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ClassGrowth 5% 3.55% 1.9% - 0.45% -2%

2 5 4 3 1

OverviewThe nucleus during Interphase:

-nuclear membrane separates fluid of nucleus from cytoplasm

-DNA is in form of chromatin (not densely coiled into chromosomes)

-enzymes are present: helicase, polymerase, ligase, primase, gyrase

-free nucleotides are present (technically called nucleoside triphosphates)

DNA Replication 7.2

• DNA replication occurs in a 5’ to 3’ direction–5’ end of the free DNA nucleotide is

added to the 3’ end of the chain of nucleotides which is already synthesized

• Replication begins at a particular sequence of nucleotides called the origins of replication

7.2– in eukaryotic

chromosomes, replication can be initiated at several different points depending on what gene or genes need to expressed, while in prokaryotes the entire genome is replicated at once

7.2.2• It is at these points that a special enzyme called

helicase splits the DNA into its two antiparallel strands

• Another enzyme DNA gyrase is inserted in the helix to relieve strain from the coil

• On the strand running in the 3'--->5' direction (called the leading strand), another enzyme called DNA polymerase III latches on at one end of the opening, called the replication bubble

• DNA Polymerase III begins to continuously lay a new DNA strand from free nucleotides in the nucleus( Lay 5’-3’)

7.2.2• At the same time DNA polymerase III is laying

new DNA, helicase is continuing to split the strands, thus allowing replication to continue uninterrupted

• On the opposite strand running 5'--->3‘ (called the lagging strand), replication is not so simple

• Because new strands have to be laid in the 5'---3' direction, DNA polymerase III cannot lay continuously as it can on the other strand.

7.2.2

• Instead, RNA primase lays short segments of RNA primer nucleotides at many points along the strand

• When one segment of primer comes in contact with another, DNA polymerase I attaches and replaces the primer with DNA.

• These segments of DNA are called Okazaki fragments

• Once the Okazaki fragments have been laid, they are joined by yet another enzyme known as DNA ligase

• DNA ligase attaches DNA into the gaps between fragments and completes the new strand.

http://nobelprize.org/educational_games/medicine/dna/a/replication/replication_ani.html

·Complimentary base pairing rules are used- A bonds with T- C bonds with G

·This ensures - the correct base is incorporated into the new

DNA strand- conservation of the base sequence- the newly formed strand is identical to the old

strand it is replacing

DNA replication is semiconservative.·the 2 strands of the original molecule are each used as a template for the formation of the new strands.·The result is two newly formed strands that each consist of one old strand and one new strand

animation

further reading on experiments to confirm semiconservative replication: p 250-251