DNA:(Deoxyribonucleic Acid) Aim: Why is DNA such an important molecule?

Genetic material of cells… GENES – units of genetic material that CODE FOR A SPECIFIC TRAIT DNA is made up of repeating molecules called NUCLEOTIDES

From cell to DNA… http://www.pbs.org/wgbh/nova/genome/dna.h tml# (a quick animation showing the structure of genetic material)

DNA Nucleotide O O=P-O N CH2 O C1 C4 C3 C2 Phosphate Group Nitrogenous base (A, G, C, or T) CH2 O C1 C4 C3 C2 5 Sugar (deoxyribose)

The Discovery of DNA Fredrich Mieshcer (1869) – the 1st scientist to isolate DNA However… many scientists still thought PROTEINS carried genetic instructions

Frederick Griffith Griffith was a bacteriologist who studied streptococcus pneumonia (the bacteria responsible for Strep throat and pneumonia) He demonstrated the transformation of DNA from one bacteria to another He called his discovery “The Transforming Principle”

Griffith’s Transformation Experiment

Griffith’s conclusion Something from the heat-killed virulent bacteria (S-strain) must have been transformed (moved into) to the non- virulent bacteria (R strain) to make them disease-causing However…. What caused the transformation – was it protein or DNA???

Avery, Macleod & McCarty - 1944 Wanted to determine the exact genetic material of a cell – what causes the transformation? They used enzymes to destroy specific parts of the cell – ie lipids, proteins, DNA, carbohydrates, etc) Transformation occurred in EVERY case EXCEPT tests with DNA-destroying enzymes

Conclusion?? DNA is the genetic material of cells and is responsible for transformations

Application for DNA transformation Transformation experiments cause bacteria (E.coli) to “take up” a gene resistant to penicillin as well as a luminescent “green” gene from a jellyfish.

Hershey & Chase In 1952, Alfred Hershey and Martha Chase used a new technology called radioactive labelling to show that genes are made of DNA They were able to “label” DNA with radioactive phosphorus and watch it incorporate itself into bacterial cells They also “labelled” protein with radioactive sulfur and found that the protein did NOT transform into the bacterial cells

The Hershey-Chase Experiment

The Hershey-Chase Experiment http://highered.mcgraw- hill.com/olc/dl/120076/bio21.swf

Phoebus Levene – discovery of nucleotides Levene was a biochemist who discovered that DNA and RNA are made up of long chains of NUCLEOTIDES – of which there are 5 types. DNA – adenine, thymine, cytosine, guanine RNA – adenine, uracil, cytosine, guanine

Chargaff’s Rule Adenine pairs with Thymine Guanine pairs with Cytosine Their amounts in a given DNA molecule will be about the same. T A G C

Relative proportions (%) of Bases in DNA of Several Organisms A (adenine) T (thymine) G (guanine) C (cytosine) E. Coli 26.0 23.9 24.9 25.2 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Rat 28.6 28.4 21.4 21.5 Human 30.9 29.4 19.9 19.8

Rosalind Franklin – 1950s Used x-ray photography to look “through” DNA molecules She concluded that the nitrogenous bases (A,T,C,G) were located on the inside (the “rungs”) and the sugar-phosphate backbone was located on the outside Her work was used by Watson & Crick in the discovery of the double helix model of DNA

Rosalind Franklin She graduated from Cambridge University with a PhD in Chemistry She also spent much of her short life studying molecular biology and x- ray crystallography Sadly, she succumbed to ovarian cancer at 37.

James Watson & Frances Crick James Watson and Frances Crick shared the Nobel Prize in 1962 for their discovery of the DNA double helix http://www.youtube.com/watch?v=OiiFVSvLfGE http://www.youtube.com/watch?v=sf0YXnAFBs8&feature=related

Watson & Crick proposed… DNA had specific pairing between the nitrogen bases: ADENINE – THYMINE CYTOSINE - GUANINE DNA was made of 2 long stands of nucleotides arranged in a specific way called the “Complementary Rule”

DNA Double Helix “Rungs of ladder” Nitrogenous Base (A,T,G or C) “Legs of ladder” Phosphate & Sugar Backbone

DNA Double Helix P O 1 2 3 4 5 P O 1 2 3 4 5 G C T A

Nitrogenous Bases A or G T or C PURINES – 2 “rings” 1. Adenine (A) 2. Guanine (G) PYRIMIDINES - 1 “ring” 3. Thymine (T) 4. Cytosine (C) A or G T or C

BASE-PAIRINGS C G H-bonds T A

Fig. 12.03

Genetic Diversity… Different arrangements of NUCLEOTIDES in a nucleic acid (DNA) provides the key to DIVERSITY among living organisms.

The Code of Life… A T C G T A T G C G G… The “code” of the chromosome is the SPECIFIC ORDER that bases occur. A T C G T A T G C G G…

DNA is wrapped tightly around histones and coiled tightly to form chromosomes

DNA Double Helix P O 1 2 3 4 5 P O 1 2 3 4 5 G C T A

DNA strands are ANTIPARALLEL 5’ 3’ 3’ 5’ One of the DNA strands runs from 3’ to 5’ and the other runs in the opposite direction – 5’ to 3’

HISTORY OF DNA GRIFFITH’S EXPERIMENT AVERY, MACLEOD, McCARTY HERSHEY-CHASE EXPERIMENT CHARGAFF’S RULE WATSON-CRICK (FRANKLIN)

Replication of DNA

DNA REPLICATION starts at the replication origin DNA helicase “unzips” DNA double strand DNA polymerase (enzyme) adds free nucleotides to the unravelled DNA strand DNA polymerase can’t initiate a new DNA strand in a 5’ to 3’ direction – THEREFORE a short strand of RNA (called an RNA primer) serves as a starting point HOWEVER – DNA polymerase can only attach nucleotides to the 3’ end (old strand is 3’ to 5’ and new strand is 5’ to 3’)(LEADING STRAND) Again, because DNA can’t initiate a new DNA strand, an RNA primer is laid down first Because the DNA polymerase only attaches new DNA 3’ to 5’ – the replication is discontinuous and backwards Short fragments called Okazaki fragments form and DNA ligase “glues” them together (this becomes the LAGGING STRAND)

REPLICATION (DNA DNA)

DNA Replication http://www.mcgrawhill.ca/school/applets/abbio/ch18/replicationfork_dna_rep.swf (McGraw-Hill animation) http://www.mcgrawhill.ca/school/applets/abbio/ch18/dna_replication.swf (more detailed, complex explanation of DNA replication)

REPLICATION IN BACTERIA Replication takes place at several locations simultaneously Each replication bubble represents 2 replication forks moving in opposite directions along the chromosome The bubbles continue to grow until they meet

Replication in Bacteria http://www.mcgrawhill.ca/school/applets/abbio/ch18/bidirectionalrep_bidire.swf

Replication Questions 1. Why is replication necessary? 2. When does replication occur? 3. Describe how replication works. 4. Use the complementary rule to create the complementary strand: A---? G---? C---? T---?

Replication Answers So both new cells will have the correct DNA A---T 1. Why is replication necessary? So both new cells will have the correct DNA 2. When does replication occur? During interphase (S phase). 3. Describe how replication works. Enzymes unzip DNA and complementary nucleotides join each original strand. 4. Use the complementary rule to create the complementary strand: A---T G---C C---G T---A

DNA Replication Semiconservative Model: . 1. Watson and Crick showed: the two strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand. . DNA Template New DNA Parental DNA

Semi-Conservative Nature of DNA Replication http://www.mcgrawhill.ca/school/applets/abbio/ch18/ meselsonstahl_meselson_.swf

(1961) Watson & Crick proposed… …DNA controlled cell function by serving as a template for PROTEIN structure. 3 Nucleotides = a triplet or CODON (which code for a specific AMINO ACID) AMINO ACIDS are the building blocks of proteins.

Protein structure For a cell to get the proteins it needs, it must translate the codons along a stretch of mRNA into amino acid sequences. At the ribosomes, tRNA carries a particular amino acid to the correct mRNA codon site.

Several amino acids are bonded together to form a polypeptide

THE CENTRAL DOGMA DNA RNA PROTEIN