IB Topics 3 and 7

#1. DNA Structure (an overview)DNA has three main components

1. deoxyribose (a pentose sugar)2. base (there are four different ones)3. phosphate

#2. The BasesThey are divided into two groups

Pyrimidines and purinesPyrimidines (made of one 6 member ring)

ThymineCytosine

Purines (made of a 6 member ring, fused to a 5 member ring) AdenineGuanine

The rings are not only made of carbon (specific formulas and structures are not required for IB)

#3. Nucleotide StructureNucleotides are formed by the condensation

of a pentose sugar, phosphate and one of the 4 bases

The following illustration represents one nucleotide

Nucleotides are linked together by covalent bonds called phosphodiester linkage

#3. Nucleotide Structure

#4. DNA Double Helix and Hydrogen BondingMade of two strands of nucleotides that are

joined together by hydrogen bondingHydrogen bonding occurs as a result of

complimentary base pairingAdenine and thymine pair upCytosine and guanine pair upEach pair is connected through hydrogen

bondingHydrogen bonding always occurs between one

pyrimidine and one purine

Complimentary base pairing of pyrimidines and purines

#4. DNA Double Helix and Hydrogen Bonding

#4. DNA Double Helix and Hydrogen Bonding

•Adenine always pairs with thymine because they form two H bonds with each other

•Cytosine always pairs with guanine because they form three hydrogen bonds with each other

#4. DNA Double Helix and Hydrogen Bonding

The ‘backbones’ of DNA molecules are made of alternating sugar and phosphates

The ‘rungs on the ladder’ are made of bases that are hydrogen bonded to each other

#5. DNA Double Helix

#6. Antiparallel strands

The strands run opposite of each other.

The 5’ end always has the phosphate attached.

5’

3’

3’

5’

Assignment (in your notebook)1. Draw the structure of ribose and number the carbons2. Draw a schematic representation of a nucleotide. Label

the sugar, base and phosphate.3. What are the complimentary base pairs to a DNA strand

that has the following order A T A C C T G A A T?4. Draw a schematic representation of an unwound DNA

double helix using the base pairs from your answer in question 3. Include the number of hydrogen bonds between each base

pair. Be sure to label all of the bases and the 5’ and 3’ ends of the structure.

#6. When phosphodiester links are formed . . . A. When the covalent bonds are formed

between nucleotides the attach in the direction of 5’→3’

B. The 5’ end of one nucleotide attaches to the 3’ end of the previous nucleotide

#7. Nucleosome structureNucleosome are the basic unit of chromatin

organizationIn eukaryotes DNA is associated with

proteins(in prokaryotes the DNA is naked)

Nucleosomes = basic beadlike unit of DNA packingMade of a segment of DNA wound around

a protein core that is composed of 2 copies of each of 4 types of histones

Nucleosomes have:8 histones in the coreDNA wrapped twice

around the coreOne histone holding

the nucleosome together

A DNA ‘linker’ continuing towards the next nucleosome

#7. Nucleosome structure

The DNA has a negatively charged backbone (because of the phosphate groups)

The proteins (the histones) are positively charged

The DNA and proteins are electromagnetically attracted to each other to form chromatin

#7. Nucleosome structure

#8. GenesGenes=units of genetic information (hereditary

information)Order of nucleotides make up the genetic codeGenes can contain the information for one polypeptideGenes can also regulate how other genes are

expressedAll cells of an organism contain the same genetic

information but they do not all express the same genesTHIS IS CELL DIFFERENTIATIONCells differentiate by genes that are activated

Repetitive sequences-part of the non-coding section of DNAFunction-unknownCan be used in DNA profiling (DNA

fingerprinting)

#8. Genes