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BIOL 103 (103)
Chapter 19


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BIOL 103
Virginia K Walker

BOAG CHAPTER 19 (pg 463-467, 472-477) NUCLEAR GENOMES -more complex eukaryote = carries more different genes 1. genome sequences allows us to identify + characterize genes of model organisms 2. gather more info in order to identify + treat human dieases 3. genomes of agriculturally important species (eg. rice) to develop new strain sof livestock + plants that have improved traits from an agricultural perspective 4. genome sequences helps us establish evolutionary relationships (eg. phylogeny) GENOME SIZES + REPETETIVE SEQUENCES -eukaryotic genomes are usually larger than prokaryotic genomes in both number of genes + genome size -genome size: total amount of DNA; not the same as # of genes -increased amount of DNA correlated w/ increases in cell size, cell complexity, and body complexity -this rule can vary within different species, as well as closely related species -->eg. similar # of chromosomes, but one species can have nearly double the amount of DNA -->double amount of DNA usually not due to twice as many genes, but repetitive sequences: short DNA sequences that are present in many copies --> often abundant in eukaryotic species TYPES OF REPETITIVE SEQUENCES Moderately repetitive sequences -few hundred to several thousand repeats in genome -sometimes these sequences are multiple copies of the same gene -these sequences play a role in regulation of genes transcription + translation Highly repetitive sequences -found in tens of thousands to millions of repeats in genome -each copy of the sequence is short -most sequences have no known f'n + benefit -exons only make up 2% of our DNA- rest is non-coding DNA -much of repetitive DNA is derived from transposable elements --> makes up 59% of our genome TRANSPOSABLE ELEMENTS CAN MOVE FROM ONE CHROMO LOCATION TO ANOTHER -genomes made of several types of DNA sequences -much of the repetitive DNA in genome is derived from transposition: a short segment of DNA moves within a cell from its original site to a new site in the genome -transposable elements (TEs): DNA segments that transpose themselves -mobile "jumping" genes -segment of DNA that can move in/out of a gene, affecting the phenotype -transposition works via different elements: 1. DNA Transposons -TE that moves via DNA -both ends of TEs have inverted repeated: DNA sequences that are identical/very similar but run in opposite directions: 5'-CTGACTCTT-3' and 5'-AAGAGTCAG-3' -TEs have a central region that encodes transposase: enzyme facilitating transposition -cut + past mechanism -transposase recognizes inverted repeat in TE, removes the sequence from original site -transposase/TE complex moves to new location, DNA is inserted into chromo -increases TE amount in a genome 2. Retroelements -common; only in eukaryotic species -moves via RNA intermediate -element transcribed into RNA -RNA template used to made double-stranded DNA, inserted into host chromo DNA -can accumulate rapidly in a genome--> single retro-element copied into many RNA Role of Transposable Elements -Selfish DNA hypothesis: TEs exist b/c they have the characteristics that allow them to insert themselves into host cell DNA -->they resemble parasites; inhabits host w/o offering benefits -->proliferates in host as long as they dont harm them -TEs can do harm: if they jump into the middle of an important gene, disrupting f'n, the phenotype can be negatively affected -TEs can be beneficial: often carries antibiotic-resistance genes providing the organism w/ survival advantage -->TEs can cause greater genetic variability by promoting chromo arrangements -->rearrangements can cause misaligned crossover, promoting formation of a gene family GENE DUPLICATION: PROVIDES ADDITIONAL MATERIAL FOR GENOME EVOLUTION + FORMATION OF GENE FAMILIES -small chromo duplications provide raw material for addition of more genes into a species' genome -->can create homologous genes: 2+ similar genes that are derived from 1 ancestral gene -->over many gen, each version of the gene accumulates different mutations = genes w/ similar but not identical DNA How do duplications occur? -misaligned crossover: 2 homologous chromo paired during meiosis; crossover is misaligned and produces 1 chromo w/ a gene duplication, 1 chromo w/ a gene deletion, and 2 normal chromo -gene duplications can form + be transmitted to future gen -can happen @ different locations on the chromo -paralogous genes: 2+ homologous genes within a single species -multiple gene duplication + sequence divergence = gene family produced -eg. globin family: ancestral globin duplicated; additional duplication events + chromo rearrangements produced a globin gene family of different globin genes -gene families important in evolution of complex traits -accumulation of different mutations in various gene family members created genes that are more specialized in their f'n -different genes expressed during different stages in development Genome sizes among selected groups of eukaryotes.(a) Genome sizes among various groups of eukaryotes are shown
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