California Science Standards #1d,4c,5a,5b,6b,6c,7c

DNA: Replication

Copy the Blueprints

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DNA: Replication

Before a parent cell divides into two identical daughter cells (mitosis), it must copy its instruction booklet (DNA)

Making an exact copy of the DNA prior to cell division is called REPLICATION

See video, “DNA_replication,” © Glencoe, 2000

http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html

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DNA Replication

3 easy steps: 1. DNA unwinds

(“unzips”) 2. Free nucleotides

attach 3. Two copies of DNA

result

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Replication- Step 1 1. DNA unwinds (“unzips”)

Helicase enzymes separate the two strands of DNA Note: if a word ends with “ase”, it is an enzyme (a protein catalyst, a helper molecule)

Hydrogen bonds between base pairs (A-T, G-C) are broken The point of separation between strands is the “replication fork”

http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html

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Replication- Step 2

2. Free nucleotides attach DNA polymerases bind to the separated

strands of DNA A polymerase “hooks up” free-floating

nucleotides to their complements on the single strand (A~T, C~G) Covalent bonds are formed within the backbone

(between sugar and phosphate) Hydrogen bonds are formed with the other

nitrogenous bases (between strands)

http://www.mcb.harvard.edu/Losick/images/TromboneFinald.swf

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Replication- Step 3

3. Two copies of DNA result Each new copy of DNA is not

totally new Each has one strand from the old

DNA This is called “semi-conservative”

replication

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dna-rna2.swf

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Complementary Base Pairing

Problem: Predict the sequence of nucleotides on the new strand, given the sequence on one of the old strands of DNA

T A A G C G A

A T T C G C T

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Practice

What will the new strand look like?

G T A A C G C

? ? ? ? ? ? ?

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Answer

What will the new strand look like?

G T A A C G C

C A T T G C G

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Quick & Accurate DNA polymerases begin replication

simultaneously at many points (thousands) along the separated chain Broken zipper analogy

Remarkable accuracy: only one error made for every 10,000 base pairs (99.99% accurate) These errors can be fixed by DNA “proofreader”

enzymes and other means After fixing, accuracy jumps to one error for every

1,000,000,000 base pairs (99.9999999% accurate)!

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Accidents Happen With Some “Accidents” (Base Mismatches) Leading to Mutation

A mutation is a heritable change in DNA sequence.

Mutations due to replication errors only happen once in every billion replicated nucleotides.

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Mutation

Some mutation is good, too much is bad.

The effects of mutation are usually bad or neutral - only sometimes are mutations beneficial.

Cells employ elaborate mechanisms to prevent mutation – but the mechanisms aren’t perfect.

Mutations are the root cause of cancer (bad).

Mutations are the only way to introduce novel alleles into a species (good for evolution).

So, just like Goldilocks – not to hot, not too cold, just right – the optimal rate of new mutation is a balancing act.

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DNA Damage is Often the Root Cause of Mutation

DNA may be chemically altered (i.e. damaged) spontaneously and by chemicals and radiation.

DNA Damage is Often the Root Cause of Mutation

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Mutation as a Villain

Cancerous growths that result from loss of a protein that polices DNA for errors.

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Cancer Incidence Increases Sharply with Age

The increase is due at least in part to the age-related accumulation of multiple mutations in single cells.

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Poor Little Froggies Problem: Populations of Cascade frogs and western

toads (Oregon Cascade Range) are in decline Hypothesis: Ozone depletion increased UV

radiation increased mutations decrease in population in small critters like frogs

Data: Pacific tree frog populations NOT declining Data: Pacific tree frogs’ DNA repair enzymes better

able to repair mutations than those of CF and WT Conclusion: ?

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Poor Little Froggies Problem: Populations of Cascade frogs and western

toads (Oregon Cascade Range) are in decline Hypothesis: Ozone depletion increased UV

radiation increased mutations decrease in population in small critters like frogs

Data: Pacific tree frog populations NOT declining Data: Pacific tree frogs’ DNA repair enzymes better

able to repair mutations than those of CF and WT Conclusion: DNA repair enzymes from CF and WT

unable to keep up with mutations