Biological Molecules: Nucleic Acids - DNA Replication

Question 1

Why does DNA replicate?

Answer 1

• DNA copies itself before cell division so that each new cell has the full amount of DNA
• This method is called semi-conservative replication
• Half of the strands in each new DNA molecule are from the original DNA molecule
• Semi-conservative replication of DNA ensures genetic continuity between generations of cells

Question 2

Process of DNA Replication

Answer 2

• Helix unwinds to form 2 strands
• The enzyme DNA helicase breaks the hydrogen bonds between the two polynucleotide strands (bases)
• Each original strand acts as a template for the synthesis of a new strand
• Complementary base pairing means that free floating DNA nucleotides are attracted to to their complementary exposed base
• DNA polymerase catalyses the condensation reactions between the sugar and phosphate groups of adjacent nucleotides
• Hydrogen bonds reform between bases on the old and new strand
• Each new DNA molecule has an original DNA (parental) strand and a new one
• Hence, DNA replicates semi-conservatively

Question 3

The Action of DNA Polymerase

Answer 3

• Each end of a DNA strand is slightly different in its structure
• One end is called the 3’ end and one end is called the 5’ end
• During DNA replication the active site of DNA polymerase is only complementary to the 3’ end of the newly forming DNA strand
• So enzyme can only add new nucleotides at the 3’ end
• New strand is made in a 5’ to 3’ direction
• DNA polymerase moves down template strand in a 3’ to 5’ direction
• Because each strand is antiparallel, two DNA polymerase molecules working on two different template strands move in the opposite directions

Question 4

Meselson and Stahl’s Experiment: Background Information

Answer 4

• DNA bases are nitrogenous (they contain nitrogen)
• Nitrogen has two isotopes:
• Lighter nitrogen 14N
• Heavy nitrogen 15N
• Bacteria take in the nitrogen from their growth medium into the new DNA they will make

Question 5

Meselson and Stahl’s Experiment: Different Bacteria Samples & Centrifuge Results

Answer 5

• Bacteria grown in a medium containing 14N will only contains this isotope and be lighter
• Bacteria grown in a medium containing 15N will only contains this isotope and be heavier
• DNA samples can be spun in a centrifuge to confirm density
• 14N DNA will settle higher up as it is less dense
• 15N DNA will settle lower down as it is more dense

Question 6

Meselson and Stahl’s Experiment: Process

Answer 6

• Two samples of bacteria were grown for many generations - one in heavy nitrogen, one in light nitrogen
• Sample of DNA was taken from each batch of bacteria and spun in a centrifuge
• Bacteria grown in heavy nitrogen broth was transferred to a broth containing only light nitrogen
• Bacteria were left for one round of DNA replication - another DNA sample taken out and spun in centrifuge

Question 7

Meselson and Stahl’s Experiment: Evidence for Semi-Conservative Replication

Answer 7

• If replication was conservative

• The original heavy DNA, would still be together and would settle at the bottom
• The new light DNA would settle at the top

• If replication was semi-conservative

• The new bacterial DNA molecules would contain one strand of new DNA with light nitrogen, and one old strand of DNA with heavy nitrogen
• DNA would settle out in the middle between where the heavy and light nitrogen settled out

• Actual observations

• DNA settled out in the middle
• DNA contained mixture of heavy and light nitrogen
• Bacteria had replicated semi-conservatively

Question 8

Watson & Crick

Answer 8

• The double-helical structure of DNA was first proposed by Watson and Crick
• Watson and Crick also proposed the semi-conservative theory and Meselson and Stahl’s experiment confirmed this