Transcription and Translation From Gene to Protein Transcription and Translation

Black Urine Started it All Archibald Garrod hypothesized that genes code for enzymes Phenotypes are expressed via different chemical reactions in the cell Alkaptonuria = lack of an enzyme to break down alkapton

The Importance of Enzymes Eye color is determined by pigments- which are made by enzymes Most organic molecules are created and destroyed by chemical processes involving enzymes Led to one gene- one enzyme hypothesis

One Gene – One Polypeptide Almost right Each gene codes for one sub-unit of a protein Many are components of enzymes Requires 4 genes!

RNA (Ribonucleic Acid) RNA contains ribose sugar, and A,C,G,U nucleotides Almost always single stranded Can fold and pair with itself (not always linear)

Transcription Overview mRNA is “transcribed” from DNA DNA serves as a template to make a strand of messenger RNA (mRNA) that is complementary to the DNA strand

Translation Overview “Translation” of the mRNA blueprint into a functional polypeptide Takes place in ribosomes

These Processes All organisms ever studied use the same process of coding It evolved very early on, before our ancestors diverged We can use bacteria to synthesize human proteins Tobacco plant expressing firefly protein

Transcription

Initiation RNA Polymerase binds to a region of the DNA called the promoter with the help of transcription factors Ensures we only transcribe the necessary gene

RNA Elongation RNA polymerase makes an mRNA strand complementary to the DNA Only one strand is used

Termination A terminator signal in the DNA causes the end of transcription The polymerase breaks away and the mRNA is released

Eukaryotic Cells Modify RNA The product of transcription is called pre-RNA or primary transcript Gets modified or processed before leaving the nucleus

Alteration of mRNA ends The 5' end (beginning of RNA strand) has a “guanine cap” added to it Protects mRNA and attaches to ribosomes The 3' end receives a poly A tail (30-200 A nucleotides) Facilitates leaving of nucleus, as well as protection for the end of the strand

RNA Splicing Long, noncoding sequences of RNA that are not necessary called introns are removed Coding segments called exons are spliced together

Accomplished by a complex of molecules called a spliceosome Splicing continued Accomplished by a complex of molecules called a spliceosome

What is the Purpose of Introns? Different segments of the gene can be expressed at different times (some segments may be introns at times, and exons at other times) Some parts of the protein, or domains, function independently. Introns provide an opportunity for new combinations of segments Allows multiple proteins to be made from one gene instructions

Nucleotide Triplets mRNA molecules can be viewed as a “sentence” of nucleotide triplets Called a codon Each codon will code for 1 amino acid

Codons form Reading Frames …UCAUGGCAGUCAGUC… Could be read as …UCA UGG CAG UCA GUC… Or …U CAU GGC AGU CAG UC… OR …UC AUG GCA GUC AGU C…

The Start Codon sets the reading frame …UCAUGGCAGUCAGUC… Translation starts at the start codon, to ensure the correct reading frame So the mRNA should be read as AUG GCA GUC AGU C…

Translation

Translation, The Synthesis of Polypeptides Messenger RNA carries code to ribosomes Transfer RNA or tRNA carries amino acids to the ribosomes Amino acids are assembled into a polypeptide

Unique Proteins Each amino acid has slightly different properties The sequence of amino acids thus creates proteins with different structures and functions

Transfer RNA tRNA carries an amino acid on one end and an anticodon on the other The anticodon is complementary to codons on the mRNA e.g. anticodon UAC will bond to AUG (the start codon)

tRNAs carry the corresponding amino acid for each mRNA codon

Ribosomal Binding Sites mRNA bonding site P site holds the growing polypeptide A site holds the next amino acid to be added E is the exit site for tRNA Ribosomes connect mRNA and tRNA

Translation Initiation Small ribosomal subunit binds to mRNA tRNA carrying Met binds to AUG (start codon) Large ribosomal subunit attaches

Termination STOP Codons do not code for amino acids When they are reached, polypeptide is released

Protein Structure Depends on More than Genes Genes determine the primary structure (sequence of AAs) Chaperone proteins may help form particular structures Also can be modified by enzymes before becoming functional

Transcription/Translation Song! Option 1 Option 2 Work alone or with one partner Make a song parody explaining transcription and translation Read it or sing it to the class Work in a group of 3-5 people Make a song parody explaining transcription and translation Make a music video and present it to the class