NOTES FOR BIOLOGY 1001

SECTION 005

Spring 2005

DR. STEVEN POMARICO

CHAPTER 13

DNA STRUCTURE AND FUNCTION

>>>>>>The search for the genetic material

In the mid-1920's, Griffith working with Streptococcus pneumoniae demonstrated that genetic material could be transferred from one bacteria to another. (See fig 13.3)

         -Smooth cells (with capsule) =>mouse dies

         -Rough cells (no capsule) => mouse lives

         -Heat killed smooth cells => mouse lives

         -Heat killed smooth cells plus rough cells => mouse dies

                             AND PRODUCE LIVE SMOOTH CELLS!

The trait from the smooth cell was transferred to the rough cells, and the rough cell were transformed.

---Transformation is the assimilation of external genetic material by a cell.

But what was transferred??

         -Chromosomes are composed of DNA and protein.

         -Protein is denatured by heat.

         -Maybe it’s the DNA. Many doubters.

>>>Evidence that the transforming molecule is DNA

More evidence that DNA is the genetic material came from the study of bacteriophages. (See fig. 13.4)

---bacteriophage (a.k.a. phage) is a virus that infects bacteria.

-Bacteriophages quickly reprogram the bacterial cells they infect.

Hershey and Chase discovered that bacteria can be reprogrammed by the genetic material from a bacteriophage and that the genetic material was DNA. (See fig. 13.5)

         -They radioactively labeled the components of the phage.

         -Infected bacteria with the labeled phage.

         -Separated the phage from the bacteria and looked for which label was                    transferred to the bacteria.

Concluded that the DNA was the genetic material in a virus.

         BUT WHAT ABOUT EUKARYOTES??

---Nucleic acids are polymers of nucleotides linked together by dehydration synthesis reactions.

---Nucleotides are the building block molecules of a nucleic acid and are composed of a sugar, a phosphate group, and a nitrogenous base.

The sugar is a 5-carbon sugar in a ring conformation.

There are two families of nitrogenous bases:

         Pyrimidines and Purines

---Pyrimidines are nitrogenous bases characterized by a six-membered ring          made up of carbon and nitrogen atoms.

         Cytosine

         Thymine

---Purines are nitrogenous bases characterized by a five-membered ring fused to a six-membered ring where both rings are made up of carbon and nitrogen atoms.

         Guanine

         Adenine

Chargaff used paper chromatography to separate the bases of the DNA from different species

         -Found that the base composition varied between species

                   -Adenine(A) + Thymine(T) / Guanine(G) + Cytosine(C) ratio varies

                   i more diversity

-There was a relationship between the bases

                             Chargaff’s rules

                                       A = T

                                       G = C

>>>>>>Watson and Crick (and others) discovered the double helix

Remember the structure of the monomer (nucleotide) and the polymer (single strand DNA).

Understanding the structure of DNA

         -Physical structure evidence

                   X-ray crystallography - Rosalind Franklin

         -Chemical structure evidence

                   Chargaff’s rule

                   Size of the bases

Lead to the double helix model (See fig 13.7)

         -2 strands are antiparallel

         -specific pairing of complementary base pairs the key to solving the puzzle

                   => A-T and G-C pairs, explained Chargaff's rules

>>>>>>DNA replication

Three fundamental steps: (see fig 10-8)

                   1. Double helix unwinds and strands separate.

                   2. Each strand can act as a template.

                   3. A new strand winds up with an old strand

                                       semiconservative replication

         -In this model each new DNA double helix would be made of a strand of old                    DNA and a strand of new DNA.

                             2 copies old-new paired

>>>>>>The replication of DNA (see fig 10-9)

The process of DNA replication is conceptually easy, but reality of the process very complex.

                   -Requires 20+ enzymes and accessory proteins

                   -Extremely rapid - prokaryote 500 nucleotides added per second

                                              - humans 50 nucleotides added per second     

                   -Accurate - About 1 in a billion chance of a mistake

Synthesis of the new strands

         -DNA polymerase

                   -synthesis occurs in 5' => 3' direction

                             -therefore synthesis is bidirectional

Because of the bidirectional nature of the replication one strand is synthesized in a continuous manner (leading strand) the opposing strand (lagging strand) is synthesized in pieces

The fragments of the lagging strand are joined by another enzyme called DNA ligase

>>>>>>Proofreading the new DNA

DNA replication is highly accurate, but errors can occur. When the wrong bases are paired up a mismatch occurs.

Mismatches occur about 1 per 1000 nucleotides added. Most mismatches are corrected during replication by a process called mismatch repair.

         -A defect in the mismatch repair process has been linked to a form of colon cancer. In individuals with this defect, errors (mutations) accumulate in the DNA until cell cycle regulation breaks down.

Another repair mechanism is called excision repair.

This type of repair corrects mistakes from replication or damage to the DNA caused by chemicals or radiation.