Ch. 8 Cell ReproductionWhy do cells make new cells?

1. Growth2. Maintenance – cells wear out3. Repair - injury4. Regeneration – lost body part5. Reproduce asexually

8.1 Some organisms reproduce asexually• Their offspring are genetic copies of the

parent and of each other

Figure 8.1A

– All organisms can reproduce sexually• Creating variety in the offspring

Figure 8.1B

8.3 Prokaryotes reproduce by binary

fission

• One chromosome• Chromosome copies itself• Cell lengthens,

copies move apart• Cell membrane

divides cell in two

Eukaryotic nuclei contain many chromosomes

Each chromosome contains thousands of genes - must be organized before cell can dividing

One-celled protists also do fission, but must do mitosis first to divide

chromosomes evenly

Colorized TEM 32,500

Prokaryotic chromosomes

LM

34

0

8.5 The cell cycle - two phases

INTERPHASE

S(DNA synthesis)

G1

G2

Cytokin

esis

Mito

sisMITOTIC

PHASE (M)

Figure 8.5

Interphase – most of cell life - non-dividing cell

Mitotic (M) phase – dividing cell

Cell Cycle

G1 growth, normal life functions

S “synthesis” – DNA replicates

G2 final growth; prepares to divide

MITOTIC PHASE

Mitosis – chromosomes condense, organize and divide

- each new cell gets one copy of every chromosome

Cytokinesis – cytoplasm divides

INTERPHASE

Some cells divide oftenBone marrow stem cells

Plants and simple organisms

Skin cells Digestive lining cells

embryo

Some divide rarely or not at all

Muscle cells Cartilage or tendon cells

Nerve cells

BEFORE a cell divides

•DNA is ALREADY COPIED (replicated)

•coils, packs, condenses Forms dense “CHROMOSOMES”

•keeps copies organized and intact until cell splits

DNA condensation

DNA wraps around

histones

DNA

One chromosome (copied and packed

for cell division)

Groups of histones form nucleosomes

This coils and wraps until it all fitsinto the nucleus.

These group to form chromatin

CHROMOSOMES CONDENSE

BEFORE CELL DIVISION

Sister chromatidsidentical copies

Centromereholds chromatids

together

Double-stranded (replicated)

chromosome

SEM of human chromosomes

Prokaryotes have a single, circular chromosome, no histones; no nucleus

Eukaryotic chromosomes

Nucleus in non-dividing cell

Chromosome is in CHROMATIN form

Early mitosis

CHROMATIN begins to condense before a

cell can divide

Ch.10 Chromosomes of eukaryotes duplicate in cell division

Before cell divides - DNA replicates

a. Helicase “unzips” molecule

b. Original strands are templates

c. Free nucleotides in nucleus

d. Polymerase – base-pairing rules

two identical molecules

Semi-conservative replication

• Original strands serve as templates

• New molecules have one original strand and one new strand

PHASES OF MITOSISPROPHASE – cell organizes, prepares

•chromatin •nuclear membrane, nucleolus •spindle and asters •centrioles

PROMETAPHASE – chromosomes condensed•move toward middle •centrioles to opposite poles•spindle

METAPHASE – chromosomes in middle•Centromeres and spindle fibers

– The stages of cell division

INTERPHASE PROPHASE PROMETAPHASE

LM 2

50

Chromatin

Centrosomes(with centriole pairs)

Nucleolus

Nuclearenvelope

Plasmamembrane

Early mitoticspindle

Centrosome

CentromereChromosome, consistingot two sister chromatids

Spindlemicrotubules

Kinetochore

Fragmentsof nuclearenvelope

Figure 8.6 (Part 1)

ANAPHASE -chromosomes separate• spindle fibers pull • single copies to opposite poles of cell

Cleavage furrow

CYTOKINESIS - division of cytoplasm• identical daughter cells

TELOPHASE – return to normal• chromosomes relax/uncoil into chromatin• nuclear membranes form; nucleoli appear• spindle fibers disappear

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate

Daughterchromosomes

Nuclearenvelopeforming

Cleavagefurrow

Nucleolusforming

Figure 8.6 (Part 2)

8.7 Animal cytokinesis

• Microfilaments

• Cleavage furrow

• Pinch cytoplasm in two

Cleavagefurrow

SE

M 1

40

Daughter cells

Cleavage furrow Contracting ring ofmicrofilaments

Figure 8.7A

Cell division differs for plant and animal cells

Figure 8.7B

TE

M 7

,500

Cell plateforming

Wall ofparent cell

Daughternucleus

Cell wall New cell wall

Vesicles containingcell wall material

Cell plateDaughter cells

In plants

- no centrioles or asters

(has spindle)

- cell plate

- new cell wall grows

along sides of cell plate

Plant cell mitosis

Control of Cell Division

Chemical signals tell a cell when to divide

Some made by cell; some from cell environment

Normal rate: growth, repair

- asexual reproduction in some organisms

Slow rate: some cell types divide rarely (liver)

- aging slow healing, lose some cells

Fast rate: some cell types (skin, digestive lining)

- embryo, growing zones (bones, root tips)

Uncontrolled cancer

Growth factors• Proteins secreted by cells, can start or stop cell division

• When cells crowded, growth factors used up

stop dividing

After forming asingle layer,cells havestopped dividing.

Providing anadditional supply ofgrowth factorsstimulatesfurther cell division.

Figure 8.8B

Cyclins - proteins produced by cell at constant rate - when reach high level, tell cell to divide

Programmed Cell Deathapoptosis

1. Cells damaged too much to repair – self-destruct

2. Embryonic development – lose unneeded cells

8.10 Cancer – uncontrolled cell division

• Cancer cells ignore cycle controls

– Repeated and rapid cell divisions; immortal• tumor – mass of non-functioning cells

– Benign – not cancer; do not spread to nearby tissue

– May get large enough to stop tissue function

• Malignant tumor – cancer

– Will invade and destroy neighboring tissue

• Metastatic tumor – spreads to other body parts– Starts new cancers

Cumulative DNA damageCells begin to divide abnormally

Continue dividing, invade healthy tissue

(malignant)

Spread through blood or lymph stream, can start new tumors

(metastatic)

Tumor

Glandulartissue

A tumor grows from asingle cancer cell.

Cancer cells invadeneighboring tissue.

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

Lymphvessels

Bloodvessel

Figure 8.10

Cells look abnormal – in structure; size of nucleus :: cytoplasm

• If not treated early, cancer will spread

• kills by destroying organ function

BE CAREFUL when sunning• Wear sunscreen, sunglasses

• Avoid brightest part of the day

• Don’t use tanning beds

The most common cancer - skin

Melanoma -deadliest

Basal cell

Squamous Cell

Melanoma

Cancer treatmentRadiation – high-energy, carefully aimed at tumor

Chemotherapy – drugs specific for tumor types

• interfere with cell division

• some normal cells destroyed, too

– Skin (lose hair); digestive lining (nausea)

Surgery – remove tumor and nearby cells

Boost immune system – healthy diet, reduce stress

Asexual Reproduction

New organism from one parent

– offspring identical genetically

Advantages:

1) no mate – good for solitary, sessile organisms or when population density is low

2) continues a successful gene makeup

3) usually fast, large numbers

Disadvantage: no genetic variation

Asexual Reproduction One-celled organisms, some simple multicelled Hydra with bud vegetative propagation in plants

LM

10

Yeast, sponges

can also bud

Regeneration – replace lost body part

starfish can replace an arm flatworms can regrow a body

Lizards can replace a tail

Stem cells and differentiation

Differentiate – cells specialize as embryo develops

Stem cell – unspecialized cell

- can differentiate to form many kinds of cells

- depends on chemical signals from nearby cells

Multipotent or totipotent

Stem cells for cloning tissues

Meiosis for Sexual Reproduction

Sexual Reproduction - two parents

a. Offspring are genetic mix of both parents

b. Have a NEW combination of genes

Advantage – genetic variation in offspring

a. Some may have traits that favor survival

b. Can pass these traits on to offspring

c. Darwin’s theory - “ survival of the fittest”

d. Variation in individuals allows a species to evolve 

CONJUGATION

a. Recipient cell gets new genes b.

Bacteria and protists

Sexual Reproduction in bacteria and protists

Complex organisms – make special cells

a. gametes – sperm and egg

b. Gametes combine in fertilization

- make a zygote new organism

Chromosome Number: Diploid and Haploid

Homologous chromosomes

a. matched chromosome pairs

b. one member of pair from each parent

c. carry genes for the same traitsd. 22 autosome pairs; one pair sex chromosomes X, Y

Gene for one trait

Cells with paired chromosomes are diploid

a. 2n (n = number)

b. Human: 2n = 46 (23 pairs)

b. Somatic (body) cells are diploid

Fruit fly 2n = 8

2 sets of chromosomes

- 2 of every gene

Locus – location of gene on a

chromosome

Meiosis is “Reduction Division”

gametes – sperm and egg

Haploid (n) - one set of chromosomes

- one member from each homologous pair

human: n = 23

Meiosis Reduces the Chromosome Number

2n parent cell

DNA replicates in interphase

First division – pairs separate

Second division – sister chromatids separate

4 haploid daughter cells

Homologous pairs separate in MEIOSIS

TWO cell divisions

- Daughter cells have ½ parent

chromosome number

Diploid cell - Has pairs

(2n=2)

Haploid cells - (n = 1)

Meiosis I - Pairs separate

(n = 1)

Meiosis II - copies separate

(n = 1)

Crossing over – only in meiosis

a. In Prophase I

b. Homologous chromatids trade pieces

c. Increases genetic variation

Meiosis I: homologous pairs separate - makes two daughter cells, but sister chromatids are still attached

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes (with centriole pairs)

Sites of crossing over

Spindle

Microtubulesattached to kinetochore

Metaphaseplate

Sister chromatids remain attached

Nuclearenvelope Chromatin

Sisterchromatids Tetrad

Centromere(with kinetochore)

Homologouschromosomes separate

2n parent cell synapsis pairs line up pairs separate disjunction

Meiosis II: sister chromatids separate 4 haploid cells

PROPHASE II METAPHASE II ANAPHASE II

TELOPHASE IAND CYTOKINESIS

TELOPHASE IIAND CYTOKINESIS

Cleavagefurrow

Haploid daughter cellsforming

Sister chromatidsseparate

MEIOSIS II: Sister chromatids separate

2n n two daughter cells chromatids 4 daughters one chromosome set each separate one set

two copies (sisters) disjunction single copies

8.15 Review: Comparing mitosis and meiosis

Mitosis Meiosis

Parent cell(before chromosome replication)

Chromosome replication

Chromosome replication

Chromosomes align at themetaphase plate

Tetradsalign at themetaphase plate

Sister chromatidsseparate during anaphase

Homologous chromosomesseparate duringanaphase I;sister chromatidsremain together

No furtherchromosomalreplication; sisterchromatidsseparateduringanaphase II

Prophase

Metaphase

AnaphaseTelophase

Duplicated chromosome(two sister chromatids)

Daughter cellsof mitosis

2n 2n

Daughtercells of

meiosis I

n n nn

2n = 4

Tetrad formedby synapsis ofhomologouschromosomes

Meiosis i

Meiosis ii

Prophase I

Metaphase I

Anaphase ITelophase I

Haploidn = 2

Daughter cells of meiosis II

2n copies

2n copies

2n

single

2n

copies

2n

copies

n

copies

n

single

Causes of genetic variation

1. Homologous pairs have different genes

• same traits, but may be different forms

2. Crossing over – homologs trade pieces before

separating new gene combinations

3. Pairs position in Metaphase I - independent

• n pairs 2n possible combinations

4. Random fertilization of eggs by sperm

• Any egg or sperm is equally likely to be used

5. Gene or chromosome mutation

- Error in replication or cell division

– Many different gene combinations in haploid gametes

Combination 1 Combination 2 Combination 3 Combination 4

Gametes

Metaphase II

Two equally probablearrangements of chromosomes at

metaphase I

Possibility 1 Possibility 2

Figure 8.16

8.16 Chromosomes line up randomly in meiosis

Making sperm and egg

Sperm:2n parent cell 4 haploid sperm

Ovum:2n parent cell 1 haploid egg+ haploid polar bodies

Ovum needs all the cytoplasm

Ovum and polar body (0.1mm)Sperm needs only DNA

- and flagellum

- and mitochondria for power

- and acrosome to penetrate ovum

When meiosis goes wrong

Nondisjunction

- do not separate correctly

In mitosis defective nucleus, cell usually dies

In meiosis defective gamete

wrong number of chromosomes in zygote

8.21 Accidents during meiosis can alter chromosome number

Nondisjunction in meiosis I

Normal meiosis II

Gametes

n 1 n 1 n 1 n 1

Number of chromosomes

Nondisjunction in meiosis II

Normal meiosis I

Gametes

n 1 n 1 n n

Number of chromosomes

Nondisjunction in meiosis I Nondisjunction in meiosis II All gametes abnormal Some gametes normal

• Fertilization after nondisjunction trisomy in zygote

Sperm cell

Egg cell

n (normal)

n + 1

Zygote2n + 1

Abnormal chromosome number = aneuploidy

Trisomy = 3

Wrong chromosome number in zygote wrong number in every cell in organism

KARYOTYPE picture of a person’s chromosomes

Photographed during mitosis

- sorted into homologous pairs

- largest-to-smallest

- sex chromosomes last

Abnormalities visible:

- missing or extra

- pieces broken or moved

- pieces added or lostautosomes sex chrom.

Trisomy 21

Normal male karyotype

Normal female karyotype

Down SyndromeTrisomy chromosome # 21

8.22 Abnormal number of sex chromosomes usually do not affect survival in humans

• Nondisjunction of large chromosomes is usually lethal

• Down Syndrome - # 21 is very small, carries few genes

• In sex chromosomes, leads to varying degrees of

malfunction, but usually not lethal

Turner Syndrome XO

Figure 8.22B

Characteristic facialfeatures

Web of skin

Constrictionof aorta

Poor breastdevelopment

Under developedovaries

Turner Syndrome

Klinefelter Syndrome XXY

Klinefelter Syndrome

Abnormalities of Sex Chromosomes in Humans

8.23 Other chromosome changes can cause birth defects and cancer

Chromosomes break – pieces lost or rearranged

- in somatic cells increases cancer risk

- in gametes genetic disorders

Deletion

Duplication

Inversion

Homologouschromosomes

Translocation

Translocation

Activates a cancer-causing gene