DNA, Chromosomes & genes 2007-2008

1953 article in Nature Watson and Crick

Watson and Crick Discovered that DNA was a double helix Based on x-ray crystallography photographs of DNA taken by Franklin X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and diffracts into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their disorder and various other information. (http://en.wikipedia.org/wiki/X-ray_crystallography)

DNA Deoxyribonucleic acid Carries the genetic information of an organism Identical copies are found in every cell of that particular organism

DNA nucleotide Made of repeating subunits called nucleotides OH CH2 O PO4 N base ribose nucleotide DNA Made of repeating subunits called nucleotides Sugar, phosphate & nitrogenous bases

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

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

Directionality of DNA You need to number the carbons! nucleotide PO4 N base 5 CH2 This will be IMPORTANT!! O 4 1 ribose 3 2 OH

The DNA backbone Putting the DNA backbone together 5 The DNA backbone PO4 Putting the DNA backbone together refer to the 3 and 5 ends of the DNA the last trailing carbon base CH2 5 O 4 1 C 3 2 O –O P O O base CH2 5 O 4 1 3 2 OH 3

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) 5 3 3 5

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) 5 3 3 5

Bonding in DNA 5 3 3 5 hydrogen bonds covalent phosphodiester ….strong or weak bonds? How do the bonds fit the mechanism for copying DNA?

Base pairing in DNA Purines Pyrimidines Pairing adenine (A) guanine (G) Pyrimidines thymine (T) cytosine (C) Pairing A : T 2 bonds C : G 3 bonds

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

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

Chromosomes A series of nucleosomes coil into chromatin fibres These fibres then fold into the final chromatin and are supercoiled

Chromosomes See DNA Packaging ppt!

What is a gene? A specific sequence of nucleotides in a section of DNA that performs a specific function

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.

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