BIOLOGY 1A03 Lecture Notes - Gene Duplication, Alec Jeffreys, Meiosis

9 views5 pages
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
Unlock document

This preview shows pages 1-2 of the document.
Unlock all 5 pages and 3 million more documents.

Already have an account? Log in
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
Unlock document

This preview shows pages 1-2 of the document.
Unlock all 5 pages and 3 million more documents.

Already have an account? Log in

Document Summary

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. Ac, giving the sequence acacacac : 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: these highly repetitive stretches often align incorrectly when homologous chromosomes synapse and cross over during prophase of meiosis i. 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.

Get access

Grade+
$10 USD/m
Billed $120 USD annually
Homework Help
Class Notes
Textbook Notes
40 Verified Answers
Study Guides
Booster Classes
Class+
$8 USD/m
Billed $96 USD annually
Homework Help
Class Notes
Textbook Notes
30 Verified Answers
Study Guides
Booster Classes