How do chromosomes, DNA and genes all fit together? We can use the analogy of a city to better understand the relationship between DNA molecules. HIGH-FIDELITY chromosome segregation is important for normal cell growth .. the motif-based sequence analysis software MEME (wagtailfarm.info). .. The kinetochore and the centromere: a working long distance relationship. PDF | The development of silver-stained synaptonemal complexes (SCs) and of chromatid cores was analyzed in squashed and surface-spread grasshopper.
Research over the last decade has led to a growing appreciation of the existence and importance of both short-range linear and long-range three-dimensional chromatin interactions to overall regulation of gene expression Hong et al.
However, the molecular determinants underlying long-range transcriptional regulation remain poorly understood. One long-standing hypothesis of transcriptional repression is that the biochemical ability of a factor to polymerize might drive spreading of repressive complexes along the chromatin, thereby providing a mechanism of long-range repression Courey and Jia ; Roseman et al.
In vitro, the isolated SAM domains from these proteins form helical, head-to-tail polymers whose overall structural homology suggests a common mode of function. In vivo, mutations that disrupt SAM-mediated self-association have been shown to reduce or ablate repression activity of both the PcG and the ETS proteins in a variety of cultured cell and transgenic overexpression assays Roseman et al.
Comparable studies have not been performed yet for either human TEL1 or Drosophila Yan, and although it is widely inferred, it has not been demonstrated that SAM-mediated oligomerization drives the long-range PcG chromatin occupancy patterns. Here we have focused on the ETS family repressor Yan that acts downstream of receptor tyrosine kinase signaling in Drosophila to orchestrate a proper balance between proliferation and differentiation in a variety of tissues.
Thus depending on context, loss of yan leads to overproliferation or inappropriate cell fate specification, while overexpression of a constitutively active form can block the induction of a variety of neural, epithelial, and mesodermal cell fates Rebay and Rubin ; Rogge et al. These regulatory interactions have been proposed to provide a bistable switch that must be flipped for a cell to commit to a fate Graham et al.
To test the model that Yan self-association through its SAM domain can induce spreading of repression complexes over extended stretches of chromatin and to gain further insight into Yan-mediated regulation of gene expression during development, we compared the global chromatin occupancy profile of endogenous wild-type Yan to that of a recombineered genomic transgene carrying a missense mutation in the SAM domain that restricts the Yan protein to a monomeric form. Consistent with the starting chromatin spreading model, we find that wild-type Yan binds at developmentally important genes as clusters of densely packed peaks that span multiple kilobases, a pattern that is conserved between Drosophila melanogaster and D.
Condensin acts on this region to constrain the stretch, possibly by catenating the centromeric DNA.
In the absence of condensin fthe centromeric chromatin can be abnormally pulled by the attached microtubules arrows causing defects in executing mitosis. Chromosomes, white; microtubules, green. Condensin loads onto the chromosomes early in mitosis and is essential for maintaining their structure and organisation, but it is not essential for global chromatin compaction. RCA regulates mitotic chromatin compaction and its activity is mediated by protein phosphorylation.
Specialised chromosome structures and chromosome organisation in interphase. The two sister chromatids white are held together at the centromere and this defines the region where the kinetochore is assembled. The inner kinetochore red lies underneath the outer kinetochore greenwhich mediates the interaction with the spindle microtubules blue.
25+ Best Chromatin Memes | Nuclear Membrane Memes, Thicker Memes, Biology Cell Memes
Each sister chromatid contains a single linear DNA molecule and the terminal ends are called telomeres pink. Courtesy of J Dorrens, University of Edinburgh.
In a hybrid chicken cell containing one CHO Chinese hamster ovary chromosome, hybridisation with CHO genomic DNA reveals that the single CHO chromosome present in the cell f is not dispersed within the interphase nucleus e but maintains a compact organisation and a distinct territory. Journal of Cell Biology Holt, Rinehart and Winston.
Earnshaw WC and Rothfield N Identification of a family of human centromere proteins using autoimmune sera from patients with scleroderma. Nature Reviews Genetics 8: Biochemical and Biophysical Research Communications Hirano T and Mitchison TJ Topoisomerase II does not play a scaffolding role in the organization of mitotic chromosomes assembled in Xenopus egg extracts.
The evolutionary trajectory along which sex chromosomes evolve such opposite types of chromatin configurations remains unclear, as most sex chromosomes are ancient and no longer contain signatures of their transitions.
Gene expression is lower on the neo-Y than on the neo-X, which is associated with a higher level of binding of a silencing heterochromatin mark. The neo-X, on the other hand, shows no evidence of evolving hyperactive chromatin for dosage compensation.
Our results show that the Y chromosome can degenerate quickly, but the tempo and mode of chromatin evolution on the sex chromosomes may be constrained by the ancestral chromatin configuration.
Chromosomes and Chromatin
Introduction Sex chromosomes have originated independently many times from ordinary autosomes in both plants and animals [ 1 ]. A common feature of heteromorphic sex chromosomes is that while X chromosomes maintain most of their ancestral genes, Y chromosomes often degenerate due to their lack of recombination, with only few functional genes remaining for a recent review see [ 2 ].
The loss of gene function is often accompanied by an accumulation of repetitive DNA on ancient Y chromosomes, and a switch of chromatin structure from euchromatin to genetically inert heterochromatin [ 23 ].
Loss and silencing of Y-linked genes drives the evolution of dosage compensation on the X chromosome, which is often mediated by chromosome-wide epigenetic modifications.
Studies of young sex chromosomes have improved our understanding of the genomic and epigenomic mechanisms driving the divergence between X and Y [ 6 — 9 ].
Neo-sex chromosomes of Drosophila are formed by chromosomal fusions between the ancestral sex chromosomes and ordinary autosomes. The neo-Y, which is the autosome that became linked to the Y, entirely lacks recombination since it is transmitted through males only, which in Drosophila do not undergo meiotic recombination.