21.12
From DNA to Protein
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Q1: What is the central dogma and how does genetic information flow in cells?
The central dogma describes the flow of genetic information from DNA to RNA to protein. DNA contains genes—sequences of nucleotides that code for amino acids in proteins. Transcription copies DNA into mRNA, while translation decodes mRNA to synthesize amino acid chains. This systematic process ensures genetic instructions are accurately transferred and expressed as functional proteins.
Q2: How does transcription convert DNA into messenger RNA?
During transcription, RNA polymerase enzyme synthesizes an RNA copy of a gene using DNA as a template. Each new RNA base added is complementary to the original DNA strand. Some transcripts become messenger RNA (mRNA) that codes for proteins, while others like ribosomal RNA (rRNA) and transfer RNA (tRNA) participate in protein synthesis and other cellular processes.
Q3: What is a codon and why is the genetic code considered degenerate?
Codons are three-nucleotide groups that translate to specific amino acids. The genetic code is degenerate because 64 possible codons exist but only 20 standard amino acids are used in proteins, meaning multiple codons can specify the same amino acid. This redundancy protects against mutations: single-nucleotide substitutions often produce the same or chemically similar amino acids, preserving protein function.
Q4: How does transfer RNA facilitate protein synthesis at the ribosome?
Transfer RNA (tRNA) has a three-hairpin loop structure with an anticodon sequence that binds complementary mRNA codons. An amino acid corresponding to this sequence attaches to the tRNA's end, transporting it into the ribosome. The tRNA delivers the correct amino acid to the growing polypeptide chain, ensuring accurate translation of the genetic code into protein sequence.
Q5: What role do start and stop codons play in translation?
The start codon AUG initiates translation and specifies methionine as the first amino acid. It establishes the reading frame near the mRNA's 5' end, determining how subsequent nucleotides are grouped into codons. Three stop codons (nonsense codons) signal translation termination, causing the ribosome to release the completed polypeptide chain so it can fold into a functional protein.
Q6: Why is the genetic code considered universal across different organisms?
Nearly all species use the same genetic code for protein synthesis, with only minor exceptions. This conservation means mRNA from one organism can be transferred to another and produce the correct protein—for example, horse globin mRNA in a tulip cell. This universal code is powerful evidence that all life shares a common evolutionary origin.
Q7: How does the ribosome assemble and begin translation?
Initiation factors bring together the small ribosome unit, initiator tRNA, and mRNA to form a translation complex. The ribosome then glides along the mRNA searching for the start codon. When the initiator tRNA anticodon binds to the complementary start codon, the large ribosome unit attaches, and translation begins with the addition of successive amino acids.