Chapter 13 DNA, RNA and Proteins

DNA Pre-class question 4/27 If you were to take a chromosome, decompress it into chromatin, separate the nucleosomes and remove the histones, what would you have? DNA Homework – Due Tuesday (4/28) Active Reading: The Structure of DNA

DNA is Genetic Material Found in Cells 3 Major Experiments 1. Griffith’s Discovery of Transformation 2 Bacteria Types Rough (R), no capsule, broken down by the body’s immune system, does not cause pneumonia Smooth (S), capsule, protected from the body’s immune system, causes pneumonia

Griffith’s Experiment Conclusions Heat destroyed the capsule, but not the S bacteria. S bacteria has the genetic code for making the capsule. The genes of the S bacteria were taken up by the R bacteria, which then began to make the protective capsule. The genetic code of the R bacteria had been transformed.

DNA, RNA or Protein? 2. Avery’s Experiment Avery wanted to know what substance, DNA, RNA or protein, was causing the transformation in Griffith’s experiment. When Avery treated the S heat killed bacteria with enzymes to destroy both RNA and protein, transformation still occurred. When Avery treated the S heat killed bacteria with enzymes to destroy the DNA, transformation did not occur.

The Genetic Material in Viruses 2. Hershey Chase Experiment Hershey and Chase worked with bacteriophages (viruses that infect bacteria, which in turn produce more viruses). Bacteriophages are made of DNA and protein. The scientists wanted to know which substance the genetic material in the bacteriophage was made of.

3 Phases Step 1 Proteins Contain Sulfur and DNA contain phosphorus. Two virus samples were incubated with either radioactive sulfur or radioactive phosphorus. Step 2 The virus samples were then allowed to infect bacteria. Each mixture was then separated into two portions, one containing bacteria and the other containing only viruses.

Conclusion Step 3 The infected bacteria contained radioactive phosphorus, but did not contain radioactive sulfur. Because the Bacteria contained radioactive phosphorus, and phosphorus is a component of DNA, Hershey and Chase conclude that DNA was the genetic material in viruses.

Pre-Class Question 4/28 In the Hershey-Chase experiment, where DNA was found to be the component of bacteriophages responsible for “infecting” bacteria, what radioactive substance was found in the bacteria?

The Double Helix of DNA Each strand of DNA is made of subunits called nucleotides Each nucleotide is made of 3 parts: A phosphate group A 5 carbon sugar molecule (Deoxyribose) A nitrogen containing base

The Information in DNA Nitrogen Bases Purines Adenine (A) and Guanine (G) Pyrimidines Cytosine (C) and Thymine (T) On opposite strands, A always pairs up with T and C always pairs up with G. Base pairs are held together with hydrogen bonds. The strands are said to be complimentary.

Discovering DNA’s Structure Chargaff’s Observation In every organism, the amount of thymine always equals the amount of adenine. Likewise the amount of cytosine always equals the amount of guanine. Photographs of DNA Rosalind Franklin and Maurice Wilkins developed x-ray diffraction images of strands of DNA.

Discovering DNA’s Structure, Cont. Watson and Crick’s Model of DNA By combining Chargaff’s observation and the photographs taken by Franklin and Wilkins, Watson and Crick created a 3-dimensional double helix model of DNA which won them the Nobel Peace Prize in 1962.

Replication of DNA In DNA replication, the DNA molecule unwinds, and the two sides split. Then new nucleotide sequences are added to each side until two identical sequences result. DNA replication occurs before cell division so that each new daughter cell has a complete copy of the parent DNA.

3 Steps of DNA Replication DNA helicases break the hydrogen bonds between base pairs and unwind the helix. As the strands separate, replication forks form Step 1 – Unwinding and Strand Separation Step 2 – Adding Complementary Bases DNA polymerase forms the new DNA molecule by traveling along each single strand and adding complementary nucleotides. Step 3 – Formation of Two Identical DNA Molecules Two identical DNA molecules are produced, each consisting of one new strand

Prokaryotic and Eukaryotic Replication Prokaryotic Replication Eukaryotic Replication Prokaryotes have circular DNA Replication occurs at one site, producing 2 replication forks and proceeds in opposite directions Eukaryotes have linear DNA Replication occurs at many sites, producing 2 replication forks and proceeds in opposite directions.

Eukaryotic Replication Because replication occurs at more than one site on the DNA, eukaryotic DNA can be replicated at a faster rate than prokaryotic DNA. An entire human chromosome can be replicated in about 8 hours.

Gene Expression and RNA Stage I – Transcription Takes place in the nucleus RNA is made from DNA Stage II – Translation Takes place in the cytoplasm on ribosomes Uses RNA to make proteins

How is RNA different from DNA? RNA is single stranded An RNA nucleotide contains ribose sugar, not deoxyribose. RNA contains the nitrogenous base, Uracil, but does not contain Thymine.

3 Types of RNA Messenger RNA (mRNA) Transfer RNA (tRNA) DNA is transcribed into messenger RNA. Messenger RNA transports the genetic code to ribosomes for translation. Transfer RNA (tRNA) Transfer RNA reads the mRNA sequence and translates it into amino acids which are used to make proteins. Ribosomal RNA (rRNA) Ribosomes are comprised of ribosomal RNA and proteins/

Transcription (3 Steps) Step 1 – RNA polymerase binds to a “promoter” gene or “start” location on the DNA. Step 2 – RNA polymerase unwinds the DNA Step 3 – RNA polymerase moves along the DNA adding complementary RNA bases (A-U, T-A, C-G, G-C) As RNA base pairs are added, the DNA strands close up and reform the double helix. Eventually the polymerase reaches a stop signal in the DNA.

Transcription vs. Replication In both processes, DNA is used as a template. However, transcription results in the formation of mRNA whereas replication results in the formation of DNA. During replication, both DNA strands are involved while only one DNA strand is involved in transcription.

Nitrogenous base (A,T,C,G,U) Pre-class Question 5/11 Which part of a nucleotide contains genetic information? Nitrogenous base (A,T,C,G,U) Homework Chapter 13 Vocab Quiz Wednesday Chapter 13 Test Friday

The Genetic Code: Three-Letter Words Each 3-nucleotide sequence is called a codon Each codon contains the code to make 1 of 20 amino acids (building blocks of proteins).

Translation: RNA to Proteins Step 1-Binding of tRNA to mRNA The anti-codon of tRNA binds to the mRNA start codon at the ribosome. Each tRNA carries one amino acid. The amino acid carried by this tRNA is methionine.

Translation continued Step 2 – Formation of peptide bonds A second tRNA anti-codon binds to the next sequence of mRNA. As a peptide bond is formed between the two adjacent amino acids, the first tRNA is released from the ribosome.

Translation Continued Step 3 – The ribosome moves one codon down the mRNA. The mRNA and the tRNA move as a unit. Step – The process of translating mRNA into an amino acid chain continues until stop codon is reaced

The Final Step of Translation The newly made polypeptide falls off the ribosome, the ribosome complex falls apart and the ribosome is then free to begin translation again with the same mRNA molecule or with a different transcript.