Genetics (Unit 2)

Chromatin

made out of DNA and protein, gets organized into chromosomes

Histones

• Positively charged proteins that form with negatively charged DNA in the nucleus

• Bind to DNA, creates nucleosomes, help give chromosomes their shape, and help control the activity of genes

heterochromatin

highly repeated non coding DNA that is usually tightly packed

euchromatin

coding DNA sequences that are loosely packed

centromere

heterochromatin, is essential for chromosome segregation

4 levels of organizational structure of DNA molecules in eukaryotes

nucleosomes, 30 nanometer fiber, chromatin, and metaphase chromosome

Nucleosomes

A 2 nm diameter DNA molecule coils around an octamer of histones. The resulting nucleosomes are a 6 nm by 11 nm flat disk, which includes spacer DNA plus H1 histone.

30 nanometer fiber

the nucleosome fiber continues to pack into a thicker structure consisting of multiple bundles of nucleosome cores coiled around and stacked upon one another. The bundles get separated from the next bundle by a H1, then get bundled into looped domains. 

Chromatin fiber

the looped domains condense further into a coiled structure 300 nm in diameters

Metaphase chromosome

the coiled chromatin fibers are compacted into the chromosome arms of a chromatid

histone tails

provide potential targets along chromatin fiber for chemical modifications

Acetylation

• Enzyme histone acetyltransferase (HAT)

• Addition of acetyl group to positivity charged amino acid group on side chain (lysine) 

• Changes net charge of protein by adding a positive charge 

Methylation

• Enzyme methyltransferase

• Adds methyl groups to arginine and lysine residues in histones 

• Positive correlation with gene activity 

Phosphorylation

• Enzyme Kinase 

• Adds phosphate groups to hydroxyl groups of amino acids serine and histidine 

_____ of DNA is another mechanism to regulate

gene expression

Methylation

Methylation of DNA

methylation of the base cytosine in DNA forms a 5-methyl cytosine and is usually negatively correlated with gene activity

Telomeres 

is a region of repetative DNA sequences at the end of chromosomes that protect the ends of chromosomes from becoming frayed or tangled 

Satellite DNA

is highly repetitive and consists of short repeated sequences, is found in the heterochromatic centromere regions of chromosomes. Not found in prokaryotes

Endogenous retroviruses

Ancient viral DNA inherited by a host and gets inherited like any other gene