Chapter 14.1 Structure of ribonucleic acid
The genetic code
The sequence of nucleotides in DNA forms a code that determines the sequence of amino acids in
the proteins of an organism. In eukaryotes, the DNA is confined to the nucleus. However, the
synthesis of proteins happens in the cytoplasm. So how is the code on the DNA in the nucleus
transferred to the cytoplasm? Sections of the DNA code are transcribed onto a single-stranded
molecule called ribonucleic acid (RNA).
There are many types of RNA. The one that transfers the DNA code from the nucleus to the
cytoplasm acts as a messenger and is given the name messenger RNA (mRNA for short). This mRNA
is small enough to leave the nucleus (through nuclear pores) and to enter the cytoplasm, where the
code it contains is used to determine the sequence of amino acids in the proteins that are
synthesised there.
It is the sequence of nucleotide bases on mRNA that is referred to as the genetic code. Although the
mRNA code is derived from the DNA code, it is not identical to it. The mRNA code is complementary
to the DNA code. The term codon refers to the sequence of three bases on mRNA that code for a
single amino acid.
The main features of the genetic code are:
Each amino acid is coded for by a codon
A few amino acids have only one codon
The code is a degenerate code. This means that most amino acids have more than one
codon. For example, leucine has six different codons
Three codons do not code for any amino acid, they are stop codons and mark the end of the
polypeptide chain
The code is non-overlapping, each base in a sequence is read only once (6 bases numbered
123456 are read as codons 123 and 456 rather than as codons 123, 234, 345 and 456)
It is a universal code, the same codon codes for the same amino acid in all organisms (with a
few very minor exceptions)
DNA is made of two nucleotide chains wound around each other, forming a double helix. The
structure of RNA is different.
Structure of RNA
RNA is a polymer made of repeating mononucleotide sub-units. It forms a single strand in which
each nucleotide is made up of:
The pentose sugar, ribose
One of the organic bases (Adenine, Guanine, Cytosine and Uracil)
A phosphate group
The two types of RNA that are important in protein synthesis are:
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Messenger RNA (mRNA)
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The genetic code
The sequence of nucleotides in DNA forms a code that determines the sequence of amino acids in
the proteins of an organism. In eukaryotes, the DNA is confined to the nucleus. However, the
synthesis of proteins happens in the cytoplasm. So how is the code on the DNA in the nucleus
transferred to the cytoplasm? Sections of the DNA code are transcribed onto a single-stranded
molecule called ribonucleic acid (RNA).
There are many types of RNA. The one that transfers the DNA code from the nucleus to the
cytoplasm acts as a messenger and is given the name messenger RNA (mRNA for short). This mRNA
is small enough to leave the nucleus (through nuclear pores) and to enter the cytoplasm, where the
code it contains is used to determine the sequence of amino acids in the proteins that are
synthesised there.
It is the sequence of nucleotide bases on mRNA that is referred to as the genetic code. Although the
mRNA code is derived from the DNA code, it is not identical to it. The mRNA code is complementary
to the DNA code. The term codon refers to the sequence of three bases on mRNA that code for a
single amino acid.
The main features of the genetic code are:
Each amino acid is coded for by a codon
A few amino acids have only one codon
The code is a degenerate code. This means that most amino acids have more than one
codon. For example, leucine has six different codons
Three codons do not code for any amino acid, they are stop codons and mark the end of the
polypeptide chain
The code is non-overlapping, each base in a sequence is read only once (6 bases numbered
123456 are read as codons 123 and 456 rather than as codons 123, 234, 345 and 456)
It is a universal code, the same codon codes for the same amino acid in all organisms (with a
few very minor exceptions)
DNA is made of two nucleotide chains wound around each other, forming a double helix. The
structure of RNA is different.
Structure of RNA
RNA is a polymer made of repeating mononucleotide sub-units. It forms a single strand in which
each nucleotide is made up of:
The pentose sugar, ribose
One of the organic bases (Adenine, Guanine, Cytosine and Uracil)
A phosphate group
The two types of RNA that are important in protein synthesis are:
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Messenger RNA (mRNA)
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