When the cell divides, one copy goes to each of the resulting cells. If the chromosomes do not line up properly, severe genetic abnormalities can occur, which can lead to death of the cell or cancer. Condensing the DNA into tightly packed chromosomes makes the process of chromosome alignment and separation during mitosis more efficient.
Gene expression, or the process of a gene being turned on and transcribed, is a complex process. It involves the binding of certain proteins, known as transcription factors, to the portion of the gene that regulates its expression.
Most transcription factors promote expression of a gene; however, some transcription factors prevent a gene from being expressed, in other words, turning it off. The way DNA is wrapped can affect gene expression, or which genes are turned on.
When the chromosomes are tightly condensed, the DNA is wrapped very tightly, making it difficult for transcription factors to bind to the DNA. When the DNA is less tightly wrapped around the histones, the histones themselves can affect gene expression. Modifications, such as binding of phosphate groups, can occur on the histones and these modifications can cause the DNA to bind more or less tightly to the histones.
Regions of the DNA that are only loosely bound to the histones are more accessible to transcription factors and RNA polymerase, making those genes easier to turn on. When the DNA is bound more tightly to the histones, however, it is more difficult for transcription factors and RNA polymerase to bind to the DNA, making it more likely that those genes will be turned off.
Part of the problem lies in the fact that electron microscopy is perhaps the best way to visualize packaging, but individual nucleosomes are hard to discern after the fiber has formed. In addition, it also makes a difference whether observations are made using isolated chromatin fibers or chromatin within whole nuclei. Thus, the nanometer fiber may be highly irregular and not quite the uniform structure depicted in instructive drawings such as Figure 1 Bednar et al.
Interestingly, histone H1 is very important in stabilizing chromatin higher-order structures, and nanometer fibers form most readily when H1 is present. Processes such as transcription and replication require the two strands of DNA to come apart temporarily, thus allowing polymerases access to the DNA template. However, the presence of nucleosomes and the folding of chromatin into nanometer fibers pose barriers to the enzymes that unwind and copy DNA.
Generally speaking, there are two major mechanisms by which chromatin is made more accessible:. When eukaryotic cells divide, genomic DNA must be equally partitioned into both daughter cells. To accomplish this, the DNA becomes highly compacted into the classic metaphase chromosomes that can be seen with a light microscope.
Once a cell has divided, its chromosomes uncoil again. Comparing the length of metaphase chromosomes to that of naked DNA, the packing ratio of DNA in metaphase chromosomes is approximately 10, depending on the chromosome. This can be thought of as akin to taking a rope as long as a football field and compacting it down to less than half an inch. This level of compaction is achieved by repeatedly folding chromatin fibers into a hierarchy of multiple loops and coils Figure 1. Exactly how this is accomplished is unclear, but the phosphorylation of histone H1 may play a role.
Indeed, this is just one area of DNA packaging that researchers will continue to explore in the years to come. Bednar, J. Nucleosomes, linker DNA, and linker histones form a unique structural motif that directs the higher-order folding and compaction of chromatin. Proceedings of the National Academy of Sciences 95 , — Fischle, W. Histone and chromatin cross-talk. Current Opinion in Cellular Biology 15 , — Hewish, D.
Chromatin sub-structure. The digestion of chromatin DNA at regularly spaced sites by a nuclear deoxyribonuclease.
Biochem Biophys Res Commun 52 , Kornberg, R. Chromatin structure: A repeating unit of histones and DNA. Science , — Luger, K. Crystal structure of the nucleosome core particle at 2. Nature , — link to article. Noll, M. Subunit structure of chromatin. Olins, A. Spheroid chromatin units v bodies. Olins, D. Chromatin history: Our view from the bridge. Nature Reviews Molecular Cell Biology 4 , — link to article. Oudet, P. Electron microscopic and biochemical evidence that chromatin structure is a repeating unit.
Cell 4 , — Smith, C. ATP-dependent chromatin remodeling. Current Topics in Developmental Biology 65 , — Thomas, J. Octamer of histones in chromatin and free in solution. Proceedings of the National Academy of Sciences 72 , — Van Holde, K. A model for particulate structure in chromatin. Nucleic Acids Research 1 , — Woodcock, C. A milestone in the odyssey of higher-order chromatin structure. Nature Structural and Molecular Biology 12 , — link to article.
Structural repeating units in chromatin. Evidence for their general occurrence. Experimental Cell Research 97 , — Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities. Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes. Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics. Copy Number Variation and Human Disease.
Genetic Recombination. Without replication, each cell lacks adequate hereditary fabric to give instructions for creating proteins vital for bodily purpose. How is DNA packaged in nucleus? To package DNA inside the nucleus, cells wrap their DNA strands around scaffolding proteins to form a coiled condensed structure called chromatin. Chromatin is further folded into higher orders of structure that form the characteristic shape of chromosomes. How much DNA can be packed into a cell?
The haploid human genome contains approximately 3 billion base pairs of DNA packaged into 23 chromosomes. Of course, most cells in the body except for female ova and male sperm are diploid, with 23 pairs of chromosomes. That makes a total of 6 billion base pairs of DNA per cell. How much DNA is in a chromosome?
One chromosome has 2 strands of DNA in a double helix. But the 2 DNA strands in chromosomes are very, very long. One strand of DNA can be very short - much shorter than even a small chromosome. How is DNA compacted into a chromosome? When fully compact, the pairs of identically-packed chromosomes are bound to each other by cohesin proteins. Is a chromosome a single DNA molecule? DNA molecules are long — so long, in fact, that they can't fit into cells without the right packaging. To fit inside cells, DNA is coiled tightly to form structures we call chromosomes.
Each chromosome contains a single DNA molecule. Humans have 23 pairs of chromosomes, which are found inside the cell's nucleus. Is DNA condensed in S phase? In the S phase, DNA replication results in the formation of identical pairs of DNA molecules, sister chromatids, that are firmly attached to the centromeric region.
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