UWCSEA - IB Biology 2023 - D1.1 - DNA replication

D1.1.1— What is DNA replication? Why is it necessary?

DNA replication the copying of DNA;

it is required for reproduction in all organisms;

and for growth;

and tissue replacement in multicellular organisms;

D1.1.2—What does semi-conservative mean in terms of copying DNA? Why does this lead to high levels of accuracy?

Semi-conservative means that one strand of DNA is 'old' and one strand is newly made;

the double helix is opened up and both parent strands are used as templates for copying;

DNA is unwound;

new nucleotides are attached to the template strands;

according to complementary base pairing;

where adenine bonds with thymine and cytosine with guanine;

this means the copy is exact;

allowing a high degree of accuracy in copying base

sequences.

D1.1.3—What is the role of helicase in DNA replication?

helicase is responsible for unwinding of the double helix;

and separation of strands;

by breaking hydrogen bonds between two strands;

D1.1.3 - What is the role of DNA polymerase (III)?

DNA polymerase (III) attaches at the primer:

DNA polymerase III copies DNA;

by adding the new DNA nucleotides; to the template strands;

according to complementary base pairing;

using hydrogen bonds;

adenine pairs with thymine and cytosine pairs with guanine (*NOT A and T and G and C);

D1.1.4—What is the polymerase chain reaction (PCR)? What reactants are required? What are the major steps?

PCR is used to make very large number of copies of selected DNA sequences;

Uses Taq polymerase enzyme; DNA nucleotides, primers and a section of target DNA to copy;

DNA is heated to break hydrogen bonds between strands;

the mixture is cooled;

to allow short strands of DNA, called primers, to anneal (join to) or hydrogen bond to the target sequence;

Taq polymerase copies the strand; by adding in new DNA nucleotides according to complementary base pairing;

the cycle repeats; until there are huge amounts of DNA;

D1.1.4 How does gel electrophoresis work? On what basis are molecules separated?

An electric current runs through a solution;

causing charged molecules to move through a gel;

DNA moves to the positive terminal, as it is negatively charged;

molecules are separated based on size;

the larger the molecule, the slower it moves;

the smaller, the faster and the further it moves through the gel;

D1.1.5—What are the applications of polymerase chain reaction and gel electrophoresis

for e.g. DNA profiling for paternity case (to work out who the father is);

if there are bands that match between parent and child, they are related;

genetic engineering;

crime scene analysis;

where all bands must match;

Nature of Science: Reliability is enhanced by increasing the number of measurements in an experiment or test;

In DNA profiling, increasing the number of markers used reduces the probability of a false match;

AHL Only - D1.1.6— How are polymerases directional?

DNA polymerases always add new nucleotides to the 3' end of a of a DNA strand;

the 3' end is the carbon on the bottom left of the sugar deoxyribose;

this is where a new sugar-phosphate bond is made;

AHL Only - D1.1.7—What is the leading strand? What is the lagging strand?

Because new DNA nucleotides can only be added at the 3' end of a DNA strand;

one strand can be synthesised continuously;

this is called the leading strand;

DNA polymerase binds at an RNA primer;

(and keeps going until the DNA is copied;)

whereas the other strand, the lagging strand,

DNA is synthesised discontinuously (not in one go);

in small sections called "Okazaki fragments";

each section requires a new RNA primer;

AHL Only - D1.1.8—What are the functions of DNA primase, DNA polymerase I, DNA polymerase III and DNA ligase in replication?

DNA primase adds an RNA primer on parental DNA (both leading and lagging strands);

Which allows attachment of DNA polymerase III at the primer:

DNA polymerase III copies DNA in a 5´ (prime) → 3´(prime) direction (both leading and lagging);

by adding the new DNA nucleotides to the 3' end;

of the new strand;

through hydrogen bonding;

according to complementary base pairing; to the template strand;

adenine pairs with thymine and cytosine pairs with guanine (*NOT A and T and G and C);

DNA polymerase I removes the RNA primers on the lagging strand;

and replaces them with DNA;

DNA ligase joins Okazaki fragments;

by forming covalent sugar-phosphate bonds;

AHL Only - D1.1.9—DNA proofreading

DNA Polymerase III also proof-reads to check for errors;

removing any nucleotide from the 3' terminal with a

mismatched base;

followed by replacement with a correctly matched nucleotide;