Study Questions:
1) Diagram the parts of an RNA nucleotide
2) Show how two nucleotides are linked together by dehydration synthesis reactions
3) Why does the purine - pyrimidine structure relate to the complementary nature of double-stranded DNA?
4) Draw a DNA double helix, showing three base pairs and the antiparallel nature of the helices.
Cell Biology Unit
I. Membranes - how stuff gets in/out
II. Harvesting Energy
Each human cell has 2m of DNA packaged into the nucleus of the cell, which is only 10 nm (1 x 10-8 m) across. In order to access the information, it must be unwound as a double helix... it needs to be "spread out" in the nucleus... at this time (when the cell is metabolically active and reading DNA) the chromosomes are not visible with a light microscope - they are too diffuse and thin. However, in order to move chromosomes and divide them up in an orderly manner, they need to be packaged up and condensed. Then they are easier to move around. At this point (Mitosis), the chromosomes are packaged as follows into dense bundles that we can see as chromosomes with a light microscope.1. Nucleic Acid Structure
2. Chromosome Structure
a. Genomes - Overview
b. Molecular Structure of Eukaryotic Chromosomes
1. Nucleosome formation - ‘beads on a string' - 2.5 loops of DNA wrapped around histone proteins (Core DNA) - the DNA bound to these histones can't be read... it is "off". Only the DNA between Histones can be read. So, this is one way DNA expression is regulated. During the G1 stage of interphase, when the cell is the most metabolically active, the DNA is in this conformation. As such, each chromosome is to thin to see as separate with a light microscope and the nucleus stains a uniform blue. The DNA is diffuse during the next stage of intephase, too. This is the S phase, when the DNA is being read and new DNA is synthesized. The chromosomes remain diffuse during the early portion of G@ also, preparing structures for mitosis... then, as mitosis approaches, the chromsomes are condensed - wrapped up as described below to make allocation of DNA to daughter cells more efficient and less error-prone..
2. Solenoid Formation - beaded string is coiled - 6 nucleosomes per solenoid coil. - interphase chromatin is this condensed
3. Supercoiling - coil of solenoids is itself coiled - coiled coil is then folded - mitotic chromosome - 10,000 fold reduction in length, and a dramatic increase in width - so it can now be seen with a light microscope.