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Topic last updated: 02 Feb 2022

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Chemistry of life

14  of  14

Types of RNA

Although RNA is a single stranded molecule it is found in three very different forms which enable it to carry out three very different functions.

 

Messenger RNAmessenger RNA
 The molecule which transcribes the DNA code and carries it out of the nucleus through the pores in the nuclear membrane to the ribosomes in the cytoplasm which synthesise the required proteins. (mRNA)

The genetic codegenetic code
 Sequence of bases in a DNA molecule which codes for the sequence of amino acids in a protein. Each group of three bases is called a codon and codes for an amino acid.  is held within the sequence of bases in the DNA double helix. It is based on codons – groups of three bases which code for an individual amino acidamino acid
 The basic building blocks of proteins. There are twenty amino acids used, in different combinations, to make every protein required by the human body. or for the beginning or ending of a transcription sequence when the genetic code is read. A section of DNA containing the codons for the amino acids which make a polypeptidepolypeptide
 A long chain of hundreds or even thousands of amino acids joined by peptide bonds. chain is called a gene. However DNA is a very large molecule and it cannot get out of the nucleus of eukaryoticeukaryotic
 Cells that make up animals, plants, fungi and protista. They are three-dimensional, membrane-bound sacs containing cytoplasm, a nucleus and a range of membrane-bound organelles. cells. Messenger RNA (mRNA) is the molecule which transcribes the DNA code and carries it out of the nucleus through the pores in the nuclear membranenuclear membrane
 The thin, flexible structure enclosing the contents of the nucleus in a cell.  into the cytoplasm of the cell. Here it lines up on the surface of a ribosome and directs the synthesis of a protein based on the original DNA code.

Mrna (1)

 

  1. The breakdown of the hydrogen bondhydrogen bond
     An intermolecular attractive force between hydrogen, when it is covalently bonded to a highly electronegative atom (fluorine, oxygen or nitrogen), and an oxygen, nitrogen or fluorine atom on another molecule.     s between the two strands of DNA is catalysed by the enzyme DNA-directed RNA polymeraseDNA-directed RNA polymerase
     Enzyme involved in the transcription of DNA and the production of mRNA. The enzyme is involved in the breakdown of the hydrogen bonds between the two strands of DNA in the formation of mRNA, and the build up of the mRNA strand from 5-3 end.      (RNA polymerase).

  2. The 5’ strand of DNA acts as the template strand and is transcribed to give a single strand of mRNA. It is known as the antisense strand.
    The 3’ strand of the DNA is the coding strand, known as the sense strand. The mRNA which is built up has the same base sequence as the 3’strand of DNA, with thymine replaced with uracil. The build up of the mRNA strand from the 5’ to the 3’ end is also catalysed by RNA polymerase.

Every strand of mRNA has an AUG start codon at the beginning and one of the three stop codons (UAA, UAC or UGA) at the end. When it becomes attached to a ribosome this is where the message starts and stops as the polypeptidepolypeptide
 A long chain of hundreds or even thousands of amino acids joined by peptide bonds. chain builds up.

 

Trna (1)

 

Transfer RNAtransfer RNA
 RNA molecule found in the cytoplasm of the cell which transports specific amino acids to the surface of the ribosomes. (tRNA)

Transfer RNA (tRNA) is found in the cytoplasm of the cell. Each tRNA unit is folded to give three loops which look rather like a clover leaf. Each tRNA unit has two key binding sites:

  • the anticodonanticodon
     An area with three bases on a tRNA molecule which binds to the corresponding three bases on the messenger RNA attached to a ribosome.      – an area with three bases which binds to the corresponding three bases on the messenger RNAmessenger RNA
     The molecule which transcribes the DNA code and carries it out of the nucleus through the pores in the nuclear membrane to the ribosomes in the cytoplasm which synthesise the required proteins.     attached to a ribosome. The mRNA has the same codon as the sense DNA – hence tRNA has the anticodon.
  • the amino acidamino acid
     The basic building blocks of proteins. There are twenty amino acids used, in different combinations, to make every protein required by the human body.     binding site – each molecule of tRNA carries an amino acid which corresponds to the specific anticodon and facilitates the conversion of the DNA code into active proteins in the cell.

 

Ribosomal RNARibosomal RNA
 The RNA which makes up at least 50% of the structure of a ribosome. It acts as an enzyme in the formation of peptide links between amino acids brought to the ribosome by tRNA and lined up on the strand of mRNA. (rRNA)

The ribosomes are the site of protein synthesis in the cell. They are made up of 50-60% ribosomal RNA (rRNA), which is combined with proteins. There are two rRNA subunits, one of which is larger than the other. The large RNA subunit acts as an enzyme and catalyses the formation of peptide bondpeptide bond
 The bond formed in a condensation reaction between the amino group of one amino acid and the carboxyl group of another. s between the amino acidamino acid
 The basic building blocks of proteins. There are twenty amino acids used, in different combinations, to make every protein required by the human body. s brought to the ribosome on tRNA and lined up on the strand of mRNA.

The main type of ribosomes in eukaryoticeukaryotic
 Cells that make up animals, plants, fungi and protista. They are three-dimensional, membrane-bound sacs containing cytoplasm, a nucleus and a range of membrane-bound organelles. cells are called 80S ribosomes (S stands for Svedberg, a unit used to measure the rate of sedimentation in a centrifuge). The large subunit is 60S and the smaller one is 40S and the ratio of rRNA:protein in these ribosomes is 1:1.

However, in prokaryoticprokaryotic
 A unicellular organism that lacks a membrane bound nucleus or any other membrane bound organelle.  cells, and in the mitochondriamitochondria
 Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse and chloroplasts of eukaryotic cells, we find 70S ribosomes. These have 50S and 30S subunits, and the ratio of rRNA:protein is around 2:1.

Ribosome

Activity

Make a poster presentation on one area of biochemistry. Work in groups and choose a page from this resource or another area of biochemistry that interests you. Make a poster and present it to your peers as you would at a poster session at a scientific conference.

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