Chromosome+Architecture+and+its+Roles+in+Gene+Expression

In late 1800s, Deoxyribonucleic acid (DNA) was isolated by Friedrich Miescher, and was called nuclein. Later, by the early 1950s, the work of Pheobus Levene, and followed by Alexander Todd elucidated the chemical structures of DNA, which consisted of nucleotides: sugar and phosphates bases.DNA was identified in the 1940s as a carrier of genetic information, or a hereditary information of cells. Its structure was determined by James Watson and Francis Crick who used X-ray crystallography to predict the double helix (2 complimentary polynucleotide chains), and by Chargaff’s rule: G-C and A-T. Genes are carried on chromosomes. In nineteenth century as threadlike structures in the nucleus of a eukaryotic cell that become visible as the cell begins to divide. - Bacterial chromosome: Circular - Eukaryotic chromosome: Linear or circular Different types - in mitochondrial and chloroplast have their own chromosome Unduplicated or duplicated (sister chromatids) Human: 23 in females, 24 in males. Chromosome consists of DNA (coiled) and protein (package and control the enormously long DNA molecules). The complex of DNA and protein is called chromatin (Greek: color). (petite: Small, followed by q)
 * DNA and Chromosome **
 * [[image:fafa.png]] ||
 * **Figure 1:** Nucleosomes as seen in the electron microscope. A) Chromatin isolated directly from an interphase nucleus. B) Experimentally unpacked or de-condensed chromatin after isolation to show the nucleosomes ||
 * [[image:wradsadcwc.png width="298" height="625"]] ||
 * **Figure 2: **Structural organization of the nucleosome ||

- 146 or 147 bp DNA/ histone octamer - Nucleosome = a protein core of eight histone molecules (H2A, H2B, H3, H4) - //<span style="font-family: 'Times New Roman',serif; font-size: 12pt;">Histone H1: //<span style="font-family: 'Times New Roman',serif; font-size: 12pt;"> Linker histone, with nonhistone proteins play role in structure of chromosomes and may affect gene expression. Histone H1 also plays a very important role in chromosome architecture. The protein changes the direction of the DNA to help the packing process more efficiently. <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">The position of nucleosome on DNA is determined by both DNA flexibility and other DNA-bound proteins <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">- A highly condensed form: heterochromatin: Most DNA does not contain genes in this form, or genes are resistant to being expressed, to protect good gene during mitosis and meiosis. It creates different types of compact chromatin with distinct features and roles. <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">A less condensed form: euchromatin: Drosophila experiment: position effects <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">Chromatin can move to specific sites within the nucleus to alter gene expression <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">Alberts, Bruce. //Molecular Biology of the Cell//. Grand Science, New York. 4th ed. <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">Voet, D., Voet, J., //Biochemistry.// Wiley, New Jersey. 4th ed. <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">Brooker, R. J., //Genetics Analysis & Principles.// McGraw-Hill Higher Education, Boston. 3rd ed.
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt;"> Protein ****<span style="font-family: 'Times New Roman',serif; font-size: 12pt;">: **<span style="font-family: 'Times New Roman',serif; font-size: 12pt;">The protein is called histone which has a globular domain and an amino terminal tail. They are usually basic protein with a larger number of positively charged Lys and Arg amino acids, which form electrostatic and hydrogen bonding interactions with Pi along DNA backbone (DNA is usually negatively charged).
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt;"> Histones with gene expression: **
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt;"> Modification on histones core can facilitate access to DNA: **
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt;"> Sources: **