CELLULAR CONTROL
(a) Types of gene mutations and their possible effects on protein production and function.
● To include substitution, insertion or deletion of one or more nucleotides
● the possible effects of these gene mutations (i.e. beneficial, neutral or harmful).
CAUSES
A mutation is a random change to the genetic material, either at the level of the DNA molecule, or
pertaining to the structure of number of chromosomes.
Physical mutagens, including ionising radiation such as X-rays, can break the phosphodiester
backbone of the DNA molecule. Some of these can be repaired, but mutations may occur in the
process.
Chemical mutagens, such as deaminating agents, chemically alter the bases in DNA, such as
converting cytosine to uracil in mRNA, changing the base sequence.
Biological mutagens come in three main form. Viruses can insert viral DNA into the genome,
changing the base sequence. Base analogues can be incorporated into DNA in place of the usual
base during replication, changing the base sequence. And alkylating agents methyl or ethyl are
attracted to bases resulting in incorrect pairing of bases during replication.
There may be no specific cause, however. Depurination or depyrimidination can occur
simultaneously, and the absence of a base can lead to the substitution of an incorrect base
through complementary base pairing during DNA replication. Also, free radicals can affect the
structures of nucleotides.
TYPES
A change in the sequence of DNA bases can occur through a substitution, deletion, or insertion.
Point mutations occur where one nucleotide is substituted for another. As the genetic code is
degenerate—that is, more than one codon codes for each amino acid—the mutation may be
‘silent’ causing no change in the primary structure of the protein, and thus no change in its
function. However, missense mutations occur where the altered codon codes for a different
amino acid, changing the primary structure of the protein, leading to further changes in the
secondary, and tertiary structure, preventing it from carrying out its usual function. An example
is sickle cell anaemia. If the altered codon codes for a stop codon, the protein’s primary
structure is truncated and the protein will not function—Duchenne muscular dystrophy is caused
by this.
Indel mutations cause a frameshift, whereby, because the genetic code is non-overlapping, all
subsequent codons are altered, drastically changing the primary structure of the protein when it
is translated. The protein cannot carry out its function, and is rapidly degraded in the cell.
Thalassaemia, a haemoglobin disorder, is caused by a frameshift as a result of a deletion.
However, insertions and deletions of a codon will result in the addition or loss on only one amino
acid, not causing a frameshift. Another type of insertion, that causes Huntington's disease are
expanding triplet nucleotide repeats, whereby the number of of repeating codons increases
during meiosis.
(b) The regulatory mechanisms that control gene expression at the transcriptional level, post-
transcriptional level and post-translational level.
● Transcriptional level:
