Structure of Chromatin Toyosi Coker, Maddie Irvin, Abby Keller, Ryan Kilgore

Nucleosome Histone A bead like structure which is a segment of DNA that is wrapped around a histone protein. A good comparison is the thread which is wrapped around a spool. With the thread being the DNA and the spool the histone proteins. This helps to keep the DNA compacted. A protein which DNA is wrapped around and formed into structural components called nucleosomes. Histones are the chief protein components of chromatin. It is the “spool” which DNA or “thread” is wrapped around. DNA wraps around 8 histone molecules approximately twice. “Beads on a string”- Refers to the looks of the string of DNA after nucleosomes have been formed. The nucleosomes resemble the beads on the DNA string.

Structure of Chromatin Scaffold proteins- Attached to DNA at specific regions (scaffold attachment regions). Form loops in the unwound form of the chromatin (“beads-on-a- string”). The chromatin fiber and associated scaffold proteins coil into a helical structure which may be observed as a chromosome. In looped domains, the loops are attached to a scaffold of non-histone proteins.

Chromatin Structure Higher Levels of DNA packing: There are several orders of chromatin coiling, each in order of increasing compactness. DNA- coils around histones to form a nucleosome. Look like “beads on a string” formation. The beaded string coils/folds to form chromatin fiber (30 nm). The chromatin fiber forms loops called looped domains. They are in the extended form of the chromosome. The looped domains condense tightly into small portions of the mitotic chromosome.

Chromatin Types Euchromatin (loosely-packed) Heterochromatin (tightly-packed) Euchromatin (loosely-packed) Found in regions of a chromosome where there are few to no genes Two types: constitutive and facultative Functions as a regulator of gene expression Found in regions of o chromosome where there are many genes Unfolded structure Functions in the active transcription of DNA to mRNA

DNA Methylation & Histone Acetylation Attachment of methyl groups (-CH3) Activate DNA Cellular differentiation in embryo Genomic imprinting Attachment of acetyl groups (-COCH3) to the amino acids in histone proteins Deactylation

Video http://www.youtube.com/watch?v=0D54U2CK5ek&feature=related

Review Terms Positive and Negative charges- helps form the nucleosome. Histone proteins have a positive charge due to it’s amino acids. DNA has a negative charge due to its phosphate backbone. Amino Acid- The amino acid is altered in methylation and acetylation. Protein- structural proteins attach with DNA to form the structure of the chromatin. DNA- segments form genes. Heterochromatin effects gene expression. Chromosome- the very condensed form of chromatin.

The Review begins! 1. What is the role of histones in DNA packaging (the compacting of DNA so it can fit into a smaller space) and what do they help form? 2.Describe the Nucleosome “beads”. 3. What are the levels of chromatin packing from lowest to highest order? 4. What is the most fundamental form of chromatin? 1. To provide a structure DNA can wrap around which forms nucleosomes. They help in the first levels of DNA packing. 2. The nucleosomes form the “beads” along the string of DNA in the “Beads on a String” analogy. 3. Nucleosomes - chromatin fiber-looped domains- metaphase chromosome. 4. DNA -histone complex (beads on a string formation) DNA is wound around histones.

The Review continues! 5. What is the main difference between heterochromatin and euchromatin? 6. In what areas of a chromosome would you generally find heterochromatin? Euchromatin? 7. What three significant functions of DNA methylation? 8. What is the effect of histones changing shape in histone acetylation? 5. Heterochromatin is tightly- packed chromatin within the nucleus (more dense), while euchromatin is loosely-packed chromatin within the nucleus (less dense) 6. Heterochromatin is found in areas where there are few to no genes present (specifically the in and around the centromere and the telomeres, while euchromatin is found in areas where there are many genes 7. activate DNA, cellular differentiation in embryo, and genomic imprinting. 8. They grip the DNA less tightly, and transcription proteins have easier access to genes in the acetylated region