DNA Structure IB Topics 3.3 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

The Bases They are divided into two groups Pyrimidines and purines Pyrimidines (made of one 6 member ring) Thymine Cytosine Purines (made of a 6 member ring, fused to a 5 member ring) Adenine Guanine The rings are not only made of carbon (specific formulas and structures are not required for IB)

Nucleotide Structure Nucleotides are formed by the condensation of a pentose sugar, phosphate and one of the 4 bases The following illustration represents one nucleotide

How do the nucleotide and the ribose attach? OH H2O

N-Glycosidic Bonds REMEMBER??? In Nucleic Acid…Glycosidic Bonds form between sugar and the nitrogen base!

How does the Phosphate group get on the ribose?

Nucleotide Structure Nucleotides are linked together (or POLYMERIZED) by covalent bonds called phosphodiester linkage

Phosphodiester Bonds Polymerized Double Stranded DNA Hydrogen is removed H Phosphodiester bond + H & Pyrophosphate as Products Pyrophosphate

DNA Double Helix and Hydrogen Bonding Made of two strands of nucleotides that are joined together by hydrogen bonding Hydrogen bonding occurs as a result of complimentary base pairing Adenine and thymine pair up Cytosine and guanine pair up Each pair is connected through hydrogen bonding Hydrogen bonding always occurs between one pyrimidine and one purine

#4. DNA Double Helix and Hydrogen Bonding Complimentary base pairing of pyrimidines and purines

#4. DNA Double Helix and Hydrogen Bonding

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

DNA Double Helix 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

Antiparallel strands 5’ 3’ 5’ 3’ The strands run opposite of each other. The 5’ end always has the phosphate attached. 5’ 3’

Homework…3.3 1. Draw the structure of ribose and number the carbons 2. 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.

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

Nucleosome structure Nucleosome are the basic unit of chromatin organization In eukaryotes DNA is associated with proteins (in prokaryotes the DNA is naked) Nucleosomes = basic beadlike unit of DNA packing Made of a segment of DNA wound around a protein core that is composed of 2 copies of each of 4 types of histones

Nucleosome structure Nucleosomes have: 8 histones in the core DNA wrapped twice around the core One histone holding the nucleosome together A DNA ‘linker’ continuing towards the next nucleosome

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

Genes Genes=units of genetic information (hereditary information) Order of nucleotides make up the genetic code Genes can contain the information for one polypeptide Genes can also regulate how other genes are expressed All cells of an organism contain the same genetic information but they do not all express the same genes THIS IS CELL DIFFERENTIATION Cells differentiate by genes that are activated

Genes Repetitive sequences-part of the non-coding section of DNA Function-unknown Can be used in DNA profiling (DNA fingerprinting)