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Lecture 6

Biology 1001A Lecture 6.pdf

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Biology 1001A
Beth Mac Dougall- Shackleton

Biology 1001A | 2012 LECTURE NOTES Lecture 6: Genome Replication –––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– Misconceptions with Junk DNA - the press reported that junk DNA is actually functional - the problem is that the researchers defined functional as anything that could be expressed - just because it could be expressed does not make it functional because the proteins are not necessarily usable by the human body - junk DNA is powerful evidence of evolution DNA Structure - the strands run of the double helix run antiparallel to each other - each strand has a 3’ end and a 5’ end, this gives the molecule a kind of direction - there is a 3’ end has a free hydroxyl (OH) and a 5’ end has a free phosphate group - the structure of DNA was first published by Watson and Crick - Watson and Crick also suggested that DNA replicates semi-conservatively (which it does) - the semi-conservative replication of DNA was proven by the experiment of Meselson and Stahl DNA Replication - every DNA polymerase extends the 3’ OH end of the molecule - replisomes replicate one strand continuously, one strand discontinuously - replication is semiconservative and semi-discontinuous - replication bubbles arise from two forks created at one origin - in prokaryotic cells, replication occurs in two forks - the replication bubble is just two forks (be able to draw this) - circular genomes of prokaryotes are relatively small and can be replicated from one origin - complex eukaryotes, such as humans, cannot replicate from even one replisome per chromosome due to immense size - large, linear, eukaryotic chromosomes have several origins of replication that operate simultaneously - origins occur at specific sequences in the DNA, but not in specific locations or in a specific frequency Chromatids - chromatids are identical DNA molecules attached the centromeres - G1 and S Phase is where DNA replicates - in G2, the chromosome is composed to two molecules of DNA - when these molecules condense and are held together, they look like an “X” (the metaphase chromosome) - it used to be one DNA molecule, now it is two Biology 1001A | 2012 - the image above is an electron micrograph of a condensed chromosome in G2 - this is one unreplicated chromosome - after replication, the nucleus contains the same thing: one chromosome - the number of chromosomes does not change during S-phase, the n value does not change, but the C value (amount of DNA) doubles DNA is Replicated During S-phase - the two chromatids stay together as one chromosome until a
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