BCOR 103 1st Edition Lecture 23 Outline of Last Lecture I Nuclear Transport II Regulation and RNA III Open Mitosis Outline of Current Lecture I Genomics II Alternative Splicing III Gene Families Current Lecture Key concept complexity of an organism does not correlate directly with genome size or the total number of genes in the genome Genomics the systematic analysis of entire cell genomes First human genome sequence 2000 10 years of work cost 2 7 billiion Key concept Organism complexity does correlate with the percentage of the genome that encodes protein inverse correlation o Example human dystrophin gene encodes a protein that is part of the complex that connects the cytoskeleton to the extracellular matrix dystrophin gene 2 5 million bases 79 exons final mRNA is 14 000 bases 0 61 of the pre mRNA Alternative Splicing Production of Multiple Proteins from single gene o Alternative splicing 90 of human genes Bottom line alternative mRNA processing allows the production of 100 000 distinct human proteins from 20 000 genes DSCAM each neuron expresses a single type of DSCAM mRNA and thus each neuron expresses a single type of DSCAM protein DSCAM allows neurons to distinguish self from non self each DSCAM type is functionally distinct o Note within each cluster of cassette exons there is a high degree of similarity but each cassette exon encodes a slightly different protein sequence Transposable elements DNA sequences capable of moving within the genome These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Mitochondrial DNA is composed of a single double stranded circular molecule There are several copies in each mitochondrion and there are many mitochondria in most human cells Mitochondria originated by endosymbiosis of a prokaryotic cell early in the evolution of eukaryotic cells Mitochondrial DNA is similar to prokaryotic DNA There are no histones or any other protein associated with mt DNA The genes contain no introns Because it is in a highly oxidizing environment it has a much higher rate of mutation than nuclear DNA The genes in mt DNA code for mitochondrial ribosomes and transfer RNAs Some genes code for polypeptide subunits of the electron transport chain common to all mitochondria The mitochondrion relies on nuclear gene products for replication and transcription o Key point the human mitochondrion is inherited only from the mother Dispersed gene families arising from gene duplication diversified functions o Following gene duplication additional gene copy may acquire new functions tissue specific expression developmental regulation altered enzymatic activity o Example human fetal globin genes fetal globin proteins possess a higher affinity for oxygen than adult globin proteins therefore allow the fetus to obtain sufficient oxygen from the maternal circulation Tandem gene families encode gene products needed in large quantities on short notice examples histone genes rRNA genes tRNA 5S rRNA FISH fluorescence in situ hybridization o Human telomeres 1500 repeats of the sequence AGGGTT per telomere total 92 telomeres o Telomeres enable linear eukaryotic genomes to resolve the end problem resulting from lagging strand DNA synthesis ALL DNA polymerases require a primer ALL DNA polymerases synthesize DNA 5 to 3 Telomeres solve the problem of replicating the ends of a linear DNA genome Telomerase reverse transcriptase that carries its own RNA template o Note most human cells do not express telomerase DNA repeats of no known function initially referred to as junk DNA or selfish DNA Repetitive DNA sequences likely played and continue to play an essential role in the evolution of complex organisms provide a mechanism for genetic change a prerequisite for evolution Transposable elements sequences that can move in the genome Basic types o 1 DNA transposons prokaryotic like transposable elements move by a cut and paste mechanism no net gain of transposable elements o 2 Retrotransposons move by use of an RNA intermediate net gain of transposable elements Key concept all transposable elements generate a short direct repeat 5 10 bp of target DNA sequences upon integration at a new site Transposase is a DNA endonuclease that recognizes the inverted repeats at the ends of the transposon and cleaves the transposon from the genome Transposase then makes a staggered cut in the target DNA allowing for the insertion of the transposon at a new site Key concept DNA transposons move by a cut and paste mechanism therefore they are cut out of their original position in the genome and pasted in some other position within the genome Transposase enzyme responsible for cutting the DNA transposon out of its original position and cutting the target DNA into which the DNA transposon will be pasted o Repair of DNA gap carried out by host cell DNA repair enzymes o Note most DNA transposons in the human genome are inactive Retrotransposable elements move within a genome through an RNA intermediate Enzymes required for retrotransposition o 1 RNA polymerase II o 2 reverse transcriptase o 3 integrase o 4 host cell DNA repair machinery Once inserted into the host cell genome retrotransposable elements are not excised thus retrotransposition is essentially irreversible Viral like retrotransposable elements possess the basic structural features of retroviruses o 1 long terminal repeats LTRs o 2 complete or partial gene structure gag pol env o 3 short flanking direct repeats of host DNA sequence at the site of integration HERVs human endogenous retroviruses entered the human genome through infection of germ cells at some point during human evolution Provirus term for the retrovirus genome that is integrated into the host cell genome Retrotransposition requires the synthesis of a capped and polyadenylated RNA by RNA polymerase II RNAse H degrades RNA of a hybrid RNA DNA helix Pseudogenes arise from the accidental reverse transcription of normal cellular mRNAs Following insertion into the genome pseudogenes are usually not expressed they generally lack flanking regulatory sequences necessary for transcription initiation and mRNA 3 processing However if expressed they may provide the basis for the evolution of novel proteins