DNA Structure & Function

What is DNA? DNA: Deoxyribonucleic Acid carries genetic information (genetic code) and controls cellular functions All living things have DNA that only differs in the sequence of nucleotides.

DNA - In eukaryotes, DNA is located in the nucleus. - In prokaryotes, the DNA is a loop, which is not contained in a nucleus but found in an area called the nucleoid region

History of DNA

Discovery of DNA In 1928 Fredrick Griffith did experiments trying to find a vaccine for pneumonia; he discovered that when harmless bacteria and a virulent (disease-causing) bacteria were mixed together, some of the harmless bacteria became virulent. He called this process transformation

History of DNA Transformation: the ability of one organism to be permanently changed by another through the exchange of DNA

Discovery of DNA In 1944 Oswald Avery repeated Griffith’s experiments but he used enzymes enzymes to remove proteins, lipids, RNA & DNA separately. - Transformation occurred each time except when DNA was removed, so DNA must be the transformation factor. DNA is the genetic material that stores and transfers genetic information from one generation of an organism to another

Discovery of DNA During the 1950’s, most scientists still thought that proteins carried genes. In 1952, Alfred Hershey and Martha Chase did a series of experiments with bacteriophages & discovered that DNA is the source of genetic information that proved Avery’s transformation principle.

History of DNA Bacteriophage: viruses that attack bacteria, composed of DNA or RNA core and a protein coat Hershey and Chase confirmed that DNA was the genetic molecule of inheritance in organisms and not proteins

History of DNA When Hersey & Chase used radio active markers to mark the protein coats of the bacteriophages, the infected bacteria didn’t become radio active. Repeated with radioactive markers on the DNA of bacteriophages, the infected bacteria became radioactive.

Discovery of DNA In 1952, Rosalind Franklin developed special X-ray diffraction experiments to study the structure of the DNA molecule These X-ray patterns showed an X-pattern with 2 strands of DNA twisted around each other. DNA resembled the coils of a spring.

Discovery of DNA In 1953, James Watson and Francis Crick described the shape and structure of DNA (with the help of previous X-ray experiments from Franklin) Through the construction of a model, they proposed the structure of DNA that is known as double helix.

History of DNA

Nitrogen Bases [A] & [G] – belong to a group of compounds called purines (double ring of carbon) * [T] & [C] – belong to a group of compounds called pyrimidines (single ring of carbon)

Watson & Crick discovered that it was a double helix shape, with a sugar phosphate backbone, connected in the middle with nitrogen bases. T binds with A, & G binds with C: base paring

Structure of DNA The monomer of DNA is a nucleotide A nucleotide consists of 3 main parts: 1. Phosphate group 2. 5 carbon sugar (deoxyribose) 3. Nitrogen base - Adenine [A] - Guanine [G] - Thymine [T] - Cytosine [C]

Structure of DNA Phosphate and sugar make up the backbone or sides of the DNA molecule Nitrogen bases pair up to form the center These base pairs are held together by weak hydrogen bonds

Discovery of DNA - Erwin Chargaff experimented with DNA and observed that in each organism the amount of the 4 nitrogen bases was equal. Although, the amount of DNA was different between different types of organisms His experiments resulted in the base pairing rule : [A] = [T] and [G] = [C]

Antiparallel (see how one side is upside down) Phosphate-sugar backbone linked with covalent bonds Nitrogen bases linked with hydrogen bonds

DNA DNA can hold a great deal of information because it is a very long molecule. Living things are different because the sequence (order) of the nucleotides is limitless. The more similar the sequence, the more closely related the organisms.

Chromosomes & Replication

DNA Length Very long E. coli: over 4.6 million nucleotides, but must fit in a cell 1/1000 in length Eukaryotes: varies by species, but can be 1000 times longer than prokaryotes Humans over 1 m of DNA in each cell

Chromosome Structure Prokaryotes: DNA is folded into a single loop chromosome Eukaryotes: DNA is folded around proteins (histones) and coiled to form multiple chromosomes Humans have 46 chromosomes in each cell

Replication Every cell must contain an identical copy of DNA so it must be replicated before new cells form. Prokaryotes: starts at 1 “point of origin” and continues around the loop chromosome Eukaryotes: hundreds of points of origin

DNA Replication DNA needs to replicate itself, which means it needs to make an exact copy of itself to pass along to the daughter cell during cell division 1 DNA molecule -------> 2 identical DNA molecules Occurs in the nucleus prior to cell division (in the cytoplasm of a prokaryote)

Replication Separate the DNA Stands The enzyme helicase breaks the hydrogen bonds to open the DNA strand like a zipper

DNA Replication The split where it unzips is called the replication fork

DNA Replication Replication happens in the 5’----> 3’ direction It is semiconservative, meaning that every double-stranded molecule of DNA has one strand that is “old” and one strand that is “new” Replication can occur at hundreds of different replication forks all at the same time on the same molecule

DNA Replication 2. Another enzyme called DNA polymerase pairs “free” complimentary nucleotides found in the nucleus to each side of the original unzipped strand. New hydrogen bonds form between the nitrogen bases.

Replication Nucleotides are added When the enzyme called DNA polymerase adds nucleotides, it uses the original DNA strand as a template. DNA polymerase can only add in the 5’ to 3’ direction Leading strand: (5’ to 3’) continuously adds nucleotides Lagging strand: (3’ to 5’) creates small sections called Okazaki Fragments as the DNA opens Enzyme DNA ligase joins Okazaki fragments together

DNA Replication This process takes time as the lagging strand has to wait for the DNA to unzip and then fill in backwards a little section at a time making it “grow” in the wrong direction (away from the replication fork). The Okasaki fragments are joined into a single strand by an enzyme called DNA ligase Ultimately, the overall direction of “growth” is toward the replication fork but it must be done in small segments that require the lagging strand to fill in away from the fork

DNA Replication 3. This continues until the entire strand has been copied. The results are two identical strands of DNA.

Overall Direction of DNA Replication is Towards the Replication Fork One strand, called the leading strand, grows in the 5’ to 3’ direction towards the replication fork

DNA Replication Other strand called the lagging strand, is created next to the 3’--->5’ strand this lagging strand must add nucleotides in short 5’ --->3’ away from the replicating fork in segments called Okazaki fragments

DNA Replication

DNA Replication

Replication fork opens in both directions 

Gene Regulation Not all genes are used in every cell. Genes can be turned off (silenced) In prokaryotic cells groups of genes that are turned on/off together are called operons

Gene Regulation Eukaryotic Cells Genes can be turned on by enhancer and promoter sequences Genes can be turned off by repressor proteins Cell Differentiation: cells in multicellular organisms have to change in order to do different jobs. All cells have all of the exact DNA, but only parts of the DNA are expressed.

DNA Replication http://www.youtube.com/watch?v=hfZ8o9D1tus http://www.youtube.com/watch?v=teV62zrm2P http://www.youtube.com/watch?v=4jtmOZaIvS0 http://www.johnkyrk.com/DNAreplication.html http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html# http://www.wiley.com/legacy/college/boyer/0470003790/animations/replication/replication.htm