Genetics Lecture 22, 10/22 - Eukaryotic Chromosome Structure, Problems with Packing DNA

Eukaryotic Chromosome Structure

• Dependent on the cell cycle
• Interphase --> chromatin --> less condensed form of DNA
• mitosis --> chromosome 10,000 fold condensation of chromatin to form distinct structures.
  
• overall organization of a eukaryotic chromosome is greater than the organization of the prokaryotic chromosome.
• E. Coli has a single chromosome with a length of 1,200um. Humans have multiple chromosomes (46) and the length is 19,000 to 73,000um.
• Two protiens that associate with DNA during condensation
• histones - major protein responsible for packing
• overall charge on a histone is positive. DNA has a negative charge.
• non histone proteins

(6:10) Evidence for how the packing of DNA in eukaryotic chromsomes occurs

• 1.) digestion of chromatin with an endonuclease. Yields fragments that are consistently 200bp in length. Suggests that the DNA in chromatin is in a repeating unit
  
• 2.) microscopic visualization shows that chromatin fibers are linear arrays of spherical particles. Look like "beads on a string"
• 3.) observed that histone proteins could interact with each other. In this interaction that formed a tetromere structure and proposed that one tetromere associated with 200 BP of DNA.
• 4.) refined endonuclease digestion data. 146 BP of DNA that associates with two tetromer of histones.

PACKING

• (12:30) First level of chromatin packing. Nucleosom core particle or the 11nm fber. 146 BP of DNA wrap around two tetromere of histones. End result is DNA with a diameter of 11nm.
• second level. 30nm fiber. Solenoid structure - you have a number of nucleosome core particles. These align around a central histone called the H1 histone. Creates a DNA molecule with a diameter of 30nm, chromatin exists as teh 30nm fiber.
• Mitotic chromosome with a diameter of 1400nm

(19:45) PROBLEMS WITH PACKING DNA

• packing leaves the DNA inaccessible to certain non-histone proteins. Ways to get around this problem is called Chromatin remodiling.
• (22:45) Chromatin remodeling: allows the packing of DNA to be temporarily relaxed so it can be replicated or transcribed.
• this is done by modifying amino acids on the histone proteins to weaken their association with DNA:
  
• Acetylation: add an acetyl group to lysine. this removes the positive charge on lysine.
• methylation: add a methyl group to lysine and arginine thus altering charges to allow brief access
• phosphorilation: add a phosphate to either serine or histidine. END RESULT: weaken the association between the histone and the DNA.
  
• (28:40) Within a chromosome there are different regions - two different levels of DNA packing
• Euchromatin - DNA that is undergoing normal packing. The genes in those regions can be expresed. APX. 90% of the DNA in the cell.
  
• Heterochromatin - DNA that has undergone extreme levels of packing. Highly condensed in comparison to euchromatin. Because of this the genes in these regions are not expressed.
• Unique features of heterochromatin:
• 1.) regions of DNA that are heterochromatin are gentically inactive.
• 2.) regions of heterochromatin are replicated later in the cell cycle (S phase). It is proposed that heterochromatin is important to the structural integrity of the DNA.
  
• 3.) Heterochromatin is unique to Eukaryotes.
• (34:55) What's composed of heterochromatin
• centromere, telomeres - sequences on the end of linear chromosomes, the majority of the Y chromosome
• Often see that regions adjacent to heterochromatin exhibit position effect. Genes in these regions are not expressed.

(38:35) REPETITIVE SEQUENCES ON CHROMOSOMES

• centromere - location of chromsome attachment during mitosis and meiosis, spindle fiber attachement. Within this there is a CEN sequence which is composed of three parts.
  
• 1st and 3rd - regions that are found on all chromosomes with high similarity.
• 2nd region is unique from chromosome to chromosome. BUT they're similar in homologous chromosomes.
• (42:20) Did mutational analysis on these sequences --> found that the 3rd is most critical to the function of the centromere
• (43:30) Telomere - repetitive sequence found on the end of linear chromosomes. Contains a number of repeats of the sequence GGGATT. Telomeres vary in length. All the individuals have different lenghts of telomeres. Role of telomere: protect the DNA at the end of teh chromosome from degredation. Only 5-10% of the DNA in an organism is usde in genes.

47:20 - whats on the test for 10/24