8.1 DNA, genes and the genetic code

Question 1

What is a gene?

Answer 1

• A section of DNA which contains the coded information for making polypeptides and functional RNA.
• It is a specific sequence of bases along the DNA molecule of an organism.
• it is a section of DNA located at a particular position, called a locus

Question 2

What do genes code for?

Answer 2

• the amino acid sequence of a polypeptide (and therefore enzymes)
• functional RNA, including rRNA and tRNA

Question 3

Why did scientists decide there must be at least 3 bases coding for one amino acid?

Answer 3

• only 20 amino acids regularly occur in proteins
• each amino acid must have its own code of bases on the DNA
• only 4 different bases are present in DNA
• if each base coded for an amino acid, there would only be 4 amino acids
• using 4 bases is inadequate as only 16 amino acids would be coded for
• three bases produces 64 possible codes (as there is more than enough, it follows that some amino acids are coded for by more than one triplet)

Question 4

What features were revealed about the genetic code?

Answer 4

• a few amino acids are coded for by only one triplet, the remaining are coded for by 2-6 triplets each
• this code is known as a ‘degenerate code’ because most amino acids are coded for by more than one triplet
• a triplet is only read in one direction
• the start of a polypeptide DNA sequence is always the same sequence coding for methionine. if it doesn’t form part of the polypeptide it is later removed
• three triplets do not code for any amino acid. they are ‘stop codes’ and mark the end of a sequence
• the code is non-overlapping (each base is only read once)
• the code is universal; a triplet codes for the same amino acid in most organisms

Question 5

Do all genes in eukaryotes code for polypeptides?

Answer 5

No.

• only certain sequences code for amino acids, these are called exons.
• these are separated by non-coding sequences called introns
• some genes code for rRNA and tRNA