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CHAPTER 10: DNA,RNA & Protein Synthesis. I. Discovery of DNA. Scientist originally believed PROTEINS would be the molecules which contained hereditary information. Some scientists who did experiments that proved DNA had genetic information: 1. Fredrick Griffith 2. Oswald Avery
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I. Discovery of DNA • Scientist originally believed PROTEINS would be the molecules which contained hereditary information. • Some scientists who did experiments that proved DNA had genetic information: • 1. Fredrick Griffith • 2. Oswald Avery • 3. Hershey & Chase
James Watson & Francis Crick • In 1953 DNA structure discovered • Double Helix model. • (2 chains of DNA) • Showed how DNA could replicate. • Relied on work of other scientists: • Rosalind Franklin & Maurice Wilkins took X-ray photos of DNA structure • ( Franklin died 1958 before Watson & Crick received Nobel Prize. 1962.)
II. DNA structure • Double Helix shape is formed by base pairs attached to a sugar-phosphate backbone. -
B. Parts of Nucleotides 1. 5 carbon sugar in DNA- deoxyribose (in RNA= ribose) 2. phosphate group 3. nitrogenous bases ( there are 4 different ones) • Adenine • Guanine • Cytosine • Thymine (in RNA- no thymine- Uracil is the base)
www.molecularstation.com/dna/dna-structure/ C. How Chemical Bonds hold DNA together • Covalent bonds- between sugar & phosphates of 2 nucleotides • Hydrogen bonds- between complementary nitrogenous bases –
D. Base Pairing Rules • Adenine always bonds with Thymine (AT) • Guanine always bonds with Cytosine (GC) • Note: 3 hydrogen bonds: GC • 2 Hydrogen bonds: AT
Must have 1 purine (a 2 ring shape) plus 1 pyriomidine (1 ring shape) in each pair or the would not “fit” inside ladder of DNA • 2. Purines (A, G) double C ring • 3. Pyrimidines (T, C) single C ring
III. DNA Replication A. Is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis or binary fission.
oak.cats.ohiou.edu/.../Heredity/Heredity.htm B. Steps in Replication 1. Helicase enzymes -separate hydrogen bonds in strands – create replication fork 2. Attach- DNA polymerase enzyme -adds nucleotides 3. Release –DNA polymerase enzyme – now have 2 identical DNA strands
C. Errors • DNA replication is very accurate. • Errors occur ~ 1 in 1 billion paired nucleotides. • “Proofreading” enzyme checks for “spelling” errors. *If a mistake does occur- new DNA is different: • Mutation- a change in the nucleotide sequence of a DNA molecule. • Caused by chemicals, radiation,UV rays. • Mutations can be favorable • - or harmful. (example- cancer)
IV Protein Synthesis • Flow of Information (DNA – RNA - Proteins) Before protein can be synthesized, the instructions in DNA must first be copied to another type of nucleic acid called messenger RNA. Then -a group of 3 nucleic acids codes for an amino acid & it is built at the ribosomal RNA with help from the transfer RNA
RNA differs from DNA in the following ways: • RNA is single stranded while DNA is double stranded. • RNA has a sugar called ribose while DNA has a sugar called deoxyribose. • RNA has the nitrogenous base uracil while DNA has the base thymine.
B. 3 types RNA: 1. messenger RNA(mRNA) 2. transfer RNA (tRNA) 3. ribosomal RNA (rRNA)
Messenger RNA, or mRNA. carries the code for building a protein from the nucleus to the ribosomes in the cytoplasm. It acts as a messenger. • Transfer RNA or tRNA. picks up specific amino acids in the cytoplasm & brings them into position on ribosome where they are joined together in specific order to make a specific protein. • Ribosomal RNA or rRNA place for protein synthesis
C. Steps in Transcription-making RNA 1. RNA polymerase (enzyme)– binds to promoter area on DNA 2. Nucleotides added & joined by the enzyme (RNA polymerase) • Termination signal- stop- RNA polymerase releases both DNA & new RNA molecules
D. Proteins • Review of protein structure -recall that proteins are made of amino acids joined together with peptide bonds -there are 20 different amino acids, the order they are joined determines the structure & function of the proteins. -proteins can be very large, complicated molecules
mRNA codons for specific amino acids • Each 3 nucleotide sequence (letters) in mRNA encodes for 1 specific amino acid, or a “start” or “stop” signal. • Each 3 nucleotide group is called acodon. • The genetic code- means the rules that relate how a particular sequence of nitrogenous bases corresponds to a particular amino acid.
E. Steps in Translation • Initiation- tRNA & mRNA join together. (The codon is on the mRNA, the “anticodon” is on tRNA) The tRNA has an amino acid attached to it) • Elongation- continued as ribosome moves the distance of 1 codon on mRNA • Elongation is built with new tRNAs attaching each amino acid as it reads the codons on the mRNA. • Termination- ribosome reaches “stop” codon on the mRNA • Disassembly – each piece is free. (see sequence page 208-209 in textbook)
Remember: • Replication- copying DNA from DNA • Transcription- making RNA from DNA • Translation- making proteins