Cell Division

DAY I

Cell Division: Mitosis & Meiosis

• The process by which two cells are formed from one.

• There are two types of cell division: mitosis and meiosis.

• Most of the time when people refer to cell division, they mean mitosis, the process of making new body cells.

• Meiosis is the type of cell division that creates egg and sperm cells.

MitosisLet’s consider mitosis first:

• Mitosis is a fundamental process for life.

• A cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Each of which begins the cycle again.

Cell Division - Mitosis

There are 4 phases of mitosis: Prophase Metaphase Anaphase Telophase

•The first phase and longest of mitosis; occurs when chromatin condenses becoming chromosomes and the mitotic spindle begins to form.•Spindles; fanlike microtuble structures which help to separate chromosomes.•The centrioles separate to opposite sides of the nucleus organizing spindles•Chromosomes (Paired identical “sister” chromatids)•The nuclear envelope begins to break down (dashed line)

Prophase

Spindles forming

Centrioles

Metaphase

•Typically happens very quickly•Chromosomes line up along the equator.(Note: This is a signature feature of metaphase.)•Each chromosome is connected along the poles of the spindle fiber at its centromere

http://www.biologycorner.com/bio1/cellcycle.html

Centrioles

Anaphase

•The “sister” chromatids separate into individual chromosomesand move apart to opposite poles(Note: These are signature features of anaphase.)•Phase ends when chromosomes stop moving

Individual chromosomes

•Fourth and final phase (not stage) of mitosis•Chromosomes begin to disperse into a tangle of dense material.•Nuclear membrane (envelope) forms around each group of chromosomes•The spindle begins to break apart, and a nucleolus becomes visible in each daughter nucleus.•Cytokinesis begins virtual.yosemite.cc.ca.us

Telophase

Nuclear Envelope Reforming

Meiosis Background• Each organism must inherit a single copy of

every gene from each of the parents.• Two sets of genes must be separated from

each other so that each gamete contains just one set of genes.

Show McGraw-Hill Clip:• http://highered.mcgraw-hill.com/olcweb/cgi/

pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio17.swf::Comparison%20of%20Meiosis%20and%20Mitosis

Class Activity

• Read section comparing mitosis and meiosis on Pg. 278

• Using a Venn diagram and begin comparing and contrasting the two forms of cell division.

Comparing Mitosis and Meiosis• Mitosis results in the production of two genetically identical

diploid (2N) cells• Meiosis produces four genetically different haploid cells.• Mitosis allows an organism’s body to grow and replace cells.• Asexual reproduction occurs when a new organism is

produced by mitosis of the cell or cells of the parent organism.

• Meiosis begins with a diploid cell but produces four haploid (N) cells.

• Haploid cells are genetically different from the diploid cells.• Meiosis is how sexually reproductive organisms produce

gametes.

DAY II

Diploid Cells

• Chromosomes after replication (ready for mitosis or meiosis)

• Chromosomes in early interphase (actually chromatin) If both

chromosomes of a pair are

present, the cell is diploid

Homologous Pairs

Diploid Cells cont.

• All body cells are diploid.

• Sex cells: egg and sperm are haploid cells

• Spores are also haploid cells

# of ChromosomesTotal

Chromosomes in a diploid

cell

# of homologous

pairs in a diploid cell

Total chromosomes

in haploid cell (gamete)

HUMAN 46CAT 36

23 23

1818

Meiosis• How are haploid (N) gamete cells produced from diploid

(2N) cells?

• MEIOSIS - The process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

• Involves two divisions, called Meiosis I and Meiosis II– By the end of Meiosis II, the diploid cell that entered meiosis

has become 4 haploid cells

Sex Cells• Gametes or Germ Cells

– Also know as Sex Cells

• Only contain one copy of genetic information

• Have one chromosome of each pair or half that of diploid cells.

• When only ONE chromosome of a pair is present, the cell is HAPLOID

Why is Haploid Important?

• If diploid cells are fertilized…

+

Chromosome number will keep doubling!!

2N + 2N 4N

Why is Haploid Important?

• If haploid cells are fertilized…

+

The Result is a Diploid Cell!Like all cells of the body!!

N + N 2N

Sexual Reproduction

• Reproductive process in which haploid sex cells (gametes) fuse to produce a diploid fertilized egg (zygote)

+

EGG SPERM Fertilized Egg

GAMETES

(ZYGOTE)

Chromosome Number• Ex: Adult Fruit Fly

– A body cell contains 8 chromosomes.– 4 chromosomes (male parent); 4 (female parent).– Both sets are considered homologous.

• Each of the chromosomes from male parent correspond with one of the female parent.

– A cell containing both set of homologous chromosomes is said to be diploid (2N).

• Contain two complete sets of chromosomes and two complete sets of genes.

• The diploid number is 8 (2N = 8).– The gametes (sexually reproducing) of fruit flies contain a

single set of chromosomes (single set of genes) are set to be haploid (N).

MEIOSIS• Cell division that

reduces the number of chromosomes in half

• Occurs in sex cells (germ cells: Egg and Sperm)

• Two Divisions– MEIOSIS I– MEIOSIS II

Meiosis I

Meiosis II

MEIOSIS I

• Prior to meiosis I, each chromosome is replicated.• Similar to mitosis.**Remember in mitosis, the 4 chromosomes line up

individually in the center of the cell and then the two chromatids of each chromosome separate.

Meiosis I: Prophase I

• Chromosomes condense & become distinctly visible

• Nuclear membrane and nucleolus disappear

• Synapsis – homologous chromosomes pair up forming a tetrad (4 chromatids).

Crossing Over – exchange of DNA creating genetic variability

Meiosis I: Metaphase I

• Homologous chromosomes line up at the equator

• PAIRS of chromosomes rather than INDIVIDUAL chromosomes line up!

Meiosis I: Anaphase I / Telophase I

ANAPHASE I:• Homologous chromosomes

separate and move to opposite poles

• Cells are still diploidTELOPHASE I:• Cell membrane pinches in

forming 2 HAPLOID cells.

MEIOSIS II

• The two cells produced by meiosis I now enter a second meiotic division.

• No replication.

MEIOSIS II: Prophase II

• Cells are haploid.

• Cells get ready to divide again.

MEIOSIS II: Metaphase II

• Chromosomes align at the equator

MEIOSIS II: Anaphase II / Telophase II

ANAPHASE II:• Sister chromatids move to opposite poles

TELOPHASE II:• Cell membrane pinches in• Nuclear membrane reappears.

Interphase????

Resulting Sex Cells

• Four HAPLOID sex cells are produced.

MEIOSIS

• Mitosis: http://www.biologycorner.com/bio1/cellcycle.html

• Mitosis vs. Meiosis: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio17.swf::Comparison%20of%20Meiosis%20and%20Mitosis

• How Cells Divide?: http://www.pbs.org/wgbh/nova/miracle/divi_flash.html

• Meiosis and Mitosis: http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20laboratory/mitosis/mitosis.htm