CHAPTER 12THE CELL CYCLE

IntroductionReproduction =continuity of life

In mammals, heat loss through the skin occurs naturally. Which of the following mammals would lose heat faster………... ….….Why?

#1 1cm # 2

2cm#33cmSA= 6 cm2

V= 1 cm3

SA:V = 6:1

Calculate the Surface Area:Volume Ratios for the cubes below. (SA:V)

SA= l x w x 6 V= l x w x h

SA:V = 3:1SA:V = 2:1

Now, assume each CUBE is one CELL… As the “cell” gets larger…………•Does its surface area get greater?•Does its volume get greater?•Does it’s surface area:volume ratio get greater?•Does increased size become an advantage to the cell? Why or why not?

Think back to the Elephant and the Mouse……do you want to change your answer? Which animal will lose heat the fastest?

If you said the mouse, you are right!!!

The mouse would lose heat faster due to it’s Surface Area:Volume Ratio

When do cells divide?

1) During Growth - adult/somatic ‘stem cells’ can form new cells/tissue - birth to death

2) Embryonic development of a multicellular organism following sexual reproduction (from embryonic stem cells)

3) Repair and renew cells that die from normal wear and tear or accidents.

Note:Repair can be Regeneration also

4) Reproduction by Mitosis - ASEXUAL REPRODUCTION - a way to produce offsprings in simple unicellular organisms like bacteria (binary fission), amoeba, and in multicellular PLANTS (cuttings)!

Human somatic cells (body cells) have 46 chromosomes

Human gametes (sperm or eggs) have 23 chromosomes

Reproduction =continuity of life

CELL CYCLE

Chromosome contains genes, which are long sequences of DNA

GENE = segment of DNA

Cell division distributes identical sets of chromosomes to daughter cells

GENOME = all of a cell’s DNACHROMATIN = DNA + packaging (proteins)

CHROMOSOME = DNA strands + protein; appear during mitosis as threads

3.2 billion base pairs

3 m of DNA to be divided and separated in each round of cell division!!!!!!

Chromatin - thin, active structure of DNAFirst level of DNA foldingEvery 200 nucleotides of DNA wrap around a core of Histone proteins giving a “beads on a string” look

Chromosome

Chromatin lengths of 50,000-100,000 nucleotides are looped together by nonhistone proteins

Chromosomes pack DNA into final structure measuring 5µm long x ~1µm wide

The highly folded DNA structure - an inactive form of DNA

What is the result of a successful cell division process?

FIDELITY OF DNA REPLICATION!!

Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome’s DNA.

As they condense, the region where the strands connect shrinks to a narrow area, is the centromere. The kinetochore

is where the spindle attaches. Why do chromosomes duplicate to make sister pairs? So that they can divide and distribute during Mitosis

Fig. 12.3

How many chromosomes are there in each of the three diagrams below? How many chromatids?

4 chromosomes……. 4 chromatids

4 chromosomes……. 8 chromatids

(after DNA has replicated)

8 chromosomes……. 8 chromatids

1 Centromere = 1 chromosome

Vocab review: Sister chromatids = identical ‘post-DNA replication structures’

joined by the centromere

Chromatids - arms of a chromosome

The process of the formation of the two daughter nuclei, mitosis, is usually followed by division of the cytoplasm, cytokinesis. They are both part of the MITOSIS CELL CYCLE PHASE

The mitotic (M) phase of the cell cycle alternates with the much longer interphase. The M phase includes mitosis and cytokinesis. Interphase accounts

for 90% of the cell cycle.

The mitotic phase alternates with interphase in the cell cycle

Fig. 12.4

A Cell’s lifetime of growth & division can be referred to as a Cell Cycle

M

G1

S

G2

G0

Cells notcycling

DNA synthesis

Mitosis

The Cell Cycle -Phases

For a 24 hour cycle,M Phase lasts about1 hour.

InterphaseHas 3 subphases:

G1 –first gap phase (growth)

S phase (DNA replication)

G2 –second gap phase (preparation for cell division)

Interphase – G1The cell doubles in size, and its enzymes, ribosomes, mitochondria and other cytoplasmic molecules and structures also increase in number

Interphase – S

The DNA replicates and sister chromatids (exact copies) are formed

Interphase – G2

Centriole division is completed in animal cells, cytoplasm makes proteins in preparation for mitosis

Karyotype: Chromosomes (23 pairs in humans)

The Cell Cycle -Control: 3 important checkpoints - if they are not crossed, cell cannot divide

Most important – cells can arrest here and enter G0 (skeletal muscle cells, brain cells)

Some cells arrest here - heart cell;

Before anaphase - All chromatids have to connected to the spindle fiber

The Cell Cycle -Control

G1- Most important – growth factors, Cdk (cyclin dependent kinase), Cyclins

– MPF (Mitosis promoting factor), (Kinase + Cyclins)

– APC (Anaphase promoting

complex)

How do kinases work?

Can activate many proteins/enzymes needed for each phase by phosphorylating it (phosphorylation cascade - remember this?)

Cyclin levels rise sharply throughout interphase, then fall abruptly during mitosis.

Peaks in the activity of one cyclin-Cdk complex, MPF, correspond to peaks in cyclin concentration.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 12.14aCDK is around, but Cyclin is degraded: Why? Cyclin controls Cdk

CANCER – cells divide without arresting in G1 or G2

Cells that do not divide Nerve, & Heart Cells

Cells that do not normally divide Liver Cells

Cells that divide constantly Skin cells, Sperm

Cells, Bone Marrow cell

Benign tumors are not cancerous. They:•can usually be removed  •do not come back in most cases  •do not spread to other parts of the body and the cells do not invade other tissues

Malignant tumors are cancerous. They:•can invade and damage nearby tissues and organs •metastasize (cancer cells break away from a malignant tumor and enter the bloodstream or lymphatic system to form secondary tumors in other parts of the body)

Each of us inherited 23 chromosomes from each parent: one set in an egg and one set in sperm.

46 chromosomes 46 chromosomes

23 chromosomes 23 chromosomes

46 chromosomes

MEIOSIS = GAMETE FORMATION

MITOSIS = SOMATIC CELL DIVISION

Mitosis is a continuum of changes. Mitosis is usually broken into five subphases:

prophase, prometaphase, metaphase, anaphase, and telophase.

                              

Interphase: chromosomes have been duplicated but loosely packed (euchromatin).

The centrosomes (contain centrioles in animal cells) have been duplicated and begin to organize microtubules into an aster (“star”).

Interphase

Prophase - the chromosomes are tightly coiled, with sister chromatids joined together.

The nucleoli disappear.The mitotic spindle begins

to form; centrosomes (plant)/centrioles (animal) move toward opposite ends (poles) of the cell.

Prophase

Prometaphase, the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes.

Microtubules from one pole attach to one of two kinetochores, special regions of the centromere, while microtubules from the other pole attach to the other kinetochore.

Pole to pole fibres are

also present

Metaphase: Sister chromatids are all arranged at the metaphase plate, an imaginary plane equidistant between the poles

Metaphase

Anaphase- the centromeres divide, separating the sister chromatids.

Each is now pulled toward the pole to which it is attached by spindle fibers.

Movement of sister chromatids involves elongation of the pole to pole fibers and shortening of the pole to kinetochore fibers

By the end, the two poles have same number of chromosomes.

Anaphase

Anaphase- the centromeres divide, separating the sister chromatids.

Each is now pulled toward the pole to which it is attached by spindle fibers.

Movement of sister chromatids involves elongation of the pole to pole fibers and shortening of the pole to kinetochore fibers

By the end, the two poles have same number of chromosomes.

Anaphase

Telophase: cell continues to elongate as pole to pole spindle fibers from each centriole push off each other.

Two nuclei begin for form, surrounded by the fragments of the parent’s nuclear envelope.

Chromatin becomes less tightly coiled.

Cytokinesis, division of the cytoplasm begins.

Telophase

Cytokinesis

Prophase

Anaphase

Metaphase

Interphase

Telophase

Cytokinesis

Prophase

Anaphase

MetaphaseInterphase

Telophase

Cytokinesis

Prophase

Anaphase

Metaphase

Interphase

Telophase

Microscope observations goals:a) Draw pencil pictures and Label all parts

b) Describe what you see in the phase. How many cells did you see in that phase?Make a % estimate of how long the cell spends in each phase based on how many cells you see at that phase.

c) Compare plant (onion root tip) and animal cell (fish blastula) mitosis

Has Cell wall,

No centrioles, only centrosome

Has No Cell wall,

Centrioles present

Plant cell Mitosis Animal cell Mitosis

Interphase

Nucleolus

Chromatin

Nuclear membrane intact

Prophase

Metaphase

Anaphase

Telophase and Cytokinesis

Cytokinesis-2 Daughter Cells

Fig. 12.5 right

The mitotic spindle has fibers composed of microtubules and associated proteins

The tubulin comes from partial disassembly of the cytoskeleton.

The spindle fibers elongate by incorporating more subunits of the protein tubulin.

Mitotic Spindle Movement

Assembly of the spindle microtubules starts in the centrosome. The centrosome (microtubule-organizing

center) of animals has a pair of centrioles at the center.

.

Fig. 12.7a

Microtubules disassemble at the kinetochores causing spindle fibres to shorten in length and draw sister chromatids to the poles.

Experiments support the hypothesis that spindle fibers shorten during anaphase from the end attached to the chromosome (kinetochore), not the centriole.

Nonkinetichore pole to pole microtubules are responsible for lengthening the cell along the axis defined by the poles.

Cytokinesis, division of the cytoplasm, typically follows mitosis.

In animals, the first sign of cytokinesis (cleavage) is the appearance of a cleavage furrow in the cell surface near the old metaphase plate.

Cytokinesis divides the cytoplasm:

Fig. 12.8a

On the cytoplasmic side of the cleavage furrow (a contractile ring of actin microfilaments and the motor protein myosin form).

Contraction of the ring pinches the cell in two.

Fig. 12.8a

Cytokinesis in plants, which have cell walls, involves a completely different mechanism.

During telophase, vesicles from the Golgi coalesce at the metaphase plate, forming a cell plate. The plate enlarges until its

membranes fuse with the plasma membrane.

Fig. 12.8b

Fig. 12.9

Mitosis in eukaryotes may have evolved

from binary fission in bacteria

Fig. 12.10

Fig. 12.11

Identify the following phases