DNA organization into chromosomes
- Viral and bacterial chromosomes relativley simple in comparison to eukaryotic chromosome
- typically we have a single piece of nucleic acid without associated proteins. By comparison eukaryotes typically have multiple chromsomes complexed with a number of proteins.
- Viral chromosomes: come in variety --> DNA or RNA and these can be single stranded or double stranded, furthermore they can be linear or circular.
- Size range of viral genome: 2-52 micrometers in length. The viral genetic material undergoes packing to fit into the phage head.
- (6:00) Bacterial Chromosomes - always double stranded DNA. The DNA lives in the nucleoid region (an area in the bacterial cell where the DNA congregates). No membrane surrounding the nucleus of the DNA.
- (8:35) E. Coli
- Single circular chromosomes - 1.2 mm in length. It is associated with a few DNA binding proteins. Circular chromosomes have the ability to supercoil. How was this figured out?: 1965 they took viral DNA from a mouse and did a density gradient centrifugation - Take something we want to seperate based on size and put it in a tube and the lighter the molecule the faster it travels. Upon doing this they found 3 distinct species of DNA in the centrifuge. One moved slowly and thus it was believed to be linear DNA. The other two moved more quickly. One of the factions was underwound circular DNA and the other was relaxed circular DNA.
- (17:00) Relaxed circular DNA - has the standard number of twists for that pieces of DNA. Circular DNA can be underwound.
- (18:10) Underwound DNA - DNA is unwound (fewer twists) and creates stress on DNA molecule. WHen this happens it allows for the DNA to supercoil. This supercoil relieves the stress the DNA feels of being unwound. Overwound DNA has the same effect in that it causes stress and results in supercoiling.
- (21:10) How we determine when and if supercoiling will occur?
- Linking # --> predicted # of turns for a DNA molecule. Dividing total # of BP = 10.4 # of BP/turn
- EXAMPLE: SV40 virus - 5200 BP in its genome
- 5200/10.4 = 500 (predicted)
- Actual linking number = 475
- To determine the number of supercoils you take the actual linking # and subtract the predicted linking #. This gives us 25 negative supercoils in our DNA
- Topoisomers - two DNA molecules of the same DNA with two different structures.
- (27:25) Topoisomerase - Enzyme responsible for converting DNA from one topoisomerase to the next.
- (28:50) Specialized Chromosome Structures
- Polytene chromosome: very large chromosomes readily viewed under a microscope. Found in the cells of a number of tissues. They are seen as a series of alternating bands (DNA) and interband regions.
- (31:40) How do we get polytene chromosmes?
- These chromosomes are always found as homologous chromosomes pairs. Typically chromosomes only pair during mitosis and meiosis.
- During DNA replication, these chromsomes replicate. But the DNA is not dispersed into new chromosome pairs/new cells. A chromosome with 5000 strands of DNA stacked together puffing occrus when teh DNA strands seperate to allow activity to occur
- (35:50) Lampbrush chromosome - received its name because it looks like a lampbrush (used to clean kerosene lamps in the 19th century)
- found in the oocytes of sharks and a number of vertebrates. Also in spermadocytes in insects.
- It is found during prophase 1
- Lampbrush chromosome functions to direct metabolic activities during meiosis I
No comments:
Post a Comment