DNA Structure and Variation

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DNA Structure and Variation

• Cynthia Downs
• Molecular Techniques
• September 6, 2007

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Genome

• The complete set of information in an organisms
  DNA.
• At least 2 types in a eukaryote cell
• Nuclear
• Organelle

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Human Genome
members.aol.com/chrominfo/ideogram.htm
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Chromosome

• Long threadlike structure of DNA and associated
  proteins that carries or all of the genetic
  information of an organism.

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Eukaryotes

• Linear, nuclear
• Nuclear chromosomes are packaged by proteins
  (particularly histones) into chromatin.

Wikipedia
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Interphase Chromosomes

• Euchromatin
• Active DNA, transcribed
• Less condensed
• Heterochromatin
• Highly condensed
• Transcriptionally inactive
• Typically around centromere and ends

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Prokaryotes

• Small, circular chromosomes

E. coli
Cairns (1963)
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Prokaryotes

• Also have plasmids

Wikipedia
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Gene

• A discrete unit of hereditary information
  consisting of a specific nucleotide sequence in
  DNA.
• A segment of DNA composed of a transcribed region
  and a regulatory sequence that makes
  transcription possible.

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Parts of a Gene

• Enhancers - distal control mechanisms
• Proximal control elements
• Promoter - controls the activity of a gene
• Exon - coding region
• Intron - interspersed in exonsdoes not code for
  proteins
• Terminator - terminates transcription

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www.unis.org
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Locus

• Specific place on a chromosome where a gene is
  located.

Nomenclature Gene OCA1Location 11q1.4 - q2.1
Chromosome
Band.sub-band
arm
Wikipedi.com
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Allele

• A variant of the DNA sequence at a given locus
• In a diploid organism, an individual has two
  alleles
• Genotype two alleles

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Heterozygous vs Homozygous

• Heterozygous two different alleles
• Homozygous two copies of one allele

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Dominance

• Types
• Complete dominance
• Incomplete dominance
• Codominance
• Defined by phenotype of heterozygote

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Complete Dominance

• Dominate trait - In a heterozygote, the allele
  that completely defines the phenotype
• Recessive trait - In a heterozygote, the
  allelethat is completely masked in the phenotype

Wikipedia.com
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Incomplete dominance

• Heterozygote expresses intermediate

www.emc.maricopa.edu
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Codominance

• Heterozygote expresses both phenotypes
• e.g., blood type AB expresses both A and B

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Ploidy

• The of homologous chromosomes in a cell.
• Diploid - 2 sets of chromosomes
• Most human cells
• Haploid - of chromosomes in a gamete
• Polypolidy - gt two sets of chromosomes

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Polyploidy

• Is a common speciation mechanism in plants
• 30-70 of angiosperms

2N 4N 6N
http//polyploid.agronomy.wisc.edu
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Autosomal loci

• Loci on the non-sex chromosomes
• In a diploid organism, there are two copies of a
  gene
• Normal rules of dominance can apply

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Sex-linked loci

• Genes located on sex chromosomes
• Homogametic sex - sex with 2 identical sex
  chromosomes
• Males in birds and butterflies
• Females in mammals
• Heterogametic sex - sex with 2 different sex
  chromosomes
• Females in birds and butterflies
• Males in mammals

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Sex-linked loci

• Heterogameticsex has only 1copy of each
  chromosome
• Recessive alleles are expressed more often

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No offspring of affected male are affected thus
trait skips generations
XCXc
XCY
XCXc
XcY
XCY
XcY
XCXc
But his female progeny are carriers, that can
have male affected progeny
XCXc
XcY
XcY
Example of X-linked recessive condition
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X-linked Inheritance in Humans

• Many genetic diseases are X-linked

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Other DNA in Eukaryotes

• Single, circular chromosome

Remind you of anything?
Mitochondrial DNA
Chloroplast DNA
home.nc.rr.com/ambiient/site/mtdna.htm
www.clivias.com/Articles/Article016.htm
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Endosymbiotic Theory

• Mitochondria and plastids (e.g., chloroplast)
  originated as separate prokaryotic organism.
• They were taken into cells as endosymbiots

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Endosymbiotic Theory

• Evidence
• Mitochondria and plastids DNA is different the
  nuclear DNA and is similar to bacterial DNA (its
  circular and small).
• Organelles are surrounded by 2 membranes
• Sequencing evidence
• Organelles rely on some genes from nucleus