Mitosis and Cell Division

Bio 11 September 5, 2008

Microfilaments differ from microtubules in that microfilaments  

A) are larger than microtubules.  

B) are found only in plants whereas microtubules are found in plants and animal cells.  

C) are mainly composed of actin whereas microtubules are composed of tubulin.  

D) help to anchor organelles, whereas microtubules primarily function to help cells change shape and move.  

E) form the inner core of cilia and flagella whereas microtubules regulate metabolism

Cell size is determined by physical principles

• Size limit of cells determined by surface area/volume ratio

• Large living things are made of multiple cells

• Living things divide cells in order to grow

Living things are extremely complex

• Cellular machinery is sophisticated and required for life

• Blueprints for all cellular machinery are contained in genes

• Genes are inherited from parents

• Humans have ~30,000 genes

All cells require a copy of the genome

• Genome- all the DNA of the cell

• DNA is similar in all cells• Gene- 1 DNA Molecule (+

proteins the genetic information to produce a single product (protein)

• DNA replication copies all cellular DNA

Prokaryotic genomes consist of a single circular chromsome

• Chromosome- a single molecule of DNA

Prokaryotic cells reproduce by binary fission

1. DNA is copied2. Copies migrate to

opposite ends of cell3. Cell membrane/cell

wall pinches off to form two cells

4. Each daughter cell has exactly the same DNA as the parent cell

Eukaryotic cells have multiple linear chromosomes

• Eukaryotic chromosome = DNA + extra proteins inside nucleus

• Chromosomal proteins assist in DNA compaction

• Each chromosome contains different genes

• Chromosomes not always condensed and visible like this

• Chromatin – uncondensed chromosomal DNA

• In multicelluar organisms, DNA is identical in all cells (Excl. Gametes)

Diploid eukaryotes have chromsomes in pairs

• Karyotype- organized and numbered by size

• Humans: 2 x 23 homologous pairs of chromosomes = 46 total chromosomes

• Each contains ~1000 genes

• 1 or 2 strands per chromosome

Chromosome number does not correlate to complexity

• Oriental Small-clawed Otter 38 • Pea 14 • Pig 38 • Pigeon 80 • Pine Marten 38 • Potato 48 • Rabbit 44 • Raccoon 38 • Radish 18 • Rat 42 • Red Deer (Elk/Wapiti) 68 • Red Fox 38 • Red Panda 36 • Rice 24

• Dog 78 • Wolf 78 • Pigeon 80 • Turkey 82 • Hedgehog Genus Erinaceus

(Woodland hedgehogs) 88 • Hedgehog Genus Atelerix

(African hedgehogs) 90 • Ichthyomys pittieri

(semiaquatic rodent) 94 [highest for a mammmal]

• Carp 104 • Algae 148

Figure 8.3

Chromosome structure

• Decondensed chromosome- chromatin

• After copying, chromosome consists of two sister chromatids, joined at the centromere

• Kinetochore – proteins found on surface of centromere

Figure 8.5

Chromosome structure

Loci(sing:Locus)

Chromosomes look different at different stages of a cell’s life cycle

• Chromatin- uncoiled chromosome

• Not visible at this stage- being used for protein synth.

• Before cell division, strands of DNA are copied

Which one of the following is false?  

• A) Prokaryotic chromosomes are more complex than those of eukaryotes.  

• B) Most prokaryotes reproduce by binary fission.  • C) Prokaryotic cells are generally smaller and simpler

than eukaryotic cells.  • D) In prokaryotes, most genes are carried on a circular

DNA molecule.  • E) Daughter prokaryotic chromosomes are separated by

some sort of active movement away from each other and the growth of new plasma membrane between them

Each chromosome must be copied before cells can divide

• Called “chromosome” at all stages

• Sister chromatids contain identical DNA

Mitosis vs. Meiosis

• Mitosis – asexual cellular reproduction (somatic cells)– Allows multicellular organisms to grow– Daughter cells are identical to parent cell

• Meiosis – cell division for formation of gametes (eggs and sperm) – Allows sexual reproduction to generate species

diversity– Daughter cells have half the genetic information as

parent cell

The Cell Cycle• G1 – “Growth 1” or “Gap

1” – cell growth• S phase – DNA

synthesis• G2 – interval of rest

before cell division• M – mitosis • Cytokinesis – splitting of

cell contents

• GO – A state of non-division

3. CELL CYCLE~24 hours total

• 1. Interphase - between cell div.• G1 phase - cell growth, ~12 hrs• S phase - synthesize DNA, ~6 hrs• G2 phase - prep. mitosis, ~6 hrs• 2. mitosis - ~half-hour

Prophase, Metaphase, Anaphase, Telophase

• 3. Cytokinesis - ~minutes

LE 8-5

INTERPHASE

G1

G2

S(DNA synthesis)

Cytokinesis

Mito

sis

MITOTICPHASE (M)

Mitosis

• The sorting and separation of chromosomes in nucleus somatic eukaryotic cells during cell division

• Forms 2 identical daughter cells (1/2 size)

• For growth/replacement/healing

• Associated with cancer

• In humans: ~25 million/sec.

• Divided into phases

Phases of Mitosis

• Prophase*• Metaphase• Anaphase• Telophase

* Campbell text adds “prometaphase”

• Interphase – time in between mitotic phases– Normal cell functions (G1+S+G2)

• Cytokinesis- divides cytosol/organelles (creates daughter cells)

Peripheral elements of Mitosis

• Centrosomes (2) – anchors the spindle fibers, and consist (in animal cells) of 2 centrioles each

• Spindle fibers – made of microtubules, they pull apart sister chromatids of chromosomes

LE 8-6a

INTERPHASE PROPHASE PROMETAPHASE

Kinetochore

Fragmentsof nuclearenvelope

CentrosomeEarly mitoticspindleChromatin

Centrosomes(with centriole pairs)

LM

250

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Centromere Spindle microtubules

LE 8-6b

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Metaphaseplate

Spindle Daughterchromosomes

Nuclearenvelopeforming

Cleavagefurrow

Nucleolusforming

Prophase

• mitosis - in most cells (exc. gonads)

• - produces 2 daughter cells

• identical to original but half size

• - for growth + replace., also cancer

• - humans ~25 million/sec.

• Prophase - begins mitosis

• - 2-stranded chromosomes visible

• - nuclear envelope breaks up

• M etaphase - single chromosomes line up at equator

• - carried by spindle (microtubules)

• - centrosome (pairs of centrioles) in animals, anchors the spindle

• A naphase - chromosomes divide between strands

• - 1-stranded chromosomes move to opposite poles of cell

•  

• Anaphase - chromosomes divide between strands

• - 1-stranded chromosomes move to opposite poles of cell

•  

• Telophase - 2 nuclei form around separate sets of chromosomes

•  

• ( Cytokinesis - finishes mitosis )

• - divides cytosol + other organelles

• - results in 2 daughter cells

• ( Interphase - prep. next mitosis )

• - cells return to normal cell activity

• - chromosomes spread as chromatin

• - each 1-stranded chromosome duplicates 2nd strand

Cytokinesis in animal cells is different from plant cells

• Actin and myosin filaments work to contract cell in center

• Cell furrow is formed

Plant cell walls aren’t flexible

• Vesicles containing cellulose form in center of cell

• Fusion of vesicles forms cell plate

• Cell plate forms cell wall of new cells

Cancer, Meiosis, and Mendelian genetics

Mybiology.com

• Chapter 8 Key Concepts quiz- 10 lecture points

• Complete by Tues, 10/14

• >70% correct full credit

• 40-69% correct half credit

Sister chromatids are

A) found right after a cell divides.  

B) tightly linked together at a centromere.  

C) formed when chromatids separate during cell division.  

D) made only of DNA.  

E) unique to prokaryotes

Control of the mitotic cycle is criticalG1 checkpoint

G0

G1

G2

G2 checkpoint

M checkpoint

M

S

Controlsystem

External signals can activate or deactivate the cell cycle

• Signals include hormones, growth factors

• Contact inhibition, density inhibition, anchorage dependence are important signals

• If the S phase was eliminated from the cell cycle, the daughter cells would

 

A) have half the genetic material found in the parental cell.  

B) be genetically identical.  

C) be genetically identical to the parental cell.  

D) synthesize the missing genetic material on their own.  

E) None of the choices are correct.

What happens when the system of checkpoints goes awry?

LE 8-8aCells anchor todish surface

and divide.

When cells haveformed a completesingle layer, theystop dividing(density-dependentInhibition).

If some cells arescraped away, theremaining cellsdivide to fill the dishwith a single layerand then stop(density-dependentinhibition).

Mitosis facilitates growth and repairContact- and density- dependent signals tell a cell when to start and stop dividing

LE 8-8aa

Cells anchor todish surfaceand divide.

When cells haveformed a completesingle layer, theystop dividing(density-dependentinhibition).

LE 8-8ab

If some cells arescraped away, theremaining cellsdivide to fill the dishwith a single layerand then stop(density-dependentinhibition).

LE 8-8b

After forming asingle layer,cells havestopped dividing.

Providing anadditional supply ofgrowth factorsstimulatesfurther cell division.

LE 8-10

Tumor

Glandulartissue

Lymphvessels

Blood vessel

A tumor grows from asingle cancer cell.

Cancer cells invadeNeighboring tissue.

Cancer cells spread throughlymph and blood vessels toother parts of the body.

Cancer is unrestricted mitotic cell growth

Metastatic cancer is caused by an accumulation of many errors

• Density-dependent inhibition

• Apoptosis• Cell growth • Nutrient supply

(angiogenesis)• Immune system

evasion

LE 8-10a

Tumor

Glandulartissue

A tumor grows from a single cancer cell.

Cancer cells invadeneighboring tissue.

LE 8-10b

Cancer cells invadeneighboring tissue.

Cancer cells spread throughlymph and blood vessels toother parts of the body.

Lymphvessels

Bloodvessel

DNA damage causes mutations that lead to cancer

• Chromosomes can be repaired, or misrepaired, in a living cell

• Damage to whole chromosomes causes harmful mutations

LE 8-23a

Deletion

Duplication

Inversion

Homologouschromosomes

LE 8-23b

Reciprocaltranslocation

Nonhomologouschromosomes

LE 8-23c

Reciprocaltranslocation

“Philadelphia chromosome”

Activated cancer-causing gene

Chromosome 22

Chromosome 9

• Which one of the following does not occur during mitotic anaphase?  

A) The centromeres of each chromosome divide.  

B) Sister chromatids separate.  

C) The chromatid DNA replicates.  

D) Daughter chromosomes begin to move toward opposite poles of the cell.  

E) All of the choices occur during mitotic anaphase.  

Figure 8.11A

Figure 8.11B

Meiosis

Asexual Reproduction is a primary means of propagation for many species

Sexual reproduction generates diversity

• Variety in offspring improves chances of adaptation to changing environments

The offspring of sexual reproduction inherit parental

traits• Offspring inherit

DNA from each parent

• Offspring are not the same

• DNA is from the same source

LE 8-13

Haploid gametes (n 23)

Egg cell

Sperm cell

FertilizationMeiosis

Diploidzygote

(2n 46)

n

Multicellulardiploid adults

(2n 46)

Mitosis anddevelopment

2n

nMeiosis is the process of cell division that allows gamete formationGametes are haploid (n), somatic cells are diploid (2n)

Chromosomes come in pairs

• For diploid organims (2n)

• One chromosome in a paired set comes from each parent

• #’s 1-22 are autosomes

• X and Y are sex chromosomes

LE 8-12

Homologous Chromosomes

Centromere

Sister chromatids

Homologous chromosomes• Pairs of

chromosomes are homologous

• The site for particular genes are called loci (singular: locus)

• Identical strands of the same chromosome are sister chromatids

Loci

LE 13-6

Key

HaploidDiploid

Gametesn

Diploidmulticellularorganism(sporophyte)

MitosisDiploidmulticellular

organism

FERTILIZATIONMEIOSIS

Zygote

n

n

2n 2n

Animals Plants and some algae Most fungi and some protists

n n

n

n n

nn

n

n

nFERTILIZATION

FERTILIZATION

MEIOSIS

MEIOSIS

Gametes Gametes

Zygote

ZygoteMitosis

Mitosis Mitosis Mitosis Mitosis

2n2n

2n

Spores

Haploid multicellular organism (gametophyte)

Haploid multicellular organism

Overview of Meiosis

• Meiosis consists of 2 major stages, Meiosis I and Meiosis II

• Cell copies DNA once, divides twice, creating 4 haploid cells

• In meiosis I, homologous pairs of chromosomes are grouped, recombined, and then segregated into two intermediate cells

• In meiosis II, those cells are divided

The Stages of Meiosis

• In (meiosis I), homologous chromosomes separate

• Meiosis I results in two haploid daughter cells with replicated chromosomes

• In the second cell division (meiosis II), sister chromatids separate

• Meiosis II results in four haploid daughter cells with unreplicated chromosomes

LE 8-14a

INTERPHASE PROPHASE METAPHASE ANAPHASE

MEIOSIS

Centrosomes(with centriolepairs)

Sites of crossing over

Spindle

Microtubulesattached tokinetochore

Metaphaseplate

Sister chromatidsremain attached

Homologouschromosomes separate

Centromere(with kinetochore)

TetradSisterchromatidsChromatin

Nuclearenvelope

: Homologous chromosome separate

LE 13-8aa

Centrosomes(with centriole pairs)

Nuclearenvelope

Chromatin

Chromosomes duplicate

INTERPHASE

MEIOSIS I: Separates homologous chromosomes

METAPHASE I ANAPHASE I

(Interphase precedes meiosis, of course)

• Division in meiosis I occurs in four phases:Prophase I

Metaphase I

Anaphase I

Telophase I

Prophase I• >90% of the time required for

meiosis• Chromosomes condense• Synapsis: homologous

chromosomes pair up• Crossing over: nonsister

chromatids exchange DNA segments

• Tetrad: four chromatids• Chiasmata: X-shaped regions

where crossing over occurred

Crossing-Over in Prophase I

Crossing-over in Prophase I

LE 13-8ab

Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Metaphase I

• Tetrads line up at the metaphase plate

• Microtubules from one pole are attached

• Microtubules from the other pole are attached to the kinetochore of the other chromosome

Animation: Metaphase IAnimation: Metaphase I

Independent Assortment in Metaphase I

• Random alignment of maternal/paternal chromosomes at the metaphase plate

• Produces genetic variability within populations

Independent Assortment in Metaphase I

Independent Assortment in Metaphase I

Independent Assortment of Chromosomes

• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

• Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

• The number of combinations possible when chromosomes assort independently into gametes is 2n-1, where n is the haploid number

• For humans (n = 23), there are more than 8 million (222) possible combinations of chromosomes

LE 13-8ab

Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Anaphase I

• homologous chromosomes separate

• One chromosome moves toward each pole

• Sister chromatids remain attached at the centromere

LE 8-14b

Cleavagefurrow

TELOPHASE PROPHASE METAPHASE ANAPHASE

TELOPHASE

Sister chromatidsseparate

Haploid daughtercells forming

MEIOSIS : Sister chromatids separate

AND CYTOKINESIS AND CYTOKINESIS