DNA - Name the four bases in DNA and describe the structure of DNA using the following terms: - nucleotide (sugar, phosphate, base) - complementary base pairing - double helix - hydrogen bonding

- DNA is used and copied thousands of times with very little change

- Found in every cell of every organism, deoxyribonucleic acid is the only molecule known that is able to replicate itself and correct errors, thereby allowing cell division.

- DNA provides the directions for the building of new cells and for the repair of worn cells.

- DNA is most often described as a double helix. DNA closely resembles a twisted ladder. Sugar and phosphate molecules form the backbone of the ladder, while the nitrogen bases form the rungs. Nitrogen bases from one spine of the ladder are connected by weak hydrogen bonds to the nitrogen bases on the other side of the ladder.

[The double helix structure was first described in 1953 by James Watson and Francis Crick. Their discovery is one of the most significant of the twentieth century]

- The DNA molecule is made up of individual units called nucleotides. Each nuleotide is composed of a deoxyribose sugar, a phosphate, and a nitrogen base. (see fig. 24.3 Page 464)

- Complementary base pairing (see fig. 24.4 Page 465) describes the behavior of the nitrogen bases. There are two families of nitrogenous bases: pyrimidines and purines. Pyrimidines have a single ring and include cytosine, thymine and uracil. Purines have a double ring and include adenine and guanine. A purine base always pairs with pyrimidine base. Can you suggest why?

[Using Mnemonics: "Thousands Count Pyramids" "Ads Guarantee Purity"]

- In DNA cytosine always pairs with guanine and adenine always pairs with thymine.

- If a stretch of one strand has the has the base sequence AGGTCCG, what would be the sequence of the same stretch of the other strand?

Answer: TCCAGGC

- A researcher finds a sample of DNA to have the four bases in the following percentages:

Why must there have been an error in his measurements?

Assuming the percentages of adenine and guanine are correct what should the values have been for thymine and cytosine?

- A genes meaning to the cell is encoded in its specific sequence of the four bases. The linear order of bases encoded in a gene specifies the amino acid sequence of a protein, which then specifies that protein's function in the cell.

- The two strands of the double helix are complementary, each the predictable counterpart of the other. It is this feature of DNA that makes possible the precise copying of genes that is responsible for inheritance. As a cell prepares to divide, the two strands of each gene seperate. Each existing strand serves as a template to order nucleotides into a new complementary strand.

- The genetic code is contained in 46 seperate chromosomes in your body.

Hydrogen bonding - The strands are held together by hydrogen bonds between the bases.

Three Functions of DNA - store info., replicate, undergo mutations

How is structure related to function? The double stranded structure aids replication because each strand can serve as a template for the formation of a complementary strand.

Replication is semiconservative (1 new, 1 old) because each of the new helixes after the first round of replication contain one of the original strands and one new strand. ( see fig. 24.6 Page 467)

[If replication were conservative then of the two new helixes one would consist entirely of the original strands and the other would consist entirely of two new strands.]

Semiconservative replication has the following steps:

1) The two strands of the parent DNA unwind (helicase) and unzip (weak hydrogen bonds broken between the bases).

2) New complementary nucleotides, always present in the nucleus, move into the appropriate place according to complementary base pairing.

3) The enzyme DNA polymerase joins the complemetary nucleotides so that the new DNA molecule is again double straded.

The bases seldom pair incorrectly because DNA polymerase has a proofreading function. If it finds an incorrect nucleotide it removes it and replaces it with the proper one. If an error is not corrected a gene mutation has occured.

[The biochemical capabilities of microorganisms have been exploited for a very long time. How? Wine and bread depends on the yeast cells to carry out fermentation.]

1980's - biotechnology gave rise to an industry that provides products made by genetically engineered bacteria including drugs and vaccines and nucleic acids for laboratory research. Bioengineered bacteria are also released into the environment to clean up pollutants and kill insect pests.

Recombinant DNA contains DNA from 2 or more different sources. A vector is used to introduce recombinant DNA into cells.

The most common vector is a plasmid. Plasmids are small accessory rings of DNA found in bacteria. Plasmids used as vectors have been removed from bacteria and have had a foreign gene inserted into them. Treated bacteria take up a plasmid, and after it enters, the plasmid replicates whenever the host reproduces. [gene cloning has occured because there are now many exact copies of a foreign gene.] [Viruses can also act as vectors when they incorporate a foreign gene into their DNA. Because each virus derived from a viral vector also contains the foreign gene they too allow cloning of a particular gene.]

Insulin was previously extracted from cattle and pigs. Again biotechnology allows mass production of insulin which is cheaper and less likely to cause an allergic reaction.

Other Examples of Hormones and Proteins produced for use in humans include:

• TPA (tissue plasminogen activator) to treat heart attacks by dissolving blood clots
• Clotting factor VIII for the treatment of hemophelia
• Human lung surfactant treats respiratory distress syndrome in premature infants

Biotechnology also produces safer vaccines for humans and farm animals to protect against Herpes, Hepatitis A, B, and C, Lyme disease, Whooping cough, and Clamydia. These vaccines are safer because unlike regular vaccines, which use treated bacteria or viruses and can sometimes cause the disease, genetically engineered vaccines use other bacteria to produce the surface protein of a pathogen for use as a vaccine. These surface proteins are enough to trigger the immune response but cannot afflict someone with the disease.

Some bacteria are bioengineered to perform other services such as:

• Development of frost-resistant crops. [bacteria that normally live on plants and encourage the formation of ice cyrstals have been genetically engineered to do the opposite]

• Endowing bacteria that normally inhabit the roots of a plant with genes that produce an insect toxin

• Production of hormones for animals (e.g. bGH to increase milk

production).

• Production of DNA probes (used to determine such things as paternity or

body identification at crime scenes).

• Enhancement of naturally occurring bacteria for use in environmental

cleanup. (e.g. oil spills)

• Identification of genes for cloning and mapping.

• Production of organic chemicals (various bacteria can be bioengineered to produce phenylalanine an organic chemical needed to make aspartame)

• Processing minerals (many companies already use bacteria to extract copper, uranium, and gold from low grade sources)

• Hardier more nutritious plants that require less fertilizer and are resistant to insects.

Both polynucleotides and involved in protein synthesis. One has ribose sugar other has deoxyribose sugar, DNA has CGAT in a double strand. RNA has CGAU in a double strand.