DNA Structure and Replication
Jan 08, 2018
DNA Structure and Replication
What does this cartoon mean?
DNA (Deoxyribonucleic acid)• DNA carries all of an organism’s hereditary (genetic) information
• It is the “universal language” of all organisms!
• Controls the production of proteins• Found in the nucleus of the cell (eukaryotes) or floating in cytoplasm
(prokaryotes ONLY!)• Belongs to a chemical group of compounds called nucleic acids
Structure of DNA• DNA is made up of a long chain of nucleotides.• Each nucleotide has three parts.
• a phosphate group• a deoxyribose sugar• a nitrogen-containing base
phosphate group
deoxyribose (sugar)
nitrogen-containingbase
The nitrogen containing bases are the only difference in the four nucleotides.
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.
What pattern do you see between the nitrogen bases?
• 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.
What would be their corresponding strands?•1) ATCGATCCG
•2)GATCCGATC
DNA’s shape is a double helix• “Twisted Ladder”• Sugar-Phosphate backbone• “outside of the ladder”
• Held together by covalent bonds
• Nitrogen base pairs in center• “rungs of the ladder”
• Held together by hydrogen bonds (weak!)
DNA Replication• DNA replicates before cell division (mitosis and meiosis) to ensure
that each new cell receives the correct number of chromosomes.
Why would a cell need more DNA?1. Repair2. Growth
DNA replicates like a zipper.• It unzips to replicate.
Step 1: Original DNA splits in half (“unzips”)
Step 2: 2 new strands are copied
Step 3: End result is 2 identical DNA molecules
DNA Replication is semiconservative• 2 new molecules are called “daughters”• Semiconservative: half of the daughter
strand is NEW DNA, and half of the daughter is the ORIGINAL (old) DNA
REPLICATION MAKES 2 EXACT COPIES!