Chapter 5

Cell replication

Cell Theory

• All organisms are made of cells (and the products of cells).

• All cells come from pre-existing cells.

• The cell is the smallest living organisational unit.

All embryos (eg chicken) develop from a fertilised egg (zygote). New cells are produced by mitosis.

Cell Division

Two forms: cell replication (or mitotic cell division),

which involves mitosis and cytokinesis;

Reduction division (or meiotic cell division), which involves meiosis and cytokinesis.

egg cell

Purposes of cell replication• Growth & Development

• Multicellular organisms grow in size by increasing the number of cells.• These cells become specialised – muscle cells, blood cells and bone in animals

• Maintenance & Repair• Regular death of the cells lining the gut• Starfish can produce an entire new individual from a single arm.

• Q 1 - 3

Mitosis

The production of new cells genetically identical with the original cell – an essential process in asexual reproduction.

Eukaryote cells typically have a nucleus containing DNA. DNA is found in thread-like structures - chromosomes.

Found inside the nucleus of a cell.

Only become visible during mitosis or meiosis when they condense, becoming shorter and thicker.

Chromosomes

Each chromosome is usually single-stranded (1 DNA molecule). Each body cell has two of each chromosome, forming matching

pairs. Humans have a total of 46 chromosomes Eucalyptus has 22 chormosomes

Structure of a chromosome

In duplicated chromosomes, the two threads (DNA molecules) connected by the centromere, are called chromatids.

Draw a chromosome

chro

mat

id =

1 s

tran

d

centromere

Varies – short as 20 minutes to as long as 2 weeks!

Usually 10 – 30 hours in plants and 18 – 24 hours in animals.

Cell Cycle – DRAW THIS!

G1 – cell growth, Synthesis – DNA synthesis, G2 – cell growth.

The cell grows in size

Accumulation of materials required for DNA synthesis

DNA synthesis occurs (replication of chromosomes)

* Chromosomes are too slender to be visible in a cell.

Interphase

Chromatin strands condense and become clearly visible as thick, thread-like chromosomes.

Each chromosome is composed of two identical strands called chromatids, joined together by a centromere.

Centrioles move to opposite sides of the cell. Spindle fibres (very fine microtubules of protein) begin to form

around and between the centrioles.

Early Prophase

Nuclear membrane has totally broken down and the nucleolus has disappeared.

Spindle fibres form across the centre of the cell.

Chromosomes align at the middle (equator), half-way between the two spindle poles, attached by their centromeres.

Chromosome consisting of two sister (identical) chromatids

Metaphase

Centromeres split, so that sister chromatids go to opposite ends of the cell.

Spindle fibres contract, pulling chromosomes towards the poles of the spindle.

Anaphase

Identical set of chromosomes appear at opposite ends of the cell.

Cleavage furrow forms.

Cytokinesis (division of the cytoplasm) begins.

Cleavage furrow

Late Anaphase

Spindle fibres degenerate.

Nuclear membrane and nucleolus reforms in each daughter cell.

Cytokinesis nears completion.

Each chromosome unwinds as the chromatin that forms it again becomes extended and thin.

Telophase

Two new daughter cells produced, each with an identical set of genetic material.

In plant cells, a cell plate forms between the two groups of chromosomes in a newly replicated plant cell and gives rise to the new cell wall.

Interphase

Stages in mitosis

Identify the stages in mitosis

A:B:C:D:E:F:G:H:I:J:K:L:

Key Questions 4 – 8pg 79

Specialised cells

• Cells begin to produce and release substances (growth factors) that affect the development of nearby cells.

• Cells become different from one another, specialised for particular functions.

• Specialisation (differentiation) is under the control of genes.

Some organisms - eg flatworms (planarians) and starfish - retain a population of stem cells throughout their life that can develop into any cell type in the body, giving them remarkable regenerative capacity.

Relatively undifferentiated cells important in tissue replacement.

Stem Cells

Stem cell: two types

Embryonic stem cells:obtained from embryos in the earliest stages of development.can make replacement cells for any type of tissue

Adult stem cells:exist in most mature tissues - supply them with replacement cells as required.more limited in their ability to develop into cells of different types. usually able to make cells for a particular type of tissue only.

Different levels of ability to turn into various types of specialised cells.

Great medical potential.

Controlled cell death

Common mechanism to remove excess tissue.

Plays an important role in growth and development (eg in the development of the hind limb of a chicken).

Apoptosis

Cancer cells

Divide at a faster rate than normal cells of the same type. Not affected by normal signals that control the cell cycle (eg contact

inhibition). Look different and may become less specialised. Release factors that stimulate the development of their own blood

supply. Their DNA mutates, making them different and sometimes resistant to

earlier successful treatments. Can ‘colonise’ new parts of the body and continue to grow unchecked. Continue dividing endlessly, whereas normal cells undergo a limited

number of cell cycles. Avoid proceeding to ‘natural’ death by apoptosis.

Normal Cancerous

Binary Fission

• The bacterial cell grows until it has almost doubled in size.

• The DNA molecule then replicates and the cell divides by splitting into two relatively equal halves = binary fission.

• A new cell wall and membrane material is laid down between the separating chromosomes to divide the cell in two.

• Prokaryotes replicate by binary fission.

Question 1

The identical number of chromosomes in a nucleus is maintained by:

A.mitosisB.binary fissionC.meiosisD.cytokinesis

Question 2

An example of an undifferentiated cell in a mammal is a:

A.skin cellB.stem cellC.sperm cellD.nerve cell

Question 3

In a mature adult mammal, mitosis

A. does not occur.B. occurs in the lens of the eye.C. occurs in the skin.D. occurs in nerves

Question 4

A cell undergoing division is viewed under a light microscope and a cell plate is visible. The cell would be from

A. a plant.B. an animal.C. a virus.D. a bacterium.

Question 5Examine the following diagram of a chromosome.a. Name structures A and B.

b. During what phase of mitosis does the chromosome appear in this state? Give reasons for your answer.

c. Chromosomes do not always look like the diagram depicted. Describe the changes in the appearance of chromosomes during the different phases of the cell cycle.

d. Draw a typical interphase cell from an animal. Label the key structures.

Question 7

Name some places where mitosis would be occurring in a pregnant woman.

Question 8

What phase of mitosis is the following cell in? Give reasons for your answer.

Question 9

a. Where are adult stem cells?

b. What is their role?

a. How do they differ from embryonic stem cells?

Question 11Contrast cytokinesis in plant and animal cells.

Cytokinesis is division of the cytoplasm and occurs after or towards the end of mitosis (nuclear division).

Similarities:

In animal cells the plasma membrane pinches in, forming two daughter cells.

In plant cells the presence of the cell wall prevents this. Instead a cell plate forms in the middle of the cell during telophase. It grows out from the centre and divides the cell into two daughter cells.

Question 12

Suggest how passage through the cell cycle might be different in stem cells than in fully differentiated cells.

Interphase in particular is likely to be much shorter:•no cell differentiation

•limited growth

Stem cells are likely to move through the cell cycle more rapidly than differentiated cells.