Before understanding the processes involved in the synthesis (making) of proteins, you need to know the difference between DNA and RNA
DNA
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RNA
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Protein synthesis has two major steps:
Transcription
A copy of the gene is created in the nucleus, in the form of mRNA. The mRNA then travels from the nucleus to a ribosome (either floating in the cytoplasm or attached to rough ER).
Using mRNA allows the gene (original instructions) to remain safely in the nucleus. |
Translation
The information found on the mRNA is read by the ribosome and is used by the tRNA to deliver the correct amino acids to the ribosome so it can join the amino acids together to form a polypeptide chain.
The language of nucleic acid allows the information on the mRNA to be turned into the correct series of amino acids in the polypeptide chain (protein). |
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Step 1. Transcription
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RNA Processing
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Step 2. Translation
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Transcription
When a transcript is made!
DNA needs to stay in the nucleus because it is safe there. If it were to move to the ribosome for protein synthesis, something could damage the DNA and that would be it! DNA is like the king, and the king never leaves his castle. However the king needs to give his workers in a far away factory instructions so they can complete the tasks necessary to keep the kingdom functioning properly. |
In order for the instructions/information on the DNA to reach the workers in the factory (ribosome), RNA polymerase transcribes (copies) the message into a strand of mRNA. mRNA can then travel to the factory - it is like a messenger - it collects the message from the king and takes it through the dangerous forest (cytoplasm) to the factory (ribosome).
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We need to know how the message is copied, not just that it gets copied.
A specific nucleotide sequence, called the promoter causes RNA polymerase (enzyme) to attach to the DNA. When attached, the DNA double helix begins to unwind the DNA making up the gene needed and RNA polymerase uses one side/strand of the DNA as a template in order to make a strand of mRNA. mRNA is a single strand, and consists of the bases A, U, C and G. Nucleotides are added to the growing mRNA strand until a terminator sequence is reached. This sequence tells RNA polymerase to release the single mRNA strand. DNA then winds back up like normal and the mRNA leaves the nucleus via pores in the membrane.
A specific nucleotide sequence, called the promoter causes RNA polymerase (enzyme) to attach to the DNA. When attached, the DNA double helix begins to unwind the DNA making up the gene needed and RNA polymerase uses one side/strand of the DNA as a template in order to make a strand of mRNA. mRNA is a single strand, and consists of the bases A, U, C and G. Nucleotides are added to the growing mRNA strand until a terminator sequence is reached. This sequence tells RNA polymerase to release the single mRNA strand. DNA then winds back up like normal and the mRNA leaves the nucleus via pores in the membrane.
To summarize - there are three steps for transcription
1. Initiation (attach to promoter), 2. Elongation (add nucleotides), 3. Termination (release mRNA strand)
1. Initiation (attach to promoter), 2. Elongation (add nucleotides), 3. Termination (release mRNA strand)
Videos of the process
For transcription, watch this video from the start until 7min 53sec
The promoter is also known as a TATA box. Knowing about the 5' cap and Poly-A tail might aid your understanding but are not at all necessary for exam revision. You need to understand mRNA splicing though! |
For transcription, watch this video from the start until 5min 33sec.
'Central dogma' not required for exams. You need to have an idea of splicing (removal of introns)! |
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Pictures of the process
RNA processing
Before mRNA leaves the nucleus, one more thing needs to happen. The RNA is processed. This means it is altered so that it can carry out its role efficiently. Think about it like processed food - food is modified to last longer etc. Same goes with mRNA.
The following things happen to the mRNA:
1. A 5' cap is added to the 5' end of the mRNA. This preserves the mRNA strand during its journey out of the nucleus.
2. A poly-A tail is added to the 3' end of the mRNA. A poly-A tail is lots of A's (AAAAAAAAAAA+) to again preserve and protect the mRNA.
These two you don't need to worry about. You do need to worry about RNA splicing!
3. RNA splicing - the coded region (all the 'instructions') also contains lots of extra information that is not needed. This information is cut out of the mRNA molecule.
1. A 5' cap is added to the 5' end of the mRNA. This preserves the mRNA strand during its journey out of the nucleus.
2. A poly-A tail is added to the 3' end of the mRNA. A poly-A tail is lots of A's (AAAAAAAAAAA+) to again preserve and protect the mRNA.
These two you don't need to worry about. You do need to worry about RNA splicing!
3. RNA splicing - the coded region (all the 'instructions') also contains lots of extra information that is not needed. This information is cut out of the mRNA molecule.
RNA SPLICING
A strand of mRNA is made up of two types of 'sections' - introns and exons. Think of introns as intruders and exons as what gets expressed. The mRNA needs to hold on to the exons (so they can be expressed) and get rid of the introns (intruders). So the introns are cut out (spliced) and the exons are kept and joined together. |
Translation
When the transcript is translated into a protein!
Once out of the nucleus, the mRNA travels through the cytoplasm (dangerous forest) and attaches to a ribosome (factory). A ribosome is an organelle that reads mRNA bases three at a time and has the corresponding amino acid delivered by numerous tRNA molecules (workers). We know already (hopefully we remember from Y11) that three bases on a strand of DNA is called a triplet. Well, on mRNA three bases are called a codon. Codons are read by the ribosome (factory) and tRNAs (workers) containing the corresponding three bases to the codon (=anticodon) fetch the amino acid that is coded for and adds it to the chain. Peptide bonds form between the amino acids, forming a polypeptide chain which is released once a stop codon is reached.
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A codon is a sequence of 3 bases found on an mRNA strand. An anticodon is a sequence of 3 bases found on a tRNA strand. An codon matches up with an anticodon which is how the right amino acids are brought and connected to the chain. For example, a codon of AAG would have an anticodon of UUC, and with the anticodon would come the amino acid that the codon (AAG) codes for. |
Here is a strand of mRNA. This will be taken from the castle (nucleus) to the factory (ribosome) and read - once it is read the tRNAs (workers) are sent out to fetch a particular amino acid depending on the codon. Our first codon is UUU so the tRNA with the corresponding sequence/anticodon (AAA) would go out and fetch the amino acid that UUU codes for (look it up in the translation table). The tRNA brings it back to the ribosome and hooks it up to the chain of amino acids. The next codon is GAG, so the tRNA that has the corresponding sequence/anticodon CUC would go and fetch the amino acid that is coded for by GAG and hook it up to the chain. The third codon, GUG would require the tRNA with the anticodon CAC to fetch the amino acid that GUG codes for and then add it to the chain. The chain continues to grow until a STOP codon is read, which signals the ribosome to release the chain of amino acids (also called a polypeptide chain) so that the protein can fold into its tertiary structure and become functional.
Pictures of the process
The genetic code consists of 64 triplets. Once these triplets are transcribed into mRNA they become codons. Each codon codes for one of 20 amino acids (see below). It sounds impossible for 20 amino acids to make the HUGE number of proteins we have, but it's true. Here's how to get your brain around it all.
If you have red, blue, and yellow blocks to arrange in a row of four blocks, how many different combinations can you make?
The first block could be either red, blue, or yellow. Likewise, the second, third, and fourth blocks could each be either red, blue, or yellow. As a result, you have three choices for each position. So, the number of possible combinations of blocks is 3 ×3 ×3 ×3 =34 = 81. In the problem involving proteins, there are 20 possibilities for each of the 5 positions. So, the number of possible proteins with five amino acids is 20 ×20 ×20 ×20 ×20 = 205 = 3,200,000. Because many proteins contain more than five amino acid links, it is easy to see how millions of different proteins are possible.
This information came from here.
If you have red, blue, and yellow blocks to arrange in a row of four blocks, how many different combinations can you make?
The first block could be either red, blue, or yellow. Likewise, the second, third, and fourth blocks could each be either red, blue, or yellow. As a result, you have three choices for each position. So, the number of possible combinations of blocks is 3 ×3 ×3 ×3 =34 = 81. In the problem involving proteins, there are 20 possibilities for each of the 5 positions. So, the number of possible proteins with five amino acids is 20 ×20 ×20 ×20 ×20 = 205 = 3,200,000. Because many proteins contain more than five amino acid links, it is easy to see how millions of different proteins are possible.
This information came from here.
Videos of the process
Redundancy and the genetic code
Redundancy due to the degeneracy of the genetic code
caThese two words mean very much the same thing, and you need to be able to explain the benefit of having several codons that are all able to code for the same amino acid, or having amino acids that can be coded for by several codons.
The benefit of this property of the genetic code is: if there was a mistake during the making of the mRNA strand and the genetic code was not redundant, the mistake would result in the wrong amino acids being added to the polypeptide chain. This would result in the wrong protein being produced and the gene not being correctly expressed. However because the code is redundant, this means that if a mutation occurred during the making of the mRNA strand, it is possible that although the wrong nucleotide base is on the mRNA strand, the codon it is part of would still code for the same amino acid. Using the example above for alanine - if the mRNA strand has the codon GCU, but a mutation occurs causing the last base to become a C (=GCC) this would have no effect on the protein because both GCU and GCC code for the amino acid alanine.
The benefit of this property of the genetic code is: if there was a mistake during the making of the mRNA strand and the genetic code was not redundant, the mistake would result in the wrong amino acids being added to the polypeptide chain. This would result in the wrong protein being produced and the gene not being correctly expressed. However because the code is redundant, this means that if a mutation occurred during the making of the mRNA strand, it is possible that although the wrong nucleotide base is on the mRNA strand, the codon it is part of would still code for the same amino acid. Using the example above for alanine - if the mRNA strand has the codon GCU, but a mutation occurs causing the last base to become a C (=GCC) this would have no effect on the protein because both GCU and GCC code for the amino acid alanine.