DNA, RNA, and the Flow of Information
- RNA differs from DNA in three ways: It is single-stranded, its sugar molecule is ribose rather than deoxyribose and its fourth base is uracil rather than thymine.
- The central dogma of molecular biology is DNA→RNA→protein.
- A gene is expressed in two steps: first, DNA is transcribed to RNA; then RNA is translated into protein.
- Some viruses are exceptions to the central dogma. Some viruses exclude DNA altogether, going directly from RNA to protein. In retroviruses, the central dogma is reversed: RNA→DNA.
Transcription: DNA-Directed RNA Synthesis
- RNA is transcribed from a DNA template after the bases of DNA are exposed by unwinding of the double helix.
- In a given gene, only one of the two strands of DNA (the template strand) acts as a template for transcription.
- RNA polymerase catalyzes transcription from the template strand of DNA.
- The initiation of transcription requires that RNA polymerase recognize and bind tightly to a promoter sequence on the DNA.
- RNA elongates in a 5′-to-3′direction, antiparallel to the template DNA. Special sequences and protein helpers terminate transcription.
- In prokaryotes, translation begins before transcription of the mRNA is completed. In eukaryotes, transcription occurs in the nucleus and translation occurs in the cytoplasm.
The Genetic Code
- The genetic code consists of triplets of nucleotide bases (codons). There are four bases, so there are 64 possible codons.
- One mRNA codon indicates the starting point of translation and codes for methionine. Three stop codons indicate the end of translation. The other 60 codons code only for particular amino acids.
- Because there are only 20 different amino acids, the genetic code is redundant; that is, there is more than one codon for certain amino acids. But the code is not ambiguous: A single codon does not encode more than one amino acid.
- Test-tube experiments led to the assignment of amino acids to codons.
Preparation for Translation: Linking RNAs, Amino Acids and Ribosomes
- In translation, amino acids are linked in an order specified by the codons in mRNA. This task is achieved by transfer RNAs (tRNAs), which bind to specific amino acids. Each tRNA species has an anticodon complementary to an mRNA codon. _ A family of activating enzymes attaches specific amino acids to their appropriate tRNAs, forming charged tRNAs.
- The mRNA meets the charged tRNAs at a ribosome.
- The small subunit of the ribosome checks to determine whether the tRNA anticodon and mRNA codon have formed hydrogen bonds.
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