3.4.1 DNA, genes and chromosomes

Question 1

what is a gene?

Answer 1

a base sequence of DNA that codes for the amino acid sequence of a polypeptide and a functional RNA

Question 2

what is a locus?

Answer 2

the fixed position of a gene on a particular DNA molecule

Question 3

what is an allele?

Answer 3

one of a number of alternative forms of a gene

Question 4

what is meant by a homologous pair of chromosomes?

Answer 4

they have exactly the same genes but might have different alleles

Question 5

how is DNA stored in eukaryotes?

Answer 5

• stored as chromosomes inside the nucleus
• chromosomes are linear in shape
• DNA is tightly wound around proteins called histones
• this complex of DNA wrapped around a histone is called a nucleosome

Question 6

how is DNA stored in prokaryotes?

Answer 6

• DNA molecules are shorter and circular
• DNA is not associated with a protein
• instead, DNA is supercoiled to fit in the cell

Question 7

how is DNA in mitochondria and chloroplasts stored?

Answer 7

• similar to the DNA of prokaryotes
• short, circular and not associated with proteins

Question 8

what is a start codon?

Answer 8

the three bases (triplet) at the start of every gene which initiates translation

Question 9

what is a stop codon?

Answer 9

• the three bases (triplet) at the end of every gene that do not code for an amino acid
• the stop codons mare the end of a polypeptide chain and cause ribosomes to detach
• this therefore stops translation

Question 10

what are the three features of the genetic code?

Answer 10

• degenerate
• universal
• non-overlapping

Question 11

what is meant by the genetic code being ‘degenerate’?

Answer 11

most amino acids are coded for by more than one triplet of bases

Question 12

what is the advantage of the genetic code being degenerate?

Answer 12

if a point mutation (substitution) occurs, the triplet of bases will be different but it may still code for the same amino acid and therefore will have no effect

Question 13

what is meant by the genetic code being ‘universal’?

Answer 13

the same triplet of bases codes for the same amino acid in all organisms

Question 14

what is the advantage of the genetic code being universal?

Answer 14

it means that genetic engineering is possible (e.g., inserting the human gene for insulin into bacteria)

Question 15

what is meant by the genetic code being ‘non-overlapping’?

Answer 15

• each base in a gene is only part of one triplet of bases that codes for one amino acid
• therefore each codon, or triplet of bases, is read as a discrete unit

Question 16

what is the advantage of the genetic code being non-overlapping?

Answer 16

if a point mutation occurs, it will only affect one codon and therefore one amino acid

Question 17

what are introns?

Answer 17

• sections of DNA that do not code for amino acids and therefore polypeptide chains
• are found in eukaryotic DNA but not prokaryotic NDA
• these get spliced (removed) out of mRNA molecules

Question 18

what are exons?

Answer 18

sections of DNA that do code for amino acids