CELL DIVISION

Think about it…

How many different cells make up our body?

CELL DIVISION

Some cells divide frequently(some human skin cells divide once/hour)

Some cells divide occasionally (liver cells divide about once/year)

Some cells don’t divide once they form (nerve cells)

Asexual Reproduction• Production of genetically identical offspring

from a single parent cell (CLONES)• Bacteria (binary fission), some multicellular

organisms (budding)• We do it too! (mitosis)

PRO: Fast, simpleCON: Same DNA, no diversity

Sexual Reproduction• Fusion of two gametes genetic info

PRO: Diverse DNACON: Takes longer, 2 gametes needed

95% of the time DNA is loosely coiled in nucleus(CHROMATIN)

Chromatin condenses into tight complex of visible chromosomes during cell division (5% of time)

Duplicated Chromosome: Duplicated Chromosome: A closer look…A closer look…

Chromatid

Protein DNA is coiled around

Cell DivisionCell Division

• Process in which a parent cell divides to Process in which a parent cell divides to produce daughter cellsproduce daughter cells

• It is estimated that more than 25 million cells It is estimated that more than 25 million cells undergo division each second in an adult humanundergo division each second in an adult human

Eukaryotic Cell CycleEukaryotic Cell Cycle

CELL CYCLECELL CYCLEINTERPHASEINTERPHASE• Phase in preparation for cell divisionPhase in preparation for cell division• Cell spends about 90% of its life in interphase• Divided into 3 phases:Divided into 3 phases:

G1 – gap phase 1G1 – gap phase 1- Cell growth, performs cell duties, DNA in form of - Cell growth, performs cell duties, DNA in form of

chromatinchromatin

S – synthesisS – synthesis- DNA synthesis (replication)- DNA synthesis (replication)

G2 – gap phase 2G2 – gap phase 2--Prepares to undergo mitosisPrepares to undergo mitosis

Cell Cycle checkpointsCell Cycle checkpoints

Checkpoints in G1 and G2 ensure the cell is prepared to enter into the next phase of the cell cycle

(adequate nutrition, size, not damaged…)

CELL CYCLECELL CYCLEM phase = Mitosis and CytokinesisM phase = Mitosis and Cytokinesis

MitosisMitosis• Parent cell nucleus Parent cell nucleus

undergoes cell divisionundergoes cell division• 4 Phases 4 Phases

– ProphaseProphase– MetaphaseMetaphase– AnaphaseAnaphase– TelophaseTelophase

CytokinesisCytokinesis• Process in which the Process in which the

cell finally splits into cell finally splits into two daughter cells two daughter cells

Mitotic spindleMitotic spindle

• Cells contain Cells contain centriolescentrioles made of made of microtubules microtubules and produce long fibers and produce long fibers called called spindle fibersspindle fibers

MITOSIS

Mitosis: PROPHASE

• Chromatin fibers condense into visible chromosomes

• Nuclear envelope breaks down• Mitotic Spindles form; microtubules

attach to chromosomes @ centromere to direct their movement (like puppet strings)

microtubules

Mitosis: Metaphase

• Chromosomes gather in middle of cell between the two ends of spindles

• Form a line at “equator” of cell

Mitosis: Anaphase

• Sister chromatids separate from “partner”

• Spindle microtubules shorten, bringing chromosomes closer to opposite poles (reeling them in)

Sister Chromatids

Mitosis: Telophase (and cytokinesis)• Chromosomes reach opposite poles of

spindle• 2 new nuclear envelopes reform around each

new set of daughter chromosomes• Chromosomes uncoil (prophase….in reverse)• Cytokinesis completes cell division by

dividing cytoplasm in 2, each with a nucleus

Mitosis in action!

Cancer• A disorder where body cells lose the ability to

control cell growth (defective gene)• Don’t respond to regulatory signals• Tumor- a mass of cells • benign (not cancer, doesn’t spread) • Malignant (invade and destroy surrounding

tissue metastasis)

Cancer

• Smoking, chewing tobacco, radiation, genes• Treatments

– Remove tumor– Radiation – Chemotherapy uses chemicals that target rapidly

dividing cells (downside, affects normal cells also undergoing cell division)

Embryonic Stem Cells• Have the ability to differentiate into almost

any cell in body• Possibly treat damaged tissue

(heart attacks, stroke brain tissue, spinal cord)

• Apoptosis- Cells die or it is “programmed” to die.

• Important in development

• Eukaryotes have two different types of cells

– Somatic - include skin, internal organs, bones, blood, etc. (cells produced thru MITOSIS)

– Gametes - egg or sperm (cells produced thru MEIOSIS)

–Somatic cells have 46 chromosomes or 23 pairs of chromosomes, 1 set of 23 from Mom and 1 set of 23 from Dad = DIPLOID cell (2N)

–Gametes have 23 chromosomes; HAPLOID (1N) when gametes meet during conception they “fuse” together

• HOMOLOGOUS pairs of chromosomes- Pair of corresponding chromosomes (1 from mom, 1 from dad)

• Pairs 1-22 are autosomal• 23rd pair of chromosomes are sex

chromosomes (XX or XY)

A Karyotype is an Arranged Picture of Chromosomes At Their Most Condensed State

A normal human karyotye

Boy or girl?

Note that almost all chromosomes come in homologous pairs.

MitosisMitosis – process in which a eukaryotic – process in which a eukaryotic somatic cell divides into 2 identical daughter somatic cell divides into 2 identical daughter cellscellsDiploid cells Diploid cells Diploid cells Diploid cells

MeiosisMeiosis – process in which the number of – process in which the number of chromosomes is halved to produce gametes chromosomes is halved to produce gametes (ie eggs and sperm)(ie eggs and sperm)Diploid cells Diploid cells haploid cells haploid cells

Meiosis-Prophase 1 •Homologous chromosomes pair up (TETRAD)•Chromosomes swap genetic information called Crossing over

Tetrad

MEIOSIS- METAPHASE I

Tetrads line up in center of cell

MEIOSIS- ANAPHASE IHomologous chromosomes separate and move to the opposite poles

IMPORTANT: CHROMATIDS DO NOT SEPERATE

MEIOSIS- TELOPHASE INuclear membrane reforms around each cluster of chromosomesCytokinesis follows, forming two new cells

MEIOSIS- PROPHASE IINew spindle forms around the chromosomes

MEIOSIS- METAPHASE IIChromosomes line up at the equator

MEIOSIS- ANAPHASE IIChromatids move to the opposite poles

MEIOSIS- TELOPHASE IINuclear envelope forms around each new set of chromosomes, cytokinesis. 4 haploid sex cells

Haploid cells to Sperm and EggsHaploid cells to Sperm and EggsSpermatogenesisSpermatogenesis – Each of the 4 haploid cells – Each of the 4 haploid cells

become spermatozoabecome spermatozoaOogenesisOogenesis – Only 1 of the 4 haploid cells become an – Only 1 of the 4 haploid cells become an

ovum (egg cell)ovum (egg cell)

But its just a little more But its just a little more complicated than that… complicated than that…

What happens next?!What happens next?!SpermatogenesisSpermatogenesisOogenesisOogenesis

Meiosis I begins Meiosis I begins before birthbefore birth

Puberty elicits Puberty elicits continuation of continuation of Meiosis Meiosis

Hormones play major Hormones play major rolerole

Get Excited....Meiosis Get Excited....Meiosis Animation!!Animation!!

Meiosis Contributes to Genetic Variation

• 3 Main Ways lead to genetic variation– 1. Independent Assortment– 2. Crossing Over– 3. Random Fertilization