12.2 Structure of DNA

WARM UP

• Which scientists performed experiments with viruses? – What type of viruses did these scientists use?

• Name the scientist that experimented with two different strains of bacteria and mice?

• What final conclusion did the scientists come to after their experiments.

12.2 Structure of DNA

KEY CONCEPT DNA structure is the same in all organisms.

12.2 Structure of DNADNA & RNA

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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Nucleic Acids• Ex. DNA and RNA

12.2 Structure of DNADNA Location and Function

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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

· Location - in nucleus· Function - blueprint for life = determine genes

(heredity), make proteins, cell division

12.2 Structure of DNADNA basic structure

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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Discovered structure of DNA = double helix

12.2 Structure of DNA

DNA is composed of building blocks called nucleotides.

• DNA is made up of two long chains of nucleotides.• Each nucleotide has three parts.

– a phosphate group– a deoxyribose sugar– a nitrogen-containing base (A,T,C,G)

phosphate group

deoxyribose (sugar)

nitrogen-containingbase

12.2 Structure of DNA

• The nitrogen containing bases are the only difference in the four nucleotides.

12.2 Structure of DNA

Watson and Crick determined the three-dimensional structure of DNA by building models.

• They realized that DNA is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.

12.2 Structure of DNA

• Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.

– Franklin’s x-ray images suggested that DNA was a double helix of even width.

– Chargaff’s rules stated that A=T and C=G.– Discovered even nucleotide ratios

– Adenine – Thymine– Cytosine – Guanine

12.2 Structure of DNA

TAC

G

Nucleotides always pair in the same way (Complementary Base Pairing) • The base-pairing rules show

how nucleotides always pair up in DNA.

• Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width.

– A pairs with T– C pairs with G

12.2 Structure of DNA

• The backbone is connected by covalent bonds.

hydrogen bond covalent bond

• The bases are connected by hydrogen bonds.

12.2 Structure of DNADNA Replication

• The process of copying the DNA molecule• Occurs in the nucleus during interphase (S phase)

– Is a “semi-conservative” process– DNA is used as a “Template” to make new DNA– Results in 2 strands of DNA that are half old and half

new (2 semi conservative DNA molecules)

Why do cells need to make a copy of their DNA?

12.2 Structure of DNADNA Replication: semi conservative

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1. DNA helicase uncoils and unzips DNA

2. New (daughter) nucleotides line up next to both old (parent) chains of the DNA

Figure 3.13

12.2 Structure of DNADNA Replication

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3. DNA polymerase binds the new nucleotides together and “proofreads” the new strands for errors.

4. Results in 2 semi-conservative strands of DNA

Figure 3.13

12.2 Structure of DNA

DNA Replication

• Final Result – 2 Semi-Conservative Strands of DNA

12.2 Structure of DNADNA triplets

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12.2 Structure of DNADNA: the Genetic Code is carried in Triplets

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· Genetic code= order of nitrogenous bases in triplets of DNA

· 1 DNA triplet codes for 1 amino acid in a protein

· Gene: is a DNA segment that carries the information for building proteins (or polypeptide chain)

· Process Discussed further in Protein synthesis