Nucleic Acids and their Structure

I. What criteria or properties must the hereditary material possess?A. It must be able to pass information from parent to offspring and therefore must be very large to

carry all of that information.B. It must be able to self replicate with each cell division, and this should happen with very few

mistakes or offspring would not resemble the parents.C. It must be very stable in order to last the life of the cell and the organism.D. When a mistake does occur, it must be copied faithfully otherwise there would be no variations

and no evolution.II. What evidence suggests that a chemical substance is responsible for carrying the hereditary

information?A. Griffith’s Experiment (1928)

III. Which two substances found in all cells could be responsible for controling the cell and carryingthe hereditary information?

IV. What evidence support the idea that DNA is the hereditary material?A. All cells contain DNA (some viruses contain only RNA but viruses are not considered cells)B. DNA is long enough to hold all of the information (if we took all of the DNA from all of your

cells and strung it together it would go to the sun and back five times!C. All the cells of an organism contain the same amount of DNA, except reproductive cells.

1. How much would we find here? 2.

D. Hershey/Chase experiment (1952) “Final confirmation”

E. Chargaff’s Results 1. The proportions of the four bases in DNA are the same in all cells of all of the

individuals in a given species. 2. These proportions vary from species to species.

V. What is the chemical composition of the DNA molecule?A. The structure of a nucleotide (made of three parts)

1. A nitrogenous base

2. A sugar

3. A phosphate

VI. What is the exact chemical structure of this molecule?A. Watson, Crick & Franklin B. It is a polymer made of many repeating units called nucleotides.C. Structure of a single nucleotide

D. Several nucleotides

E. Note the 5’ and 3’ ends. F. Note that the double strands are head to tail (antiparallel).G. Principle of Base Pairing (Chargaff)

1. Adenine always pairs with thymine and guanine with cytosine.H. The overall shape of DNA is described as a double helix (a twisted ladder).I. The two halves of the ladder are held together by hydrogen bonds between the base pairs.

VII. How does this DNA control living systems?A. Every three bases codes for one amino acid (triplet code)B. Remember these are messenger RNA codes.

VIII. The other nucleic acid: RNA A. What does RNA stand for?B. Differences between DNA and RNA .

1. RNA contains ribose instead of deoxyribose.2. RNA contains uracil instead of thymine.

3. RNA is found both in the nucleus and cytoplasm.4. RNA is not necessarily in the form of a double helix.

C. There are three kinds of RNA.1. Messenger RNA (m-RNA)

a. copies the DNA code in the nucleus and carries the message to the ribosomes.b. single stranded

2. Transfer RNA (t-RNA) page 283a. Each t-RNA consists of ~90 nucleotides.b. transfers amino acids to the ribosomes and places them in the correct position in the

protein.3. Ribosomal RNA (r-RNA)

a. Part of the ribosome and involved in the synthesis of t-RNA

DNA Replication

I. How does an organism pass on its genetic information to the next generation?II. How many times does a cell replicate its DNA.

III. Meselson-Stahl experiments

IV. The process of replicationA. Just prior to cell division, promoter proteins are produced.

1. These proteins bind to DNA at several sites. B. Enzymes called helicase are produced and attach to DNA, where the promoter proteins are

located.1. These enzymes break the hydrogen bonds linking the bases and the helix opens up

and many sites along the DNA molecule.C. Now another enzyme, RNA primase reads the exposed nucleotides on both halves and produces

a RNA primer (about 10 nucleotides long).D. DNA polymerase now starts to replicate the DNA (base pairing) forming the new strand in the 5’

- 3’ direction. 1. So it reads the parent DNA in which direction?

E. There is a directional problem.1. One molecule of DNA polymerase produces a continuous (leading) strand of DNA.2. The problem is that DNA polymerase only forms DNA in the 5’-3’ direction.

a. So what happens on the other side of the molecule? Page 269 3. On the lagging strand, new primers have to form at many sites and DNA polymerase

forms many small (Okazaki) fragments (1,000- 2,000 long in prokayotes and 100 -200 long in eukaryotes).

F. This process occurs at many sites (replication bubbles) along the DNA molecule.1. Why not just start at one end and go to the other end?

a. Takes too much time.b. In humans, DNA polymerase only incorporates 50 nucleotides per second and

prokaryotes 500 per second.G. When DNA polymerase reaches the 5’ end of the RNA primer it is released and other enzymes

remove the RNA primers and replace them with DNA nucleotides.H. Next, DNA ligase joins together all of the large leading fragments and the many Okazaki

fragments to form the two new daughter strands.I. DNA polymerase also checks and corrects any mistakes in base pairing.J. Topoisomerase prevents kinks as the parent DNA is unzipped.