DNA (Deoxyribonucleic Acid) : Structure and Function

Structure of DNA  Discovery of Double Helix: 1962  James Watson, Francis Crick and Maurice Wilkins received a NOBEL PRIZE  Watson, Crick and Wilkins used unpublished data from Rosalind Franklin’s research obtained without her knowledge and used without her consent. - She was misrepresented and unrecognized - She died of cancer in 1958  We know now: DNA is a double helix made of nucleotides - A, T, G, C - in 2 polynucleotide strands - strands run antiparallel - [5'-----3'] - held together via weak H-Bonds & complimentary base pairing (A-T and C-G)

#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 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)

#4. 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 (A & T) Guanine and Cytosine pair up (G & C) Each pair is connected through hydrogen bonding Hydrogen bonding always occurs between one pyrimidine and one purine

Complimentary base pairing of pyrimidines and purines #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

G G A T T A A C T G C A T C

DNA Structure To crack the genetic code found in DNA we need to look at the sequence of bases. The bases are arranged in triplets called codons. A G G - C T C - A A G - T C C - T A G T C C - G A G - T T C - A G G - A T C

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’ 5’

DNA Structure In eukaryotes it is always associated with proteins NUCLEOSOMES = DNA wrapped around 8 histones (proteins) Help compact DNA Help control DNA transcription Chromosomes = made of DNA, which contains the cell’s genetic information

#8. 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

DNA’s job Stores information It has the code for all 20 amino acids Mutation: changes in the base sequence  primary structure of a protein is altered (changing its shape) Can be harmful, neutral or beneficial Important: create variation – basis of natural selection

RNA Discovered after DNA Single strand shorter than DNA Contains Uracil instead of Thymine A + U G + C Sugar = ribose (DNA contains deoxyribose) Types: - mRNA: messenger  blueprint for protein - rRNA: ribosomal  makes up ribosomes - tRNA: transfer  delivers the proper amino acid to the ribosome