Which of the following statements is true regarding eukaryotic cells?
a) They have polycistronic mRNA
b) Each mRNA can code for one protein product per mRNA
c) Gene expression is not very diverse
d) Each mRNA can code many protein products per mRNA
B is correct. Each mRNA can code for one protein product per mRNA. Eukaryotic mRNA is mostly monocistronic, meaning each mRNA molecule can code for a single protein product. Prokaryotes, on the other hand, have polycistronic mRNA, meaning that each mRNA molecule can code for multiple protein products. Coding in this way is efficient for prokaryotes because they would only have to synthesize a single mRNA molecule to produce all of the proteins necessary for a particular biochemical pathway. In eukaryotes, however, every biochemical pathway requires multiple mRNA molecules to produce the many proteins involved in the pathway, which may not be as efficient as prokaryotic cells. However, having each protein product under the control of separate mRNA molecules allows for more diversity in gene expression.
Eukaryotic gene expression refers to the process of using the information found in genes to create gene products. It is a process that takes place in two different locations in eukaryotes, unlike in prokaryotes, where gene expression happens only in the cytoplasm. In eukaryotes, part of the process occurs in the nucleus and the other part in the cytoplasm. Also, eukaryotic messenger RNA, or mRNA, is mostly monocistronic. Monocistronic mRNA is mRNA that can only code for one single protein product at a time. Prokaryotes, on the other hand, have polycistronic mRNA, which is mRNA that can code for multiple protein products at one time.
In this way, the process of gene expression in eukaryotes is more complicated than in prokaryotes. In order to carry out a particular biochemical pathway, eukaryotes have to produce separate mRNA molecules for each protein that will be required in the pathway. Some benefits to this approach include that, by having each protein product under the control of a separate mRNA and promoter, different cells can express different combinations of proteins more efficiently. Ultimately, this process allows for more diversity in gene expression. However, the downside of this system is that gene expression regulation is much more involved in eukaryotes than in prokaryotes. In terms of transcription regulation, there are many ways this can be done in eukaryotes.
One way of altering gene transcription levels is through the use of transcription factors. Transcription factors are proteins that can bind to DNA sequences called promoters or enhancer regions. When transcription factors bind to DNA, they can either decrease or increase gene expression. Figure 1 shows an example of transcription factors. Upon binding the promoter region, the transcription factors will increase the affinity of RNA polymerase for the promoter. By doing so, gene transcription will be increased.
Another method of transcription regulation is through the regulation of chromatin structure. Recall that chromatin is DNA wrapped around histone proteins. One way of altering chromatin structure is by the addition or removal of acetyl groups on histone proteins. This process can alter the accessibility of DNA by RNA polymerase, which will alter the ability of the cell to transcribe genes. Interestingly, histone acetylation is involved in several types of lung diseases such as asthma, by regulation of inflammatory genes. Moreover, research has shown that histone acetylation can contribute to cancer development as well.
A third method of transcription regulation is by altering histone structure through histone methylation. In histone methylation, instead of adding or removing acetyl groups from histone proteins, methyl groups are being added or removed. This process can result in either an increase or a decrease in gene expression. It is important to note that histone methylation is different from DNA methylation. DNA methylation typically silences gene expression by adding methyl groups to DNA.
Gene duplication, also known as gene amplification, involves duplicating a region of DNA (Figure 2). If the duplicated DNA region includes both a promoter and as well as the entire coding region, this can lead to increased protein expression because now there are twice as many mRNA molecules and proteins being produced. It is important to note that gene duplication does not occur very often, and may even be a result of errors in DNA replication. Also, if the duplicated genes are oncogenes, which are cancer-causing genes, gene duplication can cause cancer. Some such cancers that can be caused through the amplification of oncogenes are breast cancer, cervical cancer, gastric cancer, or even lung cancer.
Alternative splicing refers to the fact that different combinations of introns and exons can be capped or removed in a pre-mRNA molecule. Figure 3 illustrates how alternative splicing works. One DNA sequence is transcribed to pre-mRNA and then spliced in several different ways to produce many different protein products from the same gene. By having the ability to produce multiple protein products from one gene, alternative splicing increases the efficiency of gene expression in eukaryotes.
Non-coding RNAs are RNA molecules that do not code for proteins. In other words, non-coding RNA are not mRNA. There are several types of non-coding RNA, and it is important to understand what they are for the MCAT exam. Transfer RNA, or tRNA, are non-coding RNA responsible for bringing amino acids to ribosomes during translation. Ribosomal RNA, or rRNA, are components of ribosomes. Small nuclear RNA, or snRNA, are components of spliceosomes used in RNA splicing. Finally, microRNA, are short fragments of RNA that will hybridize to complementary sequences on mRNA. When they bind to complementary mRNA sequences, they block translation of that gene. In this way, microRNAs are one way of silencing gene expression.