CELL DIVISION © 2006 Mesoblast Limited - Students know meiosis is an early step in sexual reproduction in which the pairs of chromosomes separate and segregate randomly during cell division to produce gametes containing one chromosome of each type. - Students know only certain cells in a multi cellular organism undergo meiosis.

Essential Content Questions  Explain the differences between sex chromosomes and autosomes.  Distinguish between diploid and haploid cells.  Summarize the events of interphase.  Describe the stages of mitosis.

Chromosome Numbers:  Human somatic cells (or body cells) have 23 pairs of chromosomes or 46 chromosomes (diploid or 2n number)  The 2 chromatids of a chromosome pair are called homologous (have genes for the same trait at the same location)  Human reproductive cells or gametes (sperm & eggs) have 23 chromosomes (haploid or n number)  Every organism has a specific chromosome number.

Chromosomes  Fertilization: joining of the egg & sperm, restores the diploid chromosome number in the zygote (fertilized egg cell)  Sex chromosomes, either X or Y, determine the sex of the organism  XX will be female  XY will be male  All other chromosomes, except X & Y, are called autosomes

Karyotype  Chromosomes from a cell may be arranged in pairs by size starting with the longest pair and ending with the sex chromosomes to make a karyotype  A human karyotype has 22 pairs of autosomes and 1 pair of sex chromosomes (23 total)

Karyotype

Cell division and reproduction  It is necessary for reproduction in unicellular or multicellular organisms Microbus © Microscope-Microscope.orgMicroscope-Microscope.org © 2007 Paul Billiet ODWSODWS

Prokaryotes  Prokaryotes have no nucleus  They have a single circular chromosome  Prokaryotes simply divide their cells in two by binary fission © 2007 Paul Billiet ODWSODWS

Eukaryotes  Eukaryotes must divide their nucleus (and other organelles such as mitochondria) in preparation for cell division (mitosis or meiosis)  Before the nucleus divides the genetic material replicates (duplicates) © 2007 Paul Billiet ODWSODWS

Mitosis  When a living organism needs new cells to repair damage, grow, or just maintain its condition, cells undergo mitosis.  Cells go through phases or a cell cycle during their life before they divide to form new cells  Cell division includes mitosis (nuclear division) and cytokinesis (division of the cytoplasm).

The Cell Cycle The Cell cycle consists of the folowing steps:The Cell cycle consists of the folowing steps: G1 (Gap 1) Phase - Cell performs its normal function (cells which do not divide stay in this stage for their entire life span)G1 (Gap 1) Phase - Cell performs its normal function (cells which do not divide stay in this stage for their entire life span) S (Synthesis) Phase - Here the cell actively duplicates its DNA in preparation for divisionS (Synthesis) Phase - Here the cell actively duplicates its DNA in preparation for division G2 (Gap 2) Phase - Amount of cytoplasm (including organelles) increases in preparation for division.G2 (Gap 2) Phase - Amount of cytoplasm (including organelles) increases in preparation for division. Mitosis - Actual division occursMitosis - Actual division occurs

Interphase  Cell Replicates its DNA/Chromosomes in preparation of upcoming division (G1, S, and G2 phases). Animal Cell Plant cell

DNA Replication

Prophase 1.Chromosomes Shorten and become visible. 2. Centrioles move to opposite sides of the cell 3. Nuclear envelope disappears 4. Spindle Fibers & Astral Fibers both together are known as the Spindle Apparatus begin to form Animal Cell

Chromatin / Chromosomes

Metaphase  Chromosomes line up along center of cell called the Metaphase Plate  Chromosomes attach to spindle fibers  Spindle & Astral fibers are now clearly visible Plant Cell

Anaphase  Centromeres break up separating chromosome copies  Chromosomes are pulled apart to opposite sides of cell  Spindle & Astral fibers begin to break down Animal Cell

Telophase (cytokenesis)  Nuclear envelope forms around both sets of chromosomes  DNA uncoils  Spindle & Astral fibers completely disappear Cytokenesis happens with most (but not all) cells Cytokenesis happens with most (but not all) cells Cytoplasm & organelles move (mostly equally) to either side of the cell.Cell Membrane “pinches” to form 2 separate cells Cytoplasm & organelles move (mostly equally) to either side of the cell.Cell Membrane “pinches” to form 2 separate cells Plant Cell

Animal Cytokeneisis  Cytokenesis differs significantly between Animal & Plant cells.  With animals, the membranes pinch together to form a Cleavage Furrow, which eventually fuses to form two daughter cells

Plant Cytokenesis  With Plants, a cell wall must be formed between the 2 daughter cells.  Vessicles containing Cellulose form and fuse between the tow daughter cells, eventually forming a complete cell wall.

Overview of Mitosis

Meiosis  Similar in many ways to mitosis  Several differences  Involves 2 cell divisions  Results in 4 cells with 1/2 the normal genetic information

Vocabulary  Diploid (2N) - Normal amount of genetic material  Haploid (N) - 1/2 the genetic material.  Meiosis results in the formation of haploid cells.  In Humans, these are the Ova (egg) and sperm.  Ova are produced in the ovaries in females  Process is called oogenesis  Sperm are produced in the testes of males.  Process is called spermatogenesis

Meiosis Phases  Meiosis occurs in 2 phases; Meiosis I, & Meiosis II.  Meiosis I. Prior to division, amount of DNA doubles

Crossing Over  During metaphase 1 homologous chromosomes line-up along the metaphase plate

Crossing over contd.  Crossing Over of genes occurs now Segments of homologous chromosomes break and reform at similar locations. Segments of homologous chromosomes break and reform at similar locations. Results in new genetic combinations of offspring. Results in new genetic combinations of offspring. This is the main advantage of sexual reproduction This is the main advantage of sexual reproduction

Chromosome reduction  During anaphase 1, each homologous chromosome is pulled to opposite sides of the cell. Unlike mitosis, THE CENTROMERES DO NOT BREAK.

Meiosis I continued  Nuclei may or may not reform following division.  Cytokenesis may or may not occur

Meiosis II  DNA does not double  Chromosomes randomly line-up along metaphase plate like regular mitosis.  During anaphase 2, CENTROMERES BREAK and each chromosome is pulled to opposite sides of the cell.  Nuclei reform and cytokenesis usually occurs (although it is often unequal).

Overview of Meiosis

Comparison of Mitosis & Meiosis