Structure of a Gene

A eukaryotic gene contains coding regions called exons and non-coding regions called introns that are transcribed together into pre-mRNA. During RNA processing, introns are removed and exons form the mature RNA translated into protein. Genes also include regulatory sequences like promoters, terminators, enhancers, and silencers that control gene expression without encoding proteins.

The promoter sequence is located upstream of a gene where RNA polymerase binds to initiate transcription. The terminator sequence at the 3' end acts as a stop signal for RNA polymerase, ending transcription. Together, these regulatory elements define the boundaries of the transcribed region and control when transcription begins and ends.

Enhancers are regulatory sequences located thousands of base pairs upstream or downstream of a gene where activator proteins bind to upregulate transcription. Silencers are bound by repressor proteins to downregulate gene expression. Both elements allow cells to fine-tune when and how much a gene is expressed without altering the gene's coding sequence.

Only 1% of human DNA encodes proteins; the remaining 99% is non-coding DNA that performs regulatory functions. Non-coding DNA provides binding sites for transcription factors and regulatory proteins that activate or repress gene activity. This extensive regulatory DNA allows cells to control which genes are expressed at specific times and locations, enabling different cell types to perform distinct functions despite containing identical genes.

Prokaryotic cells primarily control gene expression at the transcriptional level, continuously transcribing RNA until a protein is needed. Eukaryotic cells regulate gene expression at multiple levels: epigenetic, transcriptional, post-transcriptional, translational, and post-translational. This complexity arises because transcription occurs in the nucleus while translation occurs in the cytoplasm, allowing eukaryotes greater control over protein production.

Although most people contain the same genes, small differences in DNA base sequences create different alleles. These sequence variations contribute to different phenotypes among individuals. Despite having identical sets of genes, allelic differences account for the genetic variation observed in human populations.

Gene regulation at the epigenetic level involves unwinding and loosening DNA from nucleosomes to allow transcription factors to bind. Chromatin packaging and solenoid model organization determines whether genes are accessible for transcription. By controlling chromatin structure through histone modification acetylation and methylation, cells can activate or silence genes without changing DNA sequences.