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Chapter 12: DNA
12.1 The Role of DNA
• DNA has 3 functions:– Storing information– Copying information– Transmitting information
12.2 The Structure of DNA
• DNA – deoxyribonucleic acid• Nucleotides – monomers that
make up a DNA molecule– 3 parts:
• 5 Carbon sugar (deoxyribose)• Phosphate group• Nitrogenous base
– Adenine (A)– Guanine (G)– Cytosine (C)– Thymine (T)
Solving the Structure of DNA
Edwin Chargaff (1949)Chargaff’s Rule
[A]=[T][C]=[G]
Adenine Thymine Guanine Cytosine35%
45%
• Rosiland Franklin used X-ray diffraction to visualize DNA structure (1952).
• Watson and Crick were the first to model DNA as a double helix in 1953.
The Double Helix Model
• All the previous discoveries show us the structure of DNA and how DNA can function as a carrier of genetic information.
• Characteristics of the double helix model:– Antiparallel strands– Hydrogen Bonding– Base Pairing
Antiparallel StrandsRun in opposite directionsBase PairingBases are held together by hydrogen bonds.Base PairingA bonds with T, and C bonds with G
Complimentary base pairs
hydrogen bond covalent bond
Practice Base Pairing
• If the sequence of bases on one strand of a DNA molecule is TCGAACTGA, the sequence on the other (complimentary) strand is:
12.3 DNA Replication• Why does DNA replicate?
(Hint: Think back to Mitosis)– DNA is replicated in the S phase of
the cell cycle, before the cell divides.
Copying the Code
• Step 1– The DNA double
helix unwinds • DNA helicase
(enzyme).– breaks the H
bonds between the bases.
• The areas where the double helix separates are called replication forks.
• Step 2– DNA polymerase
(enzyme) moves along each DNA strand adding complimentary bases according to the base pairing rules.
– DNA polymerase also proofreads the DNA molecule to reduce errors.
• Step 3– The process
continues until all of the DNA has been copied.
– DNA polymerase detaches.
• This process produces two DNA molecules each composed of one new and one original strand.
• Both DNA molecules produced are identical to each other.
• DNA replication takes place at many points on a eukaryotic chromosome.– Allows replication to happen more
quickly.