Created by: CK-12/Adapted by Christine Miller

This sketch illustrates some of the variability in human cells. The shape and other characteristics that make each type of cell unique depend mainly on the specific proteins that particular cell type makes. Proteins are encoded in genes. All the cells in an organism have the same genes, so they all have genetic instructions for the same proteins. Obviously, different types of cells must use (or express) different genes to make different proteins.

Using a gene to make a protein is called gene expression. It includes the synthesis of the protein by the processes of transcription of DNA into mRNA,  and translation of mRNA into a protein. It may also include further processing of the protein after synthesis.

Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any point in the expression of a gene, from the start of the transcription phase of protein synthesis to the processing of a protein after synthesis occurs. The regulation of transcription is one of the most complicated parts of gene regulation in eukaryotic cells, and it is the focus of this concept.

As shown in Figure 5.9.2, transcription is controlled by regulatory proteins. These proteins bind to regions of DNA, called regulatory elements, which are located near promoters. The promoter is the region of a gene where RNA polymerase binds to initiate transcription of the DNA to mRNA. After regulatory proteins bind to regulatory elements, the proteins can interact with RNA polymerase. Regulatory proteins are typically either activators or repressors. Activators are regulatory proteins that promote transcription by enhancing the interaction of RNA polymerase with the promoter. Repressors are regulatory proteins that prevent transcription by impeding the progress of RNA polymerase along the DNA strand, so the DNA cannot be transcribed to mRNA.

Although regulatory proteins and elements are typically the key players in the regulation of transcription, other factors may also be involved. Regulation of transcription may also involve enhancers. Enhancers are distant regions of DNA that can loop back to interact with a gene’s promoter. They can also increase the likelihood that transcription of the gene will occur.Enhancers

The TATA Box

Different types of cells have unique patterns of regulatory elements that result in only the necessary genes being transcribed. That’s why a blood cell and nerve cell, for example, are so different from each other. Some regulatory elements, however, are common to virtually all genes, regardless of the cells in which they occur. An example is the TATA box, which is a regulatory element that is part of the promoter of almost every eukaryotic gene. A number of regulatory proteins bind to the TATA box, forming a multi-protein complex. It is only when all of the appropriate proteins are bound to the TATA box that RNA polymerase recognizes the complex and binds to the promoter so transcription can begin.

The regulation of gene expression is extremely important in an organism’s early development. Regulatory proteins must “turn on” certain genes in particular cells at just the right time, so the individual develops normal organs and organ systems. Homeobox genes are important genes that regulate development.

Homeobox genes are a large group of similar genes that direct the formation of many body structures during the embryonic stage. In humans, there are an estimated 235 functional homeobox genes. They are present on every chromosome and generally grouped in clusters. Homeobox genes contain instructions for making chains of 60 amino acids, called homeodomains. Proteins containing homeodomains are transcription factors that bind to and control the activities of other genes. The homeodomain is the part of the protein that binds to the target gene and controls its expression.

Some types of cancer occur because of mutations in the genes that control the cell cycle. Cancer-causing mutations most often occur in two types of regulatory genes: proto-oncogenes and tumor-suppressor genes. Both are shown in Figure 5.9.4.

• Proto-oncogenes are genes that normally help cells divide. When a proto-oncogene mutates to become an oncogene, it is continuously expressed, even when it is not supposed to be. This is like a car’s accelerator pedal being stuck at full throttle. The car keeps racing at top speed. A cell, in this case, keeps dividing out of control, which can lead to cancer.
• Tumor suppressor genes are genes that normally slow down or stop cell division. When a mutation occurs in a tumor suppressor gene, it can no longer control cell division. This is like a car without brakes. The car can’t be slowed or stopped. A cell, in this case, keeps dividing out of control, which can lead to cancer.

Attributions

Figure 5.9.1

Stem_cell_differentiation.svg by Haileyfournier on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 5.9.2

Activators and Repressors by Christine Miller is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 5.9.3

TATA_box_description by Luttysar on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 5.9.4

Pathways to cancer by CK-12 Foundation is used under a CC BY-NC 3.0 (http://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under  • Terms of Use • Attribution

References

Brainard, J/ CK-12 Foundation. (2012). Figure 3 Flow chart (series of mutations leading to cancer) [digital image]. In CK-12 College Human Biology (Section 5.8) [online Flexbook]. CK12.org. https://www.ck12.org/c/physical-science/concentration/?referrer=crossref

ndsuvirtualcell.(2008). Regulated transcription. YouTube. https://www.youtube.com/watch?v=vi-zWoobt_Q&feature=youtu.be

TED-Ed. (2012, December 5). How do cancer cells behave differently from healthy ones? – George Zaidan. YouTube. https://www.youtube.com/watch?v=BmFEoCFDi-w&feature=youtu.be

TED-Ed. (2015, April 15). What is leukemia? – Danilo Allegra and Dania Puggioni. YouTube. https://www.youtube.com/watch?v=Z3B-AaqjyjE&feature=youtu.be