Genetics 13
HC 10
13.1
Proteins are essential for cell structure and function; the structural genes (or protein-
encoding genes) are genetic material which encode the polypeptide chain and regulate in
which cells, when and in which amount protein/polypeptides are produced. These structural
genes are first transcribed into mRNA, which forms the translation template; translation is
the process in which mRNA coding sequences are converted into an amino acid chain that
makes up a polypeptide. Usually, many polypeptide chains add up to make one protein.
Note that within the mRNA, there is a starting sequence (start codon) that determines the
reading frame for the remaining codons.
Archibald Garrod conducted an experiment from which he concluded that there must be a
connection between a gene and the production of an enzyme; if the enzyme would be
produced incorrectly, there is an inborn error of metabolism, being in the genes. Beadle and
Tatum later as well conducted another experiment, from which the one-gene/one-enzyme
hypothesis was concluded; this nowadays has been modified in several ways:
- Enzymes are a category of proteins, not all proteins are enzymes.
- Some proteins exist out of more polypeptides, so it is not just a single polypeptide or
in that sense, a single gene.
- Many genes do not code for proteins, some for example code for special RNA
functions.
- One gene can encode different variants of polypeptides due to alternative splicing
and RNA editing.
13.2
Polypeptide chains are made from mRNA coding via the genetic code. The sequence of
bases code for specific amino acids; every three bases, one codon, is one amino acid (sense
codons, like the start codon). Nonsense or termination codons are those that code for a
stop codon. Moreover, there are also anticodons: three nucleotide sequences that are
complementary and antiparallel to codons in the mRNA. They are supplied by tRNA
molecules (if they carry an amino acid, they are said to be charged), which as well connect
an amino acid to the right anticodon (which thus forms a bridge between a codon and an
amino acid). Besides, the genetic code is degenerate; more than one codon can specify an
amino acid, they are called synonymous codons. This is especially the case for the third base
in a codon, the wobble base; this enables the toleration of silent mutations. The code is
nearly universal, aside from a few exceptions: mitochondrial genetics code is slightly
different and sometimes there are ‘new’ enzymes coded, which should actually have been
stop codons.
A polypeptide has directionality parallel to the codons of the mRNA. The covalent bonds
between peptides are peptide bonds; carboxyl groups are added to amino acid ends. The
first amino acid has a free amino group (N-terminal), whilst the last has a free carboxyl
group (C-terminal). Each amino acid has the same basis, but a different side chain or ‘R-
group’, which gives chemical distinct properties and influences the overall structure of the
chain/protein. Nonpolar amino acids are hydrophobic, so found on the inside of a protein,
HC 10
13.1
Proteins are essential for cell structure and function; the structural genes (or protein-
encoding genes) are genetic material which encode the polypeptide chain and regulate in
which cells, when and in which amount protein/polypeptides are produced. These structural
genes are first transcribed into mRNA, which forms the translation template; translation is
the process in which mRNA coding sequences are converted into an amino acid chain that
makes up a polypeptide. Usually, many polypeptide chains add up to make one protein.
Note that within the mRNA, there is a starting sequence (start codon) that determines the
reading frame for the remaining codons.
Archibald Garrod conducted an experiment from which he concluded that there must be a
connection between a gene and the production of an enzyme; if the enzyme would be
produced incorrectly, there is an inborn error of metabolism, being in the genes. Beadle and
Tatum later as well conducted another experiment, from which the one-gene/one-enzyme
hypothesis was concluded; this nowadays has been modified in several ways:
- Enzymes are a category of proteins, not all proteins are enzymes.
- Some proteins exist out of more polypeptides, so it is not just a single polypeptide or
in that sense, a single gene.
- Many genes do not code for proteins, some for example code for special RNA
functions.
- One gene can encode different variants of polypeptides due to alternative splicing
and RNA editing.
13.2
Polypeptide chains are made from mRNA coding via the genetic code. The sequence of
bases code for specific amino acids; every three bases, one codon, is one amino acid (sense
codons, like the start codon). Nonsense or termination codons are those that code for a
stop codon. Moreover, there are also anticodons: three nucleotide sequences that are
complementary and antiparallel to codons in the mRNA. They are supplied by tRNA
molecules (if they carry an amino acid, they are said to be charged), which as well connect
an amino acid to the right anticodon (which thus forms a bridge between a codon and an
amino acid). Besides, the genetic code is degenerate; more than one codon can specify an
amino acid, they are called synonymous codons. This is especially the case for the third base
in a codon, the wobble base; this enables the toleration of silent mutations. The code is
nearly universal, aside from a few exceptions: mitochondrial genetics code is slightly
different and sometimes there are ‘new’ enzymes coded, which should actually have been
stop codons.
A polypeptide has directionality parallel to the codons of the mRNA. The covalent bonds
between peptides are peptide bonds; carboxyl groups are added to amino acid ends. The
first amino acid has a free amino group (N-terminal), whilst the last has a free carboxyl
group (C-terminal). Each amino acid has the same basis, but a different side chain or ‘R-
group’, which gives chemical distinct properties and influences the overall structure of the
chain/protein. Nonpolar amino acids are hydrophobic, so found on the inside of a protein,
