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Chapter 19 - Chapter 19 Cell Cycle Inter phase is not part...

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Chapter 19 Cell Cycle Inter phase is not part of the mitosis, Inter phase consists of the sets of steps a cell has to go through before it can start dividing in mitosis. Mitosis and cytokinesis is two different things, mitosis is the separation of chromosomes but, cytokinesis is division of cytoplasm. M stage consists of all the different phases of mitosis and cytokinesis (where cell is finally divided). About 90% of the cell’s life in spent in interphase. Stages of Inter phase G1 cells grows like normally, usually double in size S stage replicates its DNA semi-conservitely (each new strand of DNA has one old strand). And Cell is committed to dividing. Chromosomes are divided. G2 cell resumes its growth again and prepare for mitosis FIG 19-4 & 19-5 Replication of DNA occurs bi-directionally, same DNA grows from two parts, both in linear and circular DNA, the origin of growth are called replication forks. DNA always replicates form 5’ to 3’ end. It adds hydroxyl group to 3’ end. There are mistakes that occur during DNA replication and even after replication, but there are ways to repair those mistakes. (by DNA polymerase). FIG 19-13 DNA polymerases are important in making DNA they can only add to 3’ end, but there is no place on 3’ end to add to in the initial step. So, RNA is added to 5’ end, after this DNA polymerase can be added. Lagging strands are made of lots of little pieces of DNA. FIG 19-15 End Replication Problem Because of this problem (you have to add RNA to 5’end), when you have linear chromosomes, and you have many origin of replication. 1. A chromosome grows in two direction from the origin of the growth, the replication forks moves in the opposite direction. 2. Finally only one primer remains on the each daughter DNA molecules
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3. Those primers are removed by exonuclease but, no DNA polymerase can fill this “gap” because we don’t have any hydroxyl group and as a result the 5’ ends are little shorter than the 3’ end. 4. Each time DNA replicates the ends get shorter and shorter, This is the reason why animals age, we don’t replicate our DNA very fast and therefore, we don’t shorten our DNA strands very fast and age very fast. Sperm and Egg cells overcome this negative effect by using an enzyme called Telomerase. Telomerase can add back to the telomere, telomerase is a ribosomal enzyme that has RNA and proteins associated with it.
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