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How does a gene’s encoded message become
a trait?
DNA (genotype) proteins  phenotype
DNA directs “protein synthesis” – also known
as… “gene expression”
Each gene specifies a certain protein.
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RNA (ribonucleic acid) is the link between
DNA and protein synthesis.
 DNA –> RNA –> Protein
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Key differences:
1. single strand
2. uses the base U (uracil) instead of T
(thymine)… so U pairs with A
3. has a ribose sugar
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DNA is made of nucleotides.
Proteins are made of amino acids.
To transition from one language to the other,
2 major steps are required: transcription and
translation.
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Transcription = the synthesis of RNA under
the direction of DNA
 An RNA copy (mRNA) of the DNA carries the
gene’s instructions to the protein-synthesizing
machinery (ribosomes)
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Translation = the synthesis of a polypeptide,
under the direction of RNA
 Ribosomes facilitate the orderly linking of amino
acids into peptide (protein) chains
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Why an intermediate (RNA)?
 1. Protection – original DNA copy stays inside the
nucleus
 2. Efficiency – many copies of a protein can be
made simultaneously, and RNA transcripts can be
used repeatedly
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After the initial transcription of RNA, it must
also go through RNA processing.
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How do 4 nucleotide bases specify 20 amino
acids?
Triplicate code:
 The genetic instructions for a polypeptide chain
are written in the DNA as a series of
nonoverlapping, three-nucleotide words.
 Each set of 3 bases specifies a certain amino acid.
▪ “Codons” = mRNA base triplicates
▪ Genetic code was decoded by Marshall Nirenberg (1961)
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Only one strand of DNA is used as the template for
a given gene.
The mRNA strand is complementary to the DNA.
For a given codon, it is generally understood to be
written in the 5’3’ direction, but may be specified
Example: if a DNA strand is 3’-ACC-5’ then the RNA
strand would read 5’-UGG-3’.
During translation, the sequence of mRNA codons is
decoded, “translated,” into a sequence of amino
acids making up a polypeptide chain.
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Start and Stop codons begin and end
translation.
 AUG = Start
There is redundancy but not ambiguity in the
genetic code.
 Multiple codons result in the addition of the
same amino acid, but a given codon will always
result in the addition of the same amino acid.
 Cells read codons as three-letter words
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 Reading frame is important
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The genetic code is nearly universal!
Genes from one organism can be
transplanted into another…
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https://www.youtube.com/watch?v=nHM4U
UVHPQM