CHAPTER 12 DNA

12.1 Identifying the Substance of Genes Griffith was trying to find a vaccine for Streptococcus pneumoniae. Worked with two types or strains. 1. Capsule composed of polysaccharides so called smooth- edged (S). 2. No capsule so called rough-edged (R). Griffith knew that mice infected with S bacteria grew sick and died, while mice infected with R bacteria were not harmed.

Somehow the harmless R bacteria had changed into virulent S bacteria. Transformation: Is a change in genotype caused when cells take up foreign genetic material. Scientist were trying to figure out if genes are made of DNA or protein. Proteins has 20 different amino acid blocks DNA has 4 different nucleotide blocks.

Avery’s Experiment Used protein–destroying enzymes to destroy protein. Transformation occurred. Used DNA-destroying enzymes to destroy DNA. Transformation did not occur. Avery discovered that DNA is the material responsible for transformation and that the genetic material is composed of DNA.

Hershey-Chase Experiment Virus: A package of nucleic acid wrapped in a protein coat. Viruses are not living, and can only reproduce with a host. Bacteriophage: A virus that infects bacteria.

Hershey-Chase Experiment 1. T2 phages were labeled with radioactive isotopes. Virus’s protein coat with sulfur 35 S and Virus’s DNA core with phosphorus 32 P. 1. The phages were used to infect two separate E. coli bacterial cells P moved into cells. DNA must carry genetic information.

What is the role of DNA in heredity? DNA is capable of storing, copying, and transmitting the genetic information in a cell. Storing information. The genetic material stores information needed by every living cell. Copying information. Before a cell divides, its genetic information must be copied. Transmitting information. When a cell divides, each daughter cell must receive a complete copy of the genetic information.

12.2 The Structure of DNA DNA (Deoxyribonucleic acid): is a nucleic acid made up of nucleotides joined into long strands of chains by covalent bonds. Molecule where heritable genetic information of an organism is stored. DNA is a nucleic acid, a polymer built from many monomers called nucleotides. Double stranded Sugar= deoxyribose Bases=A-T-C-G

DNA-Twisted ladder The two sides of the ladder are made of alternating sugar parts and phosphate parts. (backbone) The rungs of the ladder are made of a pair of bases.

Nucleotide: Building blocks (the monomers) of nucleic acid polymers. Structure: 1. Ring-shaped sugar called deoxyribose 2. Phosphate group 3. A nitrogenous base  Adenine A  Guanine G  Thymine T  Cytosine C

Pyrimidines: Single ring structures such as T and C Purines: Double ring structures such as A and G

Discovering DNA’s Structure Chargoff discovered the amount of adenine always equaled the amount of thymine A=T, same for guanine and cytosine G=C.

Wilkins and Franklin Wilkins and Franklin developed X-ray diffraction photographs on DNA. (a beam of X-rays is directed at an object. The X-rays bounce off the object and are scattered in a pattern on a piece of film.)

Watson and Crick Used Franklin and Wilson’s X-ray diffraction research. Found the three-dimensional structure called the double helix. Built a DNA model configured in a double helix “twisted ladder” of two strands of nucleotides.

DNA Model

Base Pairing Rules Adenine always pairs with Thymine. Cytosine always pairs with Guanine. Held together by H bonds. Complimentary base pairs: Sequence of bases on one strand is paired to the sequence of bases on another strand. 1A. TCGAACT 1B. AGCTTGA

12.3 DNA Replication When is the DNA copied? Before a cell can divide, its DNA must be copied. HOW is the DNA copied? DNA replication - process of making a copy of DNA.

The Replication of DNA Step 1: DNA helicase opens the double helix by breaking the hydrogen bonds. Area where double helix separates is called the replication fork.

Step 2: DNA polymerase add nucleotides to the exposed nitrogen bases. Step 3: Process continues until all DNA has been copied, DNA polymerase detaches. Identical DNA strands.

Checking for Errors DNA polymerase proofreads while making new DNA. It can only add new nucleotides if the previous pair is correct.

Replication Video

Telomeres: DNA at the tips of the chromosomes. DNA is particularly difficult to replicate. Cells use a special enzyme, called telomerase to add short, repeated DNA sequences to the telomeres. In rapidly dividing cells, such as stem cells and embryonic cells, telomerase helps prevent genes from being damaged or lost during replication. Telomerase is often turned off as adults. In cancer cells, telomerase may be activated, enabling these cells to grow and divide rapidly.

Replication in Living Cells What phase of the cell cycle does DNA replication happen? Interphase, S phase. Prokaryotic DNA replication: Replication in most prokaryotic cells starts from a single point and proceeds in two direction until the entire chromosome is copied.

Replication in Living Cells Eukaryotic DNA Replication: In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.