Abstract
Genes are regions on DNA that contain the instructions for making specific proteins. In humans, genes vary in size from hundreds of DNA bases to over 3 million base pairs. From DNA to proteins, two steps are involved. Transcription is accessing the gene and reading the instructions therein in the nucleus producing as a single strand of RNA called messenger RNA (mRNA). Translation is reading the instructions on mRNA to assemble the specified proteins on the surface of ribosomes. Genetic mutations are heritable, small-scale alterations in one or more base pairs that damage DNA. Although new mutations introduce new variation, these are constantly removed from populations. Mutations can arise naturally during DNA replication or can be caused by environmental factors like chemicals or radiation. They can be harmful, neutral, or beneficial to the organism and are generally of five types: point mutations, frameshift mutations, transposons, transitions, and transversions. This chapter explores this aspect of genes.
TopChapter Outline
8.1 Connecting Genes and Proteins
8.2 Making a Transcript from DNA
8.3 How Enzymes Read DNA: Genetic Code
8.4 How the mRNA Transcript is Translated
8.5 When Genes Change: Genetic Mutations
Chapter Summary
TopLearning Outcomes
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Illustrate how genes were connected to proteins by experimentation.
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Explain the process of transcription to make messenger RNA.
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Summarize the elements of the genetic code.
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Explain the process of translation.
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With examples, demonstrate how mutations occur, types and their effects.
Top8.1 Connecting Genes And Proteins
In the mid-1800s, Charles Darwin proposed the theory of evolution which explained a mechanism that allowed organisms to evolve as their environments change (section 1.4). This is what we call adaptation and it is heritable—allowing advantageous changes to accumulate and benefit the population. Genetic adaptation occurs because favorable genetic changes (mutations) accrue in on organism allowing it to adapt to existing conditions. The experimental work of Gregor Mendel on garden peas (Chapter 5, section 5.3) in the latter half of 1800 showed that genetic traits (like flower colour) are passed on to offspring in distinct packages we today call genes. In the early 1900s scientists continued working to determine how genes worked. How do genes allow organisms to adapt? How do genes pass traits they carry from parent to offspring?
Early experiments by Beadle and Tatum (Cairns, 2003) provided experiential evidence connecting genes and enzymes working on the bread mold Neurospora crassa. They demonstrated that mutant strains bombarded with X-rays were unable to grow in growth medium that was minimal in nutrients. However, when the mutant strains were put in a complete growth medium, they thrived. The normal strain (unmutated or wild-type) can make all the nutrients it requires and missing from the minimal medium. They hypothesized that the mutant strains had mutations in specific genes required in the synthesis of the missing nutrients. To find out exactly what genes had mutated in the mutants, follow-up experiments on the biochemical synthesis of the amino acid arginine were performed by Srb and Holowitz on N. crassa (Horowitz, 1995). In the 3-step metabolic pathway required for arginine synthesis (see below) each mutant required the addition of a specific amino acid in the medium. It was hypothesized that the mutant lacked the gene coding for the enzyme required to make the added amino acid (see Box 1)
The conclusion from these experiments was that the mutants had incurred faults in the genes required for making the enzymes needed at each step in the pathway giving way for the one gene-one enzyme hypothesis. As more information became available from research in later years—that not all proteins were enzymes, this hypothesis was modified to read one gene–one polypeptide hypothesis. Some proteins are used in the body for other purposes like building body structure (muscles), in hair, hormones, antibodies, etc.
Key Terms in this Chapter
Peptidyl Transferase: Enzyme that catalyzes the formation of peptide bonds in a growing polypeptide chain.
Homeotic Genes: Highly conserved DNA sequences that code for specific transcription factors that regulate gene expression of specific anatomical structures.
Genetic Mutations: Heritable alterations in one or more base pairs that damage DNA and can affect a single nucleotide pair or larger segments of a chromosome.
Codons: A set of 3 nucleotides or triplets on the mRNA that codes for a specific anticodon.
Mature mRNA: mRNA that has been spliced, processed, and is ready for exportation to the cytoplasm to begin the process of translation.
Spliceosome: An enzyme that catalyzes esterification reactions in the removal of introns and the joining of exons.
Transversions: A type of mutation in which pyrimidines are changed to purines, and vice versa.
Precursor Messenger RNA (Pre-mRNA): A primary transcript that contains both exons and introns, and thus precedes the formation of mRNA.
Small Nuclear Ribonucleoprotein Particles (snRNPs): Uridine-rich RNA-proteins that interact with unmodified pre-mRNA and other proteins to form a larger complex known as the spliceosome.
Polyadenylation: The process by which poly-A polymerase adds on adenine nucleotides are adding onto the 3’OH end of pre-mRNA.
Promoter: A specific region upstream of the DNA sequence that regulates transcription by binding to RNA polymerase II.
7-Methylguanosine Cap: A cap that is added to the 5’ end of the hRNA molecule that protects the mRNA from degradation.
Genetic Code: Standard set of rules and meanings of nucleotide triplets in which all life forms share.
60S Ribosomal Subunits: In eukaryotes, the larger subunit of the ribosome complex that is responsible for peptide formation.
Missense Mutation: A type of point mutation that results in the substitution of one amino acid for another in the final polypeptide.
Exons: Coding regions of mRNA that are spliced together by spliceosomes during post-translational modification and will exit the nucleus.
Poly-A Polymerase: Enzyme that catalyzes the addition of adenine nucleotides during post-transcriptional modification.
Human Genome Project: An international project created in 1990 with the goal of sequencing and identifying the entire human genome.
RNA Polymerase Ii: Enzyme complex that catalyzes the transcription of DNA by binding to the promoter.
mRNA Ribonucleoprotein (mRNP) Complex: Protein complex containing the appropriate export receptors that is translocated through the nuclear pore to the cytoplasm.
Point Mutations: Mutations that occur from one nucleotide change for another in DNA.
Nonsense Mutation: A type of point mutation that results in the substitution of a stop codon for an amino acid in the final polypeptide.
Introns: Non-coding regions of mRNA that are removed by spliceosomes during post-translational modification.
Repetitive DNA: Short repeating sequences of DNA.
Adaptor Proteins: Proteins that facilitate the binding of a mRNA transcript by recruiting appropriate signal components such as receptors. They form complexes with other proteins to regulate signal transduction pathways.
Anticodon: A set of 3 nucleotides on a tRNA that correspond to a complementary codon in mRNA.
Frameshift Mutations: Mutations that occur from the insertion or deletion of nucleotides and results in a shift in the reading frame.
Transposons: Repetitive DNA sequences that can insert or remove themselves from one location to another in the genome.
Polyribosomes: A complex of several ribosomes that attach to mRNA and translate in sequence.
Genetic Adaptation: Traits that result from natural selection and random variation, which give rise to favorable genetic changes in an organism.
Poly-A Tail: A set of approximately 200 adenine nucleotides at the 3’ OH end of a pre-mRNA.
Aminoacyl-tRNA Synthase: Enzyme that catalyzes the addition of a corresponding amino acid to its tRNA.
Transitions: A type of mutation in which pyrimidines are changed to another (i.e., C to T) or purines are changed to another (i.e., A to G). The transition rate is higher than the transversion rate in animal genomes.
TATA Binding Protein: Protein that recognizes and binds to a specific region of repeating thymine and adenine bases.
Transcription Factors: Family of proteins that regulate (activate or repress) gene expression and thus transcription.
Junk DNA: Non-coding DNA sequences in the genome.