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Biology Chapter 20.docx

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Lovaye Kajiura

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Biology Chapter 20: Genomics Repeated Sequences and DNA Fingerprinting  In addition to containing repeated sequences from transposable elements, eukaryotic genomes have several thousand loci called simple tandem repeats (STRs)  These are small sequences repeated one after another down the length of a chromosome  There are two major classes of STRs o Repeating units that are just 1 to 5 bases long are known as microsatellites or simple sequence repeats o Repeating units that are 6 to 500 bases long are called minisatellites or variable number tandem repeats (VNTRs) o Both type of repeated sequences make up 3% of the human genome o The most common type of microsatellite is a repeated stretch of the dinucleotide AC, giving the sequence ACACACAC… o Microsatellite sequences are thought to originate when DNA polymerase skips or mistakenly adds extra bases during replication  Soon after these sequences were first characterized, Alec Jeffreys and co-workers established that microsatellite and minisatellite loci are “hypervariable” meaning that they vary among individuals much more than any other type of sequence does  One hypothesis for why both have so many different alleles o These highly repetitive stretches often align incorrectly when homologous chromosomes synapse and cross over during prophase of meiosis I o Instead of lining up in exactly the same location, the two chromosomes pair in a way that matches up bases in different repeated segments o Due to this misalignment, unequal crossover occurs o Chromosomes produced by unequal crossover contain different number of repeats o The key is that if a particular microsatellite or minisatellite locus has a unique number of repeats, it represents a unique allele o Each allele has a unique length o As with any allele, microsatellite and minisatellite alleles are transmitted from parents to offspring o Misalignments or errors by DNA polymerase are so common at these loci that in most eukaryotes, the genome of virtually every individual has at least one new allele o This variation in repeat number among individuals is the basis of DNA fingerprinting  DNA fingerprinting refers to any technique for identifying individuals based on the unique features of their genomes  Because microsatellite and minisatellite loci vary so much among individuals, they are now the loci of choice for DNA fingerprinting  To fingerprint an individual, researchers obtain a DNA sample and perform PCR using primers that flank a region containing an STR  Once many copies are available, they can be analyzed to determine the number of repeats present  Primers are now available for many different STR loci so researchers can analyze the alleles present at many STRs efficiently  Research on repeated sequences has revealed that the probability of getting a new allele is higher for shorter repeats than for longer repeats  For some two base-pair repeats, the number of repeats present changes so quickly over time that only very close relatives are likely to share any of the same alleles  This observation has practical implications o Ex. DNA fingerprinting of blood or semen found at crime scenes has been used to show that people who were accused of crimes were actually innocent o DNA fingerprinting has also been used as evidence to convict criminals or assign paternity in birds, humans and other species that have well-characterized microsatellite or minisatellite sequences Gene Duplication and the Origin of Gene Families  In eukaryotes, the major source of new genes is the duplication of existing genes  Biologists infer that genes have been duplicated recently when they find groups of similar genes clustered along the same chromosome  Genes are usually similar in structural aspects, such as arrangement of exons and introns  Within a species, genes that are extremely similar to each other in structure and function are considered to be part of the same gene family  Genes that make up gene families are hypothesized to have arisen from a common ancestral sequence through gene duplication  When gene duplication occurs, an extra copy of a gene is added to the genome  The most common type of gene duplication results from crossing over during meiosis  Gene-sized segments of chromosomes can be deleted or duplicated if homologous chromosomes misalign during prophase of meiosis I and an unequal crossing over occurs  The duplicated segments resulting from unequal crossing over arranged in tandem, one after the other  Gene duplication is important because the original gene is still functional and produces a normal product  As a result, the new, duplicated stretches of sequences are redundant  In some cases, the duplicated genes retain their original function and provide additional quantities of the same product  But if mutations in the duplicated sequence alter the protein product and if the altered p
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