Mitosis• The form of cell divisioncell division by which a

eukaryotic somatic cellsomatic cell duplicates.

• MitosisMitosis is asexual reproduction.asexual reproduction.

• Cell divisionCell division is the continuation of life based on the reproduction of cells.

Somatic ReproductionSomatic Reproduction

• Most eukaryotic cells reproduce asexually by mitosismitosis.

• Somatic cells Somatic cells are all body cellsbody cells (like nerve, liver, (like nerve, liver, etc...) etc...) except sperm and ova (egg).

• All Somatic cells have the same number of chromosomeschromosomes.

Cell DivisionCell Division Cell Division: • All cells are derived from

preexisting cells (Cell Theory) • Cell division is the process by

which cells produce new cells • Cell division differs in

prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals)

• Some tissues must be repaired often such as the lining of gut, white blood cells, skin cells with a short lifespan 

• Other cells do not divide at all after birth such as muscle & nerve 

Reasons for Cell Division:

• Cell growth • Repair & replacement of

damaged cell parts • Reproduction of the species

Copying DNACopying DNA• Since the instructions for

making cell parts are encoded in the DNA, each new cell must get a complete set of the DNA molecules

• This requires that the DNA be copied (replicated, duplicated) before cell division

Chromosomes & Their StructureChromosomes & Their Structure • The plans for making cells are coded in DNA • DNA is a long thin molecule that stores genetic information • DNA is organized into giant molecules called chromosomes • Chromosomes are made of protein & a long, single, tightly-

coiled DNA molecule visible only when the cell divides • When a cell is not dividing the DNA is less visible & is

called chromatin • DNA in eukaryotic cells wraps tightly around proteins called

histones to help pack the DNA during cell division • Centromeres hold duplicated chromosomes together before

they are separated in mitosis • When DNA makes copies of itself before cell division, each

half of the chromosome is called a sister chromatid 

Chromosome NumbersChromosome Numbers • Humans somatic or body cells

have 23 pairs of chromosomes or 46 chromosomes (diploid or 2n number)

• The 2 chromatids of a chromosome pair are called homologues (have genes for the same trait at the same location)

• Human reproductive cells or gametes (sperms & eggs) have one set or 23 chromosomes (haploid or n number)

• Every organism has a specific chromosome number

Organism Chromosome Number (2n)

Human 46

Fruit fly 8

Lettuce 14

Goldfish 94

• Fertilization, is the joining of the egg & sperm cell

• Sex chromosomes, either X or Y, determine the sex of the organism

• Two X chromosomes, XX, will be female and XY will be male

• All other chromosomes, except X & Y, are called autosomes

• 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)

Human Male Karyotype

Cell division in ProkaryotesCell division in Prokaryotes DNA of prokaryotes (bacteria) is one,

circular chromosome attached to the inside of the cell membrane

• Prokaryotes divide into two identical new cells by the process of binary fission

• Binary fission is an asexual method of reproduction

Occurs in 3 steps: The chromosome, attached to cell

membrane, makes a copy of itself The cell grows to about twice its normal

size Next, a cell wall forms between the

chromosomes & the parent cell splits into 2 new identical daughter cells (clones)

Binary FissionBinary Fission• Bacteria, cyanobacteria, and most single celled

organisms reproduce by binary fissionbinary fission.

Asexual Asexual ReproductionReproduction

chromosome

plasma membrane

bacteria

Cell CycleCell Cycle

Mitosis-prophase-metaphase-anaphase-telophase

G1 G2

Sphase

interphase

Cell Cycle:Cell Cycle:• The phases of life of a cell are called THE CELL

CYCLE• The cell cycle includes 2 main parts --- interphase

and cell division • Cell division includes mitosis (nuclear division)

and cytokinesis (division of the cytoplasm) • Interphase is the longest part of a cell's life cycle

and is called the "resting stage" because the cell isn't dividing

• Interphase consists of 3 parts: G1, S, & G2 phases

InterphaseInterphase1. G1. G11 (gap) phase: (gap) phase: First growth stage

Cell increases in size

Cell prepares to copy its DNA

2. S phase:2. S phase: Copying of all of DNA’s instructions

Chromosomes duplicated

3. G3. G22 (gap) phase: (gap) phase: Time between DNA synthesis & mitosis

Cell continues growing

Needed proteins produced

Cell CycleCell Cycle

Stages in growth & division

G1 PhaseS PhaseG2 PhaseM PhaseCytokinesis

Cell Division in Eukaryotes:Cell Division in Eukaryotes:• Eukaryotes have a nucleus &

membrane-bound organelles which must be copied exactly so the 2 new cells formed from division will be exactly alike

• The original parent cell & 2 new daughter cells must have identical chromosomes

• Both the nucleus (mitosis) and the cytoplasm (cytokinesis) must be divided during cell division in eukaryotes

Stages of Mitosis:• Division of the nucleus or

mitosis occurs first • Mitosis is an asexual method of

reproduction • Mitosis consists of 4 stages ---

Prophase, Metaphase, anaphase, & Telophase

ProphaseProphase• Longest phaseLongest phase

• Nucleolus disappears

• Chromatin material condenses into chromosomeschromosomes, consisting of two sister chromatidssister chromatids.

• CentriolesCentrioles move apart (not found in plants).

• Spindle fibers Spindle fibers form and attach from centriolescentrioles to centromeres by kinetochores.centromeres by kinetochores.

• Nuclear envelope fragments and disappears.

ProphaseProphase

late prophaselate prophaseearly prophaseearly prophase

centrioles

spindle fibers

centromere

nuclear envelopedisappearing

asterfibers

MetaphaseMetaphase

• Shortest phase

• CentriolesCentrioles are at opposite ends of the cell and attached with aster fibersaster fibers.

• ChromosomesChromosomes move to the metaphase platemetaphase plate (equatorial plateequatorial plate - center of cell).

MetaphaseMetaphase

metaphase plate

centrioles

spindle fibers

asterfibers

AnaphaseAnaphase• Sister chromatids Sister chromatids separate and move apart.• After separation, chromatidschromatids are now considered

chromosomeschromosomes.

• During this phase, the cell contains twice the normal number of chromosomeschromosomes.

• Cell begins to elongateelongate.

• At the end, there are equal numbers of chromosomeschromosomes at the poles.

AnaphaseAnaphase

No longer sister chromatidssister chromatids, now chromosomeschromosomes

centriolesspindle fibers

asterfibers

TelophaseTelophase

• Nuclear membrane & nucleolus reappears.

• ChromosomesChromosomes uncoil.

• In the end, two genetically identical nuclei are present.

TelophaseTelophase

cleavage furrow (cytokinesis)

nuclear membrane reforming nucleolus reappears

CytokinesisCytokinesis

• Cytoplasmic division

• Cleavage furrowfurrow develops in animal cells (CytokinesisCytokinesis begins).

• Cell plate develops in plantsplants (no cleavage furrows in plants).

• Nucleolus reappears.

cell platecell plate

2 daughter cells identical to original

Parent cell

Chromosomes are copied and double in number

Chromosomes now split

Question:Question:

• A cell containing 20 chromosomes at the beginning of mitosis would, at its completion, produce cells containing how many chromosomes chromosomes eacheach?

Answer:Answer:

• 20 chromosomes20 chromosomes

• Cancer is Uncontrolled Mitosis:

• Mitosis must be controlled, otherwise growth will occur without limit (cancer)

• Control is by special proteins produced by oncogenes