AP Biology DNA, Chromosomes & genes

AP Biology Watson and Crick 1953 article in Nature

AP Biology 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 DNA  Made of repeating subunits called nucleotides  Sugar, phosphate & nitrogenous bases OH CH 2 O PO 4 N base ribose nucleotide

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 material.”Watson & Crick

AP Biology 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 number the carbons!  it matters! OH CH 2 O PO 4 N base ribose nucleotide This will be IMPORTANT!!

AP Biology The DNA backbone  Putting the DNA backbone together  refer to the 3 and 5 ends of the DNA  the last trailing carbon OH O 3 PO 4 base CH 2 O base O P O C O –O–O CH

AP Biology Anti-parallel strands  Nucleotides in DNA backbone are bonded from phosphate to sugar between 3 & 5 carbons  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 3 rd carbon in the sugar ring)  5’ end, ends with a phosphate group (attached to the 5 th carbon in the sugar ring)

AP Biology Bonding in DNA ….strong or weak bonds? How do the bonds fit the mechanism for copying DNA? covalent phosphodiester bonds hydrogen bonds

AP Biology 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 in/tour/ Tour of the cell animation- do on your own time! Also check out DNAi.org website as well!