2.3.2 How DNA replicates

2.3.2 How DNA replicates

1. DNA is a self-replicating molecule

   • All DNA within a cell (genome), carries coded instructions to make and maintain the organism
   • Every time a cell divides, each DNA molecule must be copied so each new daughter cell receives a full set of instructions
   • The replication takes place during interphase, before the cell divides
     • In eukaryotes it results in each chromosome (molecule of DNA) having an identical copy
       • At first they join at the centromere and form 2 sister chromatids
   • DNA within mitochondria and chloroplasts also replicate before division, each time they divide

2. Semi-conservative replication

   • To make a new copy of itself, each DNA molecule:
     • Unwinds - the double helix is untwisted bit by bit, catalysed by gyrase
     • Unzips - H bonds between nucleotide bases break, catalysed by DNA helices, results in 2 single DNA strands with exposed nucleotide bases
   • Next:
     • Free phosphorylated nucleotides, present in nucleoplasm, bond to exposed bases following complementary base pairing rules
     • DNA polymerase catalyses the addition of new nucleotide bases, in the 5 to 3 direction, to single strands of DNA
       • It uses each single strand of unzipped DNA as a template
     • The leading strand is synthesised continuously whereas the lagging strand is in fragments that are joined later by ligase
     • Hydrolysis of activated nucleotides, to release extra phosphate groups, supplies energy to make phosphodiester bonds between sugar residue of 1 nucleotide and the phosphate group of the next
   • The product of replication is 2 DNA molecules identical to each other and the parent molecule
     • Each molecule contains an old strand and a new one = semi-conservative replication
   • The loops of DNA in prokaryotes, mitochondria and chloroplasts also replicate semi-conservatively
     • A bubble sprouts from the loop and unwinds and unzips, complementary nucleotides join to exposed nucleotides and eventually the whole loop is copied

3. Did you know?

   • In the 1950s scientists knew DNA was a self-replicating molecule but didn't know how. There were 3 theories:
     • Conservative- the original molecule acts as a template and a new molecule is made
     • Dispersive- the original molecule breaks up into nucleotides, each one joins to complementary nucleotide and new ones join up again
     • Semi-conservative: the new molecule consists of 1 original strand and 1 newly formed strand
     • 2 scientists carried out an experiment in 1958 which showed DNA replication is semi-conservative:
       • They grew bacteria, E. coli, for 14 generations in a medium with the heavy 15-N isotope. This contains an extra neutron in every atomic nucleus. After 14 generations, most DNA in the bacteria would be heavy as they contain 15-N
       • They then transferred some of these bacteria into a medium with normal 14-N and left to undergo one replication
       • The DNA from the bacteria after 1 division was found to be hybrid DNA which shows DNA doesn't replicate conservatively as it should have 2 bands of DNA, one heavy and one light
       • The bacteria were allowed to divide one more time and their DNA was extracted and centrifuged. It produced 2 bands of DNA, 1 hybrid and 1 light, showing replication is semi-conservative and not dispersive

4. Mutations

   • During DNA replications, errors can occur and wrong nucleotides can be inserted
     • Could change genetic code= point mutation
   • During replication there are enzymes that can proofread and edit out incorrect nucleotides reducing rate of mutations
   • Many genes have changes to their nucleotide sequence = alleles = different versions of the same gene
   • Not all mutations are harmful, some can be advantageous e.g. a white coat in animals during winter

5. Did you know?

   • A useful mutation arose in FOXP2 gene allowing humans to speak
     • The gene encodes a protein called forehead box that is involved in human speech and language

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