Physical Physical Structure and Structure and Replication of Replication of

DNADNA

Property 1Property 1 - it must contain, in a - it must contain, in a stable form, information encoding stable form, information encoding the organismthe organism’’s structure, function, s structure, function, development and reproductiondevelopment and reproduction

Property 2Property 2 - it must replicate - it must replicate accurately so progeny cells have the accurately so progeny cells have the same genetic makeupsame genetic makeup

Property 3Property 3 - it must be capable of - it must be capable of some variation (mutation) to permit some variation (mutation) to permit evolutionevolution

Nature of the Genetic Nature of the Genetic MaterialMaterial

DNA double helixDNA double helix1. Sugar-phosphate backbone2. Base-pair "rungs" of ladder3. Nucleotides attached to S-P molecules4. Strands antiparallel (run in opposite

directions, 5'-->3')5. Each base-pair "rung" has a purine (A or G)

and pyrimidine (C or T)6. Strands held together by hydrogen bonds

between nucleotides7. Chemical structures of nucleotides discourage

"incorrect" pairing8. G-C pair has 3 hydrogen bonds, A-T only 2--

>former is stronger

How DNA Replicates

DNA ReplicationDNA Replication1. Semiconservative = replication results in two DNA

molecules each with two strands, one original and one new.

2. Sequence of eventsa) Helix unwindsb) Both strands replicate simultaneously, during unwinding processc) "Leading" strand replicates continuously from 3' end of existing strand, with newest end of forming strand facing into replication forkd) "Lagging" strand replicates by a series of fragments placed end-to-end, with newest ends of fragments facing away from fork; fragments later "ligated"e) During replication, 2 polymerases "proofread" for mismatched bases

Speed of DNA replication:Speed of DNA replication: 3,000 nucleotides/min in human 3,000 nucleotides/min in human 30,000 nucleotides/min in 30,000 nucleotides/min in E.coliE.coli

Accuracy of DNA replication: Accuracy of DNA replication: Very precise (1 error/1,000,000,000 nt)Very precise (1 error/1,000,000,000 nt)

Replication of DNA and Replication of DNA and ChromosomesChromosomes

Process of DNA Replication Hydrogen bonds between base pairs Hydrogen bonds between base pairs

are broken and the two sides of the are broken and the two sides of the ladder unzipladder unzip

Semi-conservative replication: the Semi-conservative replication: the original molecule is no longer present original molecule is no longer present but each new molecule will have but each new molecule will have one original strandone original strand

PolymerasesPolymerases (enzymes) link free- (enzymes) link free-floating nucleotides to their floating nucleotides to their matching base on the parent strandmatching base on the parent strand

There are estimated to be 3 billion There are estimated to be 3 billion bases in a human DNA strandbases in a human DNA strand

Mistakes are rare but do happenMistakes are rare but do happen Proofreader enzymes Proofreader enzymes fix most fix most

mistakesmistakes

DNA has a direction. It consists of two antiparallel strands with distinguishable 5′ and 3′ ends

(the numbers refer to the number of the carbon in the ribose sugar).Carbon-1 → base Carbon-3 → downstream PO4 Carbon-5 → upstream PO4

The original strand of DNA is read in the 3’ to 5’ direction

The new strand is assembled in the 5’ to 3’ direction

If both sides were synthesized at once, you'd need two different DNA polymerases:

one for 5′ → 3′; and one for 3′ → 5′.

However, experimentally, we find that all DNA polymerases synthesize the new strand 5′ → 3′

The original strand of DNA is read in the 3’ to 5’ direction,The new strand is assembled in the 5’ to 3’ direction

Since DNA synthesis only occurs in Since DNA synthesis only occurs in the 5′ and 3′, so DNA polymerases the 5′ and 3′, so DNA polymerases must move in antiparallel directions must move in antiparallel directions to synthesise the two daughter to synthesise the two daughter helices. helices.

Because the synthesis of DNA only occurs in one direction, different processes must occur on the two strands.

These two strands are termed the leading and lagging strands.

The leading strand is synthesised continuously 5′→3′.

However, the other, 'lagging' strand is still synthesised 5′→3′ but in discrete chunks called Okazaki fragments, from the replication fork back towards the origin.

Gene RecombinationsGene Recombinations The process of cutting out damaged The process of cutting out damaged

or foreign segments of DNA and or foreign segments of DNA and repairing themrepairing them

Restriction enzymes: The scissors Restriction enzymes: The scissors that cut certain segments out of the that cut certain segments out of the DNA strandDNA strand

DNA Ligase: The glue that restores DNA Ligase: The glue that restores the damaged segmentthe damaged segment

ResourcesResources From DNA to Protein DNA and Protein Synthesis in the Ce

ll Genetics Handbook Genetics Handbook Animations at Virtual Cell Biology Animations at Virtual Cell Biology

ClassroomClassroom Say it with DNA Protein synthesis Say it with DNA Protein synthesis

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