3.3 7.1 Dna Structure

Topics 3.3 & 7.1

Nucleic Acids

IB Biology

Where can you find it?

Inside the cell- nucleus, chloroplasts and mitochondria

Chromosomes are made of DNA

Nucleic AcidsPolymers:

DNA (deoxyribonucleic acid) RNA (ribonucleic acid)

Units: nucleotidesNucleotide: sugar + nitrogenous base + phosphate

Parts of a NucleotidePhosphateSugar: Ribose or Deoxyribose (both pentoses)Nitrogenous Bases:

- Purines (double rings): Adenine + Guanine- Pyrimidines (single ring): Cytosine, Thymine, Uracil (RNA only)

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received a NOBEL PRIZE Watson, Crick and Wilkins used unpublished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... - 4 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)

DNA is double-stranded, with complementary base pairing

Observe - hydrogen bonds

- complementary base pairing :

A – TC – G

- covalent bonds between 2 nucleotides

(formed through condensation)

Counting Carbons...

Anti-parallel strandsOne side 5’ 3’Other side 3’ 5’

DNA StructureIn eukaryotes it is always associated with proteinsNUCLEOSOMES = DNA wrapped around 8 histones (proteins)Help compact DNA Help control DNA transcription

Single copy x Highly repetitive sequences

Much of DNA in eukaryotes = repetitive base sequences, which are not translated (satellite DNA) = 5-300 bases / repeated as many as 10,000x

5-45% of DNA = function not clear

Single copy/unique genes = actually code for something (small proportion)

DNA’s jobStores informationIt has the code for all 20 amino acidsMutation: 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

RNADiscovered after DNASingle strand shorter than DNAContains Uracil instead of ThymineSugar = ribose (DNA contains deoxyribose)DNA: nucleus, mitochondria, chloroplastsRNA: nucleus, cytoplasm, part of ribosomes

Types:RNAm: messenger blueprint for proteinRNAr: ribosomal makes up ribosomesRNAt: transfer delivers the proper amino acid to the ribosome