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How Are Genes Expressed?
Chapter11
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.
• How can patterns of 4 bases hold
information?
• The same way your computer works!
• 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”)
• We can make 64 different codons, but we
only use 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.
• 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.
DNA
vs.
RNA
Uses A,T, C and G
Uses A, U, C and G
Use deoxyribose
Uses ribose
Double stranded
Single stranded
• 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.
• 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.
• 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
• The mRNA leaves the nucleus through the
pores in the nuclear envelope, and finds a
ribosome in the cytosol or on rough E.R.
• 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.
Ribosome
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.”
Changing from the “language” of nucleic
acids to the “language” of proteins is
called TRANSLATION.
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.
• 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.
• 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.
Elongation
The same three steps are repeated until the
“stop” codon is read.
1. An amino acid is placed in position on
the “A” site of the ribosome
2. The peptide bond is formed.
3. 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.)
• 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.
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.
Play with DNA
Now its your turn !
• Newly made proteins have a signal
peptide – 4 to 12 amino acids – that direct
the protein where to go.
• A typical eukaryotic cell transcribes only
about 20 % of its DNA into RNA.
Regulation of protein synthesis
• To form each peptide bond requires 3
molecules of ATP. Since each protein can
have from 50 to thousands of amino acids,
much of the cell’s energy goes into protein
synthesis.
• Protein synthesis is regulated at every
step of the process. The most energy
efficient, is to control protein synthesis by
controlling transcription.
We can also have:
• Posttranscriptional control
• Translational control
• Posttranslational control
Genes can be of two types:
• Structural genes – codes for proteins
used outside the nucleus
• Regulatory genes – help govern the
amount of polypeptide made by the cell.
– Transcription factors
– Regulatory site