dna part ii: the "stuff" of life its structure, replication, & unending repair

DNA Part II:

The "Stuff" of LifeIts Structure, Replication,

& Unending Repair

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Structure of a Nucleotide

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Sides of the Ladder

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Hydrogen Bonding and Nitrogenous Bases

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Hydrogen Bonding and Nitrogenous Bases

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Hydrogen Bonding and Nitrogenous Bases

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Hydrogen Bonding and Nitrogenous Bases

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Forming the Double Helix

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DNA Forming Chromosomes

Structure in eukaryotes.

• the DNA is wrapped around proteins called histones forming nucleosomes.

• This forms a fiber known as chromatin.

• This forms a coil within a coil.

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Replication of DNA

Possible Scenarios for DNA Replication

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Matt Meselson (left) and Frank Stahl (right) in 1958 demonstrated that replication was semi-conservative using radioactive nucleotides with dividing bacteria supporting Watson’s and Crick’s hypothesis

Experiment of Meselson and Stahl

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Proof for DNA Replication

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Simple Model Proposed for DNA Replication

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The prokaryotic chromosome attaches to the plasma membrane.

The DNA is then replicated in both directions.

Prokaryotic DNA Replication

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DNA replication occurs simultaneously in many locations along the very long eukaryotic chromosomes.

Three replication bubbles are visible along the DNA within this cultured Chinese hamster cell. The arrows indicate the directions of DNA replication at the two ends of the bubble.

Eukaryotic DNA Replication

Helicases are enzymes responsible for the unwinding of the DNA molecule. They unwind the DNA in both directions

Helicases

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Since DNA is a double helix, there will be tension in the DNA strand that causes it to tangle as it is unwound by the helicase.

The enzymes topoisomerase I and II are responsible for relieving that stress by clipping one or two strands of the DNA.

Releasing Stress in the DNA Molecule

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Nucleotides are always added on as triphosphates. When the nucleotides are added then two phosphates are cleaved off making a pyrophosphate.

Adding Nucleotides as Triphosphates

When a DNA molecule is being synthesized , the nucleotides are added as triphosphates, and two phosphates are removed. Nucleotides are always added to the 3 end!

Synthesis ALWAYS occurs in the 5 to 3 direction!

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DNA polymerase must always attach the com-plementary nucleotide to a 3 end of the deoxyribose sugar molecule.

So, in the very beginning a small RNA primer must be laid down in order to start the process of DNA replication. Primase is the enzyme responsible for this.

In Need of a Primer!

RNA nucleotides (red pentagons) are being laid down by primase before DNA polymerase begins DNA replication.

Putting Down a RNA Primer

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The Lagging Strand

This animation, shows the leading strand being synthesized followed by the lagging strand. The enzyme named ligase ties them together.

The Lagging Strand and Ligase

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Once DNA has been replicated, there is one problem. The usual replication machinery provides no way to complete the 5 ends after the RNA primer is removed, so repeated rounds of replication produce shorter and shorter DNA molecules.

To compensate for this repeated shortening process, repetitive sequences of DNA are added. These are noncoding sequences and called telomeres.

Function of Telomeres

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More About Telomeres

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In gametes, the shortening of telomeres would cause serious problems. If chromosomes of gametes became shorter each time during replication, then essential genes would eventually be missing. An enzyme complex called telomerase catalyzes the lengthening of telomeres in gametes.

Maintaining Telomere Length in Gametes and Germ Cells

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Telomerase Function

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Errors in DNA replication occur about 1 in every 10,000 base pairs. Not bad, but with 6 billion bases being replicated that amounts to 60,000 mistakes every time a cell divides.

DNA repair systems repair about 99% of these mistakes.

DNA Repair

A Recap!

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Once More With Feeling!

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Experiment of Meselson and Stahl

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Topoisomerase Animation