19.1

how is DNA packaged into chromosomes

DNA and histone molecules form beads on a string, the extended chromatin fiber seen during interphase. A nucleosome has 8 histone molecules with the amino acid end tail of each projecting outward. A different type of histone H1 can bind to DNA next to a nucleosome where it helps to further compact the 10 nm fiber. The string of nucleosomes coils to form a chromatin fiber that is 30 nm in diameter. This form is also seen during interphase. During prophase further folding of the 30 nm fiber into looped domains forms a 300 nm fiber. The loops are attached to a scaffold of non histone proteins. The chromatin folds further resulting in the maximally compacted chromosome seen at metaphase. Each metaphase chromosome consists of 2 chromatids.

In eukaryotes the DNA protein complex called chromatin is ordered into higher structural levels than the DNA protein complex in prokaryotes. Support this statement by discussing the greater levels of organization that exist in eukaryotic chromosomes. How can the structure of chromatin regulate gene expression.

DNA and histone molecules form nucleosomes that are seen during interphase. A nucleosome has 8 histone molecules with the amino acid end projecting outwards. H1 binds to DNA next to a nucleosome to further compact the 10 nm fiber. The string of nucleosomes coils to form the 30 nm fiber that is also seen during interphase. During prophase, further folding of the 30 nm fiber into looped domains creates the 300 nm fiber. The loops are attached to a scaffold of non histone proteins. The 300 nm fiber folds further to form the maximally compacted chromosome that consists of 2 chromatids and is seen during metaphase. The structure of chromatin regulates gene expression because tightly packed heterochromatin is inaccessible to transcription enzymes and cannot be transcribed. However, the looser packing of euchromatin makes it accessible to transcription enzymes and available for transcription.

in general how does dense packing of DNA in chromosomes prevent gene expression

RNA polymerase and other proteins required for transcription do not have access to the DNA in tightly packed regions of a chromosome

describe the structure of a nucleosome the basic unit of DNA packing in eukaryotic cells

a nucleosome is made up of 8 histone proteins, 2 each of 4 different types around which DNA is wound. linker DNA runs from one nucleosome to the next one

define histones

a small protein with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in its chromatin structure

what chemical properties of histones and DNA enable these molecules to bind tightly together

histones contain many basic positively charged amino acids such as lysine and arginine which can form weak bonds with the negatively charged phosphate groups on the sugar phosphate backbone of the DNA molecule

define heterochromatin

nontranscribed eukaryotic chromatin that is so highly compacted that it is visible with a light microscope during interphase

define nucleosome

the basic beadlike unit of DNA packaging in eukaryotes consisting of a segment of DNA wound around a protein core composed of two copies of each of 4 types of histone

define chromatin

the complex of DNA and proteins that makes up a eukaryotic chromosome. When the cell is not dividing, chromatin exists as a mass of very long thin fibers that are not visible with a light microscope

define euchromatin

the more open unraveled form of eukaryotic chromatin that is available for transcription