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. Human Chromosome Number.
Trisomy 21 Causes Down Syndrome. X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate. Developing the Chromosome Theory. Meiosis, Genetic Recombination, and Sexual Reproduction.
Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees. Annunziato, Ph. Citation: Annunziato, A. Nature Education 1 1 Each of us has enough DNA to reach from here to the sun and back, more than times. How is all of that DNA packaged so tightly into chromosomes and squeezed into a tiny nucleus?
Aa Aa Aa. DNA, Histones, and Chromatin. The answer to this question lies in the fact that certain proteins compact chromosomal DNA into the microscopic space of the eukaryotic nucleus. These proteins are called histones , and the resulting DNA-protein complex is called chromatin.
It may seem paradoxical that proteins are added to DNA to make it more compact. However, if you have ever tried to store a garden hose, you know that it is much easier to do so if you begin by coiling the hose.
Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus Figure 2. At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes. The histones are evolutionarily conserved proteins that are rich in basic amino acids and form an octamer. The DNA which is negatively charged because of the phosphate groups is wrapped tightly around the histone core.
This nucleosome is linked to the next one with the help of a linker DNA. This is further compacted into a 30 nm fiber, which is the diameter of the structure. At the metaphase stage, the chromosomes are at their most compact, are approximately nm in width, and are found in association with scaffold proteins. In interphase, eukaryotic chromosomes have two distinct regions that can be distinguished by staining. If stretched to its full length, the DNA molecule of the largest human chromosome would be 85mm.
Although this compaction makes it easier to transport DNA within a dividing cell, it also makes DNA less accessible for other cellular functions such as DNA synthesis and transcription.
Thus, chromosomes vary in how tightly DNA is packaged, depending on the stage of the cell cycle and also depending on the level of gene activity required in any particular region of the chromosome. For more loosely compacted DNA, only the first few levels of organization may apply. Each level involves a specific set of proteins that associate with the DNA to compact it.
First, proteins called the core histones act as spool around which DNA is coiled twice to form a structure called the nucleosome. At the next level of organization, histone H1 helps to compact the DNA strand and its nucleosomes into a 30nm fibre. Subsequent levels of organization involve the addition of scaffold proteins that wind the 30nm fibre into coils, which are in turn wound around other scaffold proteins.
Certain dyes stain some regions along a chromosome more intensely than others, giving some chromosomes a banded appearance. The material that makes up chromosomes, which we now know to be proteins and DNA, is called chromatin. Classically, there are two major types of chromatin, but these are more the ends of a continous and varied spectrum.
Euchromatin is more loosely packed, and tends to contain genes that are being transcribed, when compared to the more densely compacted heterochromatin, which is rich in repetitive sequences and tends not to be transcribed. Heterochromatin sequences include short, highly-repetitive sequences called satellite DNA , which acquired their name because their bouyant density is distictly different from the main band of DNA following ultracentrifugation.
Chromosomes also contain other distinctive features such as centromeres and telomeres. Both of these are usually heterochromatin. In most cases, each chromosome contains one centromere. These fibrous proteins also ensure that each chromosome in a non-dividing cell occupies a particular area of the nucleus that does not overlap with that of any other chromosome.
Figure 1. When a cell undergoes mitosis, the chromosomes condense even further. DNA replicates in the S phase of interphase.
After replication, the chromosomes are composed of two linked sister chromatids. When fully compact, the pairs of identically packed chromosomes are bound to each other by cohesin proteins. The connection between the sister chromatids is closest in a region called the centromere. The centromeric region is highly condensed and thus will appear as a constricted area. DNA in eukaryotes is highly structured and organized in all stages of an organisms life. Diploid organisms contain a pair of each chromosome; humans have 23 pairs for a total number of 46 chromosomes.
Pairs of chromosomes, also known as homologous chromosomes, contain the same genes though there may be differences between the version of gene on each member of the pair. DNA compacts even further in preparation for cell division. Answer the question s below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section. Skip to main content. Module 8: Cell Division.
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