DNA Ch 8 Standard 4 History of DNA 1. Hershey & Chase proved DNA is the genetic material found in cells Erwin Chargaff DNA composition all 4 bases not in equal quantity bases present in characteristic ratio A = T ~ 40% G = C ~ 60% varies from species to species Rosalind Franklin took x-ray photos of DNA that showed it’s 3-D structure Watson & Crick made the first 3-D model of DNA in 1953 Won the Nobel Prize in 1962 Why is DNA important? It stores genetic information in cells & this information is used to make proteins DNA Structure DNA is a double helix (twisted ladder) DNA is made of nucleotides 3 nucleotide parts 1. Deoxyribose sugar 2. Phosphate 3. Nitrogen base - adenine (A) - thymine (T) - guanine (G) - cytosine (C) Base Pair Rules: - - Adenine binds only with Thymine (AT) Cytosine binds only with Guanine (CG) ----Straight letters together Curvy letters together------- • Deoxyribose Sugars & phosphates make up the sides of the ladder • Phosphodiester bonds hold the sugars to the phosphates • Nitrogen bases are held together by hydrogen bonds A T = 2 hydrogen bonds G C = 3 hydrogen bonds • The Base pairs make up the steps (rungs) of the DNA Ladder Base Pair Shape Purine Double ring bases (Adenine or Guanine) Pyrimadine Single ring bases (Thymine or Cytosine) Difference in their shapes causes the DNA to twist Each Base Pair is made of one Purine and one Pyrimadine Hydrogen Bonds H Thymine H H O C Thymine H Adenine H H C C C N C C N H C N N C O Adenine C N C H N N H H Cytosine Cytosine H N H C Guanine H C N C N C C C H C N C H Guanine C N O H N O N H N Practice If one side of DNA has this base sequence, what does the other side have? ATGGACTAC TACCTGATG DNA Replication How Does DNA Replicate Process discovered by Meselson & Stahl Semi-conservative replication Half old strand, Half new strand Anti-parallel New strands are made in opposite directions When does DNA replicate? During the cell cycle before mitosis & meiosis Why does DNA replicate? So new cells will have their own copy of DNA Six Steps of DNA Replication 1) 2) 3) DNA is unzipped by the Helicase enzyme (breaks hydrogen bond) DNA polymerase enzyme binds DNA begins attaching complimentary nucleotides Same thing happens on the other side, but in opposite direction 4) Bonds Reform Helicase reforms Hydrogen bonds (between bases) Ligase reforms Phosphodiester bonds (sugar & phosphate) 5) DNA polymerase “proofreads” checking each strand for errors. 6) DNA rewinds itself into two identical double helix strands Let’s watch http://www.lpscience.fatcow.com/ jwanamaker/animations/DNA%2 0Lecture.html Once more time….. Protein Synthesis DNA RNA Proteins DNA 1.sugar=deoxyribose 2.bases = A, C, G, T 3.double strand 4.stays in nucleus RNA 1.sugar = ribose 2.Bases=A, C, G,U 3.single strand 4.leaves nucleus 3 Types of RNA 1. mRNA made from DNA in nucleus “messenger” 2. tRNA brings amino acids to the ribosomes; found in cytoplasm “transfer” 3. rRNA part of the ribosome; this is where proteins are made “ribosomal” DNA vs. RNA TRANSCRIPTION How RNA is made from DNA Transcription Steps 1. DNA is unzipped by Helicase 2. 3. 4. all genes start with the code “TAC” RNA polymerase adds RNA nucleotides to the DNA strand mRNA is complete when it reaches a stop code on DNA mRNA then leaves nucleus & carries code into the cytoplasm DNA never leaves the nucleus Don’t Confuse Replication with Transcription! DNA Replication Transcription A–T C–G G–C T-A A–U C–G G–C T-A Transcribe this…… | | | | | | | | | | I Translation Decoding the Message Steps 1. 2. 3. 4. 5. mRNA leave nucleus and enters ribosome mRNA codons read & tRNA brings matching amino acid to the ribosome Amino acids are strung together like beads on a necklace Amino Acids are held together by peptide bonds 1000 or more Amino Acids = protein WHEN WE DECODE DNA Use the “Genetic Code” Convert mRNA 3 letter groupings called codons Example: AUG= Methylamine (Start) The mRNA Code tells us what amino acid each codon codes for. Transcribe and Translate this DNA Strand without looking at your notes TACAGTACCATAATC NOW, Label the DNA, mRNA, tRNA, rRNA Which part is transcription Which is translation ? TACAGTACCATAATC mRNA AUGUCAUGGUAUUAG tRNA UACAGUACCAUAAUC DNA rRNA MET-SER-TRP-TYR-STOP DNA RNA Transcription A, T, C, G Double Strand A, U, C, G Single Strand In The Nucleus Protein Translation Amino Acids Codons & Anti-codons In Cytoplasm Is it all DNA? Entire chromosome is made of DNA Humans~ 3 billion base pairs Only part of the DNA codes for Genes Gene: The DNA that codes for a Protein. There are different parts of the chromosome Exons and Introns Introns are cut out before translation Exons are left, and get Translated to make Proteins GENETIC VARIATION There are some variations in genes ~every 1350th bp (average) Known as polymorphisms “different forms” Different forms of genes are called alleles Effects How well the protein works. EX: Melanin How the protein interacts with another protein or substrate. EX: Enzymes Proteins do the work of the cell Proteins are chains of amino acids. The amino acids are coded by the bases that make up that gene Each gene codes for a different protein. Gene Expression All cells within an organism have the same DNA and genes. What makes cells different from each other is that different genes are turned on and turned off in different cells. Ex: Pigment in eyes or skin Ex: Keratin in nails or hair How can organisms be different from each other if their DNA is made of the same nucleotides? •Two individuals DNA are different because of the order of nitrogen bases •The more closely related two organisms are, the more alike the order of nucleotides in their DNA will be.