Download Chapter 10 Nucleic Acids & Protein Synthesis After completing the

•After completing the chapter on
Genetics, we discussed the passing
on of genes, but how are genes
produced?
Brief history of DNA
Frederick Griffith – 1928
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Experimented with pneumonia bacteria called
Streptococcus pneumoniae. There were two strains of
the bacteria:
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1 formed smooth colonies and caused pneumonia
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The other formed rough colonies and was harmless.
Heat kills disease causing strain - mice don’t get pneumonia.
Adds heat killed strain to the harmless strain injected into mice. The
mice die.
He cultures the bacteria. Finds that the harmless rough bacteria had
been “transformed” or changed into the lethal strain.
Griffith’s Transformation Experiment
Conclusion
• The harmless bacteria were transformed by some factor from the
harmful bacteria = this is called transformation
• Did not know what that factor was though
Oswald Avery 1944
• Repeated Griffith’s work but used enzymes to
destroy proteins in the heat killed bacteria.
• Pneumonia and transformation still occurred.
• Then used an enzyme to destroy RNA. Still
transformed occurred.
• Finally, used an enzyme to break apart DNA.
• This stopped transformation!!!
Conclusion
• DNA is the nucleic acid that stores and transmits
genetic information.
Hershey and Chase – 1952
• Used T4 bacteriophage virus that infects E. coli
bacteria
• A virus is a non-living pathogenic particle that can’t
replicate on its own
Capsid (protein)
The Lytic Cycle of Virus infection
Attaches onto host cell
Injects DNA into host cell
Reassembly of virons
Replication of Viral parts
Lysis – bursting out
What part of a virus actually infects & causes the host cell to
become a viral factory?
Used radioactive isotopes of
phosphorus and sulfur
P32 and S35
Proteins do not have Phosphorus DNA
doesn’t contain Sulfur
Used P32
Used S35
Found that the S35 stayed outside the cell
& P32 ended up in new virons
Animation
Conclusion
The genetic material of the
bacteriophage is located in
the DNA,
not the protein
coat
Structure of DNA
Deoxyribonucleic acid
Polymer of the monomer – Nucleotides
5 carbon sugar –deoxyribose
P
A phosphate group
S
N-base
Nucleotide
A nitrogen base
Sugar & phosphate
alternate to make up
the sides of the strand
Found only in nucleus
Single nucleotide
Erwin Chargaff – 1940’s
Noticed a pattern in the amounts of the four bases: Adenine,
Guanine, Cytosine, and Thymine
• Found the number of Guanine & Cytosine nitrogen bases is always
equal in DNA
• & the number of Thymine and Adenine is always equal.
• Didn’t know why though!
4 nitrogen bases
•Guanine - Purine
•Cytosine - Pyrimidine
•Adenine - Purine
•Thymine - Pyrimidine
Follow base pairing rule
Adenine with Thymine
Guanine with Cytosine
Bases are held together by weak
hydrogen bonds
N-bases connect to sugars by
a covalent bond
Nitrogen base
Phosphate group
Covalent bond
5 Carbon sugar
Weak H bond
History of DNA:
• Rosalind Franklin took X-Ray diffraction photo of DNA.
Watson and Crick (1953)
• Using Franklin’s photo, came up with the
double helix form of DNA. Won Nobel Price
w/ Maurice Wilkins (1962).
Original DNA model.
So, how does DNA replicate?
•
Occurs during S phase of interphase – DNA makes two exact copies of the
original; if not, a mutation occurs.
STEPS:
1.
2.
The double helix unwinds and flattens out (like a zipper)
An enzyme called DNA helicase (like the zipper slide) unzips the strand at the weak
hydrogen bonds. This exposes the nitrogen bases (each tooth of the zipper).
3. Another enzyme called DNA polymerase will be responsible for rezipping the strands.
It will take free nucleotides in the nucleus and bond them to the exposed bases,
following the base pair ruling – G – C and A – T. EACH SIDE OF THE MOLECULE ACTS
AS A TEMPLATE FOR A NEW STRAND.
4. The base pairing continues until the entire strand has their complement.
5. Now there are two identical strands of DNA
http://highered.mcgrawhill.com/sites/0072943696/student_view0/chapter3/animation__dna_replication__q
uiz_1_.html
SUMMARY:
* DNA helicase unzips the
double strand
* Original (old) strands of DNA
are on the outside of the new
strands – THESE ARE THE
TEMPLATES
* Replicates takes place in
opposite directions with the
help of DNA polymerase
* Semi-conservative model
How good is DNA at replicating ?
• Accurate to about 1 error for every 1,000,000,000
base pairs.
• Why? Two reasons: complementarity and DNA
polymerase, the “proofreader!”
• Gene Mutation – error resulting from misreading
of DNA or problem in the transcription/translation
process (We’ll revisit this later)
• Mutations in genes that control cell division or
repair enzymes may cause cancer
The Flow of Genetic Information
• DNA cannot leave the nucleus – it uses a helper molecule called
RNA
• DNA is the template for RNA = transcription
• RNA then directs the synthesis of proteins on ribosomes =
translation
• DNA RNA protein
RNA
Ribonucleic acid
The other Nucleic Acid
•Acts as a messenger between DNA and the
ribosomes and carries out protein synthesis
•DNA is too large to get out of the nucleus; it is
also protected in the nucleus from DNases. The
cell uses RNA to bring its message to the rest of
the cell for protein synthesis
How DNA & RNA Differ:
* RNA is a single stranded
molecule
*RNA has ribose sugar instead
of deoxyribose
*RNA contains Uracil
in place of Thymine so
Adenine bonds with
Uracil
*RNA can be found in the
nucleus, cytoplasm
or at the ribosomes
Let’s Review!!!
• Ribosomes are small organelles that are involved with making
proteins
• They are made up of proteins and rRNA
• They consist of two subunits – large and small
• Ribosomes are found both in the cytoplasm and on the
endoplasmic reticulum
There are three different kinds of RNA
• Messenger RNA (mRNA)
Formed in the nucleus and goes to the
ribosomes; carries genetic code from DNA through
the cytoplasm to the ribosomes
• Transfer RNA (tRNA)
Shaped like T; carries amino acids to the mRNA
on the ribosomes
• Ribosomal RNA (rRNA)
Most abundant; found in globular form (like a
big glob) and makes up the ribosomes
The Process of Protein Synthesis
* Process by which DNA codes for the production of proteins
(polypeptide chains) and protein assembly
- Polypeptide chains are polymers of the 20 different
amino acids.
- Uses a genetic code – chemical letters in RNA that make
up words which code for particular amino acids
- Check your understanding: what happens if the letters
change?
Amino Acid
Polypeptide forming
Translation
Transcription
Part I. Transcription of DNA into mRNA (the message)
•
•
•
•
•
•
•
•
•
DNA flattens and is unzipped exposing its bases (template) – sound
familiar?
RNA polymerase binds free RNA nucleotides to exposed DNA bases starting
at a promoter – a specific DNA nucleotide pattern
Complementary base pairing occurs, EXCEPT THERE IS NO THYMINE IN
RNA. Instead, Adenine bonds with Uracil just as Thymine from DNA would
bond with Adenine.
Transcription continues until a termination signal is given (punctuation) to
stop the transcription process
If DNA reads: ATC GTC GAT TGG C AA
mRNA:
UAG CAG CUA ACC GUU
mRNA leaves the nucleus through a pore to go out into the
cytosol to locate a ribosome
FYI – any of the three types of RNA are made this way
http://wwwclass.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.
html
The Genetic Code:
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Where a group of 3 nucleotide bases translates into a particular
amino acid
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This 3 “letter word” is called a codon
•
Codons are groups of 3 adjacent bases on mRNA (AAA, CCC
GGG)
•
Each codon will specify a specific amino acid.
•
When the codon is recognized by the anticodon, this is called
Translation
•
There are 64 different codons with punctuation as well for start
and stop
Stop Codons
Start codon
About the genetic code…
•Codons are code words found in mRNA
•Codons code for particular amino acids
•Three of the 64 codons are stop, one is start
– AUG = methionine
•The code is degenerate – more than one
codon can code for an amino acid – why is
this important?
•The code is UNIVERSAL!!!
The questions are: what is an anticodon
and how does the amino acid get
selected?
Part II. Translation of mRNA into protein
* At the ribosome, the process of translation occurs.
Several ribosomes may undergo this process at one
time
• mRNA will temporarily bind with the two ribosomal
subunits
• tRNA is waiting in the cytoplasm with its
corresponding amino acid
• Starting with the start codon (AUG), in groups of 3,
mRNA will determine which amino acid tRNA must
bring to the ribosome.
• Animation – Virtual Cell
Polypeptide forming
Transcription
Translation
• Once tRNA brings the correct amino acid to mRNA at the
ribosome, it releases and goes back to the cytoplasm to pick
up it corresponding amino acid
• Adjacent amino acids bond together, making a peptide bond
to form a polypeptide.
• Chain could be up to 10,000 amino acids long
• This continues until the entire message is translated.
• The chain of amino acids is formed called a polypeptide
(protein). The translation ends when a STOP codon is reached
(UAA, UAG, UGA).
When things go wrong:
• Does this process ever make a mistake?
• Have you ever had to copy a large amount of
information?
• What is the likelihood of you making a mistake or more?
• What could cause these changes?
Changes in genetic material
Gene Mutations:
alters one or more genes
Chromosomal Mutations:
alter the entire chromosome or a
portion of it.
Gene Mutations
Point Mutations – affect only one amino acid
Frameshift mutations – May affect an entire amino acid
sequence.
Point mutation
• involves a change in one or a few nucleotides.
• Influences a single amino acid in the polypeptide change; caused by
a substitution of a nitrogen base.
• Sickle cell anemia is an example of this –
GUG instead of GAG
Valine instead of glutamic acid
• THE FAT CAT ATE THE RAT
• Take out “C” in Cat & substitute a “B”
• THE FAT BAT ATE THE RAT
• In this case, it does not really change the meaning to the sentence
or the protein formed
• If DNA reads: A T G G T C G A T T G G CAA
• mRNA:
U A C C A G C U A AC C GUU
• Amino Acid:
Tyrosine - Glutamine – Leucine -Threonine – Valine
• But if mRNA: U A C C A G C A A AC C GUU
• The AA:
Tyrosine – Glutamine – Glutamine – Threonine – Valine
Frameshift mutation
•involves a change in the entire protein
formed or a large portion of it.
•Caused by insertions (additions) or deletions
of nitrogen bases.
•Tay-Sachs is a disease caused by a frameshift
mutation
•THE FAT CAT ATE THE RAT
•Take out “E” in THE & group into 3’s
•THF ATC ATA TET HER AT_
This makes no sense at all!!
• If DNA reads: A T G G T C G A T T G G CAA
• mRNA:
U A C C A G C U A AC C GUU
• AA:
Tyrosine - Glutamine – Leucine -Threonine – Valine
• BUT if mRNA: U A C C A G U A A C C G U U _
• THEN Amino Acid: Tyrosine - Glutamine – STOP!!!!
• The entire sentence makes no sense. The protein formed would be
totally different
So which form of a mutation would be
more severe?
• Frameshift mutation …
since an entirely new protein would be formed
CHROMOSOMAL MUTATIONS
•involve changes in number and
structure of the chromosomes.
•Could change location of genes on the
chromosomes or the number of copies
of some of the genes.
• Deletions – part of a chromosome is missing
Duplications – Extra copies of genes are inserted
• Inversions – Reverse direction of parts of the
chromosome
Chromosomal Mutations animation
Translocations
Parts of one non-homologous chromosome breaks off and
attached onto another non-homologous chromosome