membrane, thus linking the two membranes together. Others only bind selectively to identical rivets, allowing cells to organize themselves into tissues and organs (Shubin 127). These rivets are often in the form of plasmodesmata - pieces of cellular cytoplasm that extend through cellular pores into other cell cytoplasms. More specifically, plasmodesmata can be seen in the form of spot desmosomes, tight junctions, and gap junctions (http://www.slideshare.net/musselburghgrammar/cell-molecular-biology). Spot desmosomes provide mechanical strength while tight junctions are a more selective type of rivet. Gap junctions provide transportation channels which may also play a role in cell communication. Without these rivets, organs would not be functional and would simply be a heap of incoherent cells.
ReplyDeleteOnce all cells are properly programmed, located, communicating, and bound, an organ can form and begin to function. Gene expression and cell communication are absolutely crucial to cell differentiation and tissue development. Without these mechanisms, all DNA would be expressed in all cells and our bodies would be a mess. But, thanks to these mechanisms, our cells are able to distinguish themselves by function and collaborate with other cells of similar function to form the operative organs that allow us to live.
Sami Kopinsky (sami_kopinsky@yahoo.com)