Class Notes (808,045)
Canada (493,031)
Biology (Sci) (2,417)
BIOL 200 (478)
Richard Roy (213)

11. Eukaryotic Chromosomes.pdf

4 Pages
Unlock Document

McGill University
Biology (Sci)
BIOL 200
Richard Roy

Interphase Higher-order chromosome folding Loops of scaffold "Beads on a string" Gene elements Nucleosome Introns Spacer DNA Interphase Higher-order chromosome folding Loops of scaffold "Beads on a string" Gene elements Nucleosome Introns Spacer DNANaveen Sooknanan McGill Fall 2011 Eukaryotic Chromosomes: DNA strands undergo a series of complex folds in order to compact itself. A compressed human genome can measure 10 cm in length, and therefore these compacting measures are necessary for the genome to be able to fit into the nucleus. There are 2 main levels of organization found in eukaryotes: chromatin, which is a nuclear protein complex, and chromosomes, which occur during cell division.  Mitosis is the division stage of the cell. During this time, DNA is organized into chromosomes  The cell spends rest of its life in interphase during which the cellular DNA is organized into chromatin. There are three phases in interphase during which the cell grows, producing proteins and cytoplasmic organelles: o G1, the first gap, involved cell growth o S, synthesis, involves division of chromosomes o G2, the second gap, involved more growth and preparation for mitosis When DNA is isolated with an isotonic buffer (same as intracellular salt concentration), it is observes that this DNA associates with an equal mass of proteins in order to form chromatin. There are various levels of organization which allow for the final chromatin structure to exist:  The first level of organization is the wrapping of the DNA strand around proteins called histones o This forms beads which are approximately 10 nm in diameter which are called nucleosomes. Each nucleosome is separated by “linker DNA” which are 10-90 bp in length o These “beads on a string” can be seen when the DNA is places in a solution with low salt concentration o There are 5 major types of histones: H1, H2A, H2B, H3 and H4  They are rich in positively charged amino acids which facilitate interactions with negative phosphate groups of the DNA strand  H1 is variable while the others are highly conserved (virtually the same in plants and animals) o Nucleosomes consist of a protein core with DNA measuring 147 bp in length wrapped around the surface, producing almost two full turns  The protein core is made of an octamer containing histones H2A, H2B, H3 and H4  The next level of organization is a 30nm chromatin fibre, which can be seen when the DNA is placed in a physiological salt concentration solution 1Chromosome scaffold H4' H3 H2B H2A H2A H3 H4 H2B H2A H2B H2B Phosphorylation mark Loop of 30 chromatin fiber Chromosome scaffold H4' H3 H2B H2A H2A H3 H4 H2B H2A H2B H2B Phosphorylation mark Loop of 30 chromatin fiberNaveen Sooknanan McGill Fall 2011 o The exact structure of these fibres is debated, but the most likely structure is a chain of nucleosomes stacked, every 2 one on top of the other, which form a zigzag ribbon and then a 2 start helix structure (a helix within a helix) o The histone H1 stabilizes this condensed fibre structure o The condensation of nucleosomes into histones is regulated by histone tails  These histone tails contain C terminal tails and N terminal tails, both of which are modified post-transnationally by enzymes  These tails are responsible for gene regulations using methods such as acetylation, methylation and ubiquitination  Lysine on H4 can be acetylated, which affects the regulation of condensation o Post transcriptional modification methods mentioned above all work towards loosening or tightening the DNA structure to either promote or inhibit chromatin fibre formation  These methods alter charges on the DNA strands  These methods are directly couples with gene expression and activation of transposable elements  Bound DNA cannot be expressed  Loosely bound DNA allows movement of transposable elements  The next level is less known than the chromatin fibers, but involved the binding of non- histone proteins which provide a structural scaffolding for long chromatin loops o This can be seen by treating DNA with a histone depleting detergent  The proteins involved in this structure are unknown o The loops of chromatin fibers tend to be gene rich and are 1-4 mbp in length o These loops are attached to the protein scaffold by scaffold-associated regions (SARs) in animals or matrix-attachment regions (MARs) in plants o The scaffold proteins encompass chromatin proteins such a insulators, domain boundary factors and cellular memory modules (CMMs)  These loops then undergo further folding to produce first 100-130 nm chromosome fibers, then 200-250 nm middle proph
More Less

Related notes for BIOL 200

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.