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DNA, Chromosomes & genes AP Biology 2007-2008 Watson and Crick AP Biology 1953 article in Nature Watson and Crick Discovered that DNA was a double helix Based on x-ray crystallography photographs of DNA taken by Franklin AP Biology DNA Deoxyribonucleic acid Carries the genetic information of an organism Identical copies are found in every cell of that particular organism AP Biology nucleotide PO4 DNA Made of repeating subunits called nucleotides Sugar, phosphate & nitrogenous bases N base CH2 O ribose OH AP Biology Double helix structure of DNA “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic AP Biology material. ” Watson & Crick Double helix structure of DNA One complete turn of the helix is 3.4nm and is 10 nucleotides long, therefore there are 0.34 nm between base pairs AP Biology Directionality of DNA You need to PO4 nucleotide number the carbons! it matters! N base 5 CH2 This will be IMPORTANT!! O 4 3 AP Biology 1 ribose OH 2 The DNA backbone Putting the DNA backbone together refer to the 3 and 5 ends of the DNA the last trailing carbon 5 PO4 base 5 CH2 O 4 1 C 3 O –O P O O 5 CH2 2 base O 4 1 2 3 OH AP Biology 3 Anti-parallel strands Nucleotides in DNA backbone are bonded from phosphate to sugar between 3 & 5 carbons 5 3 3 5 DNA molecule has “direction” complementary strand runs in opposite direction (antiparallel) AP Biology Anti-parallel strands 3’ end, ends with an OH group (attached to the 3rd carbon in the sugar ring) 5’ end, ends with a phosphate group (attached to the 5th carbon in the sugar ring) AP Biology 5 3 3 5 Bonding in DNA 5 hydrogen bonds 3 covalent phosphodiester bonds 3 5 ….strong or weak bonds? AP Biology How do the bonds fit the mechanism for copying DNA? Base pairing in DNA Purines adenine (A) guanine (G) Pyrimidines thymine (T) cytosine (C) Pairing A:T 2 bonds C:G 3 bonds AP Biology Chromosomes Chromosomes in interphase are called chromatin Chromatin = 40% DNA + 60% Protein A single strand of DNA have approximately 300 M nucleotide DNA is highly coiled to fit in the nucleus of a cell AP Biology Chromosomes Every 200 nucleotides, DNA is coiled around 8 positively charged proteins called histones Proteins are positive, DNA is negative Each group of histones and DNA is called a nucleosome AP Biology Chromosomes A series of nucleosomes coil into chromatin fibres These fibres then fold into the final chromatin and are supercoiled (see DNA packaging ppt) AP Biology What is a gene? A specific sequence of nucleotides in a section of DNA that performs a specific function AP Biology What is a gene? When genes (small sections of chromatin) are active, the chromatin isn’t tightly coiled or condensed, but it is in an open configuration called euchromatin. AP Biology What is a gene? Heterochromatin – are condensed portions of chromatin Some portions of chromatin are permanently condensed & their genes are never expressed Heterochromatin exists during cell division AP Biology http://learn.genetics.utah.edu/content/beg in/tour/ Tour of the cell animation- do on your own time! Also check out DNAi.org website as well! AP Biology