Chromatin packing in eukaryotic cells

Prokaryotic• Smaller genomes • DNA is in a nucleoid region

and is circular.• Prokaryotic cells may have a

separate circular DNA called a plasmid.

Eukaryotic• Larger genomes 6 billion

base pairs (bp) • Genomes are linear• More biochemical editing is

needed. Remember INTRONS and EXONS

Chromatin packing in Eukaryotic cells

• Histones- small proteins usually 100 amino acids in length with positive charges that bind to the negatively charged DNA double helix.

• Nucleosome- is the ENTIRE beadlike unit of DNA packing including histones and DNA.

This model depicts a cross section or end view.

The need to turn transcription on and off.

• All genes all cells.• What are the four basic cell types in

eukaryotes?

SUMMARY THUS FAR

• DNA is coiled around histones.• Tightly coiled DNA is inaccessible to gene reading

machinery.• Methyl groups (-CH 4) molecules bind to the

histone tails and block access to genes and inhibit transcription. This may not be bad. Genes need to be turned off.

• Acetyl (Acetylation) molecules bind to histones and loosen chromatin structure that permits transcription.

IMPLICATIONS

• Epigenetic inheritance- inheritance of traits not directly involving nucleotide sequences.

• Say again?• Molecular mechanism of heritability may not

be limited to DNA sequence differences.

Identical twins in which one acquires a genetic disease and one does not.

• Identical twins are derive form an identical zygote that splits at the two cell state.– Monozygotic twins (MZ) identical– Dizygotic twins (DZ) fraternal

Paradigm shift??????????

• A change in basic assumptions or paradigms within the ruling theory of science. IT IS A REVOLUTION! ?????

Thomas Kuhn: “The Structure of Scientific Revolution”

• A scientific revolution occurs, according to Kuhn, when scientists encounter anomalies that cannot be explained by the universally accepted paradigm within which scientific progress has thereto been made.

• 1962

Barbara McClintock and trasnsposons or transposable genetic elements

• Expression of genes can be tied to other factors besides nucleotide sequences.

• Randy Jirtle Rober Waterland of Duke University

• Diets supplemented with methyl donors such as choline, folic acid, and B-12) decreased the incidence of offspring with yellow coat color which results in REDUCING the risk of developing obesity, diabetes and cancer.

• Jirtle and Waterland, 2003 nature paper