James Watson (American) & Francis Crick (Brit) came up with the shape of the DNA molecule in 1953—double helix (spiral ladder) 5 cm long nucleic acid molecule that contains hereditary material—tells cell its function and structure Made of a 5 C sugar (deoxyribose) attached to a phosphate grp (PO4) and a nitrogen base—3 grps bond to form a nucleotide Sugar & PO4 always same, form the backbone or sides of the ladder N base can be either a purine (adenine or guanine) or a pyrimidine (thymine or cytosine) DNA

N bases held together by H bond to form the “rung” of the ladder Adenine ALWAYS bonds with thymine and cytosine ALWAYS bonds to guanine Precise arrangement of these pairings of nucleotides form genes –each DNA molecule has 1000’s of genes, which are lined up like cars on a long train DNA, cont.

DNA Replication When the cell is preparing to divide, DNA must make copies of itself to go into the new cell Many enzymes are required to unwind the double helix and make a new strand Topoisomerase unwinds the supercoil Helicase unwinds original double strand DNA polymerase attaches new nucleotides to both strands Takes place during “S” phase of Interphase—more on that later

Central Dogma of Molecular Biology DNA codes for production of mRNA during transcription—only a portion of DNA is used (gene/genes) mRNA is processed and migrates from nucleus to ribosomes Ribosomes “read” the info encoded on mRNA and use it for protein synthesis, called translation This keeps DNA pristine and protected, away from caustic chemistry of cytoplasm Gene info can be amplified by having many copies of an RNA made from one copy of DNA

RNA Same primary structure of DNA; sugar-phosphate backbone attached to a N base Sugar is ribose, instead of deoxyribose RNA is a single strand, not a double helix Uses uracil, instead of the N base thymine 3 types: mRNA—messenger RNA is the copy of the gene tRNA—transfer RNA carries the amino acids to the ribosomes from the cytoplasm rRNA—ribosomal RNA helps mRNA bind to the ribosome

Genetic Code Each amino acid is specified by a particular combination of 3 nucleotides, called codons Different species can use one codon over another for the same amino acid—giraffe use CGC for arginine over CGA

Chromosomes Prior to mitosis, DNA coils up into chromosomes that look like a fuzzy X Chromatids are half of a replicated chromosome Centromere ( protein disk) holds the 2 chromatids together Located in different location on each chromosome Each chromosome has its own size Humans have 23 pairs of different chromosomes