Lecture 16 Eukaryotic Gene Structure Flashcards
Eukaryotes: endosymbiotic theory
Eukaryotes are more complex, have organelles with own bacteria. Thought that 1-1.5 billion years ago anaerobic eukaryote engulfed an aerobic eubacterium and evolved symbiosis with it (mitochondria/chloroplasts) organelles therefore have their own genomes.
Mitochondrial genome
Size varies between species
50 to millions per cell
Mitochondria grow/divide independently of the cell - maternal inheritance
Chloroplast genome
Much larger than mitochondrial genome
Up to 60 per cell
Grow and divide independently from cell
Organelle genome DNA barcoding
Organelle genomes evolve/change more slowly than nuclear genomes so are more commonly used for DNA barcoding species
Length of DNA
one big DNA molecule per chromosome: x chromosome is ~150 million BP = 5.07cm in length h.
Average human nucleus is 6micrometres (0.000006M) in diameter
46 chromosomes = 2.3m of DNA
It requires specialist packaging
Nuclear chromatin structure
At simplest level chromatin is helical dsDNA (2nm wide)
DNA is complexed with histones to form nucleosomes. Each nucleosome has 8 histone proteins w/DNA wrapped around 1.65 times (11nm)
Nucleosomes fold to form 30 nm chromatin fibre
That forms 300nm loops
Compressed and folded to 250nm wide fibre 700nm tall
Finally compacted into a 1400nm chromosome - more condensed means less accessible
Chromatin contains 2x as much protein as DNA
Histones
Abundant basic proteins
Have positive charge
5 main types H1 H2A H2B H3 & H4
4 pairs of histone proteins= 1 nucleosome
Evolutionarily conserved
Non histones
All other proteins associated with DNA
Differ in type/ structure
Amount and mass vary
Have other functions including scaffolding
DNA needs to be loosened for access
Polytene V. Large chromosomes sometimes seen in drosophila have “puff” regions where DNA has uncoiled and there’s visible high level gene activity - product of rapid replication w/out cell division keeping up
Same can happen in regular chromosomes
Heterochromatin Vs euchromatin
Hetero/Eu
Highly packed/ uncoiled chromatin
Genetically inactive/ active (transcribed)
Only in eukary/ in eukary and prokary
Degree of condensation
Changes during cell cycle and packing changes locally during transcript & replication
Histone modification
Histone tails can be modified by addition of small molecules - reversible
Lysine residues can be acetylated (CH3COO-) or methylated (CH3-)
Serine residues can be phosphorylated (PO4²-)
Modifications are interactive and summative (they add up to give diff effects)
Acetylation can shift heterochromatin to euchromatin
Methylation can silence gene expression
Phosphorylation is necessary for meiosis/mitosis
Chromatin “remodeling” exposed binding sites for regulation proteins
Chromatin remodeling complex binds to DNA and repositions nucleosomes exposing transcription factor binding site
Transcription apparatus assembles at binding site and transcription initiated
Eukaryotic gene expression v. Complex
Eukaryote DNA packaged
Cells have compartments
Complex organisms need to respond transcriptionally w/wide range of responses to multiple diff situations relevant to cell function
Eukaryotes that are multicellular usually diff cells have diff purposes and need diff proteins
E.g. skin cell forms keratin and tongue cell does not.
Only proteins appropriate to external condition/function produced
Plants
Have rapid transcriptional control to adapt to surroundings as cannot move e.g. touch stimulus in mimosa or Venus fly trap.
Respond to drought/flood, temp., Ozone and salt
Genes may be expressed in some cells and not others and produced when necessary e.g. hormones
