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Transcription (RNA Synthesis)
DNA
→
Replication
DNA
→
Transcription
mRNA
→
Translation
protein
Transcription is an important process in the above cellular pathway. Transcription is an intermediate
process in the production of protein. Transcription is the production of RNA from a DNA template. RNA is
then used to produce protein in the ribosomes (translation). Transcription takes place in the nucleus, where
the DNA template is located.
RNA is a nucleic acid like DNA. Just as in DNA, nucleotides are connected together to form the polymer
RNA. RNA nucleotides, like DNA are made of a sugar, a phosphate group and a base. The sugar in RNA
is ribose. RNA contains guanine, cytosine and adenine. The fourth base in RNA is uracil. Uracil takes the
place of thymine in RNA. RNA is a single stranded molecule compared to the double stranded DNA.
Why do we need RNA if DNA holds all the genetic information? If DNA is damaged in any way, then the
coding sequence is changed. This is called a mutation, which could be harmful to the cell or even the whole
organism. DNA is very fragile. Chemicals, UV light or other agents can destroy it. Also DNA is unable to
leave the nucleus. If it left the nucleus some of the genetic information of the cell would be lost. Instead
messenger RNA (mRNA) carries the DNA code from the nucleus into the cytoplasm where the ribosomes
are found. The ribosomes are then able to decode the RNA to produce proteins encoded in the DNA
sequence.
Transcription is similar to the process of DNA replication. As in replication, a small section of the DNA
double helix unwinds and unzips to expose the bases on the two strands. RNA nucleotides line up in the
proper order by hydrogen bonding to their complementary bases on DNA. The nucleotides are joined
together by the enzyme RNA polymerase instead of DNA polymerase.
Directions: Place a star on the molecules found in DNA. Circle the molecules found in RNA. Some
molecules may be found in both DNA and RNA.
Procedure
1. Disassemble one of your DNA molecules from the replication activity.
2. Begin transcription by simulating the action of an enzyme by untwisting the other DNA molecule.
3. Simulate the action of an enzyme by unzipping the DNA molecule at the weak hydrogen bonds.
4. Simulate the action of the enzyme RNA polymerase by attaching complimentary nucleotides to only
one the strands of DNA (Ribose = purple pentagon; Uracil = purple tube).
5. Remove the RNA strand from the DNA molecule.
6. Rezip and retwist the DNA molecule.
Analysis
1. Where in the cell does the process of transcription occur? Explain why it must occur there.
2. Describe the steps involved in transcription.
3. What happens to the RNA molecule once it is made?
4. List 3 differences between DNA and RNA.
5. How many different parts make up the RNA molecule? List and name the parts of RNA.
6. What RNA nucleotide always pairs with a DNA nucleotide of Adenine? Guanine? Cytosine?
Thymine?
7. Why do cells have RNA if DNA carries all the genetic information of the cell?
8. If the DNA analysis of a gene shows 20% Adenine bases, what would be the percentage of
Thymine? Cytosine? Guanine? Uracil?