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.
phosphate group
• a phosphate group
• a deoxyribose sugar
• a nitrogen-containing base
deoxyribose (sugar)
nitrogen-containing
base
The nitrogen containing bases are the only
difference in the four nucleotides.
Watson and Crick determined the threedimensional structure of DNA by building
models.
• They realized that
DNA is a double
helix that is made
up of a sugarphosphate
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.
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!