Cell Division: S-phase and M-phase

DNA replication

step one of cell division and takes place in the S-phase

Structure of DNA

A (a purine) always bonds with T (a pyrimidine) 

G (a purine) always binds with C (a pyrimidine)

the two strands of DNA are held together by hydrogen bonds between nitrogenous bases 

DNA strands

the two strands run in opposite directions (anti-parallel)

5’ end and 3’ end

5’ end 

displays a phosphate group (attached to the 5th carbon of the sugar molecule) 

3’ end

has an OH (hydroxyl group) attached to the 3rd carbon of the sugar molecule 

DNA can only be built by adding nucleotides to the 3’ end 

DNA polymerase 

the enzyme responsible for building DNA

needs the 3’ OH group to add new nucleotides 

DNA replication

during cell division all the DNA within a cells nucleus must be replicated (S-phase) via semi-conservative replication

basis of DNA replication

the double helix strands of the original DNA strand are separated

each strand is used as a template to build a new DNA strand

this is semi conservative because one original strand is “conserved” in the new double stranded DNA. 

origins of replication 

DNA replication occurs at many regions along DNA simultaneously 

step one of DNA replication

hydrogen bonds are broken by helicase to form a replication fork.

Step two of DNA replication

topoisomerase unwinds DNA before the fork and single stranded binding proteins prevent re-zipping in the strands 

Step three of DNA replication

Primase adds temporary RNA nucleotides to the unzipped DNA (5’ to 3’ direction) 

The temporary RNA nucleotides are a called primer that serve as a starting point to add DNA nucleotides 

DNA nucleotides

added after step 3 of DNA replication

RNA primer provides hydroxy group for new DNA nucleotides to be added 

always added at the 5’ to 3’ end 

RNA polymerase III

the enzyme responsible for the addition of the DNA nucleotides for the new strands

can only add in the 5’3; direction 

leading strand

grows in the direction of the replication fork (5’ to 3’)

lagging strand

built in the direction away from the replication fork 

made in segments 

DNA polymerase III

can add new DNA nucleotides between the primers to generate DNA segments called Okazaki fragments 

how many average mistakes are made when making and exact DNA sequence copy

3

after S phase

cells chromosomes are fully replicated

sister chromatids 

replicated chromosomes 

M Phase

mitosis phase (where one cell becomes 2)

M phase steps

Prophase

prometaphase

metaphase

anaphase

telophase

cytokinesis 

cancer

defined by rapid and uncontrolled cell division

first treatments are chemotherapy and radiation 

however radiation can cause cancer and non cancerous cells are also subject to radiation damage 

chemotherapy

often involved transfusion with a drug that breaks down microtubules

The cell can’t build microtubules therefore it can’t preform cell division