The structure of Nucleic Acids DNA & RNA The structure of Nucleic Acids

D N A A polymer composed of repeating units (monomers) called NUCLEOTIDES DNA: Molecule found in all of your cells*, carries instructions for life. RNA: molecule used to build proteins and carry instructions *except mature red blood cells www.biologycorner.com/bio1/DNA.htm

Nucleotide Phosphate Nitrogen Base Deoxyribose Sugar* Watson and Crick discovered the double helix n 1953. They relied on info from Rosalind Franklin, x-ray photographs. Deoxyribose Sugar* A nucleotide has three components. * RNA nucleotides contains ribose sugar

Structure The “backbone” of DNA and RNA are repeating molecules of sugar and phosphate. Sugar and Phosphate are connected by covalent bonds. * Phosphate: *Sugar (notice ring shape) www.mun.ca/.../Deoxyribose_versus_Ribose.html www.emc.maricopa.edu/.../BioBookglossPQ.html

Sugar Phosphate Sugar Phosphate Sugar Phosphate Sugar Note: Sugar and Phosphate repeating along the “backbone” of the molecule. Notice the sugar is always attached to the middle molecule, a nitrogen base. PHOSPHATE DOES NOT ATTACH HERE! Sugar http://www.abpischools.org.uk/page/modules/genome/dna2.cfm?age=Age%20Range%2016-19&subject=Biology

Note: four types of bases in the middle. Pyrimidine: smaller (one ring) Purine: larger (two rings)

Notice the sugar-phosphate backbone. Note the size of nitrogen bases that pair up.

Nitrogen Bases These form the “rungs” of the DNA They are connected by weak hydrogen bonds. There are FOUR different bases in DNA. Adenine (A) * Cytosine (C ) Guanine (G) * Thymine (T) There are FOUR different bases in RNA. Guanine (G) * Uracil (U) G and A: purine T and C: pyrimidine Chargraff showed that in organisms the amount of Adenine and Thymine was always equal, and the amount of Guanine and Cytosine was always equal. http://www.abpischools.org.uk/page/modules/genome/dna2.cfm?age=Age%20Range%2016-19&subject=Biology

Complimentary Base Pair Rule Edwin Chargaff A always with T G always with C

Using Chargaff’s Ratios Organism % Adenine % Guanine % Cytosine % Thymine Grasshopper 29.3 20.5 Yeast 32.1 17.9 E. Coli 24.1 25.9 Octopus 32.4 17.6 Human 20 Can you fill in the missing amounts? *Numbers for organisms are simplified

Note: Anti-parallel One side of the DNA is arranged from 5’ (five prime) to 3’ (three prime) The sugar is oriented so the Oxygen is pointed upward. On the other side, the DNA is arranged from 3’ to 5’, and the oxygen is pointed downward. http://www.abpischools.org.uk/page/modules/genome/dna2.cfm?age=Age%20Range%2016-19&subject=Biology

Which color is the sugar? Which color is phosphate? If lime green is cytosine, what nitrogen base is the purple?? If green is adenine, what is the red nitrogen base?

What bond connects the phosphate and the sugar? What bond connects the nitrogen bases?

The mystery of DNA You need to know the following scientists and their contribution to our understanding of DNA

Frederick Griffith (1928) Provided evidence that some factor in cells passes information to other cells

Avery, McLeod, McCarty 1944 Supported griffith’s work by showing that something inside cells can transform other cells

Alfred Hershey & Martha Chase 1952 Provided evidence that DNA (not proteins) is the source of genetic information

Rosalind Franklin Maurice Wilkins X-ray crystallography Her photo 51 gave Watson the information needed to decipher the double helix shape of DNA

James Watson and Francis Crick Won Nobel Prize for discovering the structure of DNA James Watson and Francis Crick

DNA Replication replication “ It has not escaped our notice that the pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick Nature 1953

replication Replication First, ladder separated as Helicase Enzyme breaks hydrogen bonds Replication Fork

replication Replication DNA Polymerase Adds complementary nucleotides Lots of DNA polymerase

replication Replication 1 error in every 10,000 base pairs DNA polymerase proofreads the DNA Ensures all bases are correctly matched

replication Mutations DNA proofreading drops the actual number of errors to 1 in a billion nucleotide pairs. A human cell takes a few hours to replicate the 6 billion base pairs. DNA polymerase proofreads

replication Telomeres The tip of eukaryotic chromosomes Hard to copy Telomerase is a special enzyme that adds repeating DNA sequences to the ends of chromosomes DNA polymerase proofreads

replication Eukaryotic Starts in multiple locations DNA is in long strands DNA polymerase proofreads

replication Prokaryotic Replication Start at a single spot DNA is in a single circle DNA polymerase proofreads