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Gene Expression: From Gene to Protein (Genes specify proteins via…
Gene Expression: From Gene to Protein
Genes specify proteins via transcription and translation
In 1902, Archibald Garrod was the first to suggest that genes dictate phenotype through enzymes that catalyze specific chemical reactions in the cell
Beadle and Edward Tatum were able to establish the link between genes and enzymes by exploring the metabolism of a bread mold, Neurospora crassa
Wild-type Neurospora can grow on a minimal medium of agar, inorganic salts, glucose, and the vitamin biotin
During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand
Transcription of a protein-coding gene produces a messenger RNA (mRNA) molecule
Translation is the synthesis of a polypeptide, using the information in mRNA
The sites of translation are the ribosomes, complex particles that facilitate the orderly assembly of amino acids into polypeptide chains
The initial RNA transcript of any gene is called a primary transcript, while further processing yields the finished mRNA
With a triplet code, three consecutive bases specify an amino acid, creating 64 possible code words
During transcription, one DNA strand, the template strand, provides a template for ordering the sequence of nucleotide bases in an mRNA transcript
The starting point establishes the reading frame; subsequent codons are read in groups of three nucleotides
Transcription is the DNA-directed synthesis of RNA: a closer look
RNA polymerase separates the DNA strands at the appropriate point and joins RNA nucleotides complementary to the DNA template strand
RNA polymerase attaches and initiates transcription at the promoter. In bacteria, the sequence that signals the end of transcription is called the terminator.
The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit
Transcription can be separated into three stages: initiation, elongation, and termination of the RNA chain
Within the promoter is the transcription start point for the transcription of a gene
In eukaryotes, proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription
The complex of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex
Eukaryotic cells modify RNA after transcription
During RNA processing, both ends of the primary transcript are altered
The most remarkable stage of RNA processing occurs during the removal of a large portion of the RNA molecule in a cut-and-paste job of RNA splicing
Noncoding segments of nucleotides called intervening regions, or introns, lie between coding regions
The regions called exons are eventually expressed, usually by being translated into amino acid sequences
Several different snRNPs join with a variety of proteins to form a larger assembly called a spliceosome, which is about the size of a ribosome
Many genes give rise to two or more different polypeptides, depending on which segments are treated as exons during RNA processing; this is called alternative RNA splicing
Proteins often have a modular architecture with discrete structural and functional regions called domains
Translation is the RNA-directed synthesis of a polypeptide: a closer look
The interpreter is transfer RNA (tRNA), which transfers amino acids from the cytoplasmic pool to a growing polypeptide in a ribosome
At the other end of the tRNA is a nucleotide triplet called an anticodon, which base-pairs with a complementary codon on mRNA
Each amino acid is joined to the correct tRNA by aminoacyl-tRNA synthetase
Such versatility is possible because the rules for base pairing between the third base of the codon and the anticodon are relaxed. This flexible base pairing is called wobble
Initiation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits
A ribosome consists of a large and a small subunit, each made up of proteins and ribosomal RNA (rRNA)
Termination occurs when one of the three stop codons reaches the A site of the ribosome
A single mRNA may be used to make many copies of a polypeptide simultaneously as multiple ribosomes, polyribosomes or polysomes, trail along the same mRNA
Translation in all ribosomes begins in the cytosol, but a polypeptide destined for the endomembrane system or for export has a specific signal peptide region at or near the leading end
A signal recognition particle (SRP) binds to the signal peptide and attaches it and its ribosome to a receptor protein in the ER membrane
Mutations of one or a few nucleotides can affect protein structure and function
Mutations include large-scale mutations, in which long segments of DNA are affected (for example, translocations, duplications, and inversions), as well as point mutations, chemical changes in just one nucleotide pair of a gene
Some nucleotide-pair substitutions have no effect on protein function, due to the redundancy of the genetic code
In a silent mutation, a change in a nucleotide pair transforms one codon into another that is translated into the same amino acid
Nonsense mutations change an amino acid codon into a stop codon, causing premature termination of translation and nearly always leading to a nonfunctional protein
Insertions and deletions are more likely than substitutions to have a disastrous effect on the resulting protein
Unless insertion or deletion mutations occur in multiples of 3, they cause a frameshift mutation