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Chapter 19B: Regulation of Gene Expression 2 - Coggle Diagram
Chapter 19B: Regulation of Gene Expression 2
DNA is associated with proteins called histones (DNA wraps around histone to make nucleosome)
Epigenetic regulation of transcription (chromatin remodeling)
Histone modifications
acetylation (transcriptional activation)
methylation (transcriptional repression)
Acetylation or methylation of lysines in histone tails affects DNA COMPACTION and controls the ACCESSIBILITY of genes to transcription factors
euchromatin = open chromatin --> transcription
heterochromatin = compact chromatin --> no transcription
DNA modification
cytosine methylation (transcriptional repression)
CpG island: CGCGCGCGCGCGC
absence of cytosine --> transcription
heavy methylation of CpG islands --> transcription repression
ex. an increase in DNA methylation can REPRESS transcription of important genes (i.e. to regulate cell division) --> cancer
Environmental factors can promote DNA methylation
ex. diet-induced changes in DNA methylation at the Agouti gene
diet enriched in methyl donors SUPPRESSES the agouti gene --> thin, healthy rat
How is deposition of epigenetic marks regulated?
long noncoding RNA
functions
mammals:
genomic imprinting
epigenetic phenomenon in which maternally or paternally derived alleles or whole chromosome sets are silenced in somatic tissue
paternally-derived X chromosome in marsupials (i.e. kangaroos)
all paternally-derived chromosomes in coccid insects
approximately 100 genes in mammals (sperm cells or egg cells)
in gametes, both chromosomes adopt the pattern characteristic of the sex of the individual (marks are erased, appropriate marks for female or male are put on)
X inactivation
problem: how do organisms with X and Y chromosome deal with the difference between X:autosome ratio in males and females
solution (in mammals): inactivate one X chromosome in females so that males and females have the same X:autosome ratio
Random X-inactivation in the embryo occurs at about the time of implantation in the uterine wall. How?
Xist noncoding RNA is transcribed from the X inactivation center and spreads along the X chromosome
1 more item...
plants: flowering time
Post-transcriptional regulation of gene expression
micro RNA
microRNAs repress TRANSLATION and promote RNA decay
Primary miRNA transcript is folded onto itself for complementary base pairing
Dicer enzyme recognizes this small piece of paired RNA and cuts it out
this little piece of miRNA is associated with RISC (RNA induced silencing complex) and guides RISC to a site on mRNA with complementarity
can either repress translation or cause this transcript to be cut and degraded
in animals, miRNAs bind to sites in the 3' UTR
only need small seed sequence to allow for association --> translational repression
miRNAs regulate many genes in humans
tissue and developmental stage-specific miRNA
ex. Ivy development
miR156/157 represses 10 SPL transcription factors
lots of miR156/157 levels as a seedlings --> low SPL protein levels
into adulthood, there are less miRNA levels --> higher levels of SPL proteins
Because the repressive marks on the MIR156A genes INCREASE through adulthood, activation marks on the gene DECREASES
60+% of protein coding mRNAs are regulated by miRNAs
each miRNA can regulate multiple mRNA targets