6.6 Gene Expression and Cell Specialization

AP Biology Unit 6.6: Gene Expression and Cell Specialization

Transcription: Turning Genes On and Off

Gene expression is the key process that allows cells to perform specialized functions, and it all begins with transcription. During transcription, RNA polymerase and various transcription factors bind to specific regions of DNA called promoters to initiate the process of copying a gene into RNA. 🔥

Promoters are special sequences located upstream of a gene’s transcription start site, and they determine whether the gene will be expressed before RNA polymerase reaches the start site. Promoters often contain a TATA box, which is a DNA sequence recognized by the TATA-binding protein (TBP). The TBP helps RNA polymerase get into position and start transcription. Besides the TATA box, promoters also have enhancers and silencers, which are regulatory elements that control how active the promoter is—enhancers can increase transcription, while silencers can decrease it.

Negative regulation also plays an important role. Repressors are proteins that bind to promoter regions, preventing RNA polymerase from starting transcription. Transcriptional corepressors work alongside repressors, binding to transcription factors to block them from activating transcription. The balance between these positive and negative regulatory molecules helps cells precisely control gene expression, adapting their physiology according to the needs of the organism. ⚖️

Gene Regulation & Expression: Specializing Cells

Gene regulation is what makes each cell in our body different—muscle cells contract, neurons transmit signals, and immune cells fight off infections. This process of turning on different sets of genes is called differential gene expression, and it allows cells to produce the specific proteins that define their functions. 🛠️🧠

One major way that cells regulate which genes are expressed is through the use of small RNA molecules. These are short, non-coding RNAs that do not code for proteins themselves, but instead regulate the expression of other genes.

A well-known group of these molecules is microRNAs (miRNAs), which are 20-25 nucleotides long and bind to specific messenger RNAs (mRNAs) to control their activity. By binding to mRNA, miRNAs can block its translation into proteins or promote its degradation. This ensures that certain proteins are only made when needed.

Another class of small RNAs includes small interfering RNAs (siRNAs) and PIWI-interacting RNAs (piRNAs). siRNAs can target specific mRNAs for degradation, while piRNAs are involved in silencing transposable elements, which are mobile pieces of DNA in the genome. These small RNA molecules help cells regulate gene expression and maintain genetic stability.

Although the names of these small RNAs can be overwhelming, it’s enough for AP Biology to understand that cells have sophisticated systems, like small RNAs, that fine-tune gene expression at various levels, including after transcription has occurred. Understanding these regulatory processes helps us grasp how cells achieve the incredible specialization seen in multicellular organisms! 🧪