Protein Synthesis HOME
1) DNA & Protein
2) Overview of Protein Synthesis & RNA
Success Criteria & Vocabulary
Click this drop-down menu to see the Success Criteria.
I can describe the role of RNA polymerase in the process of transcription.
I can explain the process of transcription.
I can discuss why transcription is important for the maintenance of cell function.
Click this drop-down menu to see the list of Vocabulary.
mRNA: The type of RNA that carries instructions from DNA in the nucleus to the ribosome in the cytoplasm during protein synthesis.
transcription: Process that uses the DNA template strand to make a complementary mRNA strand. First stage of protein synthesis.
ribosome: Organelle located in the cytoplasm, responsible for using mRNA as the template to make a polypeptide chain.
uracil: Nitrogen base that pairs with adenine in RNA.
codon: Three consecutive bases on an mRNA strand.
promoter region: A sequence of DNA that RNA polymerase binds to, to begin transcription.
coding region: A sequence of DNA that RNA polymerase transcribes to produce an mRNA strand.
terminator region: A sequence of DNA that RNA polymerase binds to, to stop transcription.
nuclear pore: A hole in the nuclear membrane, that lets mRNA exit the nucleus.
template strand: Sequence of DNA bases that is transcribed by RNA polymerase to make mRNA.
coding strand: Sequence of DNA bases that correspond to the mRNA codons that are translated into protein.
Self-Directed Learning Tasks
Task 3: Complete Education Perfect:
Task called 'Transcription'.
Transcription of DNA is Needed to Synthesise Proteins
Information in the Nucleus is transported to the Cytoplasm
The cell is the smallest living unit. Each cell in your body contains all of the instructions for building a human body—specifically, your human body–in its DNA. Amazing! The photo shows stomach cells. The dark spots indicate the cell nuclei, where the DNA is contained.
Proteins carry out the work of the cell. Your body contains tens of thousands of different types of proteins, each with their own specific job to do. Each protein is coded for in the DNA. But you don't have enough different strands of DNA for each DNA strand to code for a single protein. You have 23 pairs of chromosomes, each one a single strand of DNA.
In human cells, the DNA is confined to the nucleus (the dark spot in the cells you saw in the picture above). It is a double stranded molecule and is literally unable to fit through the nuclear pores in the nuclear membrane to the cytoplasm.
Since proteins are made in the cytoplasm (outside of the nucleus, on structures called ribosomes), and the information for making proteins is stuck inside the nucleus - the information for making proteins must somehow get out of the nucleus and into the cytoplasm.
The Purpose of Transcription
Transcription is the first stage of protein synthesis. During transcription, a section of DNA is copied and used to make a complementary strand of mRNA (messenger RNA (mRNA), transcribed code from the DNA in the nucleus). This copying process is done by RNA polymerase enzyme. Transcription occurs in the cell’s nucleus, where DNA is housed. Therefore, the purpose of transcription is to produce mRNA transcript.
How does the cell make proteins from the information stored in the DNA?
Proteins are made through the process called protein synthesis. Protein synthesis can be split up into two distinct stages: 1) transcription, and 2) translation. This page is all about the first stage, transcription.
How does the information get out of the nucleus and to the ribosome?
Answer: messenger RNA (mRNA)! Special proteins in the nucleus unravel the DNA strands, and another enzyme (RNA polymerase) makes an RNA copy of the DNA. This process is called transcription in which a portable copy of the DNA is made.
Preparatory Step (Before Transcription)
The enzyme DNA helicase must first create a transcription bubble by breaking the hydrogen bonds between the bases of the DNA molecule.
This essentially unwinds the double helix near the gene that is getting transcribed, and makes the template and coding strand single-stranded, to make way for RNA polymerase. This region of opened-up DNA is called a transcription bubble.
Step 1: Initiation
RNA polymerase binds to the DNA of the gene at a region called the "promoter". The promoter tells the RNA polymerase where to “sit down” on the DNA and begin transcribing.
Free ribonucleotides (NOT deoxyribonucleotides) are matched and attached to the exposed bases on the template strand following the complementary base pairing rule to make mRNA.
Step 2: Elongation
RNA polymerase moves along the transcription bubble that contains the sequence of bases that are required for a protein to be built.
As RNA polymerase moves along the bases, it takes free ribonucleotides that are in the nucleus and adds them to the growing mRNA strand.
This copies the DNA template strand to produce mRNA according to the RNA base-pairing rules. This means that C pairs with G as normal, but A will bind with U (uracil) instead of T.
A single mRNA strand is made using U instead of T. One triplet is complementary to one codon. Transcription forms a single mRNA strand, with groups of 3 bases (codons) that code for the amino acids.
During transcription, one of the DNA strands is used as a template to form mRNA. This strand of DNA is known as the template strand. Nucleotides attach to the mRNA strand according to the RNA base pairing rules. This means that adenine (A) in the template strand will pair with uracil (U) in the mRNA strand instead of thymine (T).
The length of DNA that is opposite to the template strand is called the coding strand. The coding strand has the same sequence of nucleotide bases as the mRNA strand that is produced, except that the nitrogen T base is found in the coding strand instead of U.
Step 3: Termination
The strand of mRNA grows until RNA polymerase reaches a sequence of bases that signal the end point of the transcription process. This sequence of bases is called the "terminator".
The process of ending transcription is called termination, and happens once the RNA polymerase transcribes a sequence of DNA known as a terminator. Transcription is complete when mRNA detaches from RNA polymerase.
You must understand the role of the "promoter" region and the "terminator" region on DNA.
The promoter region tells RNA polymerase where to begin transcription.
The terminator region tells RNA polymerase where to end transcription.
In between the promoter and terminator regions is the RNA-coding sequence, which is the actual instructions for making a protein.
After Transcription, the mRNA Leaves the Nucleus... to undergo Translation (next topic)
The DNA goes back into its double helix form and the newly made strand of mRNA detaches from RNA polymerase. The mRNA leaves the nucleus through small holes in the nuclear membrane called nuclear pores. Then the mRNA travels to the ribosomes in the cytoplasm. Once the mRNA reaches the ribosomes in the cytoplasm, the second stage of protein synthesis can begin (translation).
... mRNA needs to be made to protect DNA
mRNA travels to the ribosomes in the cytoplasm, so that the original DNA doesn’t get damaged leaving the nucleus.
Transcription produces a copy of DNA that can leave the nucleus and carry genetic information to the ribosomes in the cell’s cytoplasm.
As a result, the DNA’s “master copy” doesn’t have to leave the nucleus, so it is less at risk of being damaged and the cells in an organism can continue to function.
... without mRNA, you DIE.
Transcription means our DNA doesn’t have to leave the nucleus where it is protected from possible damage, while both of these processes allow the cell to produce proteins it needs quickly and in large quantities.
Transcription is a vital part of protein synthesis. Without it, we would not be able to make the proteins our cells need to function properly. If the transcription process is blocked, the cells in your body will not be able to produce any new mRNA and therefore, no new proteins - our cells wouldn’t be able to carry out their normal function.