Creating Identical Body Cells Cell Cycle and Mitosis Creating Identical Body Cells

Cell Division All cells come from pre-existing cells (no spontaneous generation) New cells are produced for growth and to replace damaged or old cells GRR: Growth, Repair, Replace Different in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, and animals) End result is two identical daughter cells

Cell Division in Prokaryotes

Binary Fission Bacteria divide into 2 identical cells through the process of binary fission A single chromosome makes a copy of itself A cell wall forms between the chromosomes, dividing the cell Parent Cell Chromosomes Replicate Cell Splits 2 Identical Daughter Cells

Binary Fission

Cell Division in Eukaryotes

Chromosomes All eukaryotic cells store genetic information in chromosomes Most eukaryotes have between 10 & 50 chromosomes in their body cells (designated 2n) Human body cells have 46 chromosomes, or 23 identical pairs Diploid: 2n = 46 Haploid: n = 23 Each chromosome is composed of a single, tightly coiled DNA molecule

Chromosomes

Compacting DNA When DNA is loose (before and after division), it is called chromatin When the cell starts to divide, the DNA compacts into chromosomes The DNA is tightly coiled around proteins called histones

Compacting DNA Duplicated chromosomes are called chromatids and are held together by the centromere

The Cell Cycle

The Cell Cycle Checkpoints Cyclins and CDK Control mechanisms the act as potential stopping points if cell division is not progressing correctly Conditions of the cells are assessed There are 3 checkpoints: G1 (restriction), G2, and metaphase Cyclins and CDK Progression through the cell cycle is controlled by proteins known as cyclins (regulation) Cyclins work by activating CDK enzymes (biomolecule?) CDK = cyclin dependent kinases

The Cell Cycle Consists of three main parts Interphase – 3 stages Mitotic Phase (mitosis) – four stages Cytokinesis

Interphase (3) G1 Stage First growth after cell division Cells mature by making more cytoplasm and organelles Cell carries on its normal metabolic activities First Checkpoint (Restriction Checkpoint) This is the point at which the cell decides to divide (and enter the S phase), delay division, or enter a resting stage (also called G0)

Interphase (3) S stage Synthesis stage DNA is copied (replicated) Two identical copies of DNA Original DNA

Interphase (3) G2 Stage Second growth stage Occurs AFTER DNA has been copied All cell structures needed for division are made (ex: centrioles) Both organelles and proteins are synthesized Second checkpoint Cell checks for size and correct replication (no mutations) before being allowed to start mitosis Also a check for any damage (ex: from radiation)

Mitosis Division of the nucleus Only occurs in eukaryotes Has four stages Doesn’t occur in some cells (like brain cells)

Mitosis Prophase Metaphase Anaphase Telophase

Mitosis Prophase Chromatin in the nucleus condenses to form visible chromosomes Spindle fibers form from the centrioles The nuclear membrane breaks down

Mitosis Metaphase Chromosomes, attached to the spindle fibers at their centromere, move to the center of the cell (the equatorial plate) Third Checkpoint (Spindle Checkpoint) The tension in the sister chromatids from being pulled in opposite directions by the spindle fibers must be felt to trigger anaphase

Mitosis Anaphase Occurs very rapidly Sister chromatids are pulled apart to opposite poles of the cell by the spindle fibers Nondisjuntion Failure of chromosomes to separate properly Occurs if the metaphase checkpoint fails

Mitosis Telophase The spindles disassemble New nuclear envelopes form around each set of chromsomes at the two poles Chromosomes decondense into chromatin

Cytokinesis Division of the cytoplasm Occurs after mitosis is complete Division of the cell into two identical daughter cells In PLANT cells, a cell plate forms at the equator to divide the cell In ANIMAL cells, a cleavage furrow forms to split the cell

Mitosis in Animal Cells

Mitosis in Animal Cells

Mitosis in Plant Cells

Mitosis in Plant Cells

Daughter Cells of Mitosis Have the same number of chromosomes as each other and as the parent cell they were formed from (2n = 46) They are identical to each other, but smaller than the parent cell They must grow in size to mature cells before dividing again (Interphase – G1)

Problems with Mitosis If mitosis is not controlled, unlimited cell division occurs, causing cancer This occurs when cells do not go into a resting phase, but stay in the cell cycle, dividing over and over again The result is an overwhelming number of immature cells, which crowd out normal cells and cause a tumor

Problems with Mitosis Oncogenes are special proteins that increase the chance a normal cell develops into a cancerous cell Usually when cells detect mutations, they undergo programmed cell death (apoptosis) Cancer can be treated with Surgery: removal of a tumor Radiation: targets specific areas to shred tumor cell DNA Chemotherapy: travels throughout the body, targeting fast-dividing cells (this can cause hair loss, skin rashes, nausea, etc.)