The structure of RNA
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The structure of RNA
RNA is a type of nucleic acid called ribonucleic acid:

  • The RNA nucleotides are joined to form a single strand.
  • The nucleotides are joined by a bond between the sugar of one nucleotide and the phosphate of the next.
RNA differs from DNA in the following ways:

  • RNA is single stranded while DNA is double stranded.
  • RNA has a sugar called ribose while DNA has a sugar called deoxyribose.
  • RNA has the base uracil while DNA has the base thymine.
There are two types of RNA that need to be known for this topic. The first is messenger RNA, or mRNA. This is formed in the nucleus of the cell. It rewrites the sequence of bases of a section of DNA in a process called transcription.

mRNA carries the code for building a specific protein from the nucleus to the ribosomes in the cytoplasm. That is, it acts as a messenger.

The second is transfer RNA, or tRNA. This is found in the cytoplasm.

The tRNA picks up specific amino acids from the cytoplasm and brings them into position on the surface of a ribosome where they can be joined together in specific order to make a specific protein.

This process is called translation.

Protein synthesisThe following factors must be present for DNA replication and transcription:

  • gene (DNA) to act as a template
  • supply of free RNA nucleotides
  • enzymes
  • ATP
The base sequence in a DNA molecule, represented by the letters A T C G, make up the genetic code.

The bases hydrogen bond together in a complementary manner between strands. A will always go with T (U in RNA) and G will always go with C.

This code determines the type of amino acids and the order in which they are joined together to make a specific protein. The sequence of amino acids in a protein determines its structure and function.

The DNA code is a triplet code. Each triplet, a group of three bases, codes for a specific amino acid:

  • the triplet of bases on the DNA and mRNA is known as a codon
  • the triplet of bases on the tRNA is known as an anti-codon
The main stages of protein synthesis are transcription and translation.



The process of transcription
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The process of transcription
Transcription takes place in the nucleus:

  • the gene coding for the protein required untwists then unzips, the H-bonds between the strands break
  • free RNA nucleotides form complementary base pairs with one strand of DNA bases
  • weak hydrogen bonds form between base pairs
  • sugar phosphate bonds form between RNA nucleotides
  • mRNA strand is synthesized
  • mRNA peels off the DNA and moves out of the nucleus into the cytoplasm



The process of translation
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The process of translation
Translation takes place on the ribosomes in the cytoplasm, or found on the rough Endoplasmic Reticulum (ER):

  • the ribosomes are the sites of protein synthesis
  • the mRNA strand attaches to a ribosome
  • tRNA molecules transport specific amino acids to the ribosome
  • each mRNA codon codes for a specific amino acid
  • the anti-codons and codons match up and form complementary base pairs
  • peptide bonds form between the adjacent amino acids to form the polypeptide (protein)
It is important to note that the tRNA is reused and collects another specific amino acid. Once the protein has been synthesised mRNA may move to another ribosome to make a further protein or it can be broken down into free nucleotides to be reused.




Processing the proteins
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Processing the proteins
After translation, the protein passes into the channels of the rough endoplasmic reticulum (ER) for transportation. The protein is then passed from the rough ER to the Golgi apparatus inside tiny fluid-filled sacs, called vesicles. The Golgi apparatus is a system of membranes, which are responsible for the modification, processing, and packaging of the proteins. The protein may have a carbohydrate added, to form a glycoprotein. The Golgi apparatus packages the protein in a secretory vesicle, which fuses with the cell membrane and releases the protein from the cell.


RNA and protein synthesis exam skillsQuestions on this topic often require you to demonstrate an ability to deal with the complementary base pairings between DNA, mRNA and tRNA.

You need to understand the progression working forward from DNA to mRNA then to tRNA and to the associated amino acid may be provided in a table. Other questions may reverse this order of events. It is good practice and will make the task more manageable if you draw a diagram or template showing the base pairings. This will allow you to work forward or backwards as required.

Example 1
Part of a DNA strand has the base order A T C G T T C A G.

You are asked to identify the anti-codons associated with this strand.

Remember the sequence of events in the stages of RNA protein synthesis:

  • transcription of mRNA from DNA
  • tRNA matches the mRNA (anti-codons match the codons)
You should then work forward identifying the triplet codes as follows (make sure you substitute uracil (U) for thymine (T) when dealing with the RNA molecules):

Triplet codes in DNA, mRNA, and tRNA
DNA strand : ATC GTT CAG
mRNA : UAG CAA GUC
tRNA : AUC GUU CAG

Question
In a DNA molecule, the base sequence CAT codes for the amino acid valine. Using the initial letters of the bases, write the base sequence of the anti-codon on the tRNA molecule to which valine becomes attached.

Answer
Anti-codon : C A U