Transcript Protein Synthesis -Transcription - Mr. Lesiuk

A second major function of DNA is its role in producing structural and
functional proteins that an organism needs. This function is known as
“PROTEIN SYNTHESIS”. The full process consists of two major steps.
The first step is called “TRANSCRIPTION” the second step is called
“TRANSLATION”
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Tonight’s
homework:
Read pp. 526-528.
DNA structure Quiz
next day.
Tonight’s homework:
Read pp. 526 -528.
dna structure quiz
next day
Many times students will remember the details of each process, but they forget
the proper name for which process is which. That is big trouble! Try to
remember that to TRANSCRIBE something, means to copy it out in the same
language. See above.
The second step in Protein Synthesis is called
TRANSLATION. To “Translate” something, you convert it
from one language into another.
Now that we have established a good understanding of both terms, we
will focus in on the first process of protein synthesis known as
TRANSCRIPTION. During this process, DNA is used to make RNA.
But RNA and DNA are both the same category of organic molecule, both
are NUCLEIC ACIDS. So it is like staying in the same language.
This process (transcription), just like replication, also takes
place in the nucleus. It starts off fairly similar to replication
in that the section of DNA (GENE) that codes for a
particular protein must unwind and unzip.
But then this Transcription process begins to differ from replication.
Different enzymes called “RNA Polymerases” do the work, and rather
than DNA Nucleotides that are floating around in the nucleoplasm, it is
RNA nucleotides that will be used to build a new complimentary strand.
This strand of RNA that is
made during the
transcription of a gene is
called mRNA. This
stands for “MESSENGER
RNA”. DNA has the blue
prints for building the
protein, but the DNA
cannot leave the nucleus.
Proteins are built outside
of the nucleus out in the
cytoplasm. So mRNA acts
somewhat as a counterfeit
copy of the gene that can
carry the message out to
the protein factories.
Try practicing some transcription by using “Strand I” as the
template for building a complimentary strand of mRNA.
Strand I : ACA GGG ACC CAT TTA ACC CAG
Strand II : TGT CCC TGG GTA AAT TGG GTC
mRNA produced: _?____?____?_____?____?____?____?___
Write down your ANSWER:
UCU CCC
UGG GUA AAU UGG GUC
Every three bases represents a genetic code word called a
“CODON”. Each codon codes for a given amino acid, for
example the first codon (UCU) in our mRNA codes for the
amino acid – SERINE. Check out the Genetic Code table in
your textbook for how to read this.
Good news is
that you do not
have to
memorize this
table. It will be
provided for you
on tests. Now
either use this
one or the one in
your book to
write down the
names for the
amino acids for
the other 6
codons you
transcribed.
In a real gene, the strand of DNA is extremely long, having thousands
and thousands of bases in it. But not all parts of the Gene are important.
There are pieces of the gene that are important called “EXONS” and
pieces of the gene that are unimportant called “INTRONS”.
Before the newly made (primary) mRNA is shipped out of the nucleus it
must be processed by RIBOZYMES. These Ribozymes (a bit like
enzymes) cut out the useless Introns and splice back together the
important EXONS to form what is now called “Mature” mRNA.
Fig. 25.9
Processing mRNA and the ROLE Of
Polyadenine Tail
Primary mRNA is produced from the entire gene
during Transcription.
AUG CCG CAG CAC CCC UUA CCC GGG
Exon #1
Intron #1 Exon #2
Intron #2
Exon #3
Mature processed mRNA must be made with the
help of Ribozymes
AUG CCG CAC CCC GGG
Then a POLYADENINE TAIL is added on to
preserve this mRNA before it leaves the nucleus
AUG CCG CAC CCC GGG AAA AAA AAA AAA
AAA AAA ….
The Role of the Polyadenine tail is to protect the mRNA so
that it will be used by a protein factory to build the given
protein. Once enough of a given protein (ex Insulin) the
polyadenine tail will be cut off, telling the protein factory to
stop using that piece of mRNA. It will then be broken down
mRNA
Polyadenine Tail
Now the mature mRNA exits the nucleus and heads out to a
RIBOSOME (not a Ribozyme) protein factory where it will
be read and translated into a specific protein.