Post on 21-Dec-2015
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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