10 – 1: Cell Growth and Division

How do living things grow?

• Grow by producing more cells. (Cells do not increase in size)– A human adult has about 10 trillion – 100

trillion cells in their entire body.– About how many cells does a newborn

baby have?Answer: Much less

Cells Dividing

Blood Lily

Limits to Cell Growth

• 2 reasons why cells divide rather than grow?1. The larger cell has more trouble moving

nutrients and waste across the cell membrane.2. The larger the cell, the more demand the cell

places on its DNA.

Exchanging Material• What substances may move through the cell

membrane?Answer: Food, oxygen and water enters.

Waste leaves the cell.• The rate materials exchange depends on the

surface area of the cell• The rate materials are used depends on the

cell’s volume (size).

Ratio of Surface Area to Volume • Surface to volume ratio• Volume increases faster than

surface– The cell uses materials faster

than it can get them in

Asexual Reproduction– Asexual reproduction - a single parent

producing an offspring. The offspring produced are, in most cases, genetically identical to the parent.

– Asexual reproduction is a simple, efficient, and rapid way for an organism to produce a large number of offspring.

Sexual Reproduction– In sexual reproduction, offspring are

produced by the fusion of two sex cells – one from each of two parents.

– The offspring produced inherit some genetic information from both parents, therefore they are genetically different.

• Cells divide to form two new cells called daughter cells

• This process is called mitosis (cell division)• Before it can occur, what has to happen? The cell

replicates, or copies, all its DNA• DNA is condensed into a manageable form

(chromosome) so it can be divided precisely

10.2 The Process of Cell Division

Chromosomes• Chromosomes – bundled packages of DNA that contain

genetic information• Every organism has a specific number of chromosomes

– Fruit flies – 4– Dog - 78– Carrots – 18 – How many chromosomes do humans have? 46 (23 pairs)

Chromosomes• Chromosomes are only visible

when the cell divides.• Why is this?

DNA and protein molecules are spread throughout the nucleus in the form of Chromatin.

TEM

36

,000

Centromere

Sister chromatids

Exact copies of each other

Chromosomes (a closer look)• Before division, the chromosome (DNA) is replicated• The replicated chromosome consists of 2 identical

“sister” chromatids.– Held together near the center by centromere

• The chromosome “X” shape we usually see drawn is a duplicated chromosome made of supercoiled chromatin

Eukaryotic Cell CycleThe cell cycle represents the events in the

life of a cell.

InterphaseInterphase Growth Phase most time Growth Phase most time spent in this phasespent in this phase

G1 G1 Cell growthCell growthS S Replication of DNAReplication of DNAG2 G2 Final growth and prepare for Final growth and prepare for

divisiondivision

Mitosis Mitosis (M phase) (M phase) Division of the Division of the nucleus (can last hours to a few days)nucleus (can last hours to a few days)

4 Phases:4 Phases:1.1. ProphaseProphase2.2. MetaphaseMetaphase3.3. AnaphaseAnaphase4.4. TelophaseTelophase

• Chromatin condenses into chromosomes.• The nucleus begins to disappear• Spindle fibers begin to form at centrioles

• Chromosomes move to the center of the cell• Centrioles migrate to opposite ends of the cell• Spindle fibers attach to the chromosomes at the

centromere

• Sister Chromatids are pulled apart by spindle fibers (microtubules) connected to the centromere• Nucleus begins to re-form

• Cleavage furrow forms• Nucleus continues to form• Cytokinesis Occurs (cells actually divide)• Two diploid cells have been formed

Mitosis – 4 Stages of Cell Division

Cytokinesis• Division of the cytoplasm• Occurs at the same time as telophase

Actin (blue) and microtubules (orange) at the end of cytokinesis in a green urchin zygote.

Cytokinesis - Animal

• Animal cells are surrounded by a cell membrane

Animal Cell Formation of a cleavage furrow

Cleavagefurrow

SE

M 1

40

Daughter cells

Cleavage furrow Contracting ring ofmicrofilaments

Cytokinesis - Plant

• Plant cells are surrounded by a Plant cells are surrounded by a cell wallcell wall

Plant CellPlant Cell Formation of cell Formation of cell plateplate

TEM

7,5

00

Cell plateforming

Wall ofparent cell

Daughternucleus

Cell wall New cell wall

Vesicles containingcell wall material

Cell plate Daughter cells

Mitosis Animation

5.4.3.

2.

1.

4.3.2.1.

Cleavagefurrow

Haploid daughter cellsforming

Sister chromatidsseparate

4.3.2. 5.1.

Questions for whiteboards:

• Show how a cell looks normally while it’s doing it’s job as a tissue, muscle, bone or nerve cell. Focus on what genetic material looks like in nucleus

Questions for whiteboards:

• Show how a cell would look as it’s getting ready to divide. Again, focus on nucleus and genetic material

Questions for whiteboards:

• Using two circles, “X”s and a mitochondria show why efficiency is different between large and small cells.

Questions for whiteboards:

• List some problems that cells might encounter if they were to grow to large.