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Genetics By Samuel and Daniel Mueller What is the Study of Genetics? The study of heredity and the variation of inherited characteristics. First beginning in the mid-1800’s by Gregor Mendel, whom documented the patterns of inheritance in peas. Mendelian Genetics The simplest of genetics Dominant=functioning gene. Mendel bred true breeding pea plants, meaning one plant had a homozygous dominant genotype and the other a homozygous recessive genotype. He found that the offspring of the plants inherited their traits in a repeated pattern. With a dominant purple plant breeding with a recessive white plant, the first generation was always pink 100% and the 2nd generation was a ratio of 3:1 of purple to white offspring. Recessive=malfunctioning gene dominant genes almost always mask. What does this mean? Inherited traits can come in multiple versions Different versions of the same gene in the same location on homologous chromosomes. If an organism has a chromosome with one allele for a dominant purple color and another chromosome with a recessive white color, The expressed gene will be the dominant gene (in Mendelian genetics). Genotype VS Phenotype Genotype The organism’s actual genetics, often expressed in letters symbolizing alleles. PP=homozygous dominant purple flower Pp=heterozygous dominant purple flower Pp=homozygous recessive white flower Phenotype The organism’s “expressed” gene, the physical gene. The flower being either Purple or White How can you tell if an expressed gene is the organism’s phenotype of genotype Breeding with a homozygous recessive organism! Since organisms can have both homozygous and heterozygous genotypes and still the same genotype. If the offspring has any recessive traits, the original organism is heterozygous, if not, then it is homozygous dominant. Punnett Squares The Core of Mendelian Genetics A diagram to display the probability of an organism’s inheritance. Each of the parent’s alleles are placed next to each column/row of a square to determine the possible genotypes. Monohybrid Cross Mendel’s 1st Law of Heredity The Law of Segregation During Metaphase I of meiosis, homologous chromosomes separate and each allele is given separately to each gamete. One allele for each sperm and egg cell. Mendel’s 2nd law of Heredity The Law of Independent Assortment Again during Metaphase I, different loci, or genes, separate into gametes independently and in equal amounts. Dihybrid crosses Incomplete Dominance This is when alleles are not completely dominant. Heterozygous organisms will show a blend of phenotypes. RR=Red R’R=Pink R’R’=white Co-dominance Two alleles affect the phenotype equally and separately. Ex. Human blood types! This flower’s phenotype is a mixture of red and white but not blended. Genotype: RW Pleiotropy One gene has multiple effects on an organism. EX. Acromegaly and Dwarfism (gigantic and dwarfed respectively). <- Dwarf Sex Linked Traits Genes which lie on either one of an organism’s sex chromosomes (other 22 are autosomal chrmosomes) Discovered by T.H. Morgan. Worked with drosophila (fruit flies) An example is hemophilia in humans. When an X chromosome with this recessive gene is inherited by having a female with two of these chromosomes or a male with one, then the gene is expressed Linked Genes Linked genes are merely a combination of genes that are inherited together as they lie on the same chromosome. For example, a fruit fly's eye color and wing length lie on the same chromosome and are thus inherited together. (Not to be confused with sex-linked genes which are genes that lie on one of the sex chromosomes) X-inactivation When an organism, male or female, suffers one of its X chromosomes to become inactivated (obviously), which usually results in the early deaths of males since they only have a single X chromosome. Not every cell in an organism’s body has to have an inactivated X chromosome which is how tricolor cats form. In the cells with inactivated X chromosomes, that patch of fur may be black while another with both of its chromosomes activated would be orange. Epistasis The diagram is similar to Dihybrid crosses, however one gene has a profound effect on the other gene. One gene masks the other, in this case, with its pigment. If the mouse has the dominant CC or Cc genotype, they are allowed pigment in their fur as their BB, Bb, or bb genotypes encode. (black or brown if CC/Cc and no pigment whatsoever if recessive for c gene.) Acronyms! M M M M C Why ChoColate Hershey So Good •T.H. Morgan •Meselson and Stahl •Avery, McCarty and MacLeod •Watson and Crick •Erwin Chargaff •Hershey and Chase •Frederick Griffith Yolo because why not Moving on. T.H. Morgan Worked with Drosophila (fruit flies). Associated phenotype with chromosome. (genes located on chromosomes). White eyed males had a certain X chrmosome (sex-linked traits as well). End conclusion was that he found that either DNA or proteins are the genetic material of a cell. Frederick Griffith “Transforming Principle” Mixed Heat-killed, deadly pathogens with harmless, live bacteria The DNA from the deadly pathogens entered the harmless bacteria and changed their phenotype to make them deadly as well. “something” had been transmitted to the harmless bacteria but was it Protein or DNA. How Griffith found this out. Rough=harmless bacteria Smooth=deadly bacteria Avery, McCarty, and MacLeod DNA is the “transforming principle” Purified both the protein and DNA from the deadly pathogens and individually injected them into the harmless pathogens separately. Both became deadly, however, the offspring of the one injected with DNA had deadly offspring. Showed DNA is what is passed down. Hershey and Chase “Blender” experiment Uses radioactive isotopes, 35S and 32P (number=neutrons) to trace where proteins and DNA go. When cells multiplied, the sulfur, initially in the proteins, were found in the medium (not in the cell), while the phosphorus was found in the cell as DNA. Solidified DNA as the genetic material. Erwin Chargaff Chargaff’s Rules Showed that all 4 bases in DNA did not appear in equal amounts and varied greatly from species to species. He did find that the amount of adenine usually always equals the amount of thymine and similarly for cytosine to guanine. Watson and Crick Found double helix structure of DNA How is DNA copied (Meselson and Stahl) How did they find this out? 14N and 15N (radioactive isotopes) One strand of DNA only had 14N and the other only had 15 N changing their weights Meselson and Stahl Found Semiconservative replication was the correct theory How does DNA replicate? http://www.youtube.com/watch?list=EC3EE D4C1D684D3ADF&v=8kK2zwjRV0M&featur e=player_detailpage#t=547s Transcription Making mRNA (done by RNA polymerase) RNA is only single stranded so it is always copied off the 3’ to 5’ DNA strand so that it can easily grow in the 5’ to 3’ direction. Binding site where transcription begins is called the TATA box Introns vs Exons Introns “Junk” DNA that is not expressed when transcribed into RNA Exons The “expressed” DNA that is used for translation. mRNA splicing Molecules called Snurps combine to create spicesomes which bind to the beginning and end of an intron segment and extracts it from the pre-mRNA into regular mRNA for translation (only exons left). Translation Every 3 letters of mRNA are called triplet codons and are used to code for one of 20 amino acids. tRNA can translate mRNA by finding its anticodon which assures the gene is being read correctly and attatches the following amino acid to the primary structure of the protein being created. EPA sites mRNA enters a ribosome like a dollar bill into a vending machine, first entering through the A site (aminoacyl-tRNA site). This site carries the next amino acid to be added to the polypeptide chain Next is the P site (peptidyl-tRNA site) which holds tRNA which carries the growing polypeptide chain Finally is the E site (exit site) where the empty tRNA leaves the ribosome to be reused later. Protein Synthesis in Prokaryotes DNA in prokaryotes is located in the cytoplasm already so no 5’ cap or poly-A tail is needed. No introns are found, so no splicing occurs Transcription and translation happen simultaneously, ribosomes read mRNA as it is being transcribed.