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!