Class Notes (808,754)
Canada (493,378)
Biology (2,348)
BIOLOGY 1A03 (716)

Biology Chapter 20.docx

7 Pages
Unlock Document

McMaster University
Lovaye Kajiura

Biology Chapter 20: Genomics Bernard Ho December 3, 2010 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 pro
More Less

Related notes for BIOLOGY 1A03

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.