Download Discovering the material for heredity: DNA

Protein Synthesis:
DNA Transcription into mRNA
and
mRNA Translation into Protein
How does DNA work?
What is a gene?
You understand that a gene is a segment of DNA that codes for a certain
characteristic or trait. Ex) Blue eyes, Black hair, Dark skin, etc.
A more complete understanding of what are gene is requires you to recall a bit
of biochemistry. How do you get from a sequence of DNA to having
black hair? To have black hair, your body must produce vast amounts of the
dark pigment melanin. How do you make melanin? Your cells must have a lot
of the right basic materials to build melanin and the enzymes needed to help
carry out the synthesis of melanin. Similarly, most jobs the cell needs to carry
out require the use of enzymes or other PROTEINS.
So, to get from a gene to a specific trait requires the action of specific proteins.
Proteins are a major workhorses of the cell, carrying out all sorts of specialized
functions. The way genes work is every gene codes for the creation of a
different protein, each with its own function!
Gene X --> Protein X --> Trait X
From Gene to Protein
Where is the DNA in a eukaryotic cell? Where are proteins
made?
nucleus
Problem: DNA is in the _____________
while proteins are
cytoplasm
made in the ________________
(at structures called
ribosomes
______________).
In addition, DNA is too large to fit through the nuclear
pores…frustrating!
How do we get the information in our DNA (our genes) out
to the cytoplasm so we can make proteins?
Solution: Use a messenger!
In the cell, that messenger is RNA (mRNA).
Gene to Protein in Two Steps
Step 1: Transcription (in nucleus)
Make an copy of a segment of DNA
(a gene) in the form of mRNA.
transcription
Step 2: Translation (at ribosome)
Translate the sequence of
nucleotides in mRNA into a sequence
of amino acids (a protein!).
translation
DNA  mRNA  Protein
Transcription: Copying DNA into RNA
Steps of transcription:
• Initiation: RNA polymerase
binds to the promoter
sequence at the start of a gene.
• Elongation: RNA polymerase
moves along the template
strand, adding the RNA
nucleotide with the correct
complementary base.
– Note that RNA is synthesized
from its 5’ to its 3’ end.
• Termination: RNA
polymerase lets go of the DNA
and releases the mRNA when it
gets to the terminator
sequence at the end of the
gene.
Promoter
sequence
5’
Terminator
sequence
RNA polymerase uses complementary base-pairing to
build RNA during transcription.
(just like DNA polymerase does during DNA replication)
3’
5’
RNA
Template
DNA
Let’s review transcription…
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter15/animations.
html#
(Click on Protein Synthesis Link)
• http://www.stolaf.edu/people/giannini/biological%20anamations.
html
• http://www.biostudio.com/d_%20Transcription.htm
• http://www.dnalc.org/resources/3d/12-transcription-basic.html
• http://www.johnkyrk.com/index.htm
• http://207.207.4.198/pub/flash/26/26.html
From
nucleic acid
to
protein:
Translation
TRANSLATION: From nucleic acid to protein
• How is the information for making a protein encoded?
20 different amino acids used to build
• There are ___
4 RNA nucleotides!
proteins. But, there are only ___
• If we read the RNA one base at a time, that would
4 amino acids!
mean we could only code for ___
For example:
Base
Amino Acid
– Adenine
-->
glycine
– Cytosine
-->
tryptophan
– Guanine
-->
alanine
– Uracil
-->
phenylalanine
Then we would have run out of bases to code for amino acids!
The Genetic Code
• What if we “read” the RNA two bases at a
time? How many unique two-letter code words
could we make using four bases?
AG
AC
AU
AA
GG
GC
GU
GA
CG
CC
CU
CA
UG
UC
UU
UA
16 unique code words….Is this
– This give us 42 = ___
No!
enough to code for all 20 amino acids? _____
• So what about three-letter code words? That
64 unique code words…
should give us 43= ___
Enough? Yes, more than enough…
The Genetic Code, Revealed:
A set of three nucleotides in an RNA sequence,
called a codon, codes for the addition of an
amino acid in a polypeptide.
valine
GUG = _____
ACU = threonine
________
UUA = ________
leucine
AUG = Methionine
________or
START codon
UAA,UGA,UAG =
STOP codons
__________
The Genetic Code
(how to translate the 64 mRNA codons)
The 3 letter Amino Acid abbreviations
Phe = Phenylalanine
His = Histadine
Leu = Leucine
Gln = Glutamine
Ile = Isoleucine
Asn = Asparagine
Met = Methionine
Lys = Lysine
Val = Valine
Asp = Aspartic Acid
Ser = Serine
Glu = Glutamic Acid
Pro = Proline
Cys = Cysteine
Thr = Threonine
Trp = Tryptophan
Ala = Alanine
Arg = Arginine
Tyr = Tyrosine
Gly = Glycine
The mechanics of translation: In the
cytoplasm, three players come together…
• The mRNA molecule which carries the information
in its codons.
• Another type of RNA molecule, transfer RNA
(tRNA), matches up the right amino acid with the
right mRNA codon.
• The ribosome – composed of two subunits – is the
structure which helps the mRNA and tRNA match
up properly. It also catalyzes the synthesis of the
new peptide (covalent) bonds between the amino
acids. The ribosome is made of proteins and rRNA.
TRANSLATION
How tRNA anti-codons match up with mRNA codons
Protein translation in words
• An mRNA molecule carries the information for
building a protein (a sequence of amino acids) in the
form of a sequence of nucleotides (codons).
• Another type of RNA molecule, a tRNA molecule,
has two important parts – an anticodon and a site
where a corresponding amino acid is attached.
• The tRNA anticodon base pairs with the
corresponding mRNA codon, and then attaches its
amino acid to the growing peptide chain.
• The ribosome slides along the mRNA and the
protein is assembled, one codon for one amino acid
at a time…
Let’s look at the genetic code again…
What tells translation to start and stop?
Let’s review translation…
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter15/animations.
html#
(Click on Protein Synthesis Link)
• http://www.stolaf.edu/people/giannini/biological%20anamations.
html
• http://www.biostudio.com/d_%20Transcription.htm
• http://www.dnalc.org/resources/3d/15-translation-basic.html
(realistic animation, real-time)
• http://wwwclass.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.h
tml
DNA Mutations
Biology
What if we mess up one of the
nucleotides and change one of the
codons? We get a mutation!
• Mutations in DNA sequence:
– Point mutations
– Frame-shift mutations
• Insertions
• Deletions
• These are mutations that
cannot be seen in a
karyotype
Point Mutation
• A point mutation is a simple substitution in
one base of the gene sequence.
• This is equivalent to changing one letter in a
sentence, such as this example, where we
change the 'c' in cat to an 'h':
Original
The fat cat ate the wee rat.
Point Mutation
The fat hat ate the wee rat.
Point Mutations may or may not
cause problems…
• A point mutation can result in the wrong amino
acid being incorporated into a protein (this could be
disastrous for protein structure or not, depending on the amino acid
substitution…Why?)
Silent point mutations don’t change the
protein that is coded for by a gene
• A point mutation could also result in a
SILENT mutation, one that does not alter the
amino acid that is incorporated, due to the
redundancy of the genetic code.
Example:
The mutation that changes AUU to AUC still
codes for the same amino acid, isoleucine.
Thus, the polypeptide created would be identical
to that made by the un-mutated form of the gene.
Frame-shift mutation
• In a frame shift mutation, one or more bases
are inserted or deleted, resulting in a change
in the reading frame for the rest of the
sequence
• Because our cells read DNA in three letter
"words", adding or removing one letter
changes each subsequent word. This type of
mutation can make the DNA meaningless and
often results in a shortened protein.
Original
The fat cat ate the wee rat.
Frame Shift (deleted the ‘t’ in ‘cat’)
The fat caa tet hew eer at.
What are some genetic
disorders caused by DNA
mutations?
Breast Cancer
• Is the second major cause of cancer death in
American women.
• In 1994, two breast cancer susceptibility genes were
identified: BRCA1 on chromosome 17 and BRCA2 on
chromosome 13.
• When an individual carries a mutation in either
BRCA1 or BRCA2, they are at an increased risk of
being diagnosed with breast or ovarian cancer at
some point in their lives.
• These genes participate in repairing radiationinduced breaks in double-stranded DNA. It is thought
that mutations in BRCA1 or BRCA2 might disable this
mechanism, leading to more errors in DNA replication
and ultimately to cancerous growth.
Colon cancer
• Colon cancer is one of the most common inherited
cancers.
• Among the genes found to be involved in colorectal
cancer are: MSH2, MSH6 both on chromosome 2 and
MLH1, on chromosome 3.
• Normally, the protein products of these genes help to
repair mistakes made in DNA replication. If the MSH2,
MSH6 and MLH1 proteins are mutated and therefore
don't work properly, the replication mistakes are not
repaired, leading to damaged DNA and, in this case,
colon cancer.
PHENYLKETONURIA (PKU)
• An inherited error of
metabolism caused by a
deficiency in the enzyme
phenylalanine hydroxylase
(PAH).
• Loss of this enzyme results
in mental retardation, organ
damage, and unusual
posture.
• Is caused by mutations in
both copies of the gene for
phenylalanine hydroxylase
(PAH), found on
chromosome 12.
Other Genetic Disorders:
• Severe combined immunodeficiency
syndrome (SCID) – “The boy in the bubble”
• Duchenne muscular dystrophy
• Marfan syndrome
• Alzheimer disease (early onset)
• Epilepsy
• Parkinson’s disease (inherited form)
• Hemophilia
• Sickle-cell anemia