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 DIVISION MITOSIS 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 © 2006 Mesoblast Limited

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 Growth Growth Cells must divide to maintain maximum efficiency. Development Development Cells get old and die and must be replaced Repair Repair

PROKARYOTE CELL DIVISION BINARY 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 CELLS CHROMOSOMES 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 µm Figure 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 (including organelles) increases in preparation for division Amount of cytoplasm (including organelles) increases in preparation for division

CHROMOSOME REPLICATION (S-PHASE) Each condensed chromosome consists of 2 identical chromatids joined by a centromere. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two unduplicated chromosomes Centromere Sister chromatids Sister chromatids Duplication Non-sister chromatids Two duplicated chromosomes

During the S phase of the cell cycle, DNA replicates. Replication Centromere Chromosome arm Identical chromatidChromatid A packaged chromosome Replication Animation: ication.html CHROMOSOME REPLICATION

THE 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

 P rophase  M etaphase  A naphase  T elophase PMAT 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 complete collection 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 chromosomes Equator of the cell Poles of the cell Two daughter cells Anaphase Telophase Disappearing nuclear membrane

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THREE REASONS WHY CELLS REPRODUCE ASEXUALLY Growth Development (replacement) Repair Cancer = Uncontrolled Cell Growth