DNA Replication and the Genetic Code

Question 1

What is DNA replication?

Answer 1

• two strands of DNA double helix separate and each strand serves as template for new DNA molecule
• complementary base pairing means the two new strands are identical to the original

Question 2

What is semi-conservative replication?

Answer 2

• two new molecules of DNA are produced. Each one consists of one old strand and one new strand

Question 3

What is the process of DNA replication?

Answer 3

1. an enzyme, DNA helicase, causes the two strands of DNA to separate
2. Meanwhile, free nucleotides that have been activated are attracted to their complementary bases
3. Once the activated nucleotides are lined up, they are joined by DNA polymerase.
4. All the nucleotides are joined to form complete polynucleotide chain.

Question 4

What is the role of DNA helicase?

Answer 4

• travels along DNA backbone, catalysing the reaction that breaks the hydrogen bonds between the bases.
• separates/unwinds the two strands of DNA

Question 5

What is the role of DNA polymerase?

Answer 5

• catalyses the formation of phosphodiester bonds between nucleotides

Question 6

What is continuous replication?

Answer 6

• DNA polymerase moves along template strand in same direction. It can bind to 3’ so travels in direction of 3’ to 5’.
• as DNA unwinds in one direction, DNA polymerase has to replicate each of the template strands in opposite directions
• the strand that is unzipped from 3’ end can be continuously replicated as strands unzip.
• called leading strand and said to undergo continuous replication

Question 7

What is discontinuous replication?

Answer 7

• other strand unzipped from 5’ end, so DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand.
• results in DNA being produced in sections (Okazaki fragments) which are then joined by DNA ligase
• this is called the lagging strand and undergoes discontinuous replication

Question 8

What is the difference between continuous and discontinuous replication?

Answer 8

• continuous replication: DNA polymerase binds to end of strand and free DNA nucleotides are added without any breaks
• discontinuous replication: DNA polymerase can’t bind to end of the strand so free DNA nucleotides added in sections.

Question 9

Why does DNA polymerase not catalyse the joining of the Okazaki fragments into a single strand but a different enzyme is used?

Answer 9

• enzymes are substrate specific
• DNA polymerase catalyses joining of nucleotides
• nucleotides have different shape to Okazaki fragments

Question 10

What is a mutation?

Answer 10

• change in genetic material which may affect phenotype of an organism.
• leads to change in sequence of bases

Question 11

What is the genetic code?

Answer 11

• sequences of bases in DNA that are ‘instructions’ for sequences of amino acids in the production of proteins

Question 12

What is a gene?

Answer 12

• a section of DNA that contains complete sequences of bases to code for a protein

Question 13

Why is the genetic code ‘universal’?

Answer 13

• all organisms use same code, although sequences of bases coding for each protein may be different

Question 14

What is a codon?

Answer 14

• a 3 base sequence of DNA or RNA that codes for 1 amino acid

Question 15

Why is the genetic code described as non-overlapping?

Answer 15

• single codon signals start of sequence so DNA is read from base 1 snd not 2 or 3 and also stops at last base.

Question 16

How is the genetic code degenerate?

Answer 16

• 20 different amino acids
• more codons than amino acids
• many amino acids can be coded for by more than 1 codon

Question 17

What is meant by the triplet code?

Answer 17

• triplet code is particular sequence of 3 bases that codes for specific amino acid

Question 18

How may a genetic mutation result in an enzyme becoming non-functional?

Answer 18

• mutation in DNA changes triplet code
• means there are different amino acids in the protein/enzyme that the DNA codes for
• may change structure of active site so substrate can’t bind making enzyme non-functional

Question 19

Why are there more likely to be more differences, overall, between base sequences of DNA than between amino acid sequences of proteins?

Answer 19

• triplet code is degenerative
• 64 different codons but only 20 amino acids
• therefore amino acid can be coded by more than one codon, so more opportunity for differences in DNA sequence than amino acid sequence