DNA
KEY CONCEPT DNA structure is the same in all organisms.
DNA is composed of four types of nucleotides. DNA is made up of a long chain of nucleotides. Nucleotide: One “step” of the DNA ladder. Consisting of phosphate group, sugar, and base. Over 3 billion nucleotides in human DNA Each nucleotide has three parts. a phosphate group a deoxyribose sugar a nitrogen-containing base phosphate group deoxyribose (sugar) nitrogen-containing base
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.
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.
Nucleotides always pair in the same way. The base-pairing rules show how nucleotides always pair up in DNA. A pairs with T C pairs with G Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width. C G T A
The nitrogen containing bases are the only difference in the four nucleotides.
Nucleotides T A C G
Nucleotides Sugar (Deoxyribose) connects with the phosphate group 4 Different Bases: Adenine (A) Thymine (T) Guanine (G) Cytosine (C) Rule: A always joins with T Rule: C always joins with G C T A A T G T
The backbone is connected by covalent bonds. The bases are connected by hydrogen bonds. There are an average of 25 hydrogen bonds within each complete turn of the double helix providing a stability of binding about as strong as what a covalent bond would provide. hydrogen bond covalent bond
Human DNA Every human cell has 46 chromosomes (23 from mother and 23 from father). DNA are wound up like a Slinky to form these chromosomes.
How DNA affects our lives: DNA doesn’t actually DO anything. Just as a book has no use unless someone reads it, the info in DNA has no purpose unless it is read. The information in DNA is used to build proteins.
Proteins are the building blocks of your cells can speed up reactions when they act as enzymes perform important functions (e.g. hemoglobin – transports oxygen in your blood) consist of amino acids All proteins in your body are made by the same 20 amino acids. What separates one protein from another are shape, size, and which amino acids are in it. These amino acids come from the proteins that we eat, then are broken down to be used by the body.
The Information of DNA DNA is encoded in what is called base 4. This is because there are four possible chemicals that can be in one particular spot. We count in base 10 because we have 10 different possibilities for a number 0-9. We speak in base 26 because of our 26 letters. You might think that with only 4 “letters” DNA can’t say much but each strand of DNA has approximately 3000000000 letters. That means that there are 43000000000 (4 x 4, 3 billion times) possible combinations for humans. Which means that no human has ever looked like any other in all of human history (except of course for twins because they share the same DNA).
DNA Replication Defined: Process of copying DNA During interphase Step 1: DNA Helicase (enzyme) “unzips” the two DNA strands (breaks the H bonds) Step 2: Free floating nitrogen bases (A, T, C, G) match up with complimentary base pairs. Step 3: DNA Polymerase (enzyme) reconnects the two strands. Enzymes also “proofread” each new DNA strand to make sure it is a perfect copy.
DNA Polymerase Glues the DNA T A A T G C G C Two Identical Strands of DNA! A T C G T A A T G C G C C G T A C G A T DNA Helicase Splits the DNA
Accuracy & Repair Mutation: change in DNA sequence Mutations can be corrected Enzyme “proofreads” DNA and check/correct errors Pre-repair: 1 in 10,000 nucleotides has an error Post-repair: 1 in 1 billion nucleotides has an error Accuracy & Repair
Quick Review DNA nucleotides form long chains DNA nucleotides have 4 different bases (A, T, C, G) DNA makes exact copies of itself