Download RNA polymerase

17.4 – Protein Synthesis and Gene Expression
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gene expression – the transfer of genetic information from DNA to protein
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As described earlier, DNA is the genetic material in living things which gives the
blueprint of how an organism develops. This blueprint, however, has to be put into
a useful or structural form. In most living things, the main structural molecule is
protein. Hence, DNA provides the blueprint for all the different proteins found in
living organisms
Examples of protein structures:
1) skeletal muscle tissue 4) enzymes
2) smooth muscle tissue 5) transporters in cell membranes
3) hormones
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However, DNA is not directly used to make protein. Instead, DNA is copied to
RNA, and RNA is used to make protein. This leads us to the central dogma of gene
expression, proposed by Francis Crick.
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DNA → RNA → protein
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The use of DNA to produce RNA is called transcription.
The use of RNA to make protein is called translation.
• The Genetic Code
• Recall that in humans there are 20 amino acids (the basic units
of proteins). However, there are only 4 different nucleotides.
Therefore, if it only took 1 nucleotide to code for 1 amino acid
only 4 amino acids could be produced. If 2 nucleotides in a row
coded for 1 amino acid, you still could not code for all 20 amino
acids (only 16 possible combinations). It takes combinations of
3 nucleotide sequences to code for 1 amino acid
• Codon – the basic unit, or “word”, of the genetic code. It is a
set of 3 adjacent nucleotides in DNA or mRNA that codes for
amino acid placement on polypeptides.
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(see table 17.2, p. 590)
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Characteristics of the genetic code
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→ more than one codon can code for an amino acid
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ex. UCA, UCU, UCG, and UCC all code for serine
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→ it is continuous (no spaces or overlap)
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→ universal code – is almost the same in all living things
• Transcription (see fig 17.28, p. 591)
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The main job of transcription is to make a RNA
copy of a small section of the organism’s DNA (the
particular gene needed to make a specific protein)
• Messenger RNA (mRNA) – strand of RNA that carries
genetic information from DNA to the protein synthesis
machinery of the cell during transcription
• RNA polymerase – main enzyme that catalyses the
formation of mRNA from a DNA template
• Sense strand – strand of nucleotides containing the
instructions that direct protein synthesis. It is located
within a stretch of DNA that includes a gene (not
transcribed)
• Antisense strand – strand of nucleotides that is
complimentary to the sense strand (transcribed)
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Steps in Transcription
• 1) Initiation
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→ RNA polymerase binds to a particular sequence of
nucleotides in the sense strand
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→ RNA polymerase opens up the double helix and begins
inserting complimentary RNA nucleotides
• 2) Elongation
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→ proceeds in the 5’ to 3’ direction (no Okazaki fragments)
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→ as the polymerase molecule proceeds, the DNA helix
reforms and the mRNA molecule separates from the template
DNA strand
• 3) Termination and processing
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→ the RNA polymerase proceeds until it reaches a signal
to stop and the RNA polymerase and mRNA completely
separate from the DNA molecule
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→ a special sequence of nucleotides is added to the 5’ and
3’ ends
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→ introns are spliced out (removed), leaving only the exons
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→ the mRNA is transported out of the nucleus
• Translation (see fig 17.29, p. 593)
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Translation occurs outside the nucleus in eukaryotic cells,
and involves several elements in order to occur.
• Transfer RNA (tRNA) – RNA molecules that serve to link each
codon along a mRNA strand with its complimentary amino acid
• Transfer RNA has an unusual structure. They have a cloverleaf
shape (single-stranded), and they contain an anticodon.
• Anticodon – specialized base triplet located at one lobe of a
tRNA molecule that recognizes its complimentary codon on an
mRNA codon
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At the 3’ end of a tRNA is an amino acid transport site.
• Ribosomes – tiny two part structures found in the cell’s
cytoplasm and attaches to the rough endoplasmic reticulum
that helps to put together proteins. They bring together the
mRNA strand, tRNA molecules carrying amino acids, and the
enzymes involved in building proteins.
• Ribosomal RNA (rRNA) – most common class of RNA
molecules. During protein synthesis, these RNA molecules
supply the site on the ribosome where the polypeptide is
assembled.
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The Translation Cycle
1. mRNA binds to an active ribosome in such a way to expose
two adjacent codons.
2. The first tRNA molecule (carrying the amino acid
methionine) binds to the codon AUG (start codon).
3. A second tRNA molecule carrying an amino acid arrives at
the codon adjacent to the first tRNA.
4. Enzymes catalyze the formation of a peptide bond that joins
the amino acid carried by the first tRNA to that carried by the
second tRNA. At the same time, the polypeptide chain is
transferred from the first tRNA to the second.
5. The ribosome moves a distance of one codon along the
mRNA strand. The first tRNA molecule detaches from the
mRNA and goes to pick up another amino acid. The second
tRNA now holds a growing polypeptide chain. A third tRNA
molecule arrives at the exposed codon next to the second tRNA
and the cycle repeats.
6. When a “stop” codon is reached (UAG, UGA, UAA), the
completed protein is released and the ribosome assembly
comes apart.
Regulating Gene Expression
• The rates of transcription and translation can be
controlled to adjust to environmental conditions. For
example, artic foxes have white fur in winter, but
brown in warmer temperatures.
• Factors that affect transcription and translation in living
cells:
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1) changes in temp. or light
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2) the presence or absence of nutrients in the
environment
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3) the presence of hormones in the body