8.1 - Genes and the genetic code

Question 1

what is a gene

Answer 1

• section of DNA that contains the coded info for making polypeptides and functional RNA
• the info is in the form of a specific sequence of bases along the DNA molecule

Question 2

how do genes determine the proteins of an organism

Answer 2

• the genes contain the coded info for creating polypeptides
• these polypeptides then make up the proteins
  –> enzymes are also a protein, so are coded for by genes

Question 3

where is the gene located on the DNA

Answer 3

• a particular section of DNA called a: locus

Question 4

what does genes code for

Answer 4

• contains a base sequence of DNA that codes for:
  1. the amino acid sequence of a polypeptide
  2. a functional RNA, including ribosomal RNA and transfer RNAs

Question 5

are there only 1 gene for each DNA molecule?

Answer 5

• no
• one DNA molecule carries many genes

Question 6

in trying to discover how DNA bases coded for amino acids, what did scientists suggest

Answer 6

there must be a minimum of 3 bases that coded for each amino acid

Question 7

what was the reasoning behind the scientists suggesting ‘there must be a minimum of 3 bases that coded for each amino acid’

Answer 7

• only 20 different amino acids regularly occur in proteins
• each amino acid must have its own code of bases on the DNA
• only 4 different bases (ATGC) are present in DNA
• If each base coded for a different amino acid, only 4 different amino acids could be coded for
• Using a pair of bases 16 different codes are possible, which is still inadequate
• 3 bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids

Question 8

how many bases does each code have for each amino acid

Answer 8

3

Question 9

what are each of the bases called in a code for amino acids

Answer 9

a triplet

Question 10

can some amino acids be coded for by more than one triplet?

Answer 10

yes

Question 11

how many possible triplets are there + amino acids

Answer 11

• 64 triplets
• 20 amino acids

Question 12

what did further experiments reveal about the features of genetic code (8)

Answer 12

• a few amino acids are coded for by only a single triplet
• the remaining amino acids are coded for by between 2 and 6 triplets each
• the code is known as a ‘degenerate code’ because most amino acids are coded for by more than 1 triplet
• A triplet is always read in one particular direction along the DNA strand
• the start of a DNA sequence that codes for a polypeptide is always the same triplet. This codes for the amino acid methionine. If this first methionine molecule doesn’t form part of the final polypeptide, its later removed
• 3 triplets don’t code for any amino acid, these are called ‘stop codes’ and mark the end of a polypeptide chain. They act in much the same way a s a full stop at the end of a sentence
• the code is non-overlapping (each base in the sequence is read only once, thus 6 bases numbered 123456 are read as 123 and 456)
• the code is universal, with a few minor exceptions each triplet codes for the same amino acid in all organisms. This is indirect evidence for evolution

Question 13

what are exons

Answer 13

A sequence of DNA that codes for an amino acid sequence

Question 14

what are introns

Answer 14

• non-coding regions of the gene that do not contain codons needed to make the final protein
• found between exons

Question 15

what is a codon

Answer 15

set of 3 nucleotides in mRNA