Presentation on theme: " How does a gene’s encoded message become a trait? DNA (genotype) proteins phenotype DNA directs “protein synthesis” – also known as… “gene expression”"— Presentation transcript:
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.
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
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.
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
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.
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)
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.
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
The genetic code is nearly universal! Genes from one organism can be transplanted into another…