DNA, RNA & DNA, RNA & Protein SynthesisProtein Synthesis

GriffithGriffith’’s Transformation s Transformation ExperimentExperiment

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

GriffithGriffith’’s Resultss Results

What caused GriffithWhat caused Griffith’’s results?s results?

• The heat-killed strain passed on its disease-causing ability to the live harmless strain.

• In Griffith’s words, one strain of bacteria was TRANSFORMED into another.

• It was later demonstrated by Oswald Avery and other scientists that the transforming factor was DNA (deoxyribonucleic acid)

The Hershey-Chase ExperimentThe 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.

What did Hershey & Chase do?What did Hershey & Chase do?

• If Hershey and Chase could determine which part of the virus entered an infected cell, they would learn whether genes were made of protein or DNA.

• To accomplish this, they grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S).

• Some viruses had P-32 in their DNA, and others had S-25 in their protein coat.

• If S-35 is found in the bacteria, it would mean that viruses release their protein and if P-32 is found in the bacteria it would mean that viruses release their DNA.

Recall: Method of Bacteriophage Recall: Method of Bacteriophage InfectionInfection

o When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it.

o The viral genes replicate to produce many new bacteriophages, which eventually destroy the bacterium.

o When the cell splits open, from viral overload, hundreds of new viruses burst out and can infect surrounding cells

Hershey-Chase ResultsHershey-Chase Results

So… The genetic material in bacteriophages was the DNA,DNA, (not the protein)!!!

DNA StructureDNA Structure

• Made of monomers called nucleotidesnucleotides

• Nucleotide structure:

A nucleotide can have one of A nucleotide can have one of four bases:four bases:

Types of bases: Adenine Guanine Cytosine Thymine

A & G are bigger and are called purinesC & T are smaller and are called pyrimidines

ChargaffChargaff’’s Rule & Rosalind s Rule & Rosalind FranklinFranklin• Edwin Chargaff discovered that in almost

any DNA sample, the % G% G nearly equals the % C% C and the % A% A nearly equals the % T% 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 & CrickWatson & Crick• Using clues from Franklin’s X-ray pattern,

shown to them by Maurice Wilkins, James Watson and Francis Crick built a 3-D model that explained how DNA carried information and could be copied.

• Watson, Crick & Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine for their work.

Base-PairingBase-Pairing• Watson & Crick discovered that bonds can

only form between certain base pairs, Adenine & Thymine and Cytosine & Guanine.

• The base-pairing rule means that purinespurines only pair with pyrimidinespyrimidines, 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 DNA is a ““double-helixdouble-helix”” or twisted or twisted ladder:ladder:

oThe The ““backbonebackbone”” or sides of the DNA molecule are or sides of the DNA molecule are made up of alternating made up of alternating sugarssugars and and phosphatesphosphates and the and the ““rungsrungs”” are made up of interlocking are made up of interlocking nitrogen basesnitrogen bases..

oThe sugars and the phosphates are held The sugars and the phosphates are held together by together by covalentcovalent bonds and the nitrogen bonds and the nitrogen bases are held together by bases are held together by hydrogenhydrogen bonds. bonds.

Molecular Structure of DNA

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 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 strand4. 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

Replication Bubbles

The “Central Dogma” of Genetics

• 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

DNA RNA1. Sugar is deoxyribose2. Double-stranded3. A, T, C & G bases

1. Sugar is ribose2. Single-stranded3. 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

The Flow of Genetic Information

Protein synthesis occurs in 2 steps: transcription (DNA RNA) & translation (RNA

protein)

Transcription

• RNA is produced when RNA polymerase copies a sequence of DNA into a complementary RNA strand.

DNA: TACGGACACATT RNA: AUGCCUGUGUAA

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

RNA Processing (Editing)

Additional Details of Transcription

1. Initiation: RNA polymerase attached to promoter sequence of DNA and RNA synthesis begins

2. Elongation: RNA elongates and the synthesized RNA strand peels away from DNA template allowing the DNA strands to come back together in regions transcribed

3. Termination: RNA polymerase reaches sequence of DNA bases called a terminator signaling the end of the gene and polymerase molecule detaches

A Closer Look at tRNA

A Closer Look at Ribsomes

Steps of translation - Initiation

• After RNA is transcribed in the nucleus, it enters the cytoplasm and attaches to a small ribosomal subunit

• special initiator tRNA binds to the start codon bringing in the amino acid MET

• large ribosomal subunit binds to the small one creating a functional ribosome

Steps of Translation - Elongation

• The anticodon of an incoming tRNA molecule with AA pairs with mRNA codon in A site

• AA detaches from tRNA in P site and peptide bond forms between it and the AA in the A site

• translocation – P site tRNA leaves the ribosome and the A site complex (AA, tRNA anticodon and mRNA codon) shift s over to the P site

• Process continues until a STOP codon is reached

Steps of Translation - Termination

• Stop codons – UAA, UAG, and UGA do not code for amino acids

• These codons signal the end of translation

• The completed polypeptide is released from the last tRNA and exits the ribosome

• The ribosome splits into individual subunits

Animation

Mutations

• Mutations are changes in the genetic material– Gene Mutations: change in the nucleotide

sequence within a single gene

– Chromosomal mutations: change in an entire chromosome; may involve loss or duplication of multiple genes

• 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

Causes & Effects of Mutations• Causes: Mutagenesis can occur in many ways

– Spontaneous mutations occur during DNA replication or recombination

– Physical or chemical agents called mutagens may induce mutations (ex. High energy radiation from x-rays or UV light)

• Effects: Can be harmful, beneficial or neither– May cause of genetic disorders– May be beneficial and lead to production of proteins

with new or altered activities, which has an important role in the evolutionary process of natural selection

– Some mutations are “silent” and have no effect because the nucleotide change results in a new codon that codes for the same amino acid as the original codon

Substitutions Mutations

Frameshift Mutations

4 Types of Chromosomal Mutations

** ** PolyploidyPolyploidy = extra set of = extra set of chromosomeschromosomes