Eukaryotic Chromosome Structure The length of DNA in the nucleus is far greater than the size of the compartment in which it is contained. To fit into this compartment the DNA has to be condensed in some manner. The degree to which DNA is condensed is expressed as its packing ratio.
Leach ,1 Heather L. Chotkowski ,1 Michael G. Wotring ,1 Robert L. Dilwith ,1 and Robert L. BoxAlbany, NY Received Apr 21; Accepted May Abstract Heterochromatin is characteristically the last portion of the genome to be replicated.
In polytene cells, heterochromatic sequences are underreplicated because S phase ends before replication of heterochromatin is completed.
Truncated heterochromatic DNAs have been identified in polytene cells of Drosophila and may be the discontinuous molecules that form between fully replicated euchromatic and underreplicated heterochromatic regions of the chromosome.
In this report, we characterize the temporal pattern of heterochromatic DNA truncation during development of polytene cells. Underreplication occurred during the first polytene S phase, yet DNA truncation, which was found within heterochromatic sequences of all four Drosophila chromosomes, did not occur until the second polytene S phase.
DNA truncation was correlated with underreplication, since increasing the replication of satellite sequences with the cycE mutation caused decreased production of truncated DNAs.
Finally, truncation of heterochromatic DNAs was neither quantitatively nor qualitatively affected by modifiers of position effect variegation including the Y chromosome, Su varparental origin, or temperature. We propose that heterochromatic satellite sequences present a barrier to DNA replication and that replication forks that transiently stall at such barriers in late S phase of diploid cells are left unresolved in the shortened S phase of polytene cells.
DNA truncation then occurs in the second polytene S phase, when new replication forks extend to the position of forks left unresolved in the first polytene S phase. Eukaryotic chromosomes contain two distinct chromatin domains, euchromatin and heterochromatin.
Euchromatin typically encompasses a majority of the chromosome and contains most genes and other unique sequences.
Heterochromatin encompasses a smaller proportion of the chromosome, typically around the centromere and telomeres, and is enriched in noncoding, highly repetitive satellite sequences The presence of heterochromatin and its association with repetitive sequences is a conserved feature of eukaryotic chromosomes and is found in most, if not all, organisms including yeast, flies, and humans.
Heterochromatin is defined cytologically by its dark staining and condensed appearance and genetically by its ability to suppress gene expression. Numerous protein components of heterochromatin have now been identified, including the SIR proteins in yeast 18 ; HP1, Su varand ORC proteins in higher eukaryotes 58 ; specific acetylated isoforms of histone H4 356 ; and variant forms of histone H3 Our understanding of how these and other proteins contribute to the structure of heterochromatin and how that structure relates to the dynamic functions of heterochromatin is beginning to be broadened 4 Another conserved characteristic of heterochromatin is its late replication during S phase; it is most often the last portion of the genome to be replicated The mechanism for late replication is not known, although the condensed structure of heterochromatin and its ability to suppress transcription suggest that it might also suppress its own replication.
If heterochromatin suppresses replication origins, for example, heterochromatic sequences might be late in replicating because initiation must occur at distant euchromatic origins or because suppressed origins fire only in late S Alternatively, replication fork elongation might be substantially slower through heterochromatin than through euchromatin, resulting in longer and therefore later replication of heterochromatic sequences 17 A particularly dramatic consequence of the late replication of heterochromatic sequences is found in the polytene chromosomes of Drosophila melanogaster.
The copy number of euchromatic sequences in the polytene chromosomes of the larval salivary gland exceeds 1,c, while heterochromatic satellite sequences within the same chromosomes are severely underrepresented, remaining at or close to a copy number of 2c 1113 Underrepresentation is likely to be a consequence of the failure of satellite sequences to be replicated during the polytene cell cycle because of a shortened S phase that ends before the satellite sequences are fully replicated Consistent with this model, specific mutations in cyclin E, a regulator of S phase in both diploid and polytene cells of Drosophila 46can increase the copy number of satellite sequences in polytene chromosomes by increasing the length of S phase and allowing late replication of satellite sequences to be completed DNA molecules of discontinuous structure must occur between the fully replicated euchromatic and underreplicated heterochromatic regions of a polytene chromosome.
DNA structures consisting of nested replication forks have been proposed to populate such regions 26 ; however, truncated linear DNAs rather than fork-containing molecules have been found in these regions, prompting reconsideration of the underreplication model of heterochromatic underrepresentation 1415 In this study, we characterized the temporal pattern of heterochromatic DNA truncation during the development of polytene chromosomes.
The data suggest that replication barriers within heterochromatin inhibit fork elongation through satellite sequences. Replication forks stalled at such barriers fail to be resolved during the shortened S phase of polytene cells, causing underreplication of satellite sequences.Heterochromatin, DNA Replication, and Cancer.
Marcus B. Smolka, Molecular Biology and Genetics, is working to elucidate the mechanisms of proper heterochromatin replication to understand fundamental aspects of genome maintenance and the control of cell proliferation.
This project is centered on METTL13 (Methyltransferase-like 13). Heterochromatin Definition. Heterochromatin is a form of chromatin that is densely packed—as opposed to euchromatin, which is lightly packed—and is found in the nucleus of eukaryotic feelthefish.coms euchromatin allows the DNA to be replicated and transcribed, heterochromatin is in such a condensed structure that it does not enable DNA and RNA polymerases to access the DNA, therefore feelthefish.com · The transcription machinery requires access to the genetic information throughout the cell cycle, while replication machinery will copy the DNA during S-phase.
This added complexity is evident in key differences between euchromatin and heterochromatin, and also in the localization of chromatin within the feelthefish.com://feelthefish.com Free dna replication papers, essays, and research papers.
My Account. Your search returned over essays DNA Replication and Heterochromatin - Heterochromatin is a tightly packed DNA region where genes in such regions are usually not transcribed.
Numerous transposable elements (TEs) and repetitive DNA are found in . Another feature of heterochromatin is the presence of linker histones H1 that bind to DNA both at the entry and exit site of the nucleosome particle and which are enriched at CCs (She et al., ; Fyodorov et al., ).
The premeiotic DNA replication can be divided into three periods: In early S phase the DNA of euchromatin is replicated; in mid S phase synthesis is arrested for a minimum of 9 hours; and in late S phase the DNA of heterochromatin is replicated.