Chapter 10mmsalemscienceteacher.weebly.com/uploads/2/3/3/6/... · • Composed of chromatin –complex of DNA and protein • DNA of a single chromosome is one long continuous double-stranded

How Cells Divide

Chapter 10

2

Bacterial Cell Division

• Bacteria divide by binary fission– No sexual life cycle

– Reproduction is clonal

• Single, circular bacterial chromosome is replicated

• Replication begins at the origin of replication and proceeds in two directions to site of termination

• New chromosomes are partitioned to opposite ends of the cell

• Septum forms to divide the cell into 2 cells

3

Prior to cell division,

the bacterial DNA

molecule replicates.

The replication of the

double-stranded,

Circular DNA mole-

cule that constitutes

the genome of a

bacterium begins at a

specific site, called

the origin of replica-

tion (green area).

The replication

enzymes move out

in both directions

from that site and

make copies of each

strand in the DNA

duplex. The enzymes

continue until they

meet at another

specific site, the

terminus of replication

(red area).

As the DNA is

replicated, the cell

elongates, and the

DNA is partitioned in

the cell such that the

origins are at the ¼

and ¾ positions in the

cell and the termini

are oriented toward

the middle of the cell.

1.

2.

3.

Bacterial cell

Bacterial chromosome:

Double-stranded DNA

Origin of

replication

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4

5.

Septation then begins, in which new membrane and cell wall material

begin to grow and form a septum at approximately the midpoint of the cell.

A protein molecule called FtsZ (orange dots) facilitates this process.

When the septum is complete, the cell pinches

in two, and two daughter cells are formed,

each containing a bacterial DNA molecule.

4.

Septum


Eukaryotic Chromosomes

• Every species has a

different number of

chromosomes

• Humans have 46

chromosomes in 23

nearly identical pairs

– Additional/missing

chromosomes usually

fatal

5

950x


© Biophoto Associates/Photo Researchers, Inc.

Chromosomes

• Composed of chromatin – complex of DNA and

protein

• DNA of a single chromosome is one long

continuous double-stranded fiber

• RNA associated with chromosomes during RNA

synthesis

• Typical human chromosome 140 million

nucleotides long

• In the nondividing nucleus

– Heterochromatin – not expressed

– Euchromatin – expressed 6

Chromosome Structure

• Nucleosome

– Complex of DNA and histone proteins

– Promote and guide coiling of DNA

– DNA duplex coiled around 8 histone proteins

every 200 nucleotides

– Histones are positively charged and strongly

attracted to negatively charged phosphate

groups of DNA

7

8

DNA Double Helix (duplex) Nucleosome

Histone coreDNA


• Nucleosomes wrapped into higher order

coils called solenoids

– Leads to a fiber 30 nm in diameter

– Usual state of nondividing (interphase)

chromatin

• During mitosis, chromatin in solenoid

arranged around scaffold of protein to

achieve maximum compaction

– Radial looping aided by condensin proteins

9

10


Mitotic Chromosome

Chromatin loop

Solenoid

Scaffold

proteinScaffold protein

Chromatin LoopRosettes of Chromatin Loops

Replication

• Prior to replication, each chromosome composed of a single DNA molecule

• After replication, each chromosome composed of 2 identical DNA molecules

– Held together by cohesin proteins

• Visible as 2 strands held together as chromosome becomes more condensed

– One chromosome composed of 2 sister chromatids

13

14


Homologous chromosomes Homologous chromosomes

Cohesin

proteins

Kinetochores

Sister chromatids

Sister chromatids

Kinetochore

Centromere

Replication

Eukaryotic Cell Cycle

1. G1 (gap phase 1)– Primary growth phase, longest phase

2. S (synthesis)– Replication of DNA

3. G2 (gap phase 2)– Organelles replicate, microtubules

organize

4. M (mitosis)– Subdivided into 5 phases

5. C (cytokinesis)– Separation of 2 new cells

15

Interphase

16

Duration

• Time it takes to complete a cell cycle varies greatly

• Fruit fly embryos = 8 minutes

• Mature cells take longer to grow– Typical mammalian cell takes 24 hours

– Liver cell takes more than a year

• Growth occurs during G1, G2, and S phases– M phase takes only about an hour

• Most variation in length of G1

– Resting phase G0 – cells spend more or less time here

17


M Phase

Metaphase

Anaphase

Telophase

Prometaphase

Prophase

S

G2

G1

Interphase

M Phase

G1

Cytokinesis

Mitosis

S

G2

18

Interphase

• G1, S, and G2 phases

– G1 – cells undergo major portion of growth

– S – replicate DNA

– G2 – chromosomes coil more tightly using motor

proteins; centrioles replicate; tubulin synthesis

• Centromere – point of constriction

– Kinetochore – attachment site for microtubules

– Each sister chromatid has a centromere

– Chromatids stay attached at centromere by cohesin

• Replaced by condensin in metazoans

19


Cohesin

proteins

Centromere

region of

chromosome

Kinetochore

microtubules

Kinetochore

Metaphase

chromosome

Sister

chromatids

20

Nucleus

Nucleolus

Aster

Centrioles

(replicated;

animal

cells only)

Nuclear

membrane

• DNA has been replicated

• Centrioles replicate (animal cells)

• Cell prepares for division

Chromatin

(replicated)


21

M phase

Mitosis is divided into 5 phases:

1. Prophase

2. Prometaphase

3. Metaphase

4. Anaphase

5. Telophase

22

Prophase

• Individual condensed chromosomes first

become visible with the light microscope

– Condensation continues throughout prophase

• Spindle apparatus assembles

– 2 centrioles move to opposite poles forming spindle

apparatus (no centrioles in plants)

– Asters – radial array of microtubules in animals (not

plants)

• Nuclear envelope breaks down

23

• Chromosomes condense and

become visible

• Chromosomes appear as two

sister chromatids held together

at the centromere

• Cytoskeleton is disassembled:

spindle begins to form

• Golgi and ER are dispersed

• Nuclear envelope breaks down

Mitotic spindle

beginning to form

Condensed

chromosomes


24

Prometaphase

• Transition occurs after disassembly of nuclear

envelope

• Microtubule attachment

– 2nd group grows from poles and attaches to

kinetochores

– Each sister chromatid connected to opposite poles

• Chromosomes begin to move to center of cell –

congression

– Assembly and disassembly of microtubules

– Motor proteins at kinetochores

25

• Chromosomes attach to

microtubules at the kinetochores

• Each chromosome is oriented

such that the kinetochores

of sister chromatids are

attached to microtubules

from opposite poles.

• Chromosomes move to

equator of the cell

Centromere and

kinetochore

Mitotic

spindle


Metaphase

• Alignment of

chromosomes

along metaphase

plate

– Not an actual

structure

– Future axis of cell

division

26

Polar

microtubule

Centrioles

Metaphase

plate

Aster

Kinetochore

microtubule

Sister chromatids

57µm


© Andrew S. Bajer, University of Oregon

27

• All chromosomes are aligned

at equator of the cell, called

the metaphase plate

• Chromosomes are attached

to opposite poles and are

under tension

Polar microtubule

Chromosomes

aligned on

metaphase plateKinetochore

microtubule


28

Anaphase

• Begins when centromeres split

• Key event is removal of cohesin proteins

from all chromosomes

• Sister chromatids pulled to opposite poles

• 2 forms of movements

– Anaphase A – kinetochores pulled toward

poles

– Anaphase B – poles move apart

29

Chromosomes

• Proteins holding centromeres of

sister chromatids are degraded,

freeing individual chromosomes

• Chromosomes are pulled to

opposite poles (anaphase A)

• Spindle poles move apart

(anaphase B)

Kinetochore

microtubule

Polar

microtubule


30

Telophase

• Spindle apparatus disassembles

• Nuclear envelope forms around each set

of sister chromatids

– Now called chromosomes

• Chromosomes begin to uncoil

• Nucleolus reappears in each new nucleus

31

Polar microtubule

Nucleus reforming

• Chromosomes are clustered at

opposite poles and decondense

• Nuclear envelopes re-form around

chromosomes

• Golgi complex and ER re-form

Kinetochore

microtubule


32

Cytokinesis

• Cleavage of the cell into equal halves

• Animal cells – constriction of actin filaments produces a cleavage furrow

• Plant cells – cell plate forms between the nuclei

• Fungi and some protists – nuclear membrane does not dissolve; mitosis occurs within the nucleus; division of the nucleus occurs with cytokinesis

33


25 µmb.325 µma.a: © David M. Phillips/Visuals Unlimited; b: © Guenter Albrecht-Buehler, Northwestern University, Chicago

34


Vesicles containing

membrane components

fusing to form cell plate

Cell wall

19,000×

(top): © E.H. Newcomb & W.P. Wergin/Biological Photo Service

35

Control of the Cell Cycle

Current view integrates 2 concepts

1.Cell cycle has two irreversible points

– Replication of genetic material

– Separation of the sister chromatids

2.Cell cycle can be put on hold at specific points

called checkpoints

– Process is checked for accuracy and can be halted if

there are errors

– Allows cell to respond to internal and external signals

3 Checkpoints

1. G1/S checkpoint

– Cell “decides” to divide

– Primary point for external signal influence

2. G2/M checkpoint

– Cell makes a commitment to mitosis

– Assesses success of DNA replication

3. Late metaphase (spindle) checkpoint

– Cell ensures that all chromosomes are

attached to the spindle36

37


G2/M checkpoint Spindle checkpoint

G1/S checkpoint

(Start or restriction point)

38

Cyclin-dependent kinases (Cdks)

• Enzymes that phosphorylate proteins

• Primary mechanism of cell cycle control

• Cdks partner with different cyclins at different

points in the cell cycle

• For many years, a common view was that

cyclins drove the cell cycle – that is, the periodic

synthesis and destruction of cyclins acted as a

clock

• Now clear that Cdk itself is also controlled by

phosphorylation

39


G2/M Checkpoint

Cdc2/Mitotic Cyclin

• DNA integrity

• Replication

completed

Spindle Checkpoint

APC

• Chromosomes

attached at

metaphase plate

M

G2

G1S

G1/S Checkpoint

• Size of cell

• Nutritional state

of cell

• Growth factors

Cdc2/G1 Cyclin

• Cdk – cyclin complex

– Also called mitosis-promoting factor (MPF)

• Activity of Cdk is also controlled by the pattern of

phosphorylation

– Phosphorylation at one site (red) inactivates Cdk

– Phosphorylation at another site (green) activates Cdk

40

Cyclin-dependent kinase

(Cdk)

Cyclin

P

P


41

MPF

• Once thought that MPF was controlled solely by

the level of the M phase-specific cyclins

• Although M phase cyclin is necessary for MPF

function, activity is controlled by inhibitory

phosphorylation of the kinase component, Cdc2

• Damage to DNA acts through a complex

pathway to tip the balance toward the inhibitory

phosphorylation of MPF

42

Anaphase-promoting complex (APC)

• Also called cyclosome (APC/C)

• At the spindle checkpoint, presence of all

chromosomes at the metaphase plate and

the tension on the microtubules between

opposite poles are both important

• Function of the APC/C is to trigger

anaphase itself

• Marks securin for destruction; no inhibition

of separase; separase destroys cohesin

43

Control in multicellular eukaryotes

• Multiple Cdks control the cycle as opposed

to the single Cdk in yeasts

• Animal cells respond to a greater variety of

external signals than do yeasts, which

primarily respond to signals necessary for

mating

• More complex controls allow the

integration of more input into control of the

cycle

44


G2/M Checkpoint

Cdk1/Cyclin B

• DNA integrity

• Replication

completed

Spindle Checkpoint

APC

• Chromosomes

attached at

metaphase plate

M

G2

G1S

G1/S Checkpoint

• Size of cell

• Nutritional state

of cell

• Growth factors

Cdk2/Cyclin E

Growth factors

• Act by triggering intracellular signaling

systems

• Platelet-derived growth factor (PDGF) one

of the first growth factors to be identified

• PDGF receptor is a receptor tyrosine

kinase (RTK) that initiates a MAP kinase

cascade to stimulate cell division

• Growth factors can override cellular

controls that otherwise inhibit cell division45

46

GTP

5. MAP kinase (ERK)

activates proteins to

produce cellular

responses, including

transcription factors

that alter gene expression

4. MEK activates

MAP kinases (ERK)

1. Proteins bound to

receptor activate

Ras by exchanging

GDP for GTP.

2. Ras activates

the first

kinase (Raf)

3. Raf activates

the second

Kinase (MEK)

Growth factor

RAS

Cyclins/

proteins

for Sphase

ChromosomeP

Rb

Nucleus

E2F

Rb

ERK

MEK

P

PP

P

RAF

MEK

RAF

ERK

P

PE2F

PP

P

P

RAS


GDP

47

Cancer

Unrestrained, uncontrolled growth of cells

• Failure of cell cycle control

• Two kinds of genes can disturb the cell

cycle when they are mutated

1. Tumor-suppressor genes

2. Proto-oncogenes

48

Tumor-suppressor genes

• p53 plays a key role in G1 checkpoint

• p53 protein monitors integrity of DNA

– If DNA damaged, cell division halted and repair

enzymes stimulated

– If DNA damage is irreparable, p53 directs cell to kill

itself

• Prevent the development of mutated cells

containing mutations

• p53 is absent or damaged in many cancerous

cells

49


1. DNA damage is caused by

heat, radiation, or chemicals.

2. Cell division stops, and p53 triggers

enzymes to repair damaged region.

3. p53 triggers the destruction of

cells damaged beyond repair.

p53 allows cells with

repaired DNA to divide.

1. DNA damage is caused by

heat, radiation, or chemicals.

2. The p53 protein fails to stop cell

division and repair DNA. Cell divides

without repair to damaged DNA.

3. Damaged cells continue to divide.

If other damage accumulates, the

cell can turn cancerous.

DNA repair enzyme

Cancer cell

p53

protein

No

rma

l p

53

A

bn

orm

al

p5

3

Abnormal

p53 protein

50

Proto-oncogenes

• Normal cellular genes that become oncogenes

when mutated

– Oncogenes can cause cancer

• Some encode receptors for growth factors

– If receptor is mutated in “on,” cell no longer depends

on growth factors

• Some encode signal transduction proteins

• Only one copy of a proto-oncogene needs to

undergo this mutation for uncontrolled division to

take place

Tumor-suppressor genes

• p53 gene and many others

• Both copies of a tumor-suppressor gene

must lose function for the cancerous

phenotype to develop

• First tumor-suppressor identified was the

retinoblastoma susceptibility gene (Rb)

– Predisposes individuals for a rare form of

cancer that affects the retina of the eye

51

• Inheriting a single mutant copy of Rb means the

individual has only one “good” copy left

– During the hundreds of thousands of divisions that

occur to produce the retina, any error that damages

the remaining good copy leads to a cancerous cell

– Single cancerous cell in the retina then leads to the

formation of a retinoblastoma tumor

• Rb protein integrates signals from growth factors

– Role to bind important regulatory proteins and prevent

stimulation of cyclin or Cdk production

52

53


Proto-oncogenes

Growth factor receptor:

more per cell in many

breast cancers.

Ras protein:

activated by mutations

in 20–30% of all cancers.

Src kinase:

activated by mutations

in 2–5% of all cancers.

Tumor-suppressor Genes

Rb protein:

mutated in 40% of all cancers.

p53 protein:

mutated in 50% of all cancers.

Cell cycle

checkpoints

Ras

protein

Rb

protein p53

protein

Src

kinase

Mammalian cell

Nucleus

Cytoplasm

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Chapter 10mmsalemscienceteacher.weebly.com/uploads/2/3/3/6/... · • Composed of chromatin –complex of DNA and protein • DNA of a single chromosome is one long continuous double-stranded