cell division and DNA replication

DNA replication

happens in DNA synthesis or S phase in cell cycle

DNA replication fork

initiates at a specific location, called origin of replication (ori). enzyme helicase binds and moves along DNA unwinding and separating. this forms a Y structure. two forks formed. site of DNA replication.

prone to hairpin loops, so to prevent this, SSB binds to single stranded DNA to inhibit rebinding. exposed single stranded DNA can act as template for synthesis of complementary daughter cells

leading strand

replication is continuous and fast 5’ to 3’

lagging strand

discontinuous and relatively slower, starts a bit later. from 3’ to 5’. DNA polymerase cannot synthesise from 3’ to 5’ so DNA synthesis is carried out discontinuously from 5’ to 3’. DNA primase will synthesise multiple RNA primers as DNA unwinds. DNA polymerase will synthesise from primer to primer 5’ to 3’.

cycle is condinued on lagging strand. short fragments are called Okazaki fragments. enzyme RNase H removes the primers. another DNA polymerase will fill the space replace the removed primers. DNA polymerase cannot fill the nicks between the Okazaki fragments. joined together by DNA ligase to form discontinuous strand into continuous strand

replication in prokaryotes

replication of the circular double stranded genome begins at the origin called OriC. initiator proteins bind to Ori and initiate DNA separation. helicase is bound to each strand to form a replication bubble. unwound DNA is stabilised by SSB. the replication bubble has two replication forks on each side. DNA polymerase simulataneously synthesises leading strand and lagging strand. as unwinding continues, torsional strain happens. so type 1 topoisomerase creates a nick to allow DNA polymerase to continue. stops at the termination site. one parental strand and one newly synthesised strand, following semi-conservative replication

replication in eukaryotes

happens at multiple origins of replication bound by ORC. recruits helicase to unwind DNA, creating a replication bubble with two forks. forks move in opposite directions and disrupt nucleosomes ahead of them. nucleosomes are reassembled on the daughter strand maintaining the chromatin structure. at each fork, RNA primers provide the site for DNA polymerase to elongate the leading strand and the Okazaki fragmetns of the lagging strand. RNase enzyme removes the primers and DNA polymerase removes the primers and DNA polymerase fills the gaps. The DNA ligase seals the fragments together. as last primer is removed the lagging strand at the end of the linear chromosome, produces an overhanging stretch of the template DNA. enzyme called telomerase extends this overhanging stretch with non-coding DNA to prevent the loss of coding DNA during subsequent replication cycles. replication continues until the replication bubbles merge and the whole chromosome is duplicated

division in prokaryotes

binary fission

binary fission

division in eukaryotes

mitosis

mitosis steps

prophase, prometaphase, metaphase, anaphase, telophase

prophase

nucleic chromtid condenses into x shaped chromosomes composed of sister chromatids, attached at centromere junctions. centrosomes migrate to opposite cell sides. microtubule rods begin to grow from each, either to interior or exterior, creating weblike spindle apparatus

prometaphase

nuclear envelope dissolves exposing chromosomes to cells other content. protein structure appear on both sides of the centromere, one for every chromatid. once these kinetochores form, microtubules extend and fasten to them, sister chromatid begin tethered to a different cell pole

metaphase

spindle apparatus rearranges the chromosomes, so that they are similarly oriented in a fixed row along the cell’s equator

anaphase

kinetochore fixed microtubules fix and sister chromatids, indivually referred to as chromosomes are dragged apart. elongate the cell

telophase

chromosomes land at opposite cell sides, two spindle apparatus disbands. genetic material loosens and two nuclear envelopes one around each chromosome arises. cytokinesis cell is cytoplasmically divided.

gap phases

Ensure DNA is intact and ready for next steps. CDKs help push the cell past these checkpoints only when conditions are right

What causes a small cell

Not right environmental conditions or nutrients to grow, so stays in G phase until great enough size to be divided

G1

Cells grow, prepares for DNA replication (cyclin D + CDK4/6 - dont need to know in great detail, just know cyclin and CDK)

S

Dna is replicated (cyclin A + CDK2)

G2

Final growth, preparation for mitosis (cyclin A/B + CDK1)

M (mitosis)

Cell divides (cyclin B + CDK1)

Mitosis vs binary fission

Mitosis:

• eukaryotes

• Linear, in nucleus

• Complex (involves spindle apparatus)

• 2 indentical daughter cells

• Growth, repair, reproduction (in unicellular eukaryotes)

Binary fission:

• prokaryotes

• Circular, no nucleus

• Simple (no spindle)

• 2 identical daughter cells

• Asexual reproduction

Binary fission

Simpler and works for prokaryotes which lack complex structures like nuclei

Mitosis

Necessary for eukaryotes because of their complex cell structure - they ened to carefully divide their nucleus and organelles