DNA and RNA Honors Biology

The Discovery of the Gene 1928: Frederick Griffith experiments on bacteria that caused pneumonia He isolated 2 strains of bacteria (one harmful, one harmless)

His Experiments Hypothesis: bacteria produced a toxin that caused the disease

Results Griffith hypothesized that the “heat killed” bacteria passed “something” to the harmless strain He called this process transformation The offspring of the transformed bacteria passed the ability on to their offspring The “something” is a gene

How could Avery’s goal be tested? Next: Oswald Avery He repeated Griffith’s experiment in 1944 His goal was to determine what molecule was involved in the transformation How could Avery’s goal be tested?

He used various enzymes to destroy different molecules within the bacteria He discovered that DNA contains the information that is passed from one generation to the next

Alfred Hershey and Martha Chase Worked with Bacteriophages “bacteria eaters” Viruses that infect bacteria Consist of a DNA or RNA core surrounded by a protein coat

They determined that it was the DNA that entered the bacteria Hershey and Chase conducted an experiment to discover what part of the virus entered the bacteria They grew viruses in cultures containing radioactive isotopes: phosphorus-32 and sulfur-35 This way they could trace the DNA (32P) and the protein coat (35S) They determined that it was the DNA that entered the bacteria

The Viruses attach to bacteria and inject the genetic information The viral genes then take over the bacteria and begin to produce new virus “parts” The bacteria then splits open and hundreds of new viruses emerge to attach more cells

What is DNA made of? DNA is a polymer of subunits called nucleotides 3 parts of a nucleotide 5-C sugar = Deoxyribose Phosphate group Nitrogenous base

Nitrogenous Bases Purines: contain 2 carbon rings Adenine Guanine Pyrimidines: contain 1 carbon ring Thymine Cytosine

What does DNA look like? 1950, Erwin Chargaff discovered that the percentages of Guanine and Cytosine were almost equal in any sample of DNA The same was true for Adenine and Thymine Chargaff’s rules He had no clue why

Maurice Wilkins and Rosalind Franklin In 1952, both scientists were working independently with X- ray Diffraction

James Watson and Francis Crick Built a 3-D model of DNA In 1953, using the work of Franklin and Wilkins they determined that DNA was a double helix

DNA Replication The structure of DNA makes it very simple to replicate In eukaryotic chromosomes replication takes place in hundreds of places along the DNA in both directions= replication forks

Steps of Replication Step 1: DNA is “unzipped” by the enzyme helicase which breaks the Hydrogen bonds in several locations called replication “bubbles”

Step 2: DNA polymerase then attaches to the old strands and “plugs” in the appropriate base pairs Adds nucleotides from the 3’ to the 5’ end of the template strand One daughter stand gets copied in one continuous piece (leading strand) The other gets copied in fragments called Okazaki fragments (lagging strand) These are later linked together by DNA Ligase

Step 3: another type of DNA polymerase then “proofreads” and corrects any mistakes Error= 1 out of every billion pairs Video

Telomeres: segments of non- coding DNA found at the ends of the strands Prevents DNA from being lost or damaged Telomerase is responsible for copying these segments Often turned “off” in adults

Prokaryotes verse Eukaryotes Replication in most prokaryotes starts from a single point and proceeds in 2 directions until the entire circular chromosome is copied

RNA Polymer made up of subunits called nucleotides Nucleotide consists of: 5-C sugar ribose phosphate group nitrogenous base (Adenine, Cytosine, Guanine, Uracil)

How is rna different from dna? Single stranded Contains ribose instead of deoxyribose Contains Uracil instead of Thymine

Types of Rna Messenger RNA (mRNA) Single stranded helix Function: copies the “recipe” from the DNA so that proteins can be made

Transfer RNA (tRNA) Single stranded “hair pin” shaped Function: “transfers” amino acids to the ribosomes during protein production

Ribosomal RNA (rRNA) Single stranded globular Function: makes up ribosomes

Protein synthesis Transcription Process by which mRNA “copies” the recipe from DNA Takes place in the nucleus RNA polymerase is responsible for creating the mRNA

RNA polymerase binds to the promoter Specific sequence of nucleotides that marks the beginning of the recipe It then plugs in the complimentary base pairs until the end sequence is reached.

Introns are non-coding sequences of DNA that are not part of the “recipe” and must be cut out Exons are the coding sequences of DNA that are “expressed”

RNA processing

The Genetic code Proteins are made of 20 different amino acids What make one protein different from another? mRNA contains codons which “code” for a specific amino acid A codon consist of 3 nucleotides A codon chart is used to decipher the language of DNA

Translation Process by which the tRNA “translates” the genetic code by delivering amino acids to the ribosome Takes place on the ribosome

mRNA attaches to the P site of the small subunit of the ribsome How? The tRNA have anticodons that pair up with the codons of the mRNA AUG is the start codon that marks the beginning of the “recipe” This allows the large subunit to attach making the ribosome functional

Another tRNA comes and attaches to the next codon at the A site A peptide bond forms between the 2 amino acids Then mRNA shifts over The original tRNA is now free to leave the E site This elongation process continues until a stop codon is reached Video

How do cells determine which genes to express? Example: E. coli (bacteria) Operon: group of genes that work together In E. coli there is a lac operon which must be expressed (turned on) in order for them to use lactose as food When lactose is available the lac operon is “turned on” but when it is not available it is “turned off”

It is turned on by the presence of lactose How do you turn it off? The lac operon is turned off by repressors proteins which attach to the operator Operator: sequence of nucleotides in front of the lac operon How do you turn it on? It is turned on by the presence of lactose Lactose binds to the repressors which causes them to change shape and disconnect This leaves room for RNA polymerase to attach and begin protein synthesis

Eukaryotic gene regulation Do not have operons, instead genes are controlled individually Instead they have a TATA box sequence of about 30 nucleotides TATATA or TATAAA The TATA box binds a protein that helps RNA polymerase attach to the promoter

Various DNA binding proteins called transcription factors attach to the TATA box to either turn the gene “on” or “off”(repressors) Enhancer sequences help by looping DNA to bring transcription factors together

Transcription factors regulate gene expression in many ways Attract RNA polymerase (on) Block RNA polymerase (off) Uncoil areas of DNA so that are accessible (on) Bozeman

Eukaryotic gene regulation is much more complex than prokaryotic Why? Multicellular with different genes being expressed in different types of cells

Genes can also be regulated by controlling mRNA leaving the nucleus Manipulating the stability of RNA Breaking down proteins

RNA Interference (RNAI) Small segments of RNA (small interfering RNA siRNA) that are cut into sections called microRNA (miRNA) by Dicer Enzymes These miRNA destroy RNA with complementary sequence Page 380 Video

Differentiation: During embryonic development cells become specialized Controlled by master control genes called homeotic genes Homeobox genes code for transcription factors that activate other genes (ie. Hox genes) Mutations in these hox genes can cause mutations such as legs to grow on fruit flies where antennae should be

Differentiation also controlled by environment Ex: Metamorphosis Can be sped up by cues such as drying pond, food availability, temperature, population size