DNA: Molecule of Life History DNA Structure Protein Synthesis Gene Regulation
History of DNA H I S T O By the 1940 s, scientists knew that chromosomes consisted of both DNA and protein but did not know which was the genetic material. Scientists wanted it to be protein since they knew more about its structure. The following experiments shed some light. R Y
Animation here Heat-killed, diseasecausing bacteria (smooth colonies) Disease-causing bacteria (smooth colonies) Harmless bacteria (rough colonies) Heat-killed, diseasecausing bacteria (smooth colonies) Control (no growth) Harmless bacteria (rough colonies) Dies of pneumonia Lives Lives Live, disease-causing bacteria (smooth colonies) Dies of pneumonia Harmless, live R strain somehow became disease causing S strain transformation HOW?
Bacteriophage with phosphorus-32 in DNA Phage infects bacterium Radioactivity inside bacterium Bacteriophage with sulfur-35 in protein coat Phage infects bacterium No radioactivity inside bacterium
Hershey Chase Experiment
D N A Phosphate group Deoxyribose sugar Nitrogen bases Nucleotide: the monomer (repeating unit) of DNA S T R U C T U R E Sugar-phosphate backbone What limitations exist in our model of DNA? Nucleotide Key Hydrogen bonds-very weak Adenine (A) Thymine (T) Cytosine (C) Guanine (G)
D N A Chromosome Nucleosome Coils DNA packing (ExpLIFE 12.1) DNA double helix S T R U C T U R E Supercoils Histones Chromatin: DNA unassociated with structural proteins. Chromosome is made up of a DNA-Histone protein complex. Compacts a very long (2m) sequence of DNA.
D N A R E P L I C A T I O N Helicase unwind and separate the double helix, breaking weak H-bonds: replication fork DNA Replication Animation DNA polymerases rebuild each strand adding nucleotides to each side according to basepairing rules. Each new double helix consists of one old strand linked to one new DNA strand.
D N A R E P L I C A T I O N Errors sometimes occur (about 1 error/10,000 pairs) If a mismatch occurs, the DNA polymerase can backtrack, remove the incorrect nucleotide, and replace it with the correct one.
Prokaryotic vs. Eukaryotic What observations can you make between the Pro/Eukaryotic DNA? Eukaryotic chromosomes are so long that it would take 33 days to replicate a typical human chromosome if there were only one origin of replication. Human chromosomes are replicated in about 100 sections that are 100,000 nucleotides long, each section with its own starting point. Because eukaryotic cells have multiple replication forks working at the same time, an entire human chromosome can be replicated in about 8 hours.
What are the critical characteristics of DNA that allow both the lasting storage of information and the transfer of information through copying (replication)? What are the dis/advanages of an information transfer system that uses as physical pattern, or template?
How does the DNA store and transmit these messages? A particular sequence (a gene) along a strand of DNA codes for the production of a specific protein. In this activity, you will explore the production of one protein produced by your stomach. This protein, pepsinogen, is an inactive form of an enzyme that aids in digestion. Exp Life CD: 11.4
Ribonucleic Acid (RNA) Click to play the video. DNA RNA Double Strand Deoxyribose Thymine Single Strand Ribose Uracil
Gene Transcription and Translation Prokaryotes vs. Eukaryotes Gene processing animation RNA processing
Transcription: DNA RNA DNA template Non-template RNA strand Click to play the video. TAC AAG TTT GAC CAT ATG TTC AAA CTG GTA AUG UUC AAA CUG GUA DNA DNA unwinds RNA RNA polymerase copies at a rate of 60 nucleotides/sec
DNA RNA RNA polymerase Translation observation (BSCS DVD 3 ) Translation details Nucleus Phenylalanine trna Lysine mrna Methionine Ribosome mrna Start codon
The Polypeptide Assembly Line The ribosome joins the two amino acids methionine and phenylalanine and breaks the bond between methionine and its trna. The trna exits, allowing the ribosome to bind to another trna. The ribosome moves along the mrna, binding new trna molecules and amino acids. Growing polypeptide chain trna Ribosome Lysine trna mrna mrna Ribosome Translation direction Completing the Polypeptide The process continues until the ribosome reaches one of the three stop codons. The result is a growing polypeptide chain.
Codons in mrna Start codon
Mutation - changes in DNA DNA template 1 2 3 4 5 TAC GCA TGG AAT ACT ATG CGT ACC TTA TGA RNA transcript AUG CGU ACC UUA UGA Amino acids Met Arg Thr Leu stop Substitution codon 2 as GTA Substitution codon 2 as GCG Insertion codon 1 TATC Deletion nucleotide Point Mutation Silent Mutation Frame Shift Mutation
Normal hemoglobin GGG CTT CTT TTT Sickled hemoglobin GGG CAT CTT TTT Transcribe and translate the two DNA sequences. What kind of mutation does this represent? Protein structure As a result of this mutation, the proteins are also misshapen causing sickled cells to clog capillaries and prevent normal flow of blood to body tissues, causing severe pain. Explain why the correct sequence is important to protein synthesis. Assume the base sequence is GGG CTT CTT AAA Would this result in sickled hemoglobin? Explain.
Chromosomal Mutations (ExpLIFE 12.2) Deletion Duplication Inversion Translocation How can the movement of genes take place? Which mutation variety would be most damaging?
Regulation of Protein Synthesis Every cell in your body, with the exception of gametes, or sex cells, contains a complete copy of your DNA. Why, then, are some cells nerve cells with dendrites and axons, while others are red blood cells that have lost their nuclei and are packed with hemoglobin? Why are cells so different in structure and function? If the characteristics of a cell depend upon the proteins that are synthesized, what does this tell you about protein synthesis?
Typical Gene Structure lac operon (ExamView) Exploring a stretch of code of DNA Chromosome 11 Fly-over Regulatory sites Promoter (RNA polymerase binding site) DNA strand Start transcription Stop transcription
Complexities of Gene Expression The relationship between genes and their effects is complex. Despite the neatness of the genetic code, every gene cannot be simply linked to a single outcome. Some genes are expressed only at certain times or under specific conditions. Variations and mistakes can occur at each of the steps in replication and expression. The final outcome of gene expression is affected by the environment of the cells, the presence of other cells, and the timing of gene expression.
Section 12-5 1. Do you think that cells produce all the proteins for which the DNA (genes) code? Why or why not? How do the proteins made affect the type and function of cells? 2. Consider what you now know about genes and protein synthesis. What might be some ways that a cell has control over the proteins it produces? 3. What type(s) of organic compounds are most likely the ones that help to regulate protein synthesis? Justify your answer.