Control of Gene Expression (nucleus)
Cards (90)
- Regulation of transcription can occur via
- Histone and DNA modification
- Transcription factors
- RNA processing can also be regulated via splicing
- chromatin structure can affect genome expression
- gene is accessible then transcription will occur
- Gene:
- unit of heredity
- a section of DNA sequence encoding for a single protein
- note that this encodes for a single protein and not for a single trait as trait can be expressed by multiple geneses
- Genome: the entire set of genes in an organism
- Alleles: two genes that occupy the same position on a homologous chromosome and that cover the same trait
- Locus or Loci (multilocus): a fixed location on a strand of DNA where a gene or one of its alleles is located
- Activating the genome:
- histone acetylation
- nucleosome remodeling
- Histone acetylation: facilitated by Histone Acetyl Transferases (HATs) where it acetylates the N-terminal region of the histone, in particular the lysine
- Acetylation of the lysine-rich N-terminal region neutralizes the positive charge of lysine residue which reduced the affinity with negatively charged DNA
- Euchromatin: acetylated
- Heterochromatin: deacytelated
- Nucleosome remodeling influences the expression of individual genes
- Nucleosome remodeling is the modification or repositioning of nucleosomes within a short region of the genome
- 3 different ways nucleosomes can be remodeled:
- remodeling
- sliding / cis-displacement
- transfer / trans-displacement
- Transfer is the type of nucleosome remodeling where it is moving the histone all together
- Remodeling is the type of nucleosome remodeling where it loosens the wrapping on histones or DNA wrapped loosely
- Mechanisms that allow DNA accessibility:
- stable nucleosome
- remodeled nucleosome
- evicted nucleosome
- Silencing the genome:
- Histone deacetylation
- DNA methylation
- histone deacetylation:
- Facilitated by histone deacetylases (HDACs) which remove acetyl groups from histone tails
- histone deacetylation:
- In turn, this restores the charge of lysine (+) thereby increasing the affinity of the DNA to the histone
- DNA methylation:
- Methylation is facilitated by methyltransferases and DNA is methylated specifically at cystine residues preceding guanines (CpG dinucleotides)
- Methylation of cytosine occurs at the 5C
- DNA methylation:
- Commonly occurs at the transposable elements and it plays a key role in suppressing the movement of transposons
- 2 types of methylation:
- Maintenance methylation
- De novo methylation
- De novo methylation:
- happens when a methyl group is added to unmethylated DNA at specific CpG sites
- no replication at both/any strands
- Maintenance methylation:
- daughter strand, that is hydrogen-bonded to the methylated parent strand, becomes methylated
- facilitated by DNA methyltransferase protein (DNMT)
- Methylation can be reversed by TET family enzymes
- active genes are located in unmethylated regions
- in humans, 40%-50% of all genes are located close to CpG islands
- Housekeeping genes have unmethylated CpG islands
- Methyl-CpG-binding proteins (MeCPs) are components of both the Sin3 and NuRD histone deacetylase complexes (promotes chromatin packing)
- many transcription factors are repelled by methylation
- protein complexes can be attracted by methylation
- Mec - methyl-CpG binding protein 2
- MBD - methyl-CpG binding domain
- Epigenetic inheritance
- a transmission of information that is not contained within the sequence of DNA to daughter cells at cell division
- Epigenetic inheritance
- covers any difference in phenotype of an organism that does not result from changes in nucleotide sequence
- In epigenetic inheritance, histone modification plays an important role here since it can lead to stable patterns of modified chromatin allowing daughter cells to retain the memory for gene expression pattern observed in the parent cell
- Epigenetic inheritance:
- maintained through methylation of H3 lysine 27 by the polycomb proteins