replicating the genome

Question 1

when is DNA synthesised? what synthesises DNA?

Answer 1

during the S-phase of the cell cycle
it is synthesised by replisome which is a complex of proteins that includes the enzyme DNA polymerase that adds nucleotides in the 5’ to 3’ direction to a short RNA primer made by the enzyme DNA primase

Question 2

what is the process of DNA synthesis?

Answer 2

separation of the double strands into single-stranded regions by an unwinding enzyme, the DNA helicase
these two strands will serve as templates for DNA synthesis
one of them is copied continueously –> the leading strand
the other strand –> lagging strand
it is synthesised from multiple primers to generate multiple Okazaki fragments each with its own RNA primer that is degraded then by DNA polymerase
each Okazaki fragments are then joined to the previous one b DNA ligase

Question 3

what are the 2 phases that should be controlled? and what should be ensured that it is done properly?

Answer 3

the 2 phases are the S-phase and M-phase
ensure that the chromosomes are copied EXACTLY ONCE per S-hase
that the M-phase imitates after S-phase completion
and that the S-phase is always preceded by M-phase

Question 4

if the control of the S and M-phase is not done, what could happen to mitotic and meiotic cells?

Answer 4

mitotic cells–> may compromise somatic cell function or survival or lead to mutations that cause uncontrolled cell proliferation and cancer
meiotic cells–> aneuploid gametes leading to sterility, development defects or heritable genetic diseases

Question 5

what cells undergo a different scheme of cell cycle that mitotic cells?

Answer 5

meiotic cells
megakaryocytes
senescent cells

Question 6

how many origins are there in prokaryotes?

Answer 6

1 origin that is called the origin of replication

Question 7

what is the origin recognition complex? what forms the origin recognition complex?

Answer 7

it is a complex of proteins that serve as a “landing pad” for the assembly of proteins required for DNA synthesis
it is composed of 6 protein subunits ORC1 to ORC6

Question 8

what is origin licensing?

Answer 8

during G1, a pre-replicative complex (preRC) formed by ORC and replicative helicases made of 6Mcm proteins, is assembled

Question 9

what does the origin activation requires?

Answer 9

multiple initiation and replication proteins (e.g. DNA polymerase), phosphorylation of man of these proteins that is the result of cyclin dependent kinase (Cdk) activity