DNA: The Genetic MaterialChapter 14

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DNA Structure

DNA is a nucleic acid.

The building blocks of DNA are nucleotides, each composed of:

– a 5-carbon sugar called deoxyribose

– a phosphate group (PO4)

– a nitrogenous base

• adenine, thymine, cytosine, guanine

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DNA Structure

Nucleotides are connected to each other to form a long chain

The chain of nucleotides has a 5’ to 3’ orientation.

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DNA Structure

Determining the 3D structure of DNA involved these scientists:

– Erwin Chargaff determined that

• amount of adenine = amount of thymine

• amount of cytosine = amount of guanine

*Known as Chargaff’s Rules

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DNA Structure

Rosalind Franklin and Maurice Wilkins

– performed X-ray diffraction to identify the 3-D structure

– discovered that DNA is helical

Rosalind Franklin’s X-ray Photo (1951)

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DNA Structure

James Watson and Francis Crick, 1953

– discovred the structure of DNA using evidence from Chargaff, Franklin, and others

– proposed a double helix structure

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DNA Structure

The double helix consists of:

– 2 sugar-phosphate backbones

– nitrogen bases face interior of molecule

– bases form hydrogen bonds with complementary bases on the opposite sugar-phosphate backbone

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DNA Structure

The two strands of nucleotides are antiparallel to each other

– one is oriented 5’ to 3’, the other 3’ to 5’

Strands wrap around each other to create the helical shape

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DNA

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DNA Replication

DNA replication includes:

– initiation – replication begins at an origin of replication

– elongation – new strands of DNA are synthesized by DNA polymerase

– termination – replication is terminated

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DNA Replication

DNA replication is semidiscontinuous.

– can only add nucleotides to the 3’ end of the newly synthesized strand

– DNA strands are antiparallel to each other

leading strand is synthesized continuously (in the same direction as the replication fork)

lagging strand is synthesized discontinuously creating Okazaki fragments

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DNA Repair

Mistakes during DNA replication can lead to changes in the DNA sequence and DNA damage.

DNA can also be damaged by chemical or physical agents called mutagens.

Repair mechanisms may be used to correct these problems.

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DNA Repair

DNA repair mechanisms can be:

– specific – targeting a particular type of DNA damage

• photorepair of thymine dimers

– non-specific – able to repair many different kinds of DNA damage

• excision repair to correct damaged or mismatched nitrogenous bases

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GENE