8: Genes and Proteins
Since the rediscovery of Mendel’s work in 1900, the definition of the gene has progressed from an abstract unit of heredity to a tangible molecular entity capable of replication, expression, and mutation. Genes are composed of DNA and are linearly arranged on chromosomes. Genes specify the sequences of amino acids, which are the building blocks of proteins. In turn, proteins are responsible for orchestrating nearly every function of the cell. Both genes and the proteins they encode are absolutely essential to life as we know it.
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- 8.1: Introduction
- Since the rediscovery of Mendel’s work in 1900, the definition of the gene has progressed from an abstract unit of heredity to a tangible molecular entity capable of replication, expression, and mutation. Genes are composed of DNA and are linearly arranged on chromosomes. Genes specify the sequences of amino acids, which are the building blocks of proteins. In turn, proteins are responsible for orchestrating nearly every function of the cell.
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- 8.2: The Genetic Code
- The cellular process of transcription generates messenger RNA (mRNA), a mobile molecular copy of one or more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA template converts nucleotide-based genetic information into a protein product. Protein sequences consist of 20 commonly occurring amino acids; therefore, it can be said that the protein alphabet consists of 20 letters. Each amino acid is defined by a three-nucleotide sequence called the triplet codon.
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- 8.3: Prokaryotic Transcription
- The prokaryotes, which include bacteria and archaea, are mostly single-celled organisms that, by definition, lack membrane-bound nuclei and other organelles. A bacterial chromosome is a covalently closed circle that, unlike eukaryotic chromosomes, is not organized around histone proteins. The central region of the cell in which prokaryotic DNA resides is called the nucleoid. Prokaryotes often have abundant plasmids that are shorter circular DNA molecules that may only contain one or a few genes.
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- 8.4: Eukaryotic Transcription
- Prokaryotes and eukaryotes perform fundamentally the same process of transcription, with a few key differences. The most important difference between prokaryotes and eukaryotes is the latter’s membrane-bound nucleus and organelles. With the genes bound in a nucleus, the eukaryotic cell must be able to transport its mRNA to the cytoplasm and must protect its mRNA from degrading before it is translated.
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- 8.6: Ribosomes and Protein Synthesis
- The synthesis of proteins consumes more of a cell’s energy than any other metabolic process. In turn, proteins account for more mass than any other component of living organisms (other than water), and proteins perform virtually every function of a cell. The process of translation, or protein synthesis, involves the decoding of an mRNA message into a polypeptide product. Amino acids are covalently bonded by interlinking peptide bonds in lengths ranging from ~50 amino acid residues to >1,000.
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- 8.7: Gene Expression
- Whereas each cell shares the same genome and DNA sequence, each cell does not turn on, or express, the same set of genes. Each cell type needs a different set of proteins to perform its function. Therefore, only a small subset of proteins is expressed in a cell. In a given cell type, not all genes encoded in the DNA are transcribed into RNA or translated into protein because specific cells in our body have specific functions.
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- 8.7.1: Introduction
- 8.7.2: Regulation of Gene Expression
- 8.7.3: Prokaryotic Gene Regulation
- 8.7.4: Eukaryotic Epigenetic Gene Regulation
- 8.7.5: Eukaryotic Transcription Gene Regulation
- 8.7.6: Eukaryotic Post-transcriptional Gene Regulation
- 8.7.7: Eukaryotic Translational and Post-translational Gene Regulation
- 8.7.8: Cancer and Gene Regulation
- 8.7.9: Key Terms
- 8.7.10: Chapter Summary
- 8.7.11: Visual Connection Questions
- 8.7.12: Review Questions
- 8.7.13: Critical Thinking Questions
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- 8.11: Regulation of Gene Expression
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- 8.11.1: The lac Operon
- 8.11.2: The Use of Mutants to Study the lac Operon
- 8.11.3: Eukaryotic Gene Regulation
- 8.11.4: Regulatory Elements in Evolution
- 8.11.5: Additional Levels of Regulating Transcription
- 8.11.6: Epigenetics
- 8.11.7: Regulation of Gene Expression (Exercises)
- 8.11.8: Gene Expression Essentials
- 8.11.S: Regulation of Gene Expression (Summary)
Thumbnail: RNA Polymerase producing mRNA from a double-stranded DNA template. (CC BY-SA 3.0; Thomas Splettstoesser via Wikimedia Commons ).