Download Genetic Controls in Eukaryotes

Level of Gene Control in Eukaryotes
 Chromosome structure and levels of DNA packing
DNA is associated with proteins called histones
o Nucleosome: DNA is wrapped twice around histones…
o Chromatin: nucleosomes ordered in tight spiral…
o Chromatin fibers folded in “looped domains”…
o more folding and spiraling into final chromosome
 Regulation of chromatin structure
- Histone modification
o Histone acetylation (acetyl groups -C0CH3) are attached to +
(lysine) histone tails.
o When acetylated = neutral and cannot bind to neighboring
nucleosomes
o No binding = loose structure = easier access to genes for
transcription.
o Remove these = deacetylation. = inactive genes
 Regulation of chromatin structure
- DNA Methylation
o Inactive DNA is highly methylated (-CH3)
o Ex. inactive X chromosome in mammalian females = barr
bodies Removal of methyl groups = active genes = = genomic
imprinting = methylation permanently regulates expression of
either maternal or paternal alleles of certain genes at the
start of development.
o Duel mechanism = = DNA methylation and histone
deacetylation can stop transcription.
 Regulation at transcriptional level
- Roles of transcription factors (enhancers & repressors)
o To initiate transcription requires transcription factors.
o Bind to promoter (TATA box); RNA Pol II can bind
o “General” transcription factors leads to slow transcription.
- General = essential to initiation of transcription of all
protein
o “Specific” transcription factors leads to faster transcription
= Specific to transcription of particular protein.
 Regulation at post-transcriptional level
- RNA processing
o Alternative RNA splicing = different segments of RNA are
treated as exons and introns = different mRNA
o Controlled by regulatory proteins specific to each cell type
o Consequence = a single gene can code for more than one
polypeptide = alternative RNA splicing
o Humans can have fewer genes than proteins that are made.
- mRNA degradation
o How long mRNA stays in cytoplasm will determine how much
protein synthesis…
o Use enzymes to shorten poly-A tail = triggers enzymes to
remove 5’ cap = enzymes chew up mRNA (can last minutes to
weeks)
o Untranslated region near 3’ end (poly-A tail) determine
mRNA’s length of longevity.
o Another reason,….microRNA’s (miRNAs)
o protein + miRNA binds to complementary sequence of mRNA
= blocks translation.
 Regulation at translational level
- Initiation of Translation
o Regulatory proteins can bind to mRNA at 5’ end preventing
initiation of translation (ribosomes can’t attach)
o Stored mRNA’s in an egg lack poly-A tails = can’t initiate
translation.
- At right developmental times, A’s are added so
translation can begin
- Protein processing and degradation
o Proteins have to undergo various chemical modifications
before they are activated and functional.
- add phosphates to activate = Ex. proton pump
- Cell surface protein and others must be transported
to a target destination in the cell in order to function
o How long protein lasts in a cell…selective degradation
- Ex. cyclins = proteins that regulate cell cycle are
short lived.