Transcript DNA & CHROMSOMES

DNA, RNA &
Protein Synthesis
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.
• 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 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?
• 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 S25 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 Infection
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 Results
So… The genetic material in bacteriophages was the DNA,
(not the protein)!!!
DNA Structure
• Made 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
Chargaff’s Rule & Rosalind Franklin
• Edwin 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 Xrays on film.
Watson & 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-Pairing
• Watson & Crick discovered that bonds can only
form between certain base pairs, Adenine &
Thymine and Cytosine & Guanine.
• The 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:
oThe “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.
oThe sugars and the phosphates are held together by covalent
bonds and the nitrogen bases are held together by hydrogen
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 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
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
1. Sugar is deoxyribose
2. Double-stranded
3. A, T, C & G bases
RNA
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
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
** Polyploidy = extra set of chromosomes