Lecture Chapter 17 from gene to protein 11 17 11 The info content of DNA is in the form of specific sequences of nucleotides along the DNA strand The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins The process by which DNA directs protein synthesis gene expression includes 2 stages called transcription and translation Transcription is the synthesis of RNA under the direction of DNA produces messenger RNA mRNA Translation is the actual synthesis of a polypeptide which occurs under the direction of mRNA occurs on ribosomes The ribosome is part of the cellular machinery for translation polypeptide synthesis In prokaryotes transcription and translation occur together In eukaryotes RNA transcripts are modified before becoming true mRNA Basic principles of transcription and translation Cells are governed by a cellular chain of command DNA RNA protein The genetic code How many bases correspond to an amino acid Codons triplets of bases Genetic info is encoded as a sequence of nonoverlapping base triplets or codons During transcription the gene determines the sequence of bases along the length of an mRNA molecule Cracking the code A codon in mRNA is either translated into an amino acid or serves as a translational stop signal Codons must be read in the correct reading frame for the specified polypeptide to be produced If read starting with a different letter sentence would not make sense Evolution of the genetic code The genetic code is nearly universal shared by organisms from the simplest bacteria to the most complex animals CCG proline in bacteria and humans In lab experiments genes can be transcribed and translated after being transplanted from one species to another human genes can be inserted into bacteria to manufacture certain proteins for medical purposes Molecular components of transcription Transcription of the DNA directed synthesis of RNA RNA synthesis is catalyzed by RNA polymerase which pries the DNA strands apart and hooks together the RNA nucleotides follows the same base pairing rules as DNA except the in RNA Uracil subs for Thymine Synthesis of an RNA transcript The stages are initiation elongation termination RNA polymerase binding and initiation Promoters signal the initiation of RNA synthesis TATA box Transcription factors help eukaryotic RNA polymerase recognize Elongation of the RNA As RNA moves along the DNA it continues to untwist the double helix exposing Termination The mechanism of termination are different in prokaryotes and eukaryotes Eukaryotic cells modify RNA after transcription modify pre mRNA in specific ways before genetic messages are dispatched to the cytoplasm Alteration of mRNA ends Each end of pre mRNA molecule is modified in a particular way The 5 end receives a modified cap The 3 end gets a poly A tail Modified ends up helping to 1 export mRNA from the nucleus 2 protect the mRNA from degradation in the cytoplasm and 3 facilitate ribosome attachment Split genes and RNA splicing DNA molecules consist of coding and noncoding regions RNA splicing removes introns non coding and joins exons coding region Translation Figure 17 13 Molecular components Translation is the RNA directed synthesis of a polypeptide A cell translates an mRNA message into protein with the help of transfer tRNA Molecules of tRNA are not all identical each carries a specific amino acid on one end each has an anticodon on the other end The structure and function of tRNA A tRNA molecule consists of a single RNA strand that is only 80 nucleotides long is roughly L shaped Ribosomes facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis The ribosomal subunits Are constructed of proteins and RNA molecules named ribosomal RNA or rRNA The ribosomal has 3 binding sites the P site Peptid the A site Aminoacly tRNA binding site the E site exit site Figure 17 16b Building a polypeptide We can divide translation into 3 stages Initiation Elongation Termination Ribosome Association and initiation of translation The initiation stage bring together mRNA tRNA bearing the first amino acid of the polypeptide and 2 subunits of a ribosome Elongation of the polypeptide chain In the elongation stage of translation amino acids are added one by one to the preceding amino acid Termination of translation The final stage of translation is termination when the ribosome reaches a stop codon on the mRNA Polyribosomes A number of ribosomes can translate a single mRNA molecule simultaneously forming a polyribosome Completing and targeting the functional protein Polypeptide chains undergo modification after the translation process After translation proteins may be modified in ways that affect their 3 D shape Point mutations can affect protein structure and function Mutations are changes in the genetic material of the cell Point mutations are changes in just one base pair of a gene Point mutations within a gene can be divided into 2 general categories Base pair substitution Base pair insertions or deletions The change of a single nucleotide in the DNA s template strand leads to the production of an abnormal protein Substitutions A base pair substitution is the replacement of one nucleotide and its partner with another pair of nucleotides can cause missense or nonsense Insertions and deletions are additions or losses of nucleotide pairs in a gene may produce frameshift mutations Lecture Chapter 22 and 23 A new era of biology in 1859 when Charles Darwin published Origin of Species Origin of Species focused biologists attention on the great diversity of organisms Darwin noted that current species are descendents of ancestral Evolution can be defined by Darwin s phrase descent with modification Evolution can be viewed as both a pattern and a process In order to understand why Darwin s ideas were revolutionary We need to examine his views in the context of other western ideas about earth and its life The Greek Lecture Chapter 23 11 22 11 11 29 11 Natural Selection Note that individuals don t evolve populations evolve over time Natural Selection can only increase or decrease heritable traits in a population Adaptations vary with different environments Darwin s theory of evolution continues to be tested by how effectively it accounts for additional observations and experimental outcomes The evolution of Drug resistant HIV In humans the use of drugs selects for pathogens that through chance mutations are resistant to the drug s effects
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