Week 5 Discussion – Genetically Modified Organisms
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Choose one genetically modified organism (GMO), from this GMO Crop List, or a genetically modified microorganism or animal of your choice.
Using your GMO example, answer the following questions:
Do some research to identify the gene(s) that have been inserted into this specific GMO’s genome, and the protein(s) that this gene(s) code for. Briefly describe both.
Using what you have learned from this week’s textbook chapter about the Central Dogma of Molecular Biology, explain how the inserted gene(s) results in new trait(s) in this specific GMO. Use at least one in-text citation to this week’s textbook chapters.
Do you believe it is ethical to genetically engineer microorganisms, plants, animals, and humans? Does it depend on the intended outcome? Do you believe the risks outweigh the benefits? Briefly explain your opinion.
Include the name of your chosen GMO in the title of your post. Try to choose a GMO not yet described by other students. If you choose the same one, make sure you provide unique information.
Cite your sources using in-text citations and full references in APA format.
GM Crops List
Alfalfa (Medicago sativa)
Apple (Malus x Domestica)
Argentine Canola (Brassica napus)
Bean (Phaseolus vulgaris)
Carnation (Dianthus caryophyllus)
Cotton (Gossypium hirsutum L.)
Cowpea (Vigna unguiculata)
Creeping Bentgrass (Agrostis stolonifera)
Eggplant (Solanum melongena)
Eucalyptus (Eucalyptus sp.)
Flax (Linum usitatissimum L.)
Maize (Zea mays L.)
Melon (Cucumis melo)
Papaya (Carica papaya)
Petunia (Petunia hybrida)
Pineapple (Ananas comosus)
Plum (Prunus domestica)
Polish canola (Brassica rapa)
Poplar (Populus sp.)
Potato (Solanum tuberosum L.)
Rice (Oryza sativa L.)
Rose (Rosa hybrida)
Safflower (Carthamus tinctorius L.)
Soybean (Glycine max L.)
Squash (Cucurbita pepo)
Sugar Beet (Beta vulgaris)
Sugarcane (Saccharum sp)
Sweet pepper (Capsicum annuum)
Tobacco (Nicotiana tabacum L.)
Tomato (Lycopersicon esculentum)
Wheat (Triticum aestivum)
The Central Dogma: DNA Encodes RNA; RNA Encodes Protein
The flow of genetic information in cells from DNA to mRNA to protein is described by the central dogma (Figure 14), which states that genes specify the sequences of mRNAs, which in turn specify the sequences of proteins.
Figure 14: The Central Dogma
The central dogma states that DNA encodes RNA, which in turn encodes protein.
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The copying of DNA to mRNA is relatively straightforward, with one nucleotide being added to the mRNA strand for every complementary nucleotide read in the DNA strand. The translation to protein is more complex because groups of three mRNA nucleotides correspond to one amino acid of the protein sequence. However, as we shall see in the next module, the translation to protein is still systematic, such that nucleotides 1 to 3 correspond to amino acid 1, nucleotides 4 to 6 correspond to amino acid 2, and so on.
Transcriiption: from DNA to mRNA
Both prokaryotes and eukaryotes perform fundamentally the same process of transcriiption, with the important difference being the membrane-bound nucleus in eukaryotes. With the genes bound in the nucleus, transcriiption occurs in the nucleus of the cell, and the mRNA transcriipt must be transported to the cytoplasm. The prokaryotes, which include bacteria and archaea, lack membrane-bound nuclei and other organelles, and transcriiption occurs in the cytoplasm of the cell. In both prokaryotes and eukaryotes, transcriiption occurs in three main stages: initiation, elongation, and termination.
Initiation
Transcriiption requires the DNA double helix to partially unwind in the region of mRNA synthesis. The region of unwinding is called a transcriiption bubble. The DNA sequence onto which the proteins and enzymes involved in transcriiption bind to initiate the process is called a promoter. In most cases, promoters exist upstream of the genes they regulate. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all (Figure 15).
Figure 15: DNA Transcriiption Promoter
The initiation of transcriiption begins when DNA is unwound, forming a transcriiption bubble. Enzymes and other proteins involved in transcriiption bind at the promoter.
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Elongation
Transcriiption always proceeds from one of the two DNA strands, which is called the template strand. The mRNA product is complementary to the template strand and is almost identical to the other DNA strand, called the nontemplate strand, with the exception that RNA contains a uracil (U) in place of the thymine (T) found in DNA. During elongation, an enzyme called RNA polymerase proceeds along the DNA template, adding nucleotides by base pairing with the DNA template in a manner similar to DNA replication, with the difference being that an RNA strand is being synthesized that does not remain bound to the DNA template. As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme and rewound behind it (Figure 16).
Figure 16: Transcriiption Diagram
During elongation, RNA polymerase tracks along the DNA template, synthesizes mRNA in the 5′ to 3′ direction, and unwinds then rewinds the DNA as it is read.
Termination
Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA template and liberate the newly made mRNA. Depending on the gene being transcribed, there are two kinds of termination signals, but both involve repeated nucleotide sequences in the DNA template that result in RNA polymerase stalling, leaving the DNA template, and freeing the mRNA transcriipt.
On termination, the process of transcriiption is complete. In a prokaryotic cell, by the time termination occurs, the transcriipt would already have been used to partially synthesize numerous copies of the encoded protein because these processes can occur concurrently using multiple ribosomes (polyribosomes) (Figure 17). In contrast, the presence of a nucleus in eukaryotic cells precludes simultaneous transcriiption and translation.
Figure 17: Termination Signals
Multiple polymerases can transcribe a single bacterial gene while numerous ribosomes concurrently translate the mRNA transcriipts into polypeptides. In this way, a specific protein can rapidly reach a high concentration in the bacterial cell.
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Eukaryotic RNA Processing
The newly transcribed eukaryotic mRNAs must undergo several processing steps before they can be transferred from the nucleus to the cytoplasm and translated into a protein. The additional steps involved in eukaryotic mRNA maturation create a molecule that is much more stable than a prokaryotic mRNA. For example, eukaryotic mRNAs last for several hours, whereas the typical prokaryotic mRNA lasts no more than five seconds.
The mRNA transcriipt is first coated in RNA-stabilizing proteins to prevent it from degrading while it is processed and exported out of the nucleus. This occurs while the pre-mRNA is being synthesized by adding a special nucleotide “cap” to the 5′ end of the growing transcriipt. In addition to preventing degradation, the cap is recognized by factors involved in protein synthesis to help initiate translation by ribosomes.
Once elongation is complete, an enzyme then adds a string of approximately 200 adenine residues to the 3′ end, called the poly-A tail. This modification further protects the pre-mRNA from degradation and signals to cellular factors that the transcriipt needs to be exported to the cytoplasm.
Eukaryotic genes are composed of protein-coding sequences called exons (ex-on signifies that they are expressed) and intervening sequences called introns (int-ron denotes their intervening role). Introns are removed from the pre-mRNA during processing. Intron sequences in mRNA do not encode functional proteins. It is essential that all of a pre-mRNA’s introns be completely and precisely removed before protein synthesis so that the exons join together to code for the correct amino acids. If the process errs by even a single nucleotide, the sequence of the rejoined exons would be shifted and the resulting protein would be nonfunctional. The process of removing introns and reconnecting exons is called splicing (Figure 18). Introns are removed and degraded while the pre-mRNA is still in the nucleus.
Figure 18: Eukaryotic RNA Processing
Eukaryotic mRNA contains introns that must be spliced out. A 5′ cap and 3′ tail are also added.
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