Transcript Protein Synthesis

Cellular Metabolism
Chapter 4
Protein Synthesis
How DNA works
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.
Structure of DNA
• DNA and RNA are made up of nucleotides
(Polymers)
(Monomers)
• A nucleotide has a sugar, a phosphate
group, and a nitrogen containing base.
These nucleotides are linked
together to form chains or
strands.
In DNA two strands are joined by
hydrogen bonds.
RNA is single stranded.
Information is stored in DNA in
three base units, called codons.
• 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.
• 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 stop codon.
• Because we are eukaryotes, we modify
the pre-messenger RNA we have made.
• We cut out the intervening sequences
(Introns) and splice the expressed
sequences (Exons) back together. This
way, we can make more than one protein
from a 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.
Ribosome
There are two subunits that make up a
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.
To Review:
• To help you review transcription and translation,
go to http://www.doegenomes.org Scroll down to
the site directory and click on education. Under
quick links click on Videos, Webcasts, Graphics
and Animations. You should see three topics:
• Transcription/Translation Overview
• Transcription Detail
• Translation Detail (protein synthesis)
• I would suggest you look at all three.
There is a lot of other interesting stuff here too, FYI
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.
Mutations
• A mutation is a permanent, inheritable
change in the DNA.
• To be passed on to the next generation,
this mutation must be present in the
gametes (eggs or sperm).
Why don’t we see more mutations?
• Silent mutations can occur in non-coding
(“junk” ) DNA.
• If we change the last codon, in many
cases we get the same amino acid.
• We have pairs of chromosomes, so a
good gene may “cover” for a bad one.
• We have about 50 enzymes that “police”
our DNA looking for changes and fixing
them.
Mutations
• Mutations can be as small as a change in
one base – a point mutation
• Mutations can be small sections of DNA,
entire genes or large pieces of
chromosomes that are moved from one
place to another by transposons.
• Entire chromosomes may be duplicated
(Down syndrome – 3 copies of
chromosome 21) or lost.
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Causes of mutations
• Occur naturally
• Due to mutagens:
– Radiation
– Toxic chemicals
– Viruses