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Ch. 10 DNA, RNA, PROTEIN SYNTHESIS Discovery of DNA • In 1928, Fredrick Griffith (a British medical officer) was studying a bacterium called Streptococcus pneumoniae. – He was trying to develop a vaccine for the virulent strain of the bacterium. – The virulent strain is protected by a polysaccaharide capsule which will protect it from the body’s defense system. – He grew 2 strains in petri dishes the virulent strain was called the S strain; the non-virulent was called the R strain and lacked a capsule. © 2008 by Sinauer Associates, Inc.; http://www.nature.com/scitable/topicpage/isolating-hereditary-material-frederick- • Griffith concluded that some hereditary information could be transferred from the virulent bacteria to the non-virulent which would transform the non-virulent into the virulent strain. – Transformation- is the transfer of genetic material from one cell/organism to another cell/organism. • In 1940’s Oswald Avery, an American researcher, and his colleagues wanted to determine what the transformation agent in Griffith’s experiment was: Protein, DNA, or RNA. – The scientists used separate enzymes in heat-killed S cells to destroy the three molecules separately to determine agent. • Used protease enzyme to destroy the protein • Used RNase enzyme to destroy the RNA • Used DNase enzyme to destroy the DNA – They then mixed the individual heat-killed S cells batches separately with live R cells then injected the mixture into mice. • The researchers found the cells missing protein and RNA were able to transform the R cells into virulent S cells. • The cells absent of DNA did not transform R cells. • Conclusion: DNA was responsible for the transformation in bacteria. • In 1952, Martha Chase and Alfred Hershey, American researchers, researched whether DNA or protein was the hereditary material a virus transfers when the virus enters a bacterium. – Viruses which infect bacteria are known as bacteriophages. • They utilized E. coli (Escherischia coli) and determined DNA was the hereditary material utilized. DNA Structure • In the 1950’s, James Watson and Francis Crick teamed together to determine the structure of DNA. – By 1953 they had concluded DNA was made of 2 strands and was shaped like a spiral staircase and they illustrated their findings in a model which was correct and explained how DNA replicated. – Watson and Crick utilized other scientists work and findings, to direct their modeling research, such as X-Ray diffraction by Maurice Wilkins and Rosalind Franklin. • In 1962, Watson, Crick, Wilkins received the Nobel Peace Prize in Medicine for their work Franklin was not included as she had passed away in 1958 no one is honored with a Nobel Award post-humously. • DNA- Deoxyribonucleic Acid – Nucleic Acid made of 2 strands in the form of a double helix (spiral staircase) • Consists of nucleotides which has 3 parts: – A phosphate group – A 5-carbon sugar (Deoxyribose) – 1 of 4 different Nitrogenous bases » Adenine- Purine (double ring) always binds w/ Thymine » Thymine- Pyrimidine (single ring) always binds w/ Adenine » Guanine- Purine; always binds w/ Cytosine » Cytosine- Pyrimidine; always binds w/ Guanine • The double helix is held together by hydrogen bonds between the nitrogen bases attached to the 2 different strands. – The nitrogenous bases make the steps of the spiral staircase structure. – The rails (backbone) consists of phosphate groups bound to the 5carbon sugars; the 5-carbon sugars bond to the nitrogenous bases. http://library.thinkquest.org/27819/ch6_3.shtml X-ray diffraction image from: http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookdnamolgen.html Watson and Crick and their tin-and-wire model of DNA, image from: http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookdnamolgen.html • In 1949, the nitrogenous bases were found to be in complimentary percentages by Erwin Chargaff. – He discovered that adenine was always present in the same percentages as thymine; he also found the remaining percentage of bases consisted equally of guanine and cytosine. – These pairs of bases are known as complimentary base pairs. – The observations led to the development to base-pairing rules in DNA. • Adenine – Thymine (A – T) Cytosine – Guanine (C – G) • The order of nitrogenous bases on a DNA chain is known as base sequence. • Ex: A C C T G T G A G A C TGGACACTCTG • DNA Replication – Is the process by which DNA is copied within a cell before a cell undergoes division. – Step 1: Helicases (an enzyme) separate the DNA strands by breaking the Hydrogen bonds which connect the nitrogenous bases. Creating the replication fork (the Y-shaped region created when the DNA strands separate. – Step 2: DNA Polymerases (an enzyme) bring free floating nitrogenous bases to the now exposed DNA strands creating new hydrogen bonds between the original strands and the new bases creating 2 new DNA molecules. – Step 3: DNA Polymerase finish creating replicating the 2 exposed strands and then falls off, resulting in 2 new DNA molecules. • Replication occurs at the replication fork in two separate directions at the same time; always in the 3’ to 5’ direction. – Replication errors are rare about 1 in 1 billion paired nucleotides added (similar to typing our Biology textbook 1000 times!) • Protein Synthesis – 1st begins with the synthesis of RNA (Ribonucleic Acid) called Transcription. – Transcription is the process of RNA polymerase (an enzyme) copying certain sections of DNA. – These sections are known as genes which control particular enzymes or proteins. • The polymerase will begin transcription at a promoter and end at a termination signal. (this would be the length of a gene) – RNA is a single strand of nucleotide bases. • RNA nitrogenous bases: Cytosine – Guanine Adenine – Uracil • There are 3 types of RNA: – mRNA: MessengerRNA - carries the message to be translated. – tRNA: TransferRNA - translates the message found w/in mRNA. – rRNA: RibosomalRNA - structural component of ribosomes. • The newly formed RNA molecule (mRNA molecule) is encoded with valuable instructions called the genetic code. – The genetic code is the base sequence of an RNA molecule as it relates to the 3 base sequences called codons which represent amino acids. • Each codon represents a start or stop signal, or an amino acid. – AUG (start codon) – Methionine – UAA, UAG, UGA (stop codons) • There are 64 mRNA codons which code for 20 amino acids. – Amino acids are the subunits of polypeptides (proteins). • The # and sequence of amino acids will determine what protein is created. • Translation – The synthesis of a protein from the message carried within the mRNA molecule. – mRNA molecule attaches to ribosome where tRNA molecules w/ an anticodon subunit on one end which is complementary to a codon on the mRNA strand carries the amino acid coded for by the mRNA codon http://www.wiley.com/college/boyer/0470003790/structure/tRNA/trna_intro.htm • tRNA molecules bind to the mRNA molecule one at a time, when the 2nd attaches the first transfers its amino acid to the 2nd’s amino acid by forming a peptide bond. – After the 1st tRNA releases its amino acid it will release from the mRNA molecule. Each subsequent tRNA molecule connection will restart the events above. – The synthesis of the protein will end when the ribosome reaches a stop codon at which point the newly formed protein is released. • DNA DNA = Replication • DNA RNA = Transcription • RNA Protein = Translation DNA 5-Carbon Sugar: RNA Deoxyribose Nitrogenous Base: Adenine – Thymine Guanine – Cytosine Structure: Function: Double Helix Genetic Information (Heredity) Ribose Adenine – Uracil Guanine – Cytosine Single Strand Protein Synthesis