Naveen Sooknanan McGill Fall 2011
Organellar Chromosomes and DNA Barcoding:
DNA can not only be found in the nucleus of a cell, but also in specific organelles. In animals,
DNA can be found in the mitochondria (called mitochondrial DNA or mtDNA) whereas plant
cells contain DNA within their chloroplasts (cpDNA).
Theses chromosomes are important for respiration within the mitochondria or
photosynthesis within the chloroplasts
These genes are originally free floating and were endocytosed into the cell and
incorporated into their respective organelles
These genomes resemble prokaryotic genomes in many ways
o They are circular
o They typically lack introns
o Their genetic products resemble those of prokaryotes
There are many mitochondria or chloroplasts in one cell, and each of these organelles
may contain multiple genomes
Both organelles may have been formed from an ancestral cell which endocytosed a bacterium
capable of either respiration or photosynthesis.
We are not sure if this happened once or multiple times
Some animals, like seas slugs, eat specific algae types whose chloroplasts are endocytosed by the
slug’s epidermal cells.
As a result, the slug is able to undergo photosynthesis due to the algae it consumes
Gene exchange may also occur between the mitochondrion/chloroplast and the nucleus
Nucleus genome is much larger than Organellar DNA
Gene elimination may occur, which transports genes from the nucleus to the respective
organelles
The mitochondrion and chloroplasts may also transfer genes to the nucleus
The mitochondrion has many functions, including those listed
here. Many proteins needed in these functions are produced by
the nucleus DNA and the translated products are transferred to
the mitochondrion.
Human mitochondrial DNA (mtDNA) is a circular piece of
chromosomal DNA measuring 16,569 bp in length, which is tiny
compared to a gene within the nucleus.
Proteins can be synthesised using moving in either direction by
using each strand as a template
It only contains 37 genes because most of the genes in mtDNA
underwent gene elimination and were moved to the nucleus
o As a comparison, the e. coli chromosome contains over
5000 genes
Many encode components are needed for translation
There are no introns in mtDNA (like prokaryotes)
1Naveen Sooknanan McGill Fall 2011
Gene products stay within the mitochondrion
The genetic code is also slightly altered in the
mitochondrion:
Mutations in mtDNA can be related to aging in
animals. Mice with a mtDNA polymerase
defective for proofreading will exhibit premature
aging.
A wild type mouse can live around 1000
days
When the mtDNA polymerase is altered, the homozygous mutant mouse is only able to
live 400 days
Chloroplast DNA (cpDNA) is found in plants and is typically much larger than human
mitochondrial DNA.
They are typically 100-200 kbp In length and have a circular shape like mtDNA
They contain approximately 100-200 genes
cpDNA varies greatly in size and content depending on the plant species
DNA barcoding is a recent movement in molecular ecology which involved finding a unique
DNA sequence for every species in order to speed up identification.
There are over 2 million species which have been discovered by classical taxonomy
There are over and estimated 10 million species in existence
o DNA barcoding may be a more efficient way to figure out what these species are
mtDNA is the perfect choice of DNA for DNA barcoding because it is small and unique
to every individual
DNA barcode selection also has constraint:
The sequence differences have to be high enough to be able to distinguish between
different species, but not so high as to detect differences within the same species
The variable barcode sequence have to be surrounded by a conserved regions for primer
annealing when using PCR for gene duplication
The sequence must be easily aligned, and therefore must lack insertion/deletion regions
like introns
The PCR strategy is perfect for this technique because the primers can anneal to the conserved
regions on either side of the hypervariable region
The hypervariable region is unique to every species and is consistent in size between
organisms
The conserved region must be present because we have to know their sequence in order
to use the correct primers
Therefore, the perfect choices are mtDNA in animals a
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