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Slide 2
Recall the lesson about macromolecules where the structure of DNA and RNA was
discussed. Pause the audio lecture for a moment and study the figure shown here and see if you
can determine the difference between the two types of molecules……………. You should have
noticed two differences in the nucleotides: the type of sugar present and the base pairing. RNA
has ribose sugar, DNA has deoxyribose. Base pairing is the same in both molecules except that
the thymine base in DNA is replaced with uracil in RNA. Both thymine and uracil have the ability
to pair with adenine. You will also have noticed that DNA is represented as a double strand
whereas RNA is a single strand molecule.
Slide 3
The figure here shows three different processes where information is transferred:
1. Replication – DNA can be replicated using the existing information contained within the strands
of DNA.
2. Transcription – one of the DNA strands acts as a template for the production of RNA. Three
different types of RNA are produced: messenger RNA (mRNA), ribosomal RNA (rRNA) and
transfer RNA (tRNA).
3. Translation – when messenger RNA (mRNA) is read, the information encoded in the strand is
translated to produce polypeptides which eventually become proteins.
In eukaryotic cells both processes of replication and transcription take place in the nucleus;
translation takes place in the cytoplasm. In prokaryotes, which have no nucleus, all three
processes are completed in the same location, the cytoplasm.
This lesson will focus on the process of transcription.
Slide 4
The diagram on the left shows that transcription and translation in a prokaryote
takes place in the cytoplasm. In a eukaryotic cell, shown on the right hand side, the two processes
are separated: TRANSCRIPTION takes place in the NUCLEUS, translation in the cytoplasm.
Slide 5
Messenger RNA is the only class of RNA that carries information necessary to
build a polypeptide. Most of the genes on the DNA molecule encode for mRNA. Some genes on
the DNA strand encode ribosomal RNA, the main component of ribosomes. Recall that ribosomes
are the sub-cellular structures necessary for the assembly of polypeptide chains. Yet other genes
on the DNA encode the information necessary to produce transfer RNA. As the name implies
tRNA transfers individual amino acids to the ribosome where they are assembled into
polypeptides according to the information encoded in the mRNA.
Slide 6
The process of transcription can be divided into three phases: initiation, elongation
and termination. During the initiation phase, the enzyme RNA polymerase binds too a specific
sequence of nucleotides, the promoter, on the DNA molecule. This interaction causes the two
DNA strands to unwind in that area exposing the DNA template strand. Somewhere between 10
and 20 bases are exposed at any one time. The DNA molecule winds back into a double helix
once the bases have been transcribed.
Slide 7
During the second step, elongation of the RNA transcript, RNA polymerase moves
along the DNA template reading the strand from 3´ to 5´. The nucleoside triphosphates (found in
the nucleus of eukaryotes) are added as nucleotides to the 3´ end of the RNA strand until it grows
to a designated length. Note that the new RNA strand is being produced from its 5´ to 3´ end
making it antiparallel to the DNA template. The energy required for the reaction is provided by
the hydrolysis of pyrophosphate into two molecules of inorganic phosphate. Pyrophosphate is
produced when a nucleotide is added to the RNA transcript. How fast is this process? Estimates
show that about 60 nucleotides per second are transcribed! Think about this: at this rate of
transcription, how long will it take to make a large protein consisting of 1200 amino acids?
(Remember that 3 nucleotides encode 1 amino acid).
Slide 8
Observe the figure shown on slide 8, and note that base pairing between RNA and
DNA is the same as in the double stranded DNA helix. The exception is that the thymine base of
DNA is replaced with uracil in RNA. Both thymine and uracil have the ability to pair with
adenine.
Slide 9
The enzyme RNA polymerase will continue to read the DNA template until told to
stop. The termination phase, as in initiation, involves a specific sequence of bases forming a
termination site on the DNA template. Once these bases have been read the RNA transcript is
released. In eukaryotes, when a messenger RNA transcript is released it is referred to as “premRNA” since it requires further processing before it is ready to translate the information into a
polypeptide.
Slide 10
The first step in modifying the pre-mRNA transcript is the alteration of the ends. A
modified guanine nucleotide forms a “cap” at the 5´end. Once in the cytoplasm, this cap will help
bind the mRNA to a ribosome. At the other end of the pre-mRNA transcript, adjacent to the
termination site, a series of adenine nucleotides forming a “poly (A) tail” is attached by enzymes
to the 3´ site. Both cap and tail modifications prevent the degradation of the mRNA by hydrolytic
enzymes.
Slide 11
The pre-mRNA has one last processing step before it is considered mature mRNA
ready for translation. The pre-mRNA transcript is made up of exons and introns. Exons are
regions of the transcript that are eventually expressed during translation. Introns are non-coding
regions. This figure shows an intron sandwiched between two exons. “Snurps” bind to each end
of the intron. When the “snurps” come together with other proteins they form a splicesome,
snipping out the intron. Before the mRNA is ready to go to the cytoplasm all of the introns are
snipped out of the transcript. The remaining exons are pasted together forming mature mRNA
that is now ready for export through the nuclear pores. Introns are degraded in the nucleus.