Biology is the only subject in which multiplication is the same thing as division…

MITOSIS: Making new cells

Where it all began… You started as a cell smaller than a period at the end of a sentence… And now look at you… How did you get from there to here?

 Going from egg to baby…. the original fertilized egg has to divide… and divide… and divide… videovideo Getting from there to here…

Why do cells divide?  One-celled organisms  for reproduction  asexual reproduction (clones)  Multi-celled organisms  for growth & development  from fertilized egg to adult  for repair & replacement  replace cells that die from normal wear & tear or from injury amoeba starfish

Copying DNA  A dividing cell replicates its DNA  Creates 2 copies of all DNA  But the DNA starts loosely wound in the nucleus  If you tried to divide it like that, it could tangle & break nucleus cell DNA

Coil DNA into compact chromosomes Copying & packaging DNA Replicating DNA  When cell is ready to divide…  coil up DNA around histones like thread on a spool… making chromosomes  now DNA can move around cell without tangling & breaking

DNA must be replicated… nucleus cell DNA in chromosomes nucleus cell replicated chromosomes Before replication 4 single-stranded chromosomes After replication 4 double-stranded chromosomes

Parts of a chromosome  “Sister” Chromatid  Each identical half of a replicated chromosome  Centromere  Holds the two chromatids together Two “sister” chromatids Centromere DNA

Types of chromosomes  Homologous chromosomes  Pairs of chromosomes that carry the same traits  One from each parent  Sex chromosomes  Determine gender  XX = female, XY = male  Autosomes  All other chromosomes (not sex chromosomes)  Karyotype  Picture of all homologous pairs for an organism

Karyotype of Human Female 46 chromosomes 23 pairs

Karyotype of Human Male 46 chromosomes 23 pairs

double-stranded human chromosomes ready for mitosis

Cell Cycle: BIG picture Mitosis is just a small part of the cell’s life cycle Let’s look closer 

Interphase  3 parts  G1 = initial growth  S = DNA copied (synthesis)  G2 = More growth and prep for cell division nucleus cell DNA NOT Mitosis!

Mitosis: Prophase  Chromosomes visible  Nuclear envelope dissolves  Centrioles and spindle fibers appear cell replicated chromosomes Prepare! centrioles spindle fibers

Mitosis: Metaphase  Chromosomes line up on center “plate” Middle!

Mitosis: Anaphase  Chromosomes split apart  Sister chromatids separate  Spindle fibers move them to opposite ends of the cell Apart!

Mitosis: Telophase  Cell starts to divide  Cytokinesis begins  Nuclear envelope forms again Two cells!

Mitosis: Cytokinesis  Original cell splits in two (cyto = cell, kinesis = splitting)  Cleavage furrow = animal cells  Cell plate = plant cells Cut!

New “daughter” cells  Get 2 exact copies of original cells  same DNA  “clones”

Mitosis in whitefish embryo

Mitosis in plant cell

Overview of mitosis interphaseprophase metaphaseanaphasetelophase cytokinesis P lease M ake A nother T wo C ells