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How does DNA work? What is a gene?

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Presentation on theme: "How does DNA work? What is a gene?"— Presentation transcript:

1 Protein Synthesis: DNA Transcription into mRNA, RNA Processing and mRNA Translation into Protein

2 How does DNA work? What is a gene?
A gene is a segment of DNA that codes for a certain characteristic or trait. Ex) Blue eyes, Black hair, Dark skin, etc. How do you get from a sequence of DNA to having black hair? To have black hair, your body must produce vast amounts of the dark pigment melanin. How do you make melanin? Your cells must have the basic materials to build melanin and the enzymes needed to carry out the synthesis of melanin. Similarly, most jobs the cell needs to carry out depend upon the use of enzymes or other PROTEINS. So, to get from a gene (or genotype) to a specific trait (phenotype) requires the action of specific proteins. The way genes work is every gene codes for the creation of a different protein, each with its own function. Gene X --> Protein X --> Trait X

3 From Gene to Protein Where is the DNA in a eukaryotic cell? Where are proteins made? Problem: DNA is in the _____________ while proteins are made in the ________________ (at structures called ______________). In addition, DNA is too large to fit through the nuclear pores…frustrating! How do we get the information in our DNA (our genes) out to the cytoplasm so we can make proteins? Solution: Use a messenger! In the cell, that messenger is RNA (mRNA). nucleus cytoplasm ribosomes

4 Gene to Protein in Two Steps
Step 1: _____________ (in nucleus) Makes copie(s) of a segment of DNA (a gene) in the form of mRNA. Step 2: ____________ (at ribosome) Translates the sequence of nucleotides in mRNA into a sequence of amino acids (a protein!). Transcription transcription Translation translation

5 DNA  mRNA  Protein How is a DNA sequence transcribed into an RNA sequence? Same as DNA-DNA base pairing rules except… (look left) DNA RNA C pairs with ____ G ____ T ____ A ____ G C A U

6 TRANSLATION: From nucleic acid to protein
How is the information for making a protein encoded? There are ___ different amino acids used to build proteins. But, there are only ___ RNA nucleotides. If we read the RNA one base at a time, that would mean we could only code for ___ amino acids. For example: Base Amino Acid Adenine --> glycine Cytosine --> tryptophan Guanine --> alanine Uracil --> phenylalanine 20 4 4 We wouldn’t have enough bases to code for 20 amino acids!

7 Why a triplet code?? What if we “read” the RNA two bases at a time? How many unique two-letter code words could we make using four bases? AG GG CG UG AC GC CC UC AU GU CU UU AA GA CA UA This give us 42 = ___ unique code words….Is this enough to code for all 20 amino acids? _____ So what about three-letter code words? That should give us 43= ___ unique code words… Enough? 16 No! 64 Yes, more than enough…

8 The Genetic Code, Revealed
A set of three nucleotides in an RNA sequence, called a codon, codes for the addition of an amino acid in a polypeptide. GUG = _____ ACU = ________ UUA = ________ AUG = ________ UAA,UGA,UAG = __________

9 The Genetic Code, Revealed
A set of three nucleotides in an RNA sequence, called a codon, codes for the addition of an amino acid in a polypeptide. GUG = _____ ACU = ________ UUA = ________ AUG = ________ UAA,UGA,UAG = __________ valine threonine leucine Methionine or START codon STOP codons

10 The Genetic Code (how to translate the 64 mRNA codons)
The 3 letter Amino Acid abbreviations Phe = Phenylalanine Leu = Leucine Ile = Isoleucine Met = Methionine Val = Valine Ser = Serine Pro = Proline Thr = Threonine Ala = Alanine Tyr = Tyrosine His = Histadine Gln = Glutamine Asn = Asparagine Lys = Lysine Asp = Aspartic Acid Glu = Glutamic Acid Cys = Cysteine Trp = Tryptophan Arg = Arginine Gly = Glycine

11 Transcription Copying DNA into RNA

12 Binding & Initiation Details

13 Elongation Detail

14 Transcription: Copying DNA into RNA
Initiation: RNA polymerase binds to the promoter sequence at the start of a gene. The promoter sequence frequently includes a sequence of TATAAAA (the “TATA box”). Not all promoter sequences are identical. Elongation: RNA polymerase moves along the template strand, connecting the RNA nucleotides by bonding the sugar-phosphate backbone. Note that RNA is synthesized from its 5’ to its 3’ end. Termination: RNA polymerase lets go of the DNA and releases the mRNA when it gets to the terminator sequence at the end of the gene.

15 Let’s review transcription…
(Click on Protein Synthesis Link)

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