Cell Cycle, Division, Diversity

& Organisation• State that mitosis occupies only a small percentage of the cell

cycle.

• Describe, with the aid of diagrams and photographs, the main stages of mitosis (behaviour of the chromosomes, nuclear envelope, cell membrane and centrioles).

• Explain the meaning of the term homologous pair of chromosomes.

• Explain the significance of mitosis for growth, repair and asexual reproduction in plants and animals.

• Outline, with aid of diagrams and photographs, the process of cell division by budding in yeast.

• State that cells produced as a result of meiosis are not genetically identical (details of meiosis are not required).

• Define the term stem cell.

The Cell Cycle

• The events that take place as one parent

cell divides to produce two daughter cells

which then grow to full size.

– The daughter cells are genetically identical to

each other and to the original parent cell.

– In some cases the daughter cells are new

organisms

ChromosomesChromosome

arrangement

Stages of

the Cell

Cycle

Mitosis

• Nuclear division where 2 genetically identical nuclei are formed from one parent nucleus.

• Divided into 4 named phases:

– Prophase

– Metaphase

– Anaphase

– Telophase

Interphase

• The cell grows to full size.

• DNA replicates ready for the next round of cell division.

• The cell carries out its normal biochemical functions.

• Chromatin is not coiled & chromosomes are not visible.

Prophase

• The replicated chromosomes become supercoiled (short & thick).

• Now visible with a light microscope.

• Centriole divides and two daughters move to opposite poles.

• Spindle fibres form.

• Nuclear envelope disappears.

Metaphase

• The chromosomes line up

along the equator of the

cell.

• Each becomes attached to

a spindle thread by it’s

centromere.

Anaphase

• The centromere splits &

chromatids are pulled

apart.

• Spindle fibres shorten &

pull the chromatids towards

the poles.

Telophase

• A new nuclear envelope

forms around the

separated chromatids (now

called chromosomes).

• Spindle fibres break down.

• Chromosomes uncoil.

• Cytokinesis occurs

Timescales

• Cell cycle times vary from one species to the next.

– Some bacteria complete a whole cycle in 20 -30 minutes (given ideal conditions).

– Yeasts can take up to 4 hours.

– The fastest mammalian cell cycle is 9 – 10 hours for some intestine epithelial cells.

– Some mammalian cells can have a cycle time of over 200 hours.

– Fully differentiated cells rarely divide.

Stage G0

• G0 is the name given to the phase where a cell leaves

the cell cycle (either permanently or temporarily).

• Reasons for this:

– Differentiation

• Specialised cells no longer divide. They just do the

job they have specialised to do.

– DNA Damage

• Damaged cells do not divide.

– Aging

• Most cells only divide a certain number of times

then become senescent. Aging is the increase in

number of senescent cells.

Controlling the Cell Cycle

• Cells need to ensure they only divide

when they are ready to.

– When they have grown large enough.

– When replicated DNA is error free.

– When chromosomes are in their correct

position during mitosis.

• There are 3 checkpoints within the cell

cycle which check these process have

taken place correctly:

Checkpoints:

• Mitosis cannot proceed

unless this checkpoint is

passed

Plants v Animals v Yeasts

• Only certain plant cells (meristem cells) can divide.

• Plant cells do not have centrioles.– Spindle fibres are made in the cytoplasm.

• Plant cytokinesis starts with the formation of a cell plate across the equator of the cell.– A new cell membrane & cell wall is laid down along

this cell plate.

• Yeast cytokinesis by producing a small bud which pinches off from the cell.

Animals

Plants

Yeasts

Cleavage

Furrow

Buds

Cell plate

How long is each phase?

Why do we need mitosis?

• All organisms need to produce genetically identical cells for:– Growth

• Multicellular organisms grow by producing extra cells. Each cell must be able to carry out the same functions as existing ones.

– Repair• Damaged cells need to be replaced by new ones that are

able to perform the same functions.

– Asexual reproduction• Single celled organisms divide to produce two identical

organisms. Some multicellular organisms produce genetically identical offspring by mitosis.

– Replacement• RBCs & skin cells are constantly replaced.

Clones

• Genetically identical cells or entire

organisms.

– Bacteria produce clones by binary fission.

– Plants can produce clones by Vegetative

Propagation.

– Identical twins are clones.

– Artificial clones can be produced:

• Cuttings from plants.

• Mammalian clones (eg Dolly the Sheep)

Meiosis

• Details required for AS.Interphase, prophase 1, metaphase 1, anaphase 1, telophase 1, prophase 2, metaphase 2, anaphase 2, telophase 2, Homologous Chromosomes.

• Production of haploid daughter cells:

– Contain half the number of chromosomes as parent cell.

– Are genetically different from each other and from the parent cell.

– Used to make gametes for fusion into a zygote.

Specialised Cells

• Single celled organisms have a large

enough SA:Vol ratio to supply it with

enough oxygen & nutrients.

• Multicellular organisms have a lower

SA:Vol ratio.

– Not all cells are in contact with the outside

medium.

– They need specialised cells to perform

various functions.

Differentiation

• Where cells become specialised to

perform specific functions.

• Differentiation can involve changes to:

– Numbers of various organelles

– Shape of the cell

– Cell contents

– All three

Some Specialised Cells

• What are the functions of these cells:

– Erythrocytes

– Neutrophils

– Sperm cells

– Palisade cells

– Root Hair cells

– Guard cells

• Summarise the form and function of each.

Sperm Cells

Root Hair Cells

Tissues

• Multicellular organisms have several levels

of organisation:

– Specialised cells

– Tissues

– Organs

– Organ Systems

– Organisms

Animal Tissue Types

• There are 4 main categories of tissues:

– Nervous tissue

• Supports the transmission of electrical signals.

– Epithelial tissue

• Covers internal and external body surfaces.

– Muscle tissue

• Contracts to produce movement.

– Connective tissue

• Holds other tissues together or transports substances

between them.

• Summarise the form & function of each type.

Epithelial Tissue

• Different types:

– Squamous(small flat cells)

– Cuboidal(Cube-shaped cells)

– Columnar (Tall, thin cells)

Squamous Epithelium

• Flattened cells, thin layer.

• Ideal for lining blood vessels or forming the walls of alveoli.

– Provides very small distance for diffusion of oxygen & CO2.

• Cells held in place by a basement membrane of collagen – secreted by the epithelial cells.

Ciliated Columnar Epithelium

• Found on inner surface of

tubes (trachea, bronchi,

bronchioles, uterus,

oviducts).

• Often associated with

goblet cells.

• Cilia beat with synchrony &

rhythm to move the mucus.

Plant Tissue Types

• Plants consist of different tissue too:– Xylem tissue

• Transports water & minerals throughout the plant.

– Phloem tissue

• Transports organic nutrients throughout the plant

– Epidermal tissue

• Covers the surfaces of plants.

• Summarise the form & function of each type.

Organs

• A collection of tissues which are adapted

to perform a specific function for the

organism.

• Eg.

– Heart

– Leaf

Leaves as Photosynthetic Organs

• Photosynthesis requires:

– Light

– Water

– Carbon Dioxide

• Photosynthetic organs need to:

– Supply the requirements

– Carry out the reaction

– Remove the products

Organ Systems

• A number of organs working together to

perform a major function of the body.

• Eg:

– Digestive System,

– Cardiovascular System,

– Gas Exchange system.

Stem Cells

• Capable of becoming any one of the different cell types found in an organism.– They have a high potency

– Described as totipotent, pluripotent or multipotent

• Occur in small numbers in a body.• Bone marrow contains the multipotent stem cells for blood &

bone tissue.

• May be able to be used to repair damaged tissues without rejection problems.

Sources of Animal Stem Cells

• Embryonic Stem Cells.

– Totipotent at the 1-16 cell stage.

– Pluripotent after the blastocyst stage.

Sources of Animal Stem Cells

• Tissue (adult) Stem Cells.

– Multipotent.

– Although called Adult SC, are present from

birth.

– Found in specific areas (Eg bone marrow,

umbilical cord).

• Meristem

tissue.

– Pluripotent.

– Occur at tips

of roots &

shoots, and

in stems

(yellow areas

in diagram).

Sources of Plant

Stem Cells

Plant

Meristem• Xylem &

phloem

tissue

developing

from the

meristem

Xylem

Phloem

Uses of Stem Cells

• Research how some diseases might be treated using

stem cells:

– Heart disease/Type 1 diabetes

– Parkinson’s/Alzheimer’s diseases

– Repair of birth defects/injuries/burns

• Research also how stem cells might be used to develop

new drug treatments or further our biological knowledge.

• Summarise this work along with some of the ethical

considerations that have to be made when working with

stem cells.