Overview of Protein Synthesis & RNA
Protein Synthesis HOME
1) DNA & Protein
Success Criteria & Vocabulary
Click this drop-down menu to see the Success Criteria.
I can define protein synthesis, and identify where it occurs.
I can describe why protein synthesis is important.
I can describe the three types of RNA (i.e. mRNA, tRNA, and rRNA).
[Note: Ribosomal RNA (rRNA) is not examinable in the NCEA Level 2 Biology curriculum].
I can explain the difference between DNA and RNA.
I can define the terms triplet, codon, and anticodon.
Click this drop-down menu to see the list of Vocabulary.
Ribonucleotide: One structural unit/monomer of RNA.
Triplet: A sequence of three consecutive bases on DNA.
RNA: A single-stranded nucleic acid that is essential for protein synthesis.
mRNA: The type of RNA that carries instructions from DNA in the nucleus to the ribosome in the cytoplasm during protein synthesis.
tRNA: The type of RNA that carries amino acids to the ribosome during protein synthesis.
Complementary base pairing: C pairs with G, A pairs with T (in DNA), A pairs with U (in RNA).
Gene expression: The process of using a gene to make a fully functional gene product (not necessarily a protein).
Protein synthesis: The process of using a gene to make a fully functional protein.
Uracil: Nitrogen base that pairs with adenine (A) in RNA.
Codon: Three consecutive bases on the mRNA strand.
Anticodon: Three consecutive bases on a tRNA strand.
Self-Directed Learning Tasks
Task 3: Complete Education Perfect:
Task called 'Overview of Protein Synthesis & RNA'.
A Constant Need to Make Proteins
Proteins don’t just appear in living things, they have to be made.
After they are made, proteins can only exist in our bodies for a certain period of time before they are broken down and recycled into amino acids - the building blocks of proteins.
Some proteins can function for a whole year, but the majority of proteins in our bodies only exist for around two days before they are broken down and new ones have to be made.
There are Two Main Stages in Protein Synthesis
Protein synthesis allows cells to generate new proteins that are responsible for maintaining our cell’s structure and function. This is important because proteins are absolutely essential for the maintenance of all of our life processes.
The production of protein starts with an individual’s genetic material, which is stored within DNA in the nucleus.
Protein synthesis involves two stages:
Transcription, which makes an mRNA copy of the gene.
To create a copy of the DNA, an enzyme (RNA polymerase) must “read” the bases found in the DNA molecule in groups of three, called triplets. The mRNA then carries this copy of the genetic information to the cytoplasm for the next step, translation.
Translation, which makes a polypeptide chain.
This occurs on ribosomes in the cytoplasm. In this process, the ribosomes and tRNA operate to decode the information in the mRNA copy of the gene, linking the amino acids in the order encoded in the mRNA.
Specifically, the sequence of bases on the mRNA is read by ribosomes in groups of three, called codons. Each of these codons corresponds to one of the 20 amino acids that can be found in our cells, which are linked together during translation (by peptide bonds) to form a polypeptide chain.
After translation, the polypeptide chain folds into a 3-dimensional structure, that is a functional protein.
The difference between DNA and RNA.
Why it's important to first transcribe DNA/make a copy of it in the form or mRNA.
The difference between mRNA and tRNA.
How DNA, mRNA, and tRNA sequences all complement each other to ensure that the correct acid is added to the polypeptide chain.
The Differences Between DNA and RNA
RNA vs DNA
Before a polypeptide chain can be produced, the necessary genetic information must be transferred from nucleus to cytoplasm. The transfer of this coded information (from DNA) and its subsequent decoding involve RNA molecules. DNA and RNA are collectively called nucleic acids. Surprisingly, about 1% of the cell’s total weight is DNA (deoxyribonucleic acid), yet RNA (ribonucleic acid) can be up to 5% of the total weight of a cell! So RNA play a very important role in the cell!
RNA molecules are ribonucleic acids. This means they are similar to DNA, but there are some key differences. DNA is confined to the nucleus. Before a polypeptide chain can be produced, the necessary information must be transferred from nucleus to cytoplasm (as per slide 21). The transfer of this coded information and its subsequent decoding involve ribonucleic acid (RNA).
DNA and RNA are collectively called nucleic acids, and are very similar except that in RNA:
The sugar is ribose instead of deoxyribose
The base thymine (T) is replaced by uracil (U)
RNA is single-stranded
Finally, DNA only has one form, while RNA has three different forms! (You only need to know about mRNA and tRNA. You do not need to know about rRNA).
Types of RNA Involved in Protein Synthesis
Making a protein involves the combined action of three kinds of RNA:
Messenger RNA (mRNA). This acts as the working copy of a gene. It takes a copy of the genetic information from the DNA and carries it to the ribosomes, where proteins are made.
Transfer RNA (tRNA). This brings each amino acid into association with the mRNA at the ribosome.
Ribosomal RNA (rRNA). Combined with proteins, this is a constituent of ribosomes. Ribosomes ‘read’ the coded message in the mRNA and use it to link amino acids in the correct order. (You do not need to know about rRNA).
mRNA is Like a Photocopied Message
DNA cannot leave the nucleus.
The function of DNA is to hold (long term storage) the genetic information for the cell. The genetic information contains the instructions for development and function of living organisms.
However, all of the materials required to make proteins (amino acids, polymerase enzyme, ribosomes) are located in the cytoplasm. So in order for proteins to be made, the code for the protein must get to the ribosomes in the cytoplasm.
The problem is, DNA cannot exit the nucleus for two main reasons.
First, it could get damaged by enzymes and chemicals in the cytoplasm (i.e. it could get mutated);
Second, it is too large to fit through the pores on the nuclear membrane.
DNA cannot go to the site where proteins are made, so mRNA goes instead.
What is Messenger RNA (mRNA)?
mRNA is the molecule that carries the copy of DNA in a gene to the ribosomes in the cell’s cytoplasm, where the second stage of protein synthesis occurs (this stage is called translation). In mRNA, 3 consecutive bases are each called a codon.
You may be wondering why DNA molecules cannot be directly transcribed into a polypeptide chain, and why the bases have to copied instead. Read on!
The Importance of mRNA in Protein Synthesis
Reason #1 - mRNA keeps DNA safe
DNA is a long-lived molecule that is well protected from possible harm if it stays in a cell’s nucleus. However, the ribosomes that are used to make polypeptide chains and are not found in the nucleus. If DNA has to travel into the cytoplasm everytime a cell requires a new protein, it potentially risks getting damaged by enzymes and other chemicals in the cytoplasm.
If DNA is damaged beyond repair, it could have serious consequences for the organism. DNA provides the genetic information that is needed to build the proteins that cells require to function correctly, therefore if it is damaged, the cells in an organism will die.
On the other hand, mRNA is a short-lived molecule that is constantly produced by a cell. If mRNA is damaged, the cell can simply produce more!
Reason #2 - mRNA can fit through the nuclear pores
If DNA had to leave the nucleus, it would not be able to fit through the nuclear pores! DNA molecules are double-stranded, and are reasonably large in comparison to mRNA molecules. mRNA is only a single-stranded molecule, therefore it is easily able to leave the nucleus and move to the ribosomes in the cytoplasm.
Also because proteins are large molecules, these many not be able to leave the nucleus as they would be too large to pass through the pores of the nuclear membrane.
Reason #3 - Many many mRNA transcripts keep up with demand!
There is only a single copy of DNA in each cell, but the cell can continuously make many copies of mRNA via transcription. If translation was to occur in the nucleus directly from the DNA template strand, it would be slow as only one molecule of protein could be produced at a time by each cell as there is only one copy of the needed DNA. This means that a cell can create more proteins when needed with mRNA than what it could from a single copy of DNA. More proteins can be made simultaneously when there are multiple mRNA molecules made because the cell may have increased demands for a specific protein. Note that one mRNA can be read by multiple ribosomes!
For example, cells constantly need to produce the proteins that make up the cell membrane. These proteins play an important role in letting substances into and out of a cell. If cells had to produce these proteins directly from their DNA, they would not be able to keep up with demand and the cell membrane would be unable to maintain its function. mRNA enables cells to efficiently produce the proteins they need.
tRNA is In Charge of Bringing Along the Amino Acids
Transfer RNA (tRNA)
During translation, the genetic code on the mRNA strand is ‘read’ by small organelles called ribosomes. The ribosomes work with a molecules called transfer RNA (tRNA), to translate the genetic code on the mRNA into a specific sequence of amino acids.
tRNA attach to specific amino acids in the cytoplasm. Then the tRNA molecules carry that amino acid in the cytoplasm to the ribosome, where that amino acid is joined with other amino acids via a peptide bond to form a long polypeptide chain.
Significance of Triplets (DNA), Codons (mRNA), and Anticodons (tRNA)
Three Consecutive Bases
Triplet = 3 consecutive nucleotide bases on the DNA strand. Each triplet is a group of 3 bases and codes for a specific amino acid
Codon = 3 consecutive nucleotide bases on the mRNA strand. A start codon initiates the translation. The start codon always codes for the amino acid methionine and is AUG. A stop codon ends translation which causes the ribosome to stop translating and release the mRNA and the polypeptide chain.
Anticodon = 3 consecutive bases on a tRNA molecule that is complementary to the mRNA codon. A specific anticodon is paired with a specific amino acid.
Therefore, one triplet is complementary to one codon, and one codon is complementary to one anticodon. The codons and the anticodons match according to the RNA base-pairing rules, ensuring the correct amino acid is added to the growing polypeptide chain.
Differences in the length of DNA, mRNA, and tRNA
(There was a past NCEA exam question on this).
tRNA are short RNA molecules, made up of only ~80 nucleotides. mRNA are longer nucleic acids, ~2,200 nucleotides long. DNA is an incredibly long molecule, ~3,000,000,000 nucleotides long.
tRNA is a short molecule as it only needs to deliver an amino acid (which is also a small molecule). mRNA is longer than tRNA because it contains a code for producing a polypeptide chain. tRNA is not required to have the total coding length of the gene, but only for one anti-codon to attach one amino acid molecule, while mRNA is a longer molecule because it contains the whole code to produce a polypeptide chain (protein) which is made of a sequence of amino acids. DNA is a very long molecule because it stores all the genetic information of that individual.