Molecular Organization of Chromosomes PDF

 Composition :  The term genome refers to the sum of an organism's genetic material.  The bacterial genome is composed of chromosomal deoxyribonucleic acid or DNA and represents the bacterium's nucleoid.  Unlike the eukaryotic nucleus, the bacterial nucleoid has no nucl...

 Composition :  The term genome refers to the sum of an organism's genetic material.  The bacterial genome is composed of chromosomal deoxyribonucleic acid or DNA and represents the bacterium's nucleoid.  Unlike the eukaryotic nucleus, the bacterial nucleoid has no nuclear membrane or nucleoli.  H H1 HLP1 HU RNA RNA & Proteins center 1- Folding 2- super coiling Enzymes treatment 1- Rnase & Proteinase 2- Dnase  The nucleus contains the chromosomes of the cell.  Each chromosome consists of a single molecule of DNA complexes with an equal mass of proteins. Collectively, the DNA of the nucleus with its associated proteins is called chromatin.  Most of the protein consists of multiple copies of 5 kinds of histones.  These are basic proteins, bristling with positively charged arginine and lysine residues. (Both Arg and Lys have a free amino group on their R group, which attracts protons (H+) giving them a positive charge.) Just the choice of amino acids you would make to bind tightly to the negatively-charged phosphate groups of DNA.  Histones are some of the most conserved molecules during the course of evolution. Histone H4 in the calf differs from H4 in the pea plant at only 2 amino acids residues in the chain of 102.  Small, + charged  5 types (H1, H2A, H2B, H3, H4)  Conserved amongst eukaryotes  Distributed specifically throughout chromosome  conserved throughout generations Function:  Chromosome packaging  Protect DNA from heat and radiation  Chromatin also contains small amounts of a wide variety of nonhistone proteins  No conservation  Dispersed irregularly throughout chromosome  Multiple functions  Structure  Aid in DNA activities  Regulatory role in gene expression  Transcription factors The levels of chromatin organization Nucleosomes : Two copies of each of four kinds of histones H2A ,H2B ,H3 and H4 form a core of protein, the nucleosome core.  Around this is wrapped about 147 base pairs of DNA. From 20–60 bp of DNA link one nucleosome to the next.  Each linker region is occupied by a single molecule of histone 1 (H1). The binding of histones to DNA does not depend on particular nucleotide sequences in the DNA but does depend critically on the amino acid sequence of the histone. 1. Nucleosome 200 bp 400 bp 600 bp 2. Solenoid 2. Solenoid Types of chromatin  is found in parts of the chromosome that contain many genes.  Euchromatin is "active" chromatin, containing DNA sequences that are being transcribed into RNA.  is loosely-packed in loops of 30-nm fibers.  These are separated from adjacent heterochromatin by insulators  is found in parts of the chromosome where there are few or no genes, such as centromeres and telomeres  is densely-packed; more condensed (150 μm) (stains more intensely)  is replicated late in S phase of the cell cycle;  has reduced crossing over in meiosis.  Those genes present in heterochromatin are generally inactive. Heterochromatin is classified into the following two types: 1- Constitutive heterochromatin: In most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres. 2- Facultative heterochromatin : It represents the inactivated and condensed segments of euchromatin ; it is expressed under certain condtions. heterochromatinizatin of one of the two X chromosome of human females is a common.  The nucleosome is a core particle consisting of DNA wrapped about two times around an octamer of eight histoneproteins (two copies each of H2A, H2B, H3, and H4),much like thread wound around a spool.  The DNA in direct contact with the histone octamer is between145 and 147 bp in length, coils around the histones in a left-handed direction, and is supercoiled. It does not wrap around the octamer smoothly; there are four bends, The fifth type of histone, H1, is not a part of the core particle but plays an important role in the nucleosome structure. The precise location of H1 with respect to the core particle is still uncertain. The traditional view is that H1 sits outside the octamer and binds to the DNA where the DNA joins and leaves the octamer.