DNA Chapter 4 – Biology 12 textbook Molecule of Life

Deoxyribonucleic Acid DNA... Deoxyribonucleic Acid

DNA contains all the information for the formation of an organism Chromosomes – are made of condensed DNA DNA controls the production of proteins – which form the structure of the cell & control chemical reactions

Gregor Mendel – 1865 Father of Genetics Discovered that organisms pass their characteristics to their offspring Also: genes are found in pairs Using experiments with pea plants, he determined the laws of inheritance – without knowing anything about DNA

Friedrich Miescher – 1871 Isolation of DNA Isolated DNA for the 1st time – found in the nucleus Called it “nuclein” Did not realize that it was genetic information DNA = carbon, hydrogen, oxygen, nitrogen & phosphorous

Other scientists improved DNA extraction & found that DNA is an acidic long chain molecule (polymer) made of nucleotides 1 nucleotide: Pentose sugar Phosphate group Nitrogenous base Nucleotide

Sugar-phosphate Backbone 2 components of the backbone: Pentose sugar: deoxyribose Phosphate group: phosphoric acid

Deoxyribose sugar 5 carbons Carbons are numbered clockwise 1’ – 5’ (1 prime to 5 prime) 3’ & 5’ carbons links to phosphate groups with ester bonds 1’ carbon links to the nitrogenous base with a glycosyl bond

Phosphate group Negatively charged Free Oxygen links to the sugar

4 Nitrogenous Bases Adenine Guanine Thymine Cytosine

Purines  double ringed molecules: Adenine Guanine Pyrimidines  single ringed molecules Thymine Cytosine

Phoebus Levine discovered that each of the nitrogenous bases is attached to the sugar & the sugar is attached to the phosphate group These 3 components make up nucleotides

Nucleotide structure

Watson & Crick In 1953, thanks to an X-ray image by Rosalind Franklin, Watson & Crick discovered that DNA is in the form of a spiral Called a double helix

An unrolled helix looks like a ladder The sugar phosphate backbone forms the 2 sides The nitrogenous bases form the rungs The bases are linked to each other with hydrogen bonds

In a molecule of DNA, the quantities of adenine & thymine are equal & guanine = cytosine Watson & Crick found that the structure of the bases determines which other base they bond with Complimentary bases are: A & T G & C

Hydrogen bonds link complimentary bases The 2 strands are complimentary & antiparallel One strand runs 3’- 5’, the other strand runs 5’ – 3’

If you know the sequence of one strand, you also know the sequence of the other strand 5’ – TCGAATTGCGCGATA – 3’ 3’ - _________________ - 5’ Purines bond with pyrimidines so the diameter of DNA is a constant 2nm DNA is twisted clockwise – “right-handed helix” One complete turn (3.4 nm) every 10 nucleotides

Hydrogen bonds: individually weak, collectively strong 3 H bonds between G & C 2 H bonds between A & T G cannot bond with T & A cannot bond with C because the molecule would be unstable – too few H bonds

Homework questions: Page 216 #1-10