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Unit 6: Gene Expression

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    • 6.1: One Gene - One Enzyme Theory
      This page discusses Neurospora crassa, a fungus that reproduces both asexually and sexually, making it ideal for genetic research due to its rapid growth and haploid lifecycle. It highlights the "One Gene - One Enzyme" theory proposed by Beadle and Tatum, established through UV-induced mutations, which posits that each gene corresponds to a specific enzyme. This theory has since evolved to include all proteins produced by genes across organisms.
    • 6.2: The Transcription of DNA into RNA
      This page outlines the central dogma of molecular biology, detailing DNA transcription to RNA and subsequent translation to proteins. It describes the roles of various RNA types, including mRNA for polypeptides and non-coding RNAs for regulation. Different RNA polymerases perform transcription, while pre-mRNA processing converts it into mature mRNA through intron removal and addition of a 5' cap and poly(A) tail.
    • 6.3: Genetic Code
      This page discusses the genetic code, which consists of 64 codons encoding 20 amino acids with some redundancy. It highlights AUG as the start codon and notes codon usage bias affecting translation efficiency. While most of the genetic code is universal, there are exceptions in mitochondrial genes and some unicellular eukaryotes. The page also mentions the incorporation of nonstandard amino acids, such as selenocysteine and pyrrolysine, through specific codons.
    • 6.4: The Translation of RNA into Proteins
      This page covers the structure and role of alanine transfer RNA (tRNA) in protein synthesis, detailing its helical regions, anticodons, and the translation process, which includes initiation, elongation, and termination. It emphasizes the concept of codon bias, which affects protein synthesis efficiency, and addresses issues like premature termination codons (PTCs) and nonstop transcripts.
    • 6.5: RNA Editing
      This page discusses RNA editing, which creates differences between gene sequences and RNA, influencing protein synthesis. It details two mechanisms: Substitution Editing (changing nucleotides) and Insertion/Deletion Editing (altering sequences with guide RNA). An example is the human APOB gene, where a CAA codon becomes a stop codon, affecting protein production. Additionally, defects in RNA editing are associated with human cancers and diseases like ALS, underlining its importance in biology.
    • 6.6: Expressed Sequence Tags
      This page discusses vertebrate DNA, emphasizing that only a small percentage encodes proteins, while much consists of non-coding regions. Identifying genes is complex, even with complete genomic data. Transcriptome analysis, which examines mRNA from various cell types, aids in gene identification. Expressed Sequence Tags (ESTs) are used to determine gene activity by isolating mRNA, converting it to cDNA, and sequencing it for genomic matches.
    • 6.7: Ribosomal RNA (rRNA) Gene Cluster
      This page depicts an electron micrograph of rRNA transcription in a newt egg cell's nucleolus, highlighting eukaryotes' many identical rRNA genes. It shows DNA with proteins and rRNA fibers, indicating simultaneous transcription as RNA lengthens from the gene's start. Areas of inactive DNA lack RNA.

    This page titled Unit 6: Gene Expression is shared under a CC BY 3.0 license and was authored, remixed, and/or curated by John W. Kimball via source content that was edited to the style and standards of the LibreTexts platform.