STRUCTURE OF DNA Biology:

DNA and Genes How do genes work? How do they determine the characteristics of organisms? To truly understand genetics, biologists first had to learn the chemical composition of a gene. It took the work of many scientists over several years to identify DNA as the genetic material, and to discover its structure.

Griffith and Transformation Frederick Griffith was studying the mechanism by which bacteria cause disease. He used two strains of the same type of bacteria: one that caused pneumonia (pathogenic), and one that did not. He discovered that even when the pathogenic bacteria were killed by heating them, they could pass their disease-causing ability to the harmless strain. Griffith called this transformation, since one strain of bacteria(harmless) had been changed into another (disease-causing) strain.

Avery and DNA Avery and a group scientists repeated Griffith’s experiments, making an extract from the heat-killed bacteria. They used enzymes to remove carbohydrates, proteins, lipids, and other molecules (but not DNA). Transformation still occurred. When enzymes were used to break down DNA, transformation did not happen. They concluded that DNA is the nucleic acid that stores and transmits genetic information.

Hershey and Chase Experiment Alfred Hershey and Martha Chase studied bacteriophages, viruses that infect bacteria by injecting its DNA into the bacterial cell. They wanted to determine if it was the protein coat, or DNA that entered the infected cell. They used radioactive phosphorus ( 32 P) to mark the DNA, and radioactive sulfur ( 35 S) to mark the protein. When the virus infected the bacterial cell, 32 P was found. Hershey and Chase concluded that the bacteria was infected with DNA, the genetic material.

Structure of DNA DNA is made up of units called nucleotides. Each nucleotide is made up of three basic parts: 1)A 5-carbon sugar, deoxyribose 2)A phosphate group 3)Nitrogenous base: adenine and guanine are purines, cytosine and thymine are pyrimidines

Structure of DNA, cont. Chargaff discovered that in a DNA sample, the percentage of cytosine and guanine is equal, and the percentage of thymine and adenine is equal. Chargaff’s rules: [C]=[G] and [A]=[T] The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide.

X-ray Evidence In the early 1950s, Rosalind Franklin studied DNA using X-ray diffraction. The patterns in her pictures showed that the strands in DNA formed a coil shape (helix) Her studies indicated that there were two strands, and that the nucleotides were toward the center of the molecule.

X-ray Diffraction of DNA

Watson and Crick James Watson and Francis Crick were working on the structure of DNA in the 1950s. Using information from Chargaff, Franklin, and other scientists, they put together a 3-D model of DNA. Watson and Crick’s model was a double helix, with hydrogen bonds between the nitrogen bases holding the strands together. They won the Nobel Prize for their work.

DNA Replication During DNA replication, the double helix is separated into two individual strands. The rules of base pairing are applied, and the result is two identical new DNA strands: each with one side from the original.

Each strand of the double helix serves as a template, or model, for the new strand. The new strands are called “complementary” strands. DNA replication is carried out by a series of enzymes. One enzyme “unzips” the original DNA strand. DNA polymerase brings nucleotides together to produce the new strand and proofreads it to make sure it is an exact copy.

RNA and Protein Synthesis The structure of RNA has several differences from DNA: 1.The sugar is RNA is ribose. DNA has deoxyribose. 2.RNA is single-stranded. DNA is a double helix 3. RNA contains uracil instead of thymine. Uracil pairs with adenine in RNA.

Types of RNA There are three types of RNA, and all are involved in protein synthesis: 1. Messenger RNA (mRNA)- carries messages from DNA for assembling amino acids into proteins

Types of RNA, cont. 2. Ribosomal RNA (rRNA)- Proteins and rRNA make up ribosomes. Ribosomes are sites of protein synthesis. 3. Transfer RNA (tRNA)- transfers each amino acid to the ribosome as specified by codes in the mRNA. These amino acids are assembled into proteins.

Transcription RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA. Transcription requires an enzyme called RNA polymerase that is similar to DNA polymerase.

Transcription Adenine (DNA and RNA) Cystosine (DNA and RNA) Guanine(DNA and RNA) Thymine (DNA only) Uracil (RNA only) RNA polymerase DNA RNA

During transcription, RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses one strand of DNA as a template to assemble nucleotides into a strand of RNA. The new RNA strand is edited before they become functional. Parts removed are called introns, and the remaining parts are exons.

The Genetic Code Proteins are made by joining amino acids into long chains called polypeptides. Each polypeptide contains combinations of the 20 amino acids. The “language” of the instructions in mRNA is called the Genetic Code.

RNA contains four bases: adenine, uracil, cytosine, and guanine. The code for these bases is read in groups of three called a codon. Each codon specify for a specific amino acid to be added to the polypeptide. Example: UCGCAC is read UCG-CAC, and represents the amino acids serine and histidine.

The Genetic Code

Translation The order of the amino acids in a protein are coded for in the mRNA. This “translation” of the mRNA takes place on the ribosomes. mRNA is transcribed in the nucleus and released into the cytoplasm. There the mRNA attaches to a ribosome and translation begins.

Messenger RNA Messenger RNA is transcribed in the nucleus. Transfer RNA The mRNA then enters the cytoplasm and attaches to a ribosome. Translation begins at AUG, the start codon. Each transfer RNA has an anticodon whose bases are complementary to a codon on the mRNA strand. The ribosome positions the start codon to attract its anticodon, which is part of the tRNA that binds methionine. The ribosome also binds the next codon and its anticodon. mRNA Start codon Ribosome Methionine Phenylalanine tRNA Lysine Nucleus Translation mRNA Go to Section:

Most genes contains instructions for making proteins. Proteins are essential for all living things: many are enzymes that catalyze and regulate chemical processes. Importance of Proteins

Proteins determine many characteristics of living things, and control their growth and development. Proteins are key to everything that living cells do!