WHY CELLS DIVIDE

THE CELL CYCLE

WHAT DO ALL CELLS REQUIRE TO SURVIVE?

• A complete set of genetic instructions

• Genetic instructions are coded in the DNA of cells

• Needed to produce the required molecules that drive life functions (metabolism)

CELL DIVISIONMITOSIS

• Results in genetically identical daughter cells

• Cells duplicate their genome before they divide

• Ensuring that each daughter cell receives an exact copy of the genetic material, DNA

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WHY DO CELLS DIVIDE?

• About 2 trillion cells are produced by an adult human every day.

• Cell division, or reproduction occurs in all organisms

• WHY?

• It depends on the cell type and the organism

WHY DO CELLS DIVIDE?ASEXUAL REPRODUCTION

• GrowthGrowth• Cells must divide to

maintain maximum efficiency.

• DevelopmentDevelopment• Cells get old and

die and must be replaced

• RepairRepair

PROKARYOTE CELL DIVISIONBINARY FISSION

• Prokaryotic cells have a circular DNA molecule attached to the inner cell membrane

• They divide through a simple form of division called Binary Fission

E. coli dividing by binary fission

PROKARYOTIC CELL DIVISION

• 3 step process• DNA strand splits and

forms a duplicate of itself.

• The two copies move to opposite sides of the cell

• Cell membrane forms between two new and identical cells.

EUKARYOTIC CELLSCHROMOSOMES

• 1,000 times more DNA then an average prokaryotic cell.

• Organized into several linear chromosomes.

• Organization is much more complex than the single, circular DNA molecule in a prokaryotic cell

50 µmFigure 12.3

CHROMOSOMES

• All eukaryotic cells store genetic information in chromosomes.• Most eukaryotes have

between 10 and 50 chromosomes in their body cells.

• Human cells have 46 chromosomes.

• 23 pairs

• Complex of DNA and protein called chromatin

• Condenses during cell division

• DNA exists as a single, long, double-stranded fiber

• Organized in coils• DNA is wrapped around

proteins called histones.

STRUCTURE OF CHROMOSOMES

THE CELL CYCLE

• Some cells divide constantly: • cells in the embryo,

skin cells, gut lining cells, etc.

• Other cells divide rarely or never: only to replace themselves.

• Actively dividing cells go through a cycle: a repeating sequence of cellular growth and division.

THE CELL CYCLE

•There are 4 stages•G1•S•G2•Mitosis

THE CELL CYCLE• The first 3 phases are

collectively called Interphase

• The G1 phase• Most time spent• Growth and normal function.

• Control of cell division. • The S or Synthesis phase

• DNA is replicated. • The chromosome goes from

having one chromatid to two.

• G2 phase• Cell prepares for mitosis • Amount of cytoplasm Amount of cytoplasm

(including organelles) (including organelles) increases in preparation for increases in preparation for divisiondivision

CHROMOSOME REPLICATION (S-PHASE)

• Each condensed chromosome consists of 2 identical chromatids joined by a centromere.

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Two unduplicatedchromosomes

Centromere

Sisterchromatids

Sisterchromatids

Duplication

Non-sisterchromatids

Two duplicated chromosomes

• During the S phase of the cell cycle, DNA replicates.

ReplicationCentromere

Chromosome arm

Chromosome arm

Identical chromatidChromatid

A packaged chromosome

Replication Animation:http://www.johnkyrk.com/DNAreplication.html

CHROMOSOME REPLICATION

THE CELL CYCLE CHECKPOINTSTHE CELL CYCLE CHECKPOINTS

• Normal function

• ATP is synthesized.

• Damaged parts are repaired.

• Wastes are excreted.

• Proteins are made.

• Organelles are formed.

• Specialized tasks are performed.

INTERPHASE SUMMARY

Some cells stay in Interphase all of their lives!

Mitosis is normal cell division

The process by which the nucleus of a cell divides to form two new nuclei

Mitosis has 4 phases

MITOSIS FOURTH PHASE OF THE CELL CYCLE

MITOSIS

• Prophase• Metaphase• Anaphase• Telophase

PProphaserophase

MMetaphaseetaphase

AAnaphasenaphase

TTelophaseelophase

PMAPMATT

REMEMBER!!!!!

PROPHASE• Chromatin

condenses and chromosomes become visible

• Nuclear membrane disappears.

• Centrioles migrate to opposite sides propelled by the lengthening of microtubules

• Mitotic spindle begins to form

Centrioles

METAPHASE• Metaphase is the

longest stage of mitosis, lasting about 20 minutes.

• The centrosomes are now at opposite ends of the cell.

• Chromatids migrate to equator of spindle.

• Spindle fibres attach to the chromatids.

• The entire apparatus of microtubules is called the spindle because of its shape.

ANAPHASE

• Anaphase is the shortest stage of mitosis, lasting only a few minutes.

• Anaphase begins when the sister chromatids of each pair suddenly part.

• Each chromatid thus becomes a full-fledged chromosome.

• The chromosomes begin moving toward opposite ends of the cell, as microtubules shorten.

ANAPHASE

• The cell elongates as structural microtubules lengthen.

• By the end of anaphase, the two ends of the cell have an equal and completecollection of chromosomes.

• Chromatids are now called chromosomes.

ANAPHASE

Daughter chromosomes

TELOPHASE• Cytoplasm divides: cytokinesis• Nuclear membrane reappears. There are now

two nuclei.• Chromosomes uncoil (they are no longer

visible) and the DNA is now called chromatin• Mitosis, the division of one nucleus into two

genetically identical nuclei, is now complete.

TELOPHASE THE DIFFERENCE IN PLANT AND ANIMAL CELLS

• Plant Cells• The Golgi body

forms a cell plate between the two nuclei.

• The plasma membrane reforms on each side of the plate which grows outward becoming the new cell wall.

TELOPHASE THE DIFFERENCE IN PLANT AND ANIMAL CELLS

• Animal Cells• A cleavage appears

(groove) caused by a contractile ring of actin microfilaments

• The contraction of microfilaments is like the pulling of drawstrings

• The cell is pinched in two

CYTOKINESIS

• Occurs during telophase, generally at the same time that the nuclear membrane is reforming, (yet they are distinct processes.)

• The cytoplasm divides – the organelles are equally distributed between the two daughter cells

• Two identical daughter cells are formed.

STAGES OF MITOSIS

Interphase

Metaphase Mitotic spindle

Prophase

Mother cell

Nucleus with un-condensed chromosomes

Condensed chromosomesEquator of the cell

Poles of the cell

Two daughter cells Anaphase

Telophase

Disappearing nuclear membrane

REVIEW

How do little elephants grow up to be big elephants?

REVIEW

Why do animals shed their skin?

REVIEW

What happens when an organism is injured?

THREE REASONS WHY CELLS REPRODUCE ASEXUALLY

•Growth•Development (replacement)

•Repair

Cancer = Uncontrolled Cell Growth