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Lecture 7

Biology 1001A Lecture 7.pdf

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Biology 1001A
Beth Mac Dougall- Shackleton

Biology 1001A | 2012 LECTURE NOTES Lecture 7: Inheritance of “Sameness” –––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– Summary: Some Important Points So Far i) A-T, G-C ii) pairing is antiparallel iii) DNA polymerases extend 3’ end of properly paired bases iv) DNA polymerases cannot initiate replication, they can only continue it once it has been started v) replication bubble is just two forks vi) replisomes replicate one strand continuously, one strand discontinuously vii) chromatids are identical DNA molecules attached at their centromeres viii) replication does not start at an end of a chromosome, it starts at an origin which is somewhere along the double helix, not at the end ix) telomerase elongates the chromosome - telomerase brings its own template and uses it to add more DNA several times - this creates a region on the end of a chromosome called a telomere - the elongated strand can be primed and the space that could not otherwise have been filled is now filled by DNA polymerase - there will always be a single strand end - telomeres are not junk because they are not from outside sources such as viruses, but they are non-coding because they are structural to the DNA - good DNA replication simulation on YouTube (Howard Hughes Medical Institute) - cycling cells–when looking at inheritance, we are most interested in cells that are actively cycling - purpose of actively cycling cells - when/where are cells actively cycling in birds? - in gametogenic tissue - in bone marrow - if cells are regenerating or growing, they are cycling - any tissue that has to be renewed is actively cycling (i.e. skin, stomach lining) - system for producing blood–cells would be actively cycling - everywhere in developing organisms - wound repair - when/where are actively cycling cells in a tree? - in root tips, meristem, tissues that are making pollen, fruits, trunk - wound repair - when/where might cells be programmed to die (apoptosis)? - cells might be programmed to die so that they die before they accumulate too many mutations which could cause them to become tumour cells - developmental program - surplus, disintegrate non essential components (webbed toes, tails) Biology 1001A | 2012 - xylem pipes in plants for structure are dead cells that form the cell walls Cycling Cells - most cells in mature organisms are not actively cycling - at any time, cells can divide, not divide, or die - the purpose of a cell could be to die intentionally - chromosomes shorten at every replication and telomerase extends them - the length of the telomeres allow cells to know how old they are, and specifically to track the number of times that they have replicated - when the telomeres reach a critical length, this is the signal to the cell that it is time to die - cells die so that they are removed from the organism before they accumulate too many mutations that could result in the cell becoming a tumour - the expression of telomerase is highly regulated in organisms - if the expression of telomerase is up-regulated in mice, it is found that the mice do not actually live longer, but instead they live a shorter than average life due to the occurrence of cancers - this is because the over-expression of telomerase allows cells with too many mutations to continue dividing because their telomeres never shorten to the critical length - in most somatic cells of the body, the expression of telomerase has been turned off, whereas in actively cycling cells telomerase is on - when making gametes, telomerase is very active so as to give long telomeres to offspring - when offspring develop from a single cell, they maintain their telomere length - as organisms mature, telomerase is down-regulated and telomeres are allowed to shorten, thus protecting the aging organism from diseases such are cancers Chromosome Anatomy - the centromere is a repetitive sequence of the DNA that when condensed, results in the joining of the sister chromatids - the kinetochore is a protein complex where the spindle fibres interact with the chromatids - cohesin is a reversible protein glue that sticks two sister chromatids together - it is incorrect to say “one chromatid” because they are only chromatids when in pairs - kinetochore is where the spindle fibre interacts with chromosome - microtubules are made of tubulin - they attach to the kinetochores - chromosomes are only pulled to the metaphase plate if they are attached to spindle fibres on both sides
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