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Bowie- Biology 12
Understanding Protein Synthesis
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Protein Synthesis Notes
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 To begin protein synthesis (formation), genes (the DNA structures of the cell) are first
transcribed into an RNA strand.
 The RNA is then translated by ribosomes into protein.
 The genetic material is made of nucleotides. The sequence of nucleotides in DNA
determines the order in which amino acids are assembled. Each gene codes for a specific
protein.
 Before DNA can be expressed as protein, it must be transcribed into RNA.
Activity #1: Complete Translating the Code on page 233 of Nelson Bio 12 now.
Bowie- Biology 12
Understanding Protein Synthesis
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Protein Synthesis
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DNA is contained inside the nucleus and can never leave the nucleus.
Proteins are synthesized outside the nucleus on the ribosomes. Since DNA is too valuable
to leave the nucleus, there must be another method of getting the "recipe" out to the
ribosomes.
DNA must 1st be transcribed into a messenger RNA.
Many copies of the needed DNA sequence can be created (as mRNA) and sent out to
multiple ribosomes to increase the speed at which the protein can be synthesized.
The ribosomes then translate the sequence into polypeptide chains of amino acids.
These, in turn, are processed into proteins.
Transcription is when DNA is copied into the form of messenger RNA (mRNA). Transcribe
mean "to copy".
Translation is when ribosomes use the mRNA as a blueprint to synthesize a protein made
of amino acids. Translate means to change it into another language.
RNA refresher:
 RNA is a blueprint of genetic information.
 It is made of ribose sugar, rather than deoxyribose sugar found in
DNA. This means that it has a hydroxyl group on the 2' carbon
(while DNA has lost one oxygen from this location).
 RNA has 4 nitrogenous bases, just like DNA, but instead of
thymine, RNA uses Uracil (U).
 RNA is only single stranded, whereas DNA is
double stranded.
 When transcribing DNA into messenger RNA
there is no need to waste energy creating a
double strand, only a single complementary
strand is needed with U replacing T in the
sequences.
 There are 3 major classes of RNA molecules:
messenger RNA (mRNA), transfer RNA (tRNA)
and ribosomal RNA (rRNA)
Bowie- Biology 12
Understanding Protein Synthesis
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Types of RNA
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Messenger RNA (mRNA): is the copy of the needed section
of the DNA. It varies in length. The longer the gene, the
longer the mRNA will be.
Transfer RNA (tRNA) : it transfers the appropriate amino
acid to the ribosomes to build the protein (as coded for in
the mRNA). tRNA's are relatively short (about 70-90
ribonucleotides in length).
Ribosomal RNA (rRNA): are structural components of a
ribosomes itself. It binds with proteins to provide a construction site for the assembly of
the polypeptides.
Transcription & Translation
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Transcription has 3 main steps:
initiation, elongation and termination.
Initiation: RNA polymerase binds to the
DNA at a specific site known as a
promoter, near the beginning of the
gene. Promoter regions are often sequences of A's and T's.
Elongation: the RNA polymerase adds ribonucleotides (A/U, G/C) to complement the
DNA sequence. Elongation occurs along only one of the DNA strands, called the template
strand. The other strand is referred to as the coding strand. DNA contains coding regions
(called exons) and noncoding regions (called introns)
Termination: Shortly after passing the end of the gene, the polymerase recognizes a
"stop" sequence, known as a terminator sequence. The mRNA transcript is then
completely release from the DNA and it will eventually leave the nucleus.
Some modifications to the RNA strand must be made before it leaves the nucleus. A
5'cap is added to the start of the mRNA to protect it from digestion in the cytoplasm and
to serve as a start location in translation. An enzyme is also added as a tail to the end of
the mRNA called poly-A-polymerase (poly-A tail, for short). Finally, the noncoding,
introns, must be removed so that the protein will fold correctly. Spliceosomes do the
removal of the introns.
Bowie- Biology 12
Understanding Protein Synthesis
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Translation
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Translation also has 3 main steps: initiation, elongation and termination.
Initiation: The ribosome recognizes a specific sequence on the mRNA and binds to that
site.
Elongation: The ribosome moves along the mRNA three nucleotides at a time. Each
triplet is the code for a specific amino acid. This is called the reading frame. The tRNA
delivers the appropriate amino acid and the polypeptide elongates. The 1st codon is the
START codon AUG. AUG is the code for the amino acid, methionine. Elongation continues
until a 3-base nucleotide sequence is reached that does not code for a specific amino
acid. This is a "stop" sequence.
Termination: Elongation continues until a 3-base nucleotide sequence is reached that
does not code for a specific amino acid. This is a "stop" sequence. The ribosome then falls
off the mRNA and the polypeptide chain is released.
Bowie- Biology 12
Understanding Protein Synthesis
The Genetic Code
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There are 20 amino acids, but
only 4 nitrogenous bases in
mRNA. To code for all 20 amino
acids, a sequence of 3
nucleotides must be used.
Each triplet is called a codon.
More than one codon can code
for an amino acid.
We use a "Genetic Code" to
determine which codons code
for each amino acid.
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Bowie- Biology 12
Understanding Protein Synthesis
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Control Mechanisms
 There are about 42 000 genes that code for proteins in humans.
 Housekeeping genes are genes that code for proteins that are continuously in need in
the cells.
 Not all proteins are needed all the time, however. For example, insulin is only needed
when glucose levels in the blood become high. It would be inefficient if genes were
constantly being transcribed to produce insulin, even when blood glucose levels were
low.
 Gene regulation is the process of turning genes on or off as needed.
 Cells have developed ways to control when specific proteins are transcribed.
 Transcription factors are proteins that turn genes on when they are needed.
Mutations
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Mutations are errors in the DNA sequence that are inherited.
Errors may have a negative impact, a positive impact or no impact detected.
A mutation could cause a disease, as it does in cystic fibrosis or it could be good for the
evolution of a species, as it was in the increasing size and complexity of the human brain.
Silent mutations have no effect on the operation of the cell. Usually silent mutations
occur in the noncoding regions (introns) of DNA.
Missense mutations occur when a change in the base sequence of DNA alters a codon (a
code for a specific amino acid), resulting in the wrong amino acid being placed in the
protein sequence. This can lead to disease, like sickle cell anemia.
Nonsense mutations occur when a change in the DNA sequence causes a stop codon to
replace a codon specifying an amino acid. During translation, only the part of the protein
that precedes the stop codon is produced, and the fragment may be digested by cell
proteases. These mutations are often lethal to the cell.
Missense and nonsense mutations arise from substitutions of one base pair for another.
Deletions are mutations that occur when one or more nucleotides are removed from the
DNA sequence. This shifts the reading frame leading to the wrong amino acids being put
together. This results in the production of a defective protein.
Insertions are mutations that occur when a nucleotide has been added to the sequence.
Since amino acids are read in triplets of code sequences, adding or removing one baser
pair results in frameshift mutations.
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Understanding Protein Synthesis
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Point mutations occur when there is a mutation at a specific base pair in the genome.
Translocation is a mutation that occurs when a group of base pairs from one part of the
genome relocate or switch places with a group of base pairs from another chromosome.
The result will be a fusion protein that may have an altered function. Some forms of
leukemia are associated with translocations and their respective fusion proteins.
Transposable elements are fragments that are always on the move. They are sometimes
referred to as "jumping genes". Transposable elements can jump into a coding region
and leave it inactive.
Inversions are reversals of segments of DNA within the chromosome.
Bowie- Biology 12
Understanding Protein Synthesis
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Online Simulations:
Interactive Concepts in Biochemistry - Interactive Animations
http://www.wiley.com/college/boyer/0470003790/animations/translation/translation.htm
Protein Synthesis
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120077/
micro06.swf::Protein+Synthesis
Protein Synthesis http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP1302
Drag and Drop Protein Synthesis
http://www.zerobio.com/drag_oa/protein/overview.htm
Practice Building a Protein
http://learn.genetics.utah.edu/content/molecules/transcribe/
Protein Synthesis Online Building Activity
http://learn.genetics.utah.edu/content/molecules/transcribe/
Video
http://www.teachertube.com/viewVideo.php?video_id=60707