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10. Organellar Chromosomes and DNA Barcoding.pdf

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Biology (Sci)
BIOL 200
Richard Roy

Naveen Sooknanan McGill Fall 2011 Organellar Chromosomes and DNA Barcoding: DNA can not only be found in the nucleus of a cell, but also in specific organelles. In animals, DNA can be found in the mitochondria (called mitochondrial DNA or mtDNA) whereas plant cells contain DNA within their chloroplasts (cpDNA).  Theses chromosomes are important for respiration within the mitochondria or photosynthesis within the chloroplasts  These genes are originally free floating and were endocytosed into the cell and incorporated into their respective organelles  These genomes resemble prokaryotic genomes in many ways o They are circular o They typically lack introns o Their genetic products resemble those of prokaryotes  There are many mitochondria or chloroplasts in one cell, and each of these organelles may contain multiple genomes Both organelles may have been formed from an ancestral cell which endocytosed a bacterium capable of either respiration or photosynthesis.  We are not sure if this happened once or multiple times Some animals, like seas slugs, eat specific algae types whose chloroplasts are endocytosed by the slug’s epidermal cells.  As a result, the slug is able to undergo photosynthesis due to the algae it consumes Gene exchange may also occur between the mitochondrion/chloroplast and the nucleus  Nucleus genome is much larger than Organellar DNA  Gene elimination may occur, which transports genes from the nucleus to the respective organelles  The mitochondrion and chloroplasts may also transfer genes to the nucleus The mitochondrion has many functions, including those listed here. Many proteins needed in these functions are produced by the nucleus DNA and the translated products are transferred to the mitochondrion. Human mitochondrial DNA (mtDNA) is a circular piece of chromosomal DNA measuring 16,569 bp in length, which is tiny compared to a gene within the nucleus.  Proteins can be synthesised using moving in either direction by using each strand as a template  It only contains 37 genes because most of the genes in mtDNA underwent gene elimination and were moved to the nucleus o As a comparison, the e. coli chromosome contains over 5000 genes  Many encode components are needed for translation  There are no introns in mtDNA (like prokaryotes) 1Naveen Sooknanan McGill Fall 2011  Gene products stay within the mitochondrion The genetic code is also slightly altered in the mitochondrion: Mutations in mtDNA can be related to aging in animals. Mice with a mtDNA polymerase defective for proofreading will exhibit premature aging.  A wild type mouse can live around 1000 days  When the mtDNA polymerase is altered, the homozygous mutant mouse is only able to live 400 days Chloroplast DNA (cpDNA) is found in plants and is typically much larger than human mitochondrial DNA.  They are typically 100-200 kbp In length and have a circular shape like mtDNA  They contain approximately 100-200 genes  cpDNA varies greatly in size and content depending on the plant species DNA barcoding is a recent movement in molecular ecology which involved finding a unique DNA sequence for every species in order to speed up identification.  There are over 2 million species which have been discovered by classical taxonomy  There are over and estimated 10 million species in existence o DNA barcoding may be a more efficient way to figure out what these species are  mtDNA is the perfect choice of DNA for DNA barcoding because it is small and unique to every individual DNA barcode selection also has constraint:  The sequence differences have to be high enough to be able to distinguish between different species, but not so high as to detect differences within the same species  The variable barcode sequence have to be surrounded by a conserved regions for primer annealing when using PCR for gene duplication  The sequence must be easily aligned, and therefore must lack insertion/deletion regions like introns The PCR strategy is perfect for this technique because the primers can anneal to the conserved regions on either side of the hypervariable region  The hypervariable region is unique to every species and is consistent in size between organisms  The conserved region must be present because we have to know their sequence in order to use the correct primers  Therefore, the perfect choices are mtDNA in animals a
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