11.1 Genes are made of DNA Frederick Griffith studying two strains of bacteria One strain fatal to mice, while other strain harmless Griffith’s experiment Strain 1 injected = mouse dies Strain 2 injected = mouse healthy Heat treated Strain 1 = mouse healthy Mixture of Strain 2 and Heat treated Strain 1 = mouse dies Harmless bacteria had been “transformed” becoming deadly
Avery Experiment Destroyed proteins Mice still died with mix
Avery shows DNA is the transforming factor Attention was focused on two types of chemicals: protein and DNA Scientists already knew that chromosomes consist of DNA and protein Avery took Griffith’s experiment one step farther. Treated mixture of heat treated deadly strain and live harmless strain with protein destroying enzymes Bacterial strains still transformed Conclusion was protein cannot be transforming factor Next, treated mixture with DNA destroying enzymes This time colonies failed to transform Avery concluded DNA is genetic material of the cell
Virus experiments provide more evidence Many scientists still skeptical after Avery’s findings Protein has more complexity (20 different amino acid building blocks) DNA less complexity (4 nucleotide building blocks) “too simple” Hershey and Chase experiment using viruses Virus- package of nucleic acid wrapped in protein coat Bacteriophage- (AKA-phage) virus that infects bacteria Virus (phage) they worked with had 2 basic components: DNA on inside and coat made of protein on outside Use of radioactive sulfur for protein coat and radioactive phosphorus for DNA for the phages Radioactivity was detected outside the infected bacteria for protein but inside the infected bacteria for the DNA Conclusion: DNA must carry genetic information responsible for producing new phages
11.2 Nucleic Acids store information A. People involved with discovering DNA’s structure 1. Rosalind Franklin & Maurice Wilkins –1950, photographs of the DNA molecule using X-ray crystallography which showed the shape to be a helix 2.Erwin Chargaff – 1951, proved that the % of A = T and % of G = C 3. James Watson & Francis Crick – 1953, used data from the other scientists and built models to finally figure out the exact structure of DNA…. 1962 won the Nobel prize in Medicine/Physiology
DNA Structure a. DNA is a double helix (twisted ladder) made of subunits called nucleotides b. 4 different nucleotides Adenine, Thymine, Guanine, Cytosine *Nucleotides are made of 3 parts: 5-carbon sugar (deoxyribose) phosphate group nitrogenous base c. The sides of the double helix are made of alternating sugars and phosphate groups, the rungs of the double helix are made of nitrogenous bases Complementary Base Pairs in making the double helix A pairs with T G pairs with C
11.3 DNA REPLICATION DNA replication is the process of copying the DNA molecule. During DNA copying, the two strands of the double helix separate. Each single strand acts as a “negative” or “template” for producing a new, complementary strand.
Easy steps for DNA Replication. Enzymes are protein molecules that catalyze chemical reactions in a cell – usually any protein ending in “ase” is an enzyme 1. DNA replication begins at specific sites called the origins 2. DNA helicase unwinds and separates the two strands of original DNA molecule 3. DNA polymerase 3 adds complementary nucleotides to each separated strand 4. DNA polymerase 1 checks for correct pairing of nucleotides (looks for mutations) and fixes any mispaired nucleotides
11.4 and 11.5 Protein Synthesis A. Transcription is the making of single stranded mRNA from a DNA strand within the nucleus of a cell. During transcription, RNA nucleotides base- pair one-by-one with DNA nucleotides on one of the DNA strands (called the template strand). RNA polymerase links the RNA nucleotides together.
TRANSCRIPTION (in the nucleus of the cell) B. This takes place before the RNA leaves the nucleus. First, the introns are removed and the coding regions of the RNA transcript and the exon are joined together, thus, producing the mRNA molecule with a continuous coding sequence. This process is called RNA splicing. With it complete, the “final draft” of mRNA is ready for translation. C. Base pairing is the same as DNA replication, except that RNA has uracil instead of thymine : the U in RNA pairs with A in DNA
TRANSLATION Translation is converting the nucleic language into amino acid language; much like when a reporter transcribes a speech. The language remains the same, however, the form of the message changes. This is the transfer RNA (tRNA), or interpreter. An amino acid is attached to the tRNA and will follow the three- base word to sequence for the proper “sentence”. A codon is the three-base “word” that codes for one amino acid. Several codons form a “sentence” that translates into a polypeptide. The three-base word, AUG, dictates the start of the “sentence”. The process continues until the ribosome reaches a stop codon, UAA, UAG or UGA. G. The same genetic coding system is shared by almost all organisms
Mutations can change the meaning of genes 1. A mutation is any change in the nucleotide sequence of DNA. 2.Can be divided into two categories of mutations base substitutions – replacement of one nucleotide with another Occasionally, a base substitution causes no change to a protein, but sometimes it results in a change that affects the function of a protein, sometimes drastically (ex: sickle cell disease) base insertions or base deletions – addition of an extra nucleotide or subtracting a nucleotide usually results in a more disastrous affect mRNA is read in triplets, by adding or subtracting nucleotides may alter the triplet groupings of the genetic message. Therefore, all the nucleotides that follow the mutation will be regrouped into different codons. The result will be a different, and probably a nonworking protein.
What causes mutations? Mistakes during DNA replication can cause mutations Mutagens – physical or chemical agents that cause mutations Physical mutagens – high energy radiation, X- rays, Ultraviolet light Chemical mutagens – chemicals that are similar to DNA bases and cause incorrect base- pairing Mutations are often harmful but may alter a protein in a way that is beneficial 1. if a mutation is present in the gametes of an organism it is passed on the offspring