Genotype to Phenotype - Fundametnals of Biology - Lecture Notes, Study notes of Biology

Download Genotype to Phenotype - Fundametnals of Biology - Lecture Notes and more Study notes Biology in PDF only on Docsity! 1 Chapter 13 HOW GENES WORK Genotype to Phenotype Summary  History  The Genetic Code ◦ Watson and Crick ◦ Characteristics ◦ How the code is used History  Mendel gave us insight on how traits or characteristics are inherited.  1903 – Sutton and Bovary published the Chromosome Theory of Inheritance: ◦ Meiosis causes patterns of inheritance observed by Mendel ◦ Hereditary factors are genes located on DNA.  How do genes cause certain traits to appear?  Physicians noted that certain diseases seemed to run in families.  Garrod was interested in one such disease, alcaptonuria. ◦ This disease is characterized by the accumulation of a substance that is excreted in the urine, causing it to turn black on exposure to the air. ◦ The accumulation of this substance is caused by an error in a metabolic pathway.  Metabolic pathways ◦ Conclusion: a defect or lack of an enzyme causes accumulation of one of the substrates. Docsity.com 2 ◦ Certain defects in enzymes are inherited = Biochemical Basis of Genetic Diseases  Other scientists/physicians studied the effect of altering DNA on biochemical pathways.  Beadle and Tatum – proposed that one could find out what genes do by making them defective.  Proposed the one gene one enzyme hypothesis.  Today – genes are studied by using organisms with defective or non-functioning genes = “knock out mutants”. The Genetic Code  How do genes produce certain enzymes/proteins?  Watson and Crick ◦ Because of the structure of DNA, it probably does not catalyze protein synthesis directly. ◦ DNA could act as a “code” used to assemble the 20 different amino acids into different proteins. ◦ The code is the particular sequence of bases in a gene (a part of a strand of DNA). ◦ This particular base sequence codes for a particular sequence of amino acids. ◦ When the DNA strands are separated, the code could be “read” and used to join amino acids in the proper sequence. ◦ They later decided that the DNA code is not used directly, there must be another step involved. How did they know this? Docsity.com 5 from the nucleus to the cytoplasm. Transcription  Production of a mRNA “copy” of the DNA sequence of a certain gene.  Enzyme = RNA polymerase; 1 in prokaryotes, 3 in eukaryotes.  RNA polymerase uses nucleotides with 3 phosphate groups (ATP, GTP, etc).  3 types of RNA polymerase – RNA pol I produces rRNA, RNA pol II produces mRNA, and RNA pol III produces tRNA.  The base sequence on only 1 DNA strand is used as a template = “template strand”. The other strand is the “non-template strand”, Figure 13-4.  Sequence fig 13-8. ◦ Initiation phase  RNA polymerase and other proteins (initiation factors) bind to the promoter, a stretch of DNA upstream from the start site for a particular gene.  RNA polymerase opens up a portion of the DNA helix and RNA polymerase begins transcription. ◦ Elongation and termination phase  Elongation: RNA polymerase adds nucleotides 5’ to 3’ at a rate of ~ 50 nucleotides/second beginning at start site. Active portion = transcription bubble.  Completed mRNA strand exits bubble as it is finished.  Termination: RNA polymerase stops when the RNA recognizes a termination sequence. Transcription Docsity.com 6  In prokaryotes, the mRNA feeds directly to a ribosome complex and translation begins.  Eukaryotes only - posttranscriptional modifications ◦ Addition of a 5’ cap (adenine or guanine + methyl-GTP). ◦ Addition of a “poly A tail”. Figure 16-8  Initiation phase ◦ A small sequence of rRNA on the ribosome binds to a complementary sequence on the mRNA with the help of intiation factors. ◦ *The start codon, AUG is exposed. ◦ tRNA with a sequence that is complementary to the codon (=anticodon) attaches to the codon and releases its amino acid. The first amino acid then will always be methionine or f-methionine.  Elongation and termination phase ◦ Ribosome moves down 1 codon at a time and specific tRNA’s bring their amino acids to the chain. ◦ Amino acids are joined by peptide bonds to form the protein. ◦ 3 sites on the ribosome (APE): ◦ Translation is terminated when the ribosome reaches a stop codon.  Eukaryotes only ◦ Eukaryotic genes contain sequences that do not contain codons = INTRONS; sequences that contain codons are EXONS. Docsity.com 7 ◦ mRNA sequences contain the same introns and exons. ◦ Introns are removed after mRNA synthesis and exons are joined together = RNA splicing. ◦ snRNPs = small nuclear ribonucleoproteins are the splicers. ◦ Why? Alternate splicing can produce different proteins from the same gene sequence. 30,000 genes can be used to produce 120,000 mRNA’s. Mutations  Point mutation ◦ A change in a single nucleotide can result from errors in DNA replication or from exposure to mutagenic toxins. ◦ In some cases, an abnormal protein is made that causes a disease process.  Gene duplication – extra copy of gene is added to one chromosome during cross over.  Gene inversion – piece of DNA is flipped over during cross over.  Gene deletion – loss of DNA. Differences between Prokaryotic and Eukaryotic Gene Expression  Eukaryotic genes contain introns; prokaryotic genes do not.  Eukaryotic mRNA’s code for 1 gene; prokaryotic mRNA’s code for several related genes at one time.  Eukaryotic mRNA must be moved to cytoplasm; prokaryotic mRNA is already in the cytoplasm and translation starts before transcription is complete.  Eukaryotic mRNA’s are modified for transport – RNA splicing, poly A tail; Docsity.com