12-1: DNA How was DNA discovered? Fredrick Griffith Oswald Avery Hershey & Chase Watson & Crick
Fredrick Griffith Experimented with bacteria and mice Cultured both harmless bacteria and pneumonia causing bacteria. Exposed mice to a mixture of both harmless and heat-killed pneumonia causing bacteria Mice came down with disease page 288
Griffith’s Conclusion Griffith called it transformation Cells can be transformed when coming into contact with other types of cells Harmless bacteria transformed into disease causing strains. What was this disease causing agent and how did it transform the other cell?
Oswald Avery Set out to answer the previous question. Repeated Griffith's work, took extract from heat-killed bacteria. Treated extract with enzymes that break down: Proteins CHO’s Lipids Even RNA Same results Then repeated with enzymes that break down DNA Result: No transformation
Hershey and Chase Used radioactive markers to determine if proteins or DNA was injected by viruses The tracers could be followed from the virus to the bacteria. Injected with: Phosphorus-32 (in DNA, not in proteins) Sulfur-35 (in proteins, not in DNA) Results: Only Phosphorus-32 was transferred Martha Chase and Alfred Hershey
How did DNA work? At this point, scientists knew DNA was where genes were contained. We still needed to know: 1. How did DNA carry genes generation to generation? 2. How did DNA code for traits? 3. How was copied? Text p. 291
The Components and Structure of DNA Nucleotides 5-carbon sugar Deoxyribose Ribose Phosphate Nitrogen base Example of a nucleotide: Deoxyribose (sugar) + Phosphate group + Cytosine (nitrogen base)
Chargaff’s rule Determined complementary nature of DNA.
http://fig.cox.miami_edu/~cmallery/150/gene/chargeff. Chargaff’s Rule of Base Pairing
Rosalind Franklin X-rays were used to “see” the general structure of DNA Appeared like this:
Watson & Crick Francis Crick and James Watson were working on the shape of DNA After Watson saw Franklin’s X-ray, he knew the shape had to be a double helix Two strands with complementary base pairs in between that are bonded together.
12-2: Chromosomes and DNA Replication Chromosome structure Histones Proteins that chromatin is wrapped around Chromatin Condensed DNA Nucleosome = histones + chromatin
Duplicating DNA “Replication” After mitosis, each new cell gets an exact copy
1. Steps of replication DNA is “unzipped” by a protein called DNA helicase (breaks hydrogen bonds between complimentary bases)
2. Steps in replication DNA polymerase reads the sequence and adds base pairs that complement.
3. Steps in Replication DNA polymerase also proofreads along the existing strand.
4. Steps in Replication DNA ligases “put together” chunks of copied base segments.
12-3 RNA and Protein Synthesis Structure of RNA Single-stranded Ribose-phosphate backbone Contains nitrogen base uracil instead of thymine It bonds with adenine on DNA Klug & Cummings 1997
Types of RNA mRNA – messenger RNA transcription rRNA – ribosomal RNA Make up components of the ribosomes tRNA – transfer RNA translation Text p. 300
mRNA = messenger RNA Transcription DNA gives code to RNA for making proteins Similar to replication except now code is copied to RNA (has uracil) RNA polymerase unzips the DNA strand and begins to add bases that complement one of the strands. YouTube - Transcription
How does transcription know where to start? Promoters Specific sequences of base pairs that RNA polymerase can only bind to in order to initiate transcription.
RNA editing Introns and Exons Introns Sequences that code for nothing Exons Sequences that directly code for sections of proteins Sequences that are “expressed” as protein Eventually enzymes go back and cut the introns out and splice together the exons to have a fully functioning mRNA.
The Genetic Code 20 different amino acids Each is coded for by a segment of 3 base pairs Codon Most amino acids have multiple codons Also codons for starting and stopping transcription
Translation Copying mRNA into a sequence of amino acids. mRNA attaches to ribosome Ribosome “reads” looks for a “start codon” Two tRNA with “anticodon” that complement the strand are attached to mRNA by the ribosome.
Peptide synthesis Temporary hydrogen bonds allow the tRNA to be bound to mRNA long enough to form a “peptide bond” between the two amino acids. YouTube - Translation
Genes and Proteins The proteins that are produced ultimately go to make: Structural proteins (eye color, physical features) Enzymes (control all cellular activities, ex: digesting lactose) Hormones (producing testosterone/estrogen) Combining all of these factors ultimately make us what we are.
12-4 Mutations Mutations – changes in the genetic sequence
Point mutations Occur in one (or few) base(s) of the DNA sequence Includes substitutions Sometimes little to no effect on amino acid sequence, however sometimes effect can be cataclysmic Sickle-cell anemia
Frameshift Mutations Caused by insertions or or deletions of bases, shifting the way the mRNA is read. Shifts the “reading frame” Usually has dramatic effects on the formation of the protein – often rendering it useless
Significance of mutations Most often, mutations ultimately show little to no effect on the protein that is supposed to be made. However, when it does have an effect, the new protein formed can be: 1. Detrimental - Increases organisms chance of dying and not passing the mutated gene on. 2. Beneficial - Increases the organisms chance of survival and reproducibility – therefore passing it down CREATES GENETIC VARIABILITY! This is often how asexually reproducing organisms evolve... a slow process
12-5: Gene Regulation How does an organism “know” when to turn the gene on or off? Example: How does your body turn on the lactase gene?
Regulation of the lac operon in E.Coli Operon Groups of genes that code for specific protein lac operon in E.Coli codes for protein that breaks down lactose
Transcription Again, transcription begins at the sequence called the promoter Just “below” the promoter are “operator” (O) sites These sites are areas where “repressors” can bind In most cases repressors are bound to O site, preventing transcription Turning off the gene Just like a room, when you are not in it – TURN OFF THE LIGHT!
Lac Operon When lactose is present, it binds with the repressor changing its shape, forcing it off the O site Allows RNA polymerase to begin transcription YouTube - Lac Operon YouTube - Lac Operon
TATA box How is this done in eukaryotic cells? Much more complex than lac operon. Involves sequence of base pairs near promoter called “TATA box” Sequence of TATATATA… or TATAAA…. Positions RNA polymerase All cells contain the entire Genetic Code BUT any one cell will use a small fraction of those genes. Heart cells use different genes than brain cells.
Development and Differentiation Cells that change into specific types of specialized cells Hox genes Genes that control this differentiation early in development Mutations involving hox genes can have HUGE effect on outcome of organism Pax 6 Gene found in Drosophila and mice that controls eye development Inserted mouse Pax 6 gene into the “knee” of Drosophila embryo Grew an eye on its leg YouTube - Evolution Genetic Tool Kit YouTube - Evolution Genetic Tool Kit