CHAPTER TWENTY ONE GENOMES AND THEIR EVOLUTION Linkage map a genetic map of several thousand genetic markers space throughout the o Based on recombination frequencies o Physical map distance b w markers is the of base pairs along the DNA chromosomes GENOME Bulk of the genome is DNA that does not code for protein or produces RNA 1 5 of genome codes from protein or is transcribed into rRNAs or tRNAs 24 of genome is gene related regulatory sequences o Pseudogenes former genes that have accumulated mutations over a long time and have become nonfunctional o Repetitive DNA consists of sequences that are present in multiple copies in the o Transposable elements stretches of DNA that can move from one location to genome another within the genome Transposition jumping genes intermediate Transposable elements move from one site in a cells DNA to a different target site by a type of recombination process Transposons move within a genome by means of a DNA o Cut and paste removes the element from the original site o Copy and paste leaves a copy behind Retrotransposons move by means of an RNA intermediate that is a transcript of the retrotransposon DNA o Always leave a copy of the original o To insert into another site RNA intermediate must first CHAPTER 17 FROM GENE TO PROTEIN Gene expression the process by which DNA directs the synthesis of proteins or RNAs o Transcription o Translation Transcription Genes specify proteins via transcription and translation Information is copied from one molecule to another Synthesis of RNA under the direction of DNA DNA can serve as a template for assembling a complementary strand of RNA nucleotides Messenger RNA mRNA o Sends out multiple copies of the genes protein building instructions o Carries genetic info from the DNA to the protein synthesizing machinery of the cell Occurs in the nucleus mRNA transported to cytoplasm where translation occurs Primary transcript initial mRNA transcript from a gene RNA Polymerase pries 2 strands of DNA apart and joins the RNA nucleotides as they base pair along the DNA template o Assemble polynucleotides only in 5 3 direction o Don t need a primer Promoter DNA sequence where RNA polymerase attaches and initiates transcription Terminator sequence that signals the end of transcription o Promoter upstream from terminator Transcription unit stretch of DNA that is transcribed into an RNA molecule 3 stages of RNA transcription o RNA polymerase binding and initiation of transcription Promoter of a gene includes within it the transcription of the RNA chain Promoter determines which of the 2 strands of DNA is used as the template Transcription factors mediate the binding of RNA polymerase and the initiation of transcription TATA box forms the initiation complex at a promoter o Elongation of RNA strand RNA polymerase moves along the DNA it continues to untwist the helix exposing 10 20 bases at a time for pairing w RNA nucleotides Enzyme adds nucleotides to the 3 end of the growing RNA molecule as it continues along the double helix New RNA peels away from its DNA template and the DNA double helix reforms 40 nucleotides per second o Termination of transcription Terminator sequence in DNA Transcribed terminator RNA sequence functions as the termination signal causing polymerase to detach from the DNA and release the transcript which is available for immediate use as mRNA Cells modify RNA after transcription o Enzymes in eukaryotic nucleus modify pre mRNA in specific ways before the genetic messages are dispatched to the cytoplasm translation o RNA processing both ends of the primary transcript are altered Alteration of mRNA ends o 5 cap modified guanine nucleotide added onto the 5 end after transcription of the first 20 to 40 nucleotides o 3 end of the pre mRNA molecule is also modified before the mRNA exits the o Pre mRNA is released soon after the polyadenylation signal AAUAAA is nucleus transcribed o Poly A tail adds 50 to 250 A nucleotides RNA splicing Poly A tail and 5 cap serve to facilitate the export of the mature mRNA from nucleus Protect mRNA from degradation by hydrolytic enzymes Help ribosomes attach to 5 end of the mRNA once the mRNA reaches the cytoplasm o Cut and paste of RNA o Segments of noncoding and coding codons alternate through out the RNA Introns noncoding segments of nucleic acid that lie between codon regions Exons coding segments eventually expressed and translated into amino acids exit nucleus RNA polymerase II transcribes both introns and exons from the DNA The introns are cut out of the molecule and the exons join together forming an mRNA molecule with a continuous coding sequence RNA splicing snRNPS small nuclear ribonucleoproteins recognize splicing sites o located in the cells nucleus o composed of RNA protein molecules snRNA spliceosome interacts with certain sites along an intron releasing the intron and joining together the two exons that flanked the intron o strong evidence that snRNA catalyze the process as well as participating in spliceosome assembly and splice site recognition ribozymes RNA molecules that function as enzymes Alternative RNA splicing o Many genes give rise to2 or more different polypeptides o Alternative RNA splicing is the reason humans can work with small number of genes o The number of different protein products an organism produces can be much greater than its number of genes Domains o Discrete structural and functional regions o Presence of intros in a gene may facilitate the evolution of new and potentially useful proteins as a result of exon shuffling o Introns increase the prob of potentially beneficial corssing over between exons of alleles simply by providing more terrain for crossovers w o interrupting coding sequences Exon shuffling could lead to new proteins w novel combos of functions Translation Cell interprets a genetic message and builds a polyepeptide accordingly o the message is a series of codons along an mRNA molecule o the interpreter is tRNA transfer RNA transfers amino acids from the cytoplasmic pool of amino acids to a ribosome o ribosomes add each amino acid brought to it by tRNA to a growing end of a polypeptide chain end o as the tRNA molecule arrives at a ribosome it bears a specific amino acid at one o at the other end of the tRNA is a nucleotide triplet called an anticodon anticodon complements codons on mRNA o codon by codon the genetic message is translated as tRNAs deposit amino acids in the order prescribed and the ribosome joins the amino
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