Flashcards in Exam 1: Lecture 6 Deck (18):
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Comparison of Nuclear Gene Content
-genomes of diverse organisms can harbor very different numbers of genes
-idea that more complex organisms would have dramatically higher numbers of genes than simple organisms is not true in all circumstances
-Craig Venter and Francis Collins leaders of private and public efforts sequence human genome
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Gene Duplication (What?)
-one path to acquiring new genes
-some cases individual genes are duplicated while in other situations large segments of chromosomes can be duplicated
-often occur as result of error in homologous recombination or from movement of transposable element
-rare cases entire chromosome or genome is duplicated which result from errors in chromosome segregation and cell division
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Gene Duplication (When?)
-can occur during formation of gametes during meiosis as well as mitosis
-several cancers are due to gene duplications in isolated tissues
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Gene Duplication (Happens? sub-functionalization)
-immediately after, both daughter genes are identical in sequence, structure and function
-as time proceeds random mutations knock out complementary functions in two daughter genes thereby leaving each daughter gene to execute subset of tasks that were controlled by ancestral gene
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Gene Duplication (Happens? neo-functioanlization)
-random mutations alter function of only one of two daughter genes.
-results in one daughter carrying out all ancestral functions while other daughter gene acquires new functions
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Mutatioin That Lead to Neo-functionalization
-can affect regulatory models or coding sequences
-note: in hypothetical example in note two daughter genes carrying out same function (coding sequence not altered) but they are doing it indifferent tissues.
-If coding sequence is changed then the actual function of the encoded protein will diverge
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Gene Gain and Loss
-can have profound influence-->lead to evolution of bacterial pathogens and marine phytoplankton from putative brown algae
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Comparison of Modern Primates
-indicates 100's of genes that have been lost and gained over last 48 million years
-losses and gains could be underlying causes for some of difference that are observed between various primates
-differences between the species can also be cause by differences in DNA sequences of common genes, expression patterns, transcriptional levels and protein modifications
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Organization of Human Genome
-contains approximately 30,000 genes (note each gene found in two copies)
-only 1.5% is taken up by coding portion (exons) of genes
-rest made up of non-coding segments like introns, regulatory and untranslated sequences, intergenic DNA, pseudogenes, gene fragments and satellite sequences.
-last category is made up by remnants of transposable elements
-simpler organisms have higher gene density and thus fewer stretches of non-coding DNA
-also contains mitochondrial genome
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Mitochondrial Genome
-portion of genome is efficiently organized. Contributes 37 genes and almost no non-coding DNA
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DNA Organization: Sea Urchin
-discovered that mRNA transcripts contained only a fraction of the sequences that were found within genomic DNA
-for the first time showed that the nuclear genome contained a significant portion of non-coding sequences
-presaged several discoveries including the identification of exons and introns and the process of RNA splicing
-- Sequencing of the Drosophila, mouse, and human genomes confirmed that similar relationships existed in other complex organisms
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Non-coding DNA
-the vast majority of the human genome is comprised of non-coding DNA
-this is true of nearly all eukaryotic organisms
-exception:yeast which is a single celled eukaryote
-yeast genome has high gene density and fewer non-coding DNA segments than other eukaryotes
-Bacterial genomes are even more compact: their genomes are fairly devoid of introns, transposable elements, pseudogenes and repetitive sequences
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Pseudogenes
-inactive genes that are either no longer expressed due to a mutation in regulatory element or is dysfunctional due to a mutation within coding sequences
-can be recognized since they contain sequences that are similar to functional gene
-formation is very common after gene duplication and significant portion of eukaryotic genomes is comprised of inactive pseudogenes
-arise from duplications located relatively close to sister genes while those born from retrotransposition can be located at considerable distances
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Psudeogene: mutation in regulatory element
-duplication event takes place which is then followed by accumulation of deleterious mutations in one of daughter genes
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Psudeogene: mutation within coding sequence
-messenger RNA is converted into cDNA clone by the reverse transcriptase enzyme.
-this DNA copy inserted into genome via retrotransposition.
-original mRNA lacks enhancer and promoter elements, newly integrated cDNA copy cannot be transcribed and is inactive in genome
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Example of Pseudogenes
-analysis of hemoglobin genes
-during evolution globin genes have been duplicated several times
-some genes transcribed during fetal development while others expressed only in adulthood.
-few globin genes expressed during both phases.
-in addition to these active genes several of duplication products have been inactivated by random mutations.
-sit idle within genome on chromosome 11 and 16
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Transposable Elements
-makeup significant portion of eukaryotic genomes
-ancestors of bacteriophages and viruses
-continue to exist and infect both prokaryotic and eukaryotic cells
-bacterial genomes contain very few transposbale elements
-yeast contains a few more but overall number still small
-in genomes of higher organisms (humans and maize) percentage of genome comprised by these elements increases significantly
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