Bi1_2009_Lecture7_full

Bi1_2009_Lecture7_full - Macromolecular structures •...

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Unformatted text preview: Macromolecular structures • Nucleic acids –DNA –RNA • Carbohydrates • Lipids • Proteins Proteins: Sequence --> Structure --> Function Anfinsen Experiment: Denature ribonuclease (RNase) Remove denaturant Assay for RNase activity -- does the protein regain its 3-D structure and its enzymatic activity? Molecular chaperones - Anfinsen cages for folding proteins Xu et al., 1997, Nature 388: 741 GroEL-GroES-(ADP)7 complex Clicker question: A good design for a stable folded protein is… 1) A polar/charged core with mostly nonpolar residues on the surface. 2) A nonpolar core with mostly polar/charged residues on the surface. 3) An even mix of polar/charged and nonpolar residues in the core and on the surface. 4) Fatty acids on the inside, ribonucleotides on the outside. 5) Ralph Lauren. Clicker question: A good design for a stable folded soluble protein is… 1) A polar/charged core with mostly nonpolar residues on the surface. 2) A nonpolar core with mostly polar/charged residues on the surface. 3) An even mix of polar/charged and nonpolar residues in the core and on the surface. 4) Fatty acids on the inside, ribonucleotides on the outside. 5) Ralph Lauren. The Protein Folding Problem: the sequence of a protein cannot (yet) be used to predict its 3D structure ? Protein Structure Prediction “Critical Assessment of techniques for Structure Prediction” (CASP 9) -- a competition For more information or to enter, see http://predictioncenter.org/ Winners earn an automatic “A+” in Bi 1 (retroactively, if necessary) • Given a structure (or a functionality), identify an amino acid sequence whose fold will be that structure (exhibit that functionality). • Can we make designer proteins with desired functions? Central dogma II -- Transcription • DNA -> RNA -> protein • But for retroviruses, DNA can also be made by reverse transcription of RNA. • To understand the lifecycle of a retrovirus, we need to know more about how DNA is replicated and transcribed, and how RNA is translated into protein. Vocabulary • Replication -- copying DNA before cell division • Transcription -- making an RNA copy (messenger RNA or mRNA) of DNA. Note -- Transcription involves copying in the same language (e.g., court transcription). • Translation -- making a protein from the mRNA. Note -- The nucleic acid language is being translated into the protein language. replication DNA ---------> DNA transcription translation DNA ---------> RNA ----------> Protein reverse RNA ---------> DNA transcription Transcription: DNA --> RNA • DNA replication ensures that genetic information is passed on unchanged from a cell to its descendents. • The major thing cells do with genetic information is use it to encode PROTEINS. • Every cell contains all of an organism’s genes, so each cell could (in theory) make every protein. But which proteins are made is determined by which genes are copied into RNA. Cells express appropriate levels of proteins by regulating transcription and translation Two amplification steps: one gene can make many RNA copies, and one RNA can be translated into many proteins. RNA is a nucleic acid similar to DNA, but with critical functional differences The sugar in RNA contains an additional OH group, so is ribose, not deoxyribose. Most important: RNA can basepair with RNA or with single-stranded DNA, but it is normally single-stranded rather than double-stranded. RNA uses uracil instead of thymine. Uracil can still basepair with Adenine. Single stranded RNA can fold into complicated 3D shapes resulting from intramolecular basepairing 07_05_RNA.jpg Hairpin structures result from regions of sequence that are complementary to each other (inverted repeats). Structure of a ribozyme, an RNA enzyme Clicker question When complementary RNA bases in an inverted repeat pair with each other, they form which type of structure? 1) 2) 3) 4) 5) 6) Parallel double helix Anti-parallel double helix Parallel -sheet Anti-parallel -sheet -helix Any of the above are possible Clicker question When complementary RNA bases in an inverted repeat pair with each other, they form which type of structure? 1) 2) 3) 4) 5) Parallel double helix Anti-parallel double helix Parallel -sheet Anti-parallel -sheet Any of the above are possible Clicker question 3D structures of RNA are easier to predict than 3D structures of proteins. 1) 2) True False Base-pairing makes prediction of RNA structure reasonably accurate • Many web-available structure prediction programs e,g., http://www.santafe.edu/~pth/rna.html • Can design and make DNA-based origami using single stranded DNA: http://www.dna.caltech.edu/~pwkr/ Giant DNA complexes, imaged with an atomic force microscope (measures surface topography). False color used to indicate height. Scale: 100s of nanometers. Images taken from Paul Rothemund’s website: http://www.dna.caltech.edu/~pwkr/ The predictability of Watson-Crick base pairing allows design of DNA-based digital logic circuits • Erik Winfree’s lab at Caltech has designed and demonstrated function of AND, OR, and NOT gates, signal restoration, amplification, feedback and cascading. • We may use DNA circuits someday instead of, or in addition to, silicon-based electronics. Seelig et al., 2006, “Enzyme-free Nucleic Acid Logic Circuits, Science 314: 1585-1588. DNA is transcribed into RNA by the enzyme RNA polymerase Note that RNA polymerase unwinds a short region of the DNA double helix. Transcription video from DNA interactive Clicker question It normally takes a large amount of free energy to separate DNA strands -- e.g., you would need to heat DNA to nearly boiling temperature in the lab to separate strands. RNA polymerase works at 37˚C, so how is it able to unwind the DNA in a closed circular bacterial chromosome while it transcribes? 1) 2) 3) 4) RNA polymerase has an associated helicase, which uses the energy of ATP to force the DNA to unwind. The RNA-DNA helix has a lower free energy than a DNA helix, so the formation of the RNA-DNA helix displaces the other DNA strand. RNA polymerase degrades one of the DNA strands, so that the remaining strand can pair with the RNA strand. Bacterial DNA is negatively supercoiled (i.e., underwound so that it has more than 10.4 basepairs/turn), therefore unwinding a short stretch of double helical DNA is energetically favorable. Double stranded DNA can wrap around itself to form supercoiled structures To convert between 1 & 2 requires breaking and rejoining of a bond. This can be accomplished by a topoisomerase enzyme. No breaking of bonds is required to convert between 2 & 3. 2 1 Change in the linking number No change in the linking number 3 The topology of DNA is studied by mathematicians. http://en.wikipedia.org/wiki/DNA_supercoil RNA polymerase II movie Roger Kornberg received the 2006 Nobel Prize in Chemistry for studies on the “molecular basis of eukaryotic transcription”. Comparison of RNA and DNA polymerases • Both catalyze similar chemical reactions: formation of phosphodiester bonds linking nucleotides. – RNA polymerase links ribonucleotides. Final product is single-stranded RNA. – DNA polymerase links deoxyribonucleotides. Final product is double-stranded DNA. • RNA polymerase error rate: 1/104 nucleotides DNA polymerase error rate: 1/107 nucleotides Which is potentially most serious, errors made by 1) 2) 3) 4) 5) 6) DNA polymerase RNA polymerase Ribosome Bi1 students Bi1 TAs Bi1 professors Three major types of RNA • mRNA (messenger RNA) codes for proteins. • rRNA (ribosomal RNA) forms part of the ribosome, a machine involved in translation of RNA into protein. (Next lecture) • tRNA (transfer RNA) binds amino acids and bring them to the ribosome during protein translation. (Next lecture) Signals in DNA tell RNA polymerase where to start and stop transcription • Synthesis starts at a promoter, a conserved sequence 5’ of a gene. • Chain elongation occurs until RNA polymerase encounters a termination sequence on the DNA. • RNA polymerase releases the single stranded RNA and the double stranded DNA template upon encountering a termination sequence. • This slide depicts a bacterial RNA polymerase. Transcription initiation in eukaryotic cells is more complicated. Promoter and terminator sequences in bacteria Clicker question Propose a mechanism by which the highlighted areas could cause transcription to stop. 1) 2) 3) 4) 5) The highlighted sequences contain stop codons. RNA polymerase stops at pink sequences. The highlighted sequences contain DNA mismatches, which destabilize RNA polymerase. The first highlighted sequence contains a binding site for a termination factor, which knocks RNA polymerase off the DNA. The first two sequences are inverted repeats of each other, thus a hairpin, which is a signal for termination. Clicker question Propose a mechanism by which the highlighted areas could cause transcription to stop. 5) The first two sequences are inverted repeats of each other, thus a hairpin, which is a signal for termination. A hairpin can form in the DNA after RNA polymerase separates the two DNA strands. What is the same in all* somatic cells of your body? 1) 2) 3) 4) 5) 6) DNA RNA Proteins Lipids Carbohydrates Lysosomes * Some immune cells are interesting exceptions. The DNA in differentiated cells contains all the instructions to make a new organism This is reproductive cloning -- how Dolly the Sheep was created. In theory, this procedure could be done in humans, but it would be unethical because most of the embryos are abnormal. • Cell types differentiate during development of a multicellular organism to become specialized (e.g., muscle, nerve, brain, blood cells are different from each other), but they contain the same DNA. • What is different is that they express different genes, so they accumulate different sets of RNA and protein. The DNA in differentiated cells contains all the instructions to make a new organism • In therapeutic cloning, the object is to generate embryonic stem cell lines with the desired genome (i.e., from a particular person). Cells regulate expression of proteins at many levels We will discuss transcriptional control and one form of translational control after the translation lecture. ...
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This note was uploaded on 09/25/2010 for the course BIO 1 taught by Professor Bakorman during the Spring '09 term at Caltech.

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