2.  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.

3.  RNA (ribonucleic acid) is the link between DNA and protein synthesis.  DNA –> RNA –> Protein  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

4.  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.

5.  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)  Translation = the synthesis of a polypeptide, under the direction of RNA  Ribosomes facilitate the orderly linking of amino acids into peptide (protein) chains

6.  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  After the initial transcription of RNA, it must also go through RNA processing.

7.  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)

8.  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.

9.  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  Reading frame is important

10.  The genetic code is nearly universal!  Genes from one organism can be transplanted into another…

11.  https://www.youtube.com/watch?v=nHM4U UVHPQM https://www.youtube.com/watch?v=nHM4U UVHPQM