8.2 Structure of DNA

KEY CONCEPT DNA structure is the same in all organisms.

8.2 Structure of DNA

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-containingbase

8.2 Structure of DNA

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.

8.2 Structure of DNA

• 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.

8.2 Structure of DNA

TAC

G

Nucleotides always pair in the same way.

• The base-pairing rules show how nucleotides always pair up in DNA.

• Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width.

– A pairs with T– C pairs with G

8.2 Structure of DNA

• The nitrogen containing bases are the only difference in the four nucleotides.

C

G

T

A

• 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

8.2 Structure of DNA

hydrogen bond covalent bond

• 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.

8.2 Structure of 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.

Human DNA

8.2 Structure of DNA

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.

8.2 Structure of DNA

• 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.

Proteins

8.2 Structure of DNA

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).

• 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.

• 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

Quick Review• DNA nucleotides

form long chains• DNA nucleotides

have 4 different bases (A, T, C, G)

• DNA makes exact copies of itself