Cell Growth and Division

Chapter 5

THE CELL CYCLE

5.1

Where it all began…

• You started as a cell smaller than a period at the end of a sentence…

And now look at you!!

Getting from there to here…

• Going from egg to baby…. the original fertilized egg has to divide…

and divide… and divide… and divide…

Why do cells divide?

• For reproduction – asexual reproduction

• one-celled organisms

• For growth – from fertilized egg to

multi-celled organism

• For repair & renewal – replace cells that die

from normal wear & tear or from injury

Key Concept

• Cells have distinct phases of growth, reproduction, and normal functions

The Cell Cycle

• Is a regular pattern of growth, DNA replication, and cell division

4 Main Stages

• Gap 1 (G1): – cell growth and normal

functions • DNA synthesis (S):

– copies DNA • Gap 2 (G2):

– additional growth • Mitosis (M):

– division of the cell nucleus (mitosis) and division of the cell cytoplasm (cytokinesis)

Cell Division Rates

• Rate of cell division varies with the need for those types of cells

Cell Size

• Cell size is limited – cells can only get so big!

– surface area: volume ratio is important

Surface Area

• Must allow for adequate exchange of materials – cell growth is coordinated with division

– cells that must be large have unique shapes

MITOSIS AND CYTOKINESIS

5.2

Key Concept

• Cells divide during mitosis and cytokinesis

Mitosis

• Chromosomes condense at the start of mitosis – DNA wraps around proteins

DNA double helix

DNA and histones

Chromatin Supercoiled DNA

C-words

• DNA + proteins = chromatin

• ½ chromosome = chromatid

• Sister chromatids held together by a centromere

Goal of Mitosis

• To produce two genetically identical daughter cells

• Interphase – prepares the cell

for division

– DNA is duplicated

Prophase

• Chromosomes condense and spindle fibers form

Metaphase

• Chromosomes line up in the middle of the cell

Anaphase

• Sister chromatids separate to opposite sides of the cell

Telophase

• The new nuclei form and chromosomes begin to uncoil

Cytokinesis

• In animal cells, the membrane pinches closed

• In plant cells, a cell plate forms

REGULATION OF THE CELL CYCLE

5.3

Key Concept

• Cell cycle regulation is necessary for healthy growth

Factors to Regulate Cell Division

• External factors: – physical and chemical signals

– Ex: contact inhibition

Factors to Regulate Cell Division

• Internal factors: – kinases (enzymes)

– cyclins (protein involved in the progression of cells through the cell cycle)

Apoptosis

• Programmed cell death – a normal

feature of healthy organisms

– a cell produces self-destructive enzymes

– occurs in development of infants

webbed fingers

Cancer

• Uncontrolled cell division

• Cancer cells form disorganized clumps (tumors) – Benign tumors remain clustered and can be

removed.

– Malignant tumors metastasize, or break away, and can form more tumors

Cancer

• Cancer cells do not carry out necessary functions.

• Cancer cells come from normal cells with damage to genes involved in cell-cycle regulation

Carcinogens

• Substances known to cause cancer

• Standard cancer treatments typically kill both cancerous and healthy cells

ASEXUAL REPRODUCTION

5.4

Key Concept

• Many organisms reproduce by cell division

Binary Fission

• Similar to mitosis

• Asexual reproduction

• Creation of offspring from one parent – produces two

daughter cells genetically identical to the parent cell

– occurs in prokaryotes

parent cell

DNA duplicates

cell begins to divide

daughter cells

Environment vs. Reproduction

• Environment determines what form of reproduction is most advantageous

• Asexual reproduction – an advantage in

consistently favorable conditions

• Sexual reproduction – an advantage in changing

conditions

Budding

• Forms a new organism from a small projection growing on the surface of the parent

bud

Hydra

Yeast

Fragmentation

• The splitting of the parent into pieces that each grow into a new organism

MULTICELLULAR LIFE

5.5

Key Concept

• Cells work together to carry out complex functions

Multicellularity

• Multicellular organisms depend on interactions among different cell types

CELL TISSUE ORGAN

Specialized Cells

• Cell differentiation – cells developing into their mature forms

– different genes are expressed

Outer: skin cells Middle: bone cells Inner: intestines

Stem Cells

• Unique body cells

• Have the ability to: – divide and renew themselves

– remain undifferentiated in form

– develop into a variety of specialized cell types

Stem Cells, 3 types

Stem Cells

• Come from adults and embryos

• Adult stem cells: – hard to isolate and

grow – may prevent

transplant rejection

• Embryonic stem cells: – raises ethical issues – can be grown

indefinitely in culture

Stem Cells Uses

• Treat leukemia and lymphoma

• May cure disease or replace damaged organs

• May revolutionize the drug development process