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Lecture 4

Course Code
Jane Mitchell

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Lecture 4: Genomes & Chromosomes
1. Genomes
2. Bacterial DNA packaging
3. Eukaryotic chromosomes
4. Nucleosomes
5. Chromatin structure
Human genome
oPackaged very nicely in 23 chromosomes
o3 billion base pairs per genome
o25000 genes spread out across 23 chromosomes
oSex chromosomes: XX female, XY male
But genomes can come in all sizes…
oEverything from 3 billion pairs of human genomes to 10-15 kilo base pairs of
bacterial genomes
oIt’s circular – typical structure of prokaryotic DNAs
…and contrast these sizes with organellar genomes
oMost of the mitochondrial proteins are encoded by the nucleus
oThey are circular too which reflect the prokaryotic ancestry
Comparing genome sizes
oDoes size matter with the complexity of the organism?
Brain-eating amoeba
oIts genome is incredibly huge than the human genome
oThey go into your nose when you swim in the lake and it consumes neurons in
the back of your brain
Comparing genome sizes
oAn organism’s complexity and size does not reflect in genome size
The human genome
oAbout 1.5% of your genome encodes protein
o50% of your genome is repetitive DNA – AGAGAGAG….TCTCTCTC…
oHow could you go from 25000 genes to all the way to a much more complex
transcriptome and upwards of 100000 of proteins?
You have primary transcript of a specific gene that is processed later on
during transcription into multiple transcripts from that single gene that
would produce multiple genes
You have the primary translate and the protein is made off of specific RNA
molecule but then it’s modified into variable ways, either through
proteolytic clipping, parts of it is cut off, etc.
In this way, the diversity of proteins and even RNAs doesn’t always
necessarily reflect the number of genes
oIn the genome, we have protein encoding genes and within that protein
encoding genes we have introns (non-sequence coding gene)
oThere’s also non-repetitive DNA that is neither in introns nor codons
regulatory structures, regions that might be involved in the parts that might be
transcribed but not necessarily translated so that they can be regulatory RNAs
o50% of the genome arises from transposons integrating elements that are
capable of effectively duplicating themselves over revolutionary time
oRetroviral-like elements are also found in transposons which again has the
capacity of over evolutionary time to increase in abundance
Packaging of DNA in the cell
oPackaging occurs in prokaryotes and eukaryotes
oIn prokaryotes, DNA is condensed through folding and twisting about 1000 fold
oBound together, DNA and protein form the nucleoid
oYou use positively charged polyamines help to keep the charge shielded, so
that you don’t wind up with inability of the cell to fold DNA onto itself
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