Chapter 12

Griffith’s Experiment 1928, Frederick Griffith was investigating how bacteria made people sick, specifically the cause of pneumonia He tested 2 strains of the bacteria, one disease causing( smooth strain), the other not disease causing(rough edged strain. He injected mice with both strains, those with he disease causing bacteria injection died (predictable) New hypothesis: the disease causing bacteria produced a poison New experiment: heated and killed the bacteria, then injected the heat-killed bacteria into mice. The mice survived suggesting that the bacteria DID NOT produce a poison

Griffith’s Experiment

Griffith’s Next experiment Mixed heat killed, disease causing bacteria with live, harmless bacteria and injected mice. Neither should have caused harm, but the result was that many of the mice developed pneumonia and died. Somehow the heat killed bacteria had passed on some factor (gene) to the living bacteria. That factor may have contained the information to change a harmless bacteria into a disease carrying bacteria. The concept of transformation was developed, stating that one form of bacteria had changed into another.

Avery and DNA 1944, Oswald Avery built on Griffith’s work by using enzymes to destroy proteins, lipids, carbohydrates, etc in an extract of the heat killed bacteria and repeated the experiment. The result was that transformation still occurred. Repeated the experiment but this time destroyed the DNA. Result: Transformation DID NOT occur. Conclusion: DNA is the nucleic acid responsible for storing and transmitting genetic information from on generation of organism to another.

The Hershey-Chase Experiment Hershey & Chase did an experiment to determine how viruses may be related to bacteria They worked with a virus called a “bacteriophage” which literally means “bacteria eater”. Attempted to determine if genes were made of DNA or a protein Conclusion: the genetic material of the bacteriophage was DNA, not protein

The Hershey-Chase experiment

Structure of DNA DNA is a long molecule made up of units called nucleotides A nucleotide is made of 3 basic parts- A 5-Carbon sugar (deoxyribose) A phosphate group A nitrogenous base 4 types of nitrogenous bases in 2 groups Purines include adenine and guanine Pyrimidines include cytosine and thymine

The 4 nucleotides in 2 groups

The DNA structure is complex The percentage of Adenine is almost the same as the percentage of thymine in a DNA sample. The percentage of guanine is almost the same percentage as that of cytosine in a DNA sample. Chargraff’s Rule: [A]=[T] [C]=[G] This is called “base-pairing” A pairs with T, C pairs with G, due to the chemical bonding of the molecules involved. Why does this rule fit the structure of DNA? Rosalind Franklin used X-ray to further study DNA structure, and discovered an X like pattern.

The Double Helix Watson & Crick used Franklin’s research to physically build a 3-D model of a twisting, double helix model of the DNA molecule, which explained the DNA structure once and for all.

How does the double helix work? It looks like a twisted ladder Hydrogen bonds form between certain nitrogenous bases holding the double helix together The hydrogen bonds can only form between certain bases following Chargaff’s rules A=T and C=G

Chromosomes and DNA replication Prokaryotes have relatively simple DNA structure and the DNA is free-floating throughout the cell. Eukaryotes have roughly 1000 times the DNA of prokaryotes, located on chromosomes in the nucleus Eukaryotic cells contain DNA and protein that is tightly packed together to form chromatin Chromatin consists of DNA that is tightly coiled around proteins called histones.

Nucleosomes Together the DNA and the histones form a bead-like structure called a nucleosome Nucleosomes serve to fold the great lengths of DNA into very small spaces

DNA Replication During DNA replication, the DNA molecule separates into 2 strands, Then produces 2 new complementary strands following the rules of base pairing; ie. A=T, C=G Each strand of the double helix of DNA acts as a template, or model, for the new strand. An enzyme, primarily DNA polymerase, works to “unzip” the DNA strands, Question: An unzipped strand of TACGTT would produce what complementary base? Answer: ATGCAA

RNA and Protein Synthesis 3 main differences between RNA and DNA. The sugar in RNA is ribose, instead of deoxyribose RNA is generally a single-stranded RNA contains uracil instead of thymine Most RNA has one primary job, protein synthesis The assembly of amino acids into proteins is controlled by RNA

3 types of RNA Messenger RNA (mRNA) serves as messengers from DNA to the rest of the cell Ribosomal RNA (rRNA) is the type of RNA that makes up the major part of ribosomes Transfer RNA (tRNA) is the type of RNA that transfers amino acids to ribosomes during protein synthesis

Transcription Transcription is the process in which a part of the nucleotide sequence of DNA is copied into a complementary sequence of RNA During transcription, RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA Question: How does RNA polymerase know where to start and stop making an RNA copy of DNA? Answer: The RNA polymerase enzyme will bond to areas knows as promoters, which have specific base sequences In effect the promoters act as signals in DNA that indicate to the enzyme where to bind to make RNA. View the animation of transcription: class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html

RNA editing Sometimes RNA needs a bit of editing before it can do it’s work in a cell Sometimes pieces of RNA are removed (introns or intervening sequences) and those that remain are called exons (those that remain) Not sure why this happens, but it may have some role in evolutionary changes within species.

The Genetic Code The language of mRNA instruction is the genetic code Recall the 4 different RNA bases (A, U, C, & G) A codon consists of 3 consecutive nucleotides that specify a single amino acid.

Codon-3 consecutive nucleotides that specify a single amino acid. Example: UCG-CAC-GGU Serine-Histidine-Glycine There are 64 possible 3 base codons

Translation Translation is the decoding of a mRNA message into a polypeptide chain During translation (protein synthesis) the cell uses information from mRNA to produce proteins All 3 main types of RNA are used in this process. (mRNA, tRNA, and rRNA) The cell organelle where this occurs is the ribosome. Watch the 2 videos and summarize each.

The Roles of RNA and DNA DNA is the “master plan” for a cell and is kept protected in the nucleus of the cell RNA serves as a “blueprint” for the cell to build proteins Proteins in a cell are the key to almost everything a living cell does, including catalyzing and regulating chemical reactions within the cell as well as coding for certain genes that control the genetics of a cell

Mutations Sometimes the cells makes a mistake in copying their own DNA. This can lead to a change in a gene called a mutation. A mutation is a change in the DNA sequence that can affect the genetic information A gene mutation is a change in a single gene, while a chromosomal mutation involves changes in whole chromosomes.

Gene mutations A point mutation affects just one nucleotide, it occurs at one single point in the DNA sequence. A “frameshift mutation” is a mutation that shifts the “reading frame” of the genetic message by inserting or deleting a nucleotide

Chromosomal mutations A chromosomal mutation involves changes in the number or structure of chromosomes. A chromosomal mutation may change the locations of genes on chromosomes and even the number of copies of some genes. Types of chromosomal mutations include: deletion, duplication, inversions, or translocations

Gene regulation Genes need to “know” when to turn on and off. This is controlled by an “operon” which is a group of genes that act together to control the on/off switch for genes. In E.coli bacteria, the operon is known as lac operon, because it control the bacteria’s ability to use the sugar lactose.

Eukaryotic Gene Regulation Gene regulation in eukaryotic cells is much more complex than in E.Coli. Why? Answer: eukaryotic cells are much more complex, thus more regulation is required. In developing embryos, hox genes control the organisms organ and tissue development, and thus determines an animal’s basic body plan.