How Are Genes Expressed?

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How Are Genes Expressed?

• Chapter11

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DNA codes for proteins, many of which are enzymes.

• Proteins (enzymes) can be used to make all the
  other molecules a cell needs carbohydrates,
  lipids and nucleic acids.
• A segment of DNA that carries the instructions to
  make (codes for) a protein is called a gene.

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• How can patterns of 4 bases hold information?
• The same way your computer works!
• Computer info is in the order of 1s and 0s
• DNA info is in the order of the bases
• The bases are read in groups of three bases
  called codons.
• Each codon codes for a specific amino acid
  (except for three which mean stop)

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• How does DNA use the 4 bases?
• Need to have codes for 20 amino acids
• 1 to 1 only 4 every 2 bases only 16
• 3 bases at a time makes 64 different codons, more
  than enough for 20 amino acids.
• One codon means start here
• Three codons are stop codons.
• More than one codon can be used for a particular
  amino acid.

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• Because DNA is in the nucleus, and our work
  benches ribosomes- are in the cytoplasm, we
  need a way to get just the information we need to
  the ribosome.
• We make a copy of the gene we need in messenger
  RNA. This process is called TRANSCRIPTION. (We
  have not changed the language, just the form of
  the information.

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DNA vs. RNA
Uses A,T, C and G Uses A, U, C and G Use
deoxyribose Uses ribose Double stranded Single
stranded
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• The strand of DNA that is copied is called the
  template or antisense strand.
• An enzyme called RNA polymerase looks at the DNA
  to find a special region called a promoter. This
  tells the enzyme where to start copying the DNA.
• It copies the DNA until it comes to a terminator.

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• Prokaryotes can use the mRNA right after it is
  copied.
• Eukaryotes need to go through some additional
  steps, so at this point the mRNA is called
  pre-mRNA.

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• Eukaryotes modify the pre-messenger RNA.
• The intervening sequences (Introns) are cut out
  and the expressed sequences (Exons) are spliced
  back together. This way, more than one protein
  can be made from a single gene!
• Now it is mature mRNA

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• The mRNA leaves the nucleus through the pores in
  the nuclear envelope, and finds a ribosome in the
  cytosol or on rough E.R.

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• Ribosomes are made of rRNA and proteins.
• Each ribosome is made of two subunits
• A large subunit
• A small subunit
• These come together when they are needed.

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Ribosome
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The large subunit catalyzes the formation of the
peptide bonds between the amino acids. The small
subunit acts as a reader of the RNA.The RNA must
be read correctly, or the wrong amino acid will
be used. The 3 base sequence the RNA looks at is
called the Reading frame.
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• Changing from the language of nucleic acids to
  the language of proteins is called TRANSLATION.

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Translation

• We need a means of getting the correct amino acid
  in the correct sequence. For this we use one more
  type of RNA transfer RNA (tRNA).
• tRNA is a single strand of RNA that is folded
  into the shape of a clover. It has an anticodon
  that matches the codon on the mRNA, and a spot
  for holding the amino acid that matches the codon.

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• To be sure that the correct tRNA always carries
  the correct amino acid, the two are put together
  by enzymes that match the anticodon and the amino
  acid.

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• When the mRNA binds to the small subunit of the
  ribosome it is called initiation. The ribosome
  looks for the start codon, which is always AUG.

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Elongation

• The same three steps are repeated until the
  stop codon is read.
• An amino acid is placed in position on the A
  site of the ribosome
• The peptide bond is formed.
• The peptide moves over to the P site so that
  the A site is available for the next amino
  acid. (The old tRNA is released.)

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• When a ribosome has moved far enough down the
  mRNA, a second ribosome can pick up the mRNA and
  also start reading and translating it. It may be
  passed on to a third ribosome, and so on. When we
  have several ribosomes all translating the same
  mRNA at the same time, it is called a polysome.

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Termination

• When the ribosome reads one of the stop codons,
  there is no matching tRNA. Instead, a protein
  called a release factor binds to the stop codon,
  the polypeptide is cut from the last tRNA, and
  the polypeptide (protein) is released into the
  ctyoplasm, where other proteins will help fold it.

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Termination of translation
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oach/images/translation/tlterman.gif
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Whats in your DNA

• A typical eukaryotic cell transcribes only about
  20 of its DNA into RNA.
• Genes in DNA also code for rRNAs and tRNAs.
  These get copied from the DNA but are not
  translated like mRNA.
• Much of the DNA has sequences that just repeat
  and dont have any useful information.
• 97 of human DNA doesnt code for anything
• Much so-called junk DNA may have functions

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Genes turn on, genes turn off

• Transcription and translation are expensive
• Costs ATPs preventing unnecessary transcription
  and translation saves energy.
• Different genes are needed at different times.
• Development from zygote to birth some genes
  needed that never are again.
• Differentiation different types of tissues carry
  out different functions, use different genes
• Response to external and internal environment
• Response to hormones e.g. puberty