3.4 & 7.2 DNA Replication
Pp 16 – 19 & 60 - 61
Pp 58 – 60 & 197 -200
How it works Prior to division: cell has to
duplicate its DNA DNA replication occurs during S
phase of interphase in preparation for mitosis cell division
DNA replication is semi – conservative i.e. 50% of daughter DNA molecule is new strand while 50% is old (original) strand
Helicase splits double strand DNA polymerase binds
nucleotides to create strand 2 new strands are formed Strands are identical to original
due to complementary base pairing
significance of complementary base pairing Adenine forms
hydrogen bonds with Thymine and Guanine form hydrogen bonds with Cytosine
complementary base pairing ensures proper bases are incorporated into the new DNA strand
This ensures conservation of the base sequence by making identical copies of the DNA strand
Semi-conservative DNA Replication
Direction of DNA ReplicationDNA replication occurs in a 5’ to 3’ direction!
Enzymes drive replicationHelicase:- uncoils DNA & splits
the complimentary strandsDNA polymerase III:- adds
nucleotides in 5’ → 3’ direction (leading strand) – movement towards helicase (replication fork)
RNA primase:- adds short RNA primer strand to DNA (primer = allows DNA polymerase to start on lagging strand)
DNA polymerase I:- removes RNA primer & replaces it with DNA leaving gaps between Okazaki fragments (i.e. short lengths of DNA formed between RNA primers)
DNA ligase:- seals up gap (creates sugar-P bond) between Okazaki fragments
Okazaki fragmentsIn eukaryotes,
replication is initiated at many points concurrently.
On lagging strand, DNA replication occurs away from the replication fork, forming Okazaki fragments (i.e. short lengths of DNA formed between RNA primers)
Process of DNA replication DNA replication occurs during S
phase of interphase in preparation cell division
DNA replication is semi-conservative i.e. the new DNA molecule has one new strand & one old strand
Enzyme helicase unwinds the double helix & separates the strands by breaking the hydrogen bonds between the two complimentary strands
each strand of parent DNA is used as template for synthesis of a new strand
synthesis of a new strand is continuous on leading strand but not continuous on lagging strand
Okazaki fragments (small strands of DNA) are formed on lagging strand
synthesis of new DNA strand occurs in 5’ → 3’ direction i.e. new nucleotides are added to 3’ of the growing strand
RNA primer synthesized on parent DNA using RNA primase to act as the starting point of synthesing a new strand
DNA polymerase III adds the nucleotides to the 3 end according to complementary base pairing
adenine pairs with thymine and cytosine pairs with guanine
DNA polymerase I removes the RNA primers and replaces them with DNA
DNA ligase joins Okazaki fragments
as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates are broken off releasing energy required for bond formation
Revision QuestionsExplain the significance
of complementary base pairing during DNA replication. [3]
State a role for each of five different named enzymes in DNA replication. [5]
Explain the process of DNA replication. [8]