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Chapter 17, Eukaryotic cells modify RNA after transcription, Basic…
Chapter 17
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The Genetic Code
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If each kind of nucleotide base were translated into an amino acid, only four amino acids could be specified, one per nucleotide base
Triplets of nucleotide bases are the smallest units of uniform length that can code for all the amino acids.
triplet code
Experiments have verified that the flow of information from gene to protein is based on a triplet code: The genetic instructions for a polypeptide chain are written in the DNA as a series of non overlapping, three-nucleotide words
The series of words in a gene is transcribed into a complementary series of non overlapping, three-nucleotide words in mRNA, which is then translated into a chain of amino acids
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Gene Expression
the process by which DNA directs the synthesis of proteins (or, in some cases, just RNAs)
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template strand
This strand is called the template strand because it provides the pattern, or template, for the sequence of nucleotides in an RNA transcript
codons
The mRNA nucleotide triplets are called codons, and they are customarily written in the 5'---3' direction
In the 1930s, the American biochemist and geneticist George Beadle and his French colleague Boris Ephrussi speculated that in Drosophila, each mutation affecting eye color blocks pigment synthesis at a specific step by preventing production of the enzyme that catalyzes that step
During transcription, the gene determines the sequence of nucleotide bases along the length of the RNA molecule that is being synthesized
For each gene, only one of the two strands is transcribed
The strand that is used as the template is determined by the orientation of the enzyme that transcribes genes, which in turn depends on the particular DNA sequences associated with that gene
An mRNA molecule is complementary rather than identical to its DNA template because RNA nucleotides are assembled on the template according to base-pairing rules
The pairs are similar to those that form during DNA replication, except that U pairs with A and the mRNA nucleotides contain ribose instead of deoxyribose
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