Slides in this set
What is a gene?
· Genes are sections of DNA that contain the coded information for
· The coded information is in the form of a specific sequence of bases
along the DNA molecule.
· Polypeptides combine to make proteins & so genes determine the
proteins of an organism.
· Enzymes are proteins. As enzymes control chemical reactions they
are responsible for an organism's development and activities.
· Genes determine the nature and development of all organisms.
· A gene is a sequence of DNA bases that determines a polypeptide &
a polypeptide is a sequence of amino acids.…read more
The Genetic Code
· The genetic code is the set of rules by which information encoded
within genetic material (DNA) is translated into proteins by living
· The base sequence in a DNA molecule, represented by the letters A
T C G, make up the genetic code.
The genetic 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
· 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 is known as a codon.…read more
Scientists suggested that there must be a minimum of three bases
that coded for each amino acid.
· Only 20 amino acids regularly occur in proteins.
· Each amino acid must have its own code of bases on the DNA.
· Only four different bases are present in DNA.
· If each base coded for a different amino acid, only four different
amino acids could be coded for.
· Using a pair of bases, 16 (42) different codes are possible which is still
inadequate (since there are 20 amino acids).
· Three bases produce 64 (43) different codes, more than enough to
satisfy the requirements of 20 amino acids.
As there are 64 possible codes and only 20 amino acids, it follows
that some amino acids have more than one code. In eukaryotes much
of the nuclear DNA does not code for amino acids. These sections are
called introns and can occur within genes.…read more
Features of the Triplet Code
· The codon is triplet. 61 codons code for amino acids and 3 codons
do not code for any amino acids, hence they function as stop
· The code is known as degenerate code because most amino acids
have more than one triplet code.
· The code is always read in one particular direction.
· The start of a sequence is always the same triplet code AUG known
as the start codon. This codes for the amino acid methionine.
· Three codons do not code for any amino acid. These are called stop
codons (UAA, UAG, & UGA) and mark the end of a polypeptide
· The code is non-overlapping, i.e. each base in the sequence is read
only once. Thus six bases numbered 123456 are read as triplets 123 &
456, rather than as triplets 123, 234, 345, 456.
· The code is almost universal, i.e. with a few minor exceptions it is the…read more