27-12-2012, 05:36 PM
Human genome_Mitochondrial genomes
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Mitochondrial genome
Human genome - term used to describe total genetic informn (DNA content) in human cells.
Comprises 2 genomes:
1) Complex nuclear genome - accounting for 99.9995% of total genetic informn, provides great bulk of essential genetic informn, most of which specifies polypeptide syn on cytoplasmic ribosomes.
2) Simple mitochondrial genome - accounting for remaining 0.0005% (Fig 7.1),
Mitochondria possess their own ribosomes, few polypeptide-encoding genes in them produce mRNAs which are translated on mitochondrial ribosomes.
As mitochondrial genome specifies only small portion of specific mitochondrial func’s; bulk of mitochondrial polypeptides are encoded by nuclear genes & are syntzd on cytoplasmic ribosomes, before being imported into mitochondria (Fig 1.11).
Human-mouse comparisions shows that <5% of genome is strongly conserved, including the 1.5% devoted to coding DNA & a somewhat higher percentage including conserved seq’s within untranslated seq’s regulatory elements & so on.
Majority of coding DNA used to make mRNA & hence polypeptides but a significant minority (atleast 5% & probably close to 10%) of human genes specifies noncoding (=untranslated) RNA (RNA genes).
Variety of novel RNA genes recently identified, forcing reassessment of RNA func.
Coding seq’s freq belong to families of related seq’s (DNA seq families) organized into clusters on 1 or more chromosomes or be dispersed. [Such duplicated seq’s aroused by various gene duplicn mechnsm’s, occurred during evolutn].
Mechanism’s giving rise to duplicated genes also give rise to nonfunctional gene-related seq’s (including pseudogenes & gene fragments).
Numerous defective copies of RNA genes scattered through genome, & for some polypeptide-encoding genes, too, many related pseudogenes are also found: Analyses of the finished chromosome 21 & 22 seq’s predicts a total of ~20,000 pseudogenes in genome.
As in other complex genomes, a very large component of human genome is made up of noncoding DNA.
Mitochondrial genes
Human mitochondrial genome contains 37 genes: 28 encoded by heavy strand, & 9 by light strand.
Of 37 genes, 24 specify mature RNA product:
22 mitochondrial tRNA molecules &
2 mitochondrial rRNA molecules,
23S rRNA (large subunit component) & 16S rRNA (small subunit component of mitochondrial ribosomes).
Mitochondrial genetic code
Mitochondrial genetic code decodes heavy & light chain transcripts deciphering only 13 diff mitochondrial mRNAs on mitochondrial ribosomes. [nuclear genetic code specifies ~70,000–80,000 diff mRNAs on cytoplasmic ribosomes].
It’s genome encodes all ribosomal RNA & tRNA molecules (needed for synthesizing proteins) but relies on nuclear-encoded genes to provide all other components (E.g., protein components of mitochondrial ribosomes, amino acyl tRNA synthetases, etc.).
As there are only 22 diff types of human mitochondrial tRNA, individual tRNA molecules need to be able to interpret several diff codons:
Isolation by buoyant density gradient centrifugation
When DNA is isolated from human cells by conventional methods, it is subject to mechanical shearing. Fragments are generated from the bulk DNA (with a base composition of ~42% GC) and fragments from the satellite DNA regions which may have a similar or different base composition. If the base composition is significantly different, satellite DNA sequences can be separated from the bulk DNA by buoyant density gradient centrifugation. Following centrifugation, they appear as minor (or satellite) bands of different buoyant density from a major band which represents bulk DNA. Typically, human DNA is complexed with Ag+ ions and then fractionated in buoyant density gradients containing cesium sulfate, whereupon three satellite bands are identified at different densities: satellite 1 – 1.687 gcm-3; satellite 2 – 1.693 gcm-3; satellite 3 – 1.697 gcm-3.