DNA & The Language of Life (Chapter 11)

Griffith’s Transformation Experiment 1928 – Frederick Griffith is studying how certain strains of bacteria cause pneumonia and inadvertently makes a discovery about how genetic information is passed from organism to organism His Experiment :  Grow two slightly different strains (types) of bacteria  One strain proven harmless and other deadly  Laboratory mice are injected with these strains

Griffith’s Results

What caused Griffith’s results? The heat-killed strain passed on its disease-causing ability to the live harmless strain. If Griffith’s words, one strain of bacteria was TRANSFORMED into another.

Oswald Avery Further demonstrated that DNA was the molecule that carried the genetic code Further demonstrated that DNA was the molecule that carried the genetic code Destroyed other components of virulent pneumonia (protein, lipids, carbohydrates and RNA– infection still occurred unless the nucleic acid DNA, was destroyed.) Destroyed other components of virulent pneumonia (protein, lipids, carbohydrates and RNA– infection still occurred unless the nucleic acid DNA, was destroyed.)

The Hershey-Chase Experiment Alfred Hershey & Martha Chase studied viruses, which are non-living particles smaller than a cell that can infect living organisms. Bacteriophages : specific group of viruses that infect bacteria. OBJECTIVE: To determine which part of the virus (protein or DNA) enters a bacteria it is infecting.

How do bacteriophages infect bacteria? oWhen a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it. oThe viral genes replicate to produce many new bacteriophages, which eventually destroy the bacterium. oWhen the cell splits open, from viral overload, hundreds of new viruses burst out and can infect surrounding cells

The Hershey Chase Experiment Goal : determine which part of the virus entered an infected cell to they would learn whether genes were made of protein or DNA. Method: grew viruses in cultures containing radioactive isotopes of phosphorus-32 ( 32 P) and sulfur-35 ( 35 S). –Some viruses had P-32 in their DNA, –Some had S-35 in their protein coat. If S-35 is found in the bacteria: viruses release their protein as the genetic material If P-32 is found in the bacteria: viruses release their DNA as the genetic material

Hershey-Chase Results The genetic material in bacteriophages was the _______________ (not the____________________________)!!!

Chargaff’s Rule & Rosalind Franklin % G% C% A % TEdwin Chargaff discovered that in almost any DNA sample, the % G nearly equals the % C and the % A nearly equals the % T Rosalind Franklin used x-ray diffraction to get information about the structure of DNA. She aimed an X-ray beam at concentrated DNA samples and recorded the scattering pattern of the X- rays on film.

Watson & Crick Using clues from Franklin’s pattern, James Watson and Francis Crick built a model that explained how DNA carried information and could be copied. Their model was the previously referenced “double- helix”, in which two strands of DNA were wound around each other to form a “twisted-ladder” type structure.

DNA Structure nucleotidesMade of monomers called nucleotides Nucleotide structure:

A nucleotide can have one of four bases: Types of bases:  Adenine  Guanine  Cytosine  Thymine A & G are bigger and are called purines C & T are smaller and are called pyrimidines

Base-Pairing Watson & Crick discovered that bonds can only form between certain base pairs, Adenine & Thymine and Cytosine & Guanine. purines pyrimidinesThe base-pairing rule means that purines only pair with pyrimidines, making the rungs equally spaced like a ladder. The nitrogenous bases are held together by hydrogen bonds. – A & T are held together by TWO hydrogen bonds – C & G are held together by THREE hydrogen bonds

DNA is a “double-helix” or twisted ladder: o The “backbone” or sides of the DNA molecule are made up of alternating sugars and phosphates and the “rungs” are made up of interlocking nitrogen bases. o The sugars and the phosphates are held together by covalent bonds and the nitrogen bases are held together by hydrogen bonds.

Identify the parts of the DNA double-helix

Chromosome Structure Eukaryotic chromosomes contain DNA and protein tightly packed together to from chromatin. Chromatin consists of DNA tightly coiled around proteins called histones. DNA & histone molecules form nucleosomes, which pack together to form thick fibers of chromosomes.

DNA Replication Why would a cell need to replicate (copy) it’s DNA? When in the cell cycle does this process occur? In order to maintain the integrity of the cell, what must be accomplished during DNA replication?

DNA Replication Before a cell can divide, it’s DNA must be replicated or copied in the S-phase of the cell cycle. In most prokaryotes, replication begins at a single point and continues in two directions. In eukaryotes, replication occurs in hundreds of places simultaneously and proceeds in two directions until complete. Sites of replication are called replication forks.

How does the process occur? 1.Helicase untwist DNA molecules. 2.Restriction enzymes unzip the molecule. 3.DNA polymerase brings in complementary base pairs for each strand 4.Ligase “glues” together the nucleotides Process is semi-conservative.  Each “new” strand of DNA consists of one original template strand and one newly made strand.  This allows for proofreading, using the template strand as the “master”.

Visual Summary of DNA replication ANIMATION

Meselson & Stahl Experiment

“Central Dogma” Protein synthesis occurs in 2 steps: transcription (DNA  RNA) & translation (RNA  protein)

RNA & Protein Synthesis Genes are coded DNA instructions for the construction of proteins. DNA is located in the nucleus, but proteins are made in ribosomes To avoid damage to the DNA molecules, they are first decoded into RNA which is sent to the ribosome to be the instructions for protein synthesis.

DNA v. RNA DNARNA 1.Sugar is deoxyribose 2.Double-stranded 3.A, T, C & G bases 1.Sugar is ribose 2.Single-stranded 3.Uracil instead of thymine

Three types of RNA mRNA (messenger) – carries copies of instructions for assembling amino acids into proteins rRNA (ribosomal) – makes up part of the ribosome tRNA (transfer) – carries each AA needed to build the protein to the ribosome

Transcription RNA is produced when RNA polymerase copies a sequence of DNA (gene) into a complementary RNA strand. Note: Gene sequence is only one side of DN A molecule

RNA Processing/Editing

Translation Decoding of an mRNA message into a polypeptide chain (protein) mRNA molecules are “read” in three base segments called codons Each codon specifies a particular amino acid Some AA are specified by more than one codon.

The Genetic Code

Steps of translation 1.After RNA is transcribed in the nucleus, it enters the cytoplasm and attaches to a ribosome 2.Codons are read at the “ A site ” and the appropriate AA is brought in by tRNA, which has a complimentary anticodon 3.The tRNA and amino acid shift over to the P site and “wait” for the next AA to arrive at the A site. 4.A peptide bond forms between the two AA and the molecule shifts over again 5.Empty tRNA can now be reused 6.Process continues until a STOP codon is reached. Animation

Mutations Mutations are changes in the genetic material – Gene Mutations : change is within a single gene – Chromosomal mutations : change in an entire chromosome Point mutations are gene mutations involving a change in one or a few nucleotides – Substitution: usually changes only one AA – Frameshift : addition or deletion of a nucleotide shifts the grouping of codons

Significance of Mutations Many mutations have little or no effect on gene expression Some mutations are the cause of genetic disorders Some mutations may be beneficial and lead to production of proteins with new or altered activities These beneficial mutations have an important role in the evolutionary process of natural selection

Substitution (missense)Mutations

Frameshift (nonsense) Mutations