1. DNA, Chromosomes & genes

2. 1953 article in Nature
Watson and Crick

3. 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. (

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

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

6. 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

7. 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

8. 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

9. The DNA backbone Putting the DNA backbone together5
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

10. 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

11. 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

12. 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?

13. 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

14. 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

15. 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

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

17. Chromosomes
See DNA Packaging ppt!

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

19. 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.

20. 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