Cellu

lar R

eplic

ation

What is the meaning of life?

•The purpose of life is to pass genetic information onto the next generation•How is this done?

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Cells

•The Cell Theory of Life states that:1.Cells are the smallest functional unit of life2.All living things consist of one or more cells3.New cells are produced from existing cells

Cells

•The Cell Theory of Life states that:1.Cells are the smallest functional unit of life2.All living things consist of one or more cells3.New cells are produced from existing cells

HOW?•One (parent) cell divides into two (daughter) cells = CELL DIVISION�however, one cell becomes two cells

= CELL MULTIPLICATION

Therefore, cells multiply by dividing�Let s just call it � cellular replication

Cellular Replication

•There are THREE types of Cellular ReplicationMITOSIS MEIOSIS BINARY FISSIONeukaryotes eukaryotes prokaryotes

single-celled and multi-celled

multi-celled single-celled

TWO daughter cells produced

FOUR daughter cells produced

TWO daughter cells produced

reproduction; growth of organism;

replace old cells

gamete production

reproduction

DNA replication is central to the process of cellular replication

BINARY FISSION? cellular replication in prokaryotes

one paramecium

two paramecia

red paramecium

blue paramecium

BINARY FISSION? cellular replication in prokaryotes

one paramecium

two paramecia

cell wallcell membrane

large, circular

DNA moleucle

DNA replication

attachment point

expansion of plasma membranecell division

in ideal conditions, binary fission can occur every 15-

20 minutes!

BINARY FUSSION? cellular replication in prokaryotes

• single DNA molecule is attached to the plasma membrane at a specific point before DNA replication• the two copies of the DNA molecule are separated by the expansion of the plasma membrane• plasma membrane and cell wall furrow inwards to divide the cytoplasm, resulting in two daughter cells with identical genetic material

one paramecium

two paramecia

Cell Division in Eukaryotes

•A combination of two processes:•DIVISION OF NUCLEUS (incl. DNA replication)•DIVISION OF CYTOPLASM (incl. organelle replication)

•To accomplish this task, the cell passes through a series of discrete stages, or phases, known as the cell cycle

DIVISION OF NUCLEUS = MITOSIS

DIVISION OF CYTOPLASM = CYTOKINESIS

The Cell Cycle

M

SG1G2

Growth & normal

metabolic roles

Growth & preparation for mitosis

DNA replication

first Grow

th phasese

cond

Gro

wth

pha

se

Synthesis phase

Mitotic phase

prophasem

etaphase anap

hase

telo

phas

e

NOT all parts of the cell cycle are mitosis!

The non-mitosis parts of the cell cycle are known

as interphase

cultured mammalian cells

can take 18-24 hours to complete

the cell cycle

INTERPHASE

The Cell Cycle

G1G2

S

MCell carries out its

biochemical activities

8-10 hours

DNA replication (synthesis)

6-8 hours

Cell actively prepares for cell division.

Organelles are duplicated

4-6 hours

Nuclear division (followed by cytokinesis)

40-60 minutes

Interphase

Cells spend most of their time in interphase (~95%)

Although the cell appears to be resting � � nothing could be further from

the truth!•cell growth (most cell contents are synthesised, increasing cell mass)•metabolic activity (this is when a cell does what it needs to do

i.e. secrete, engulf, receive, �respond etc) •DNA replication

tissue containing several cells in interphase.

note that individual chromosomes are not visible; DNA in the nucleus is in the

form of chromatin fibres

III

XIII

II

IV

VVI

VII

VIII

IX

X

XI

if the cell cycle was a 24 hour clock, interphase would last from 12am (0:00) until 10:55pm (22:55)

Interphase

Cells spend most of their time in interphase (~95%)

tissue containing several cells in interphase.

note that individual chromosomes are not visible; DNA in the nucleus is in the

form of chromatin fibres

Although the cell appears to be resting ? � � nothing could be

further from the truth!

• cell growth most cell contents are synthesised, increasing cell mass• metabolic activity this is when a cell does what it needs to do �i.e. secrete, engulf, receive, respond etc • DNA replication

during S phase

The G1 Phase

•Interphase starts with G1•G stands for Gap phase•G should stand for Growth phase

•Time when cell carries out its biochemical activities•Length of G1 phase varies between cells; •typically 8-10 hours•some cells sit in G1 for weeks, months, years!•cells that are arrested (stopped) in G1 are said to be in a G0 state

•MOST NERVE CELLS NEVER LEAVE G0•The decision to commit to cell division is made when the cell passes through the first checkpoint at the end of G1

M

SG1G2

The S Phase

•Once the cell is committed to cell division (leaves G1), the cell enters the S Phase•S stands for Synthesis

•Time for DNA replication•typically takes 6-8 hours

•S Phase ends when DNA content of the cell has doubled

(this is obvious at the start of the M phase when the chromosomes become visible)

•Cell is now committed to undergo mitosis•Too late to turn back now!

M

SG1G2

The G2 Phase

•Once DNA is replicated the cell enters G2•This phase has a more fixed timing•4-6 hours in most cells

•G2 is a period of high metabolic activity and protein synthesis, as the cell actively prepares for cell division•Organelles are duplicated during this time

•To ensure that everything is ready for the division of the nucleus (mitosis), the cell passes through another checkpoint at the end of G2

M

SG1G2

The M Phase

•This phase encompasses division of the nucleus (mitosis) followed by the division of the cytoplasm (cytokinesis)•The M phase lasts less than an hour •40-60 minutes

•This phase is separated into various smaller phases that are characterised by particular chromosome behaviour•explains how the two copies of chromosomal DNA formed during S Phase are separated from each other and partitioned into two daughter cells

M

SG1G2

The G1 PhaseM

SG1G2

•After cell division, the two new daughter cells enter interphase at G1•The Cell Cycle starts again!

•NB The cell cycle is highly regulated by �intracellular signalling molecules and extracellular signalling proteins called growth factors that control the rate of cell growth and division•Involves checkpoints to make sure that everything is going according to plan

interphase metaphase & cytokinesis

interphase interphase

Three Checkpoints Regulate the Cell Cycle

G1G2

S

M G1 Checkpointcheck for:

• cell size• nutrients• growth factors• DNA damage

G2 Checkpointcheck for:

• cell size• DNA damage

Spindle Assembly Checkpoint

check for:• chromosome attachments to spindle fibres

(occurs during middle of mitosis)

What happens if things go wrong???

Cell leaves the cell cycle and enters a

resting state� � G0

Then �APOPTOSIS! APOPTOSIS

APOPTOSISAPOPTOSIS