1.3 Cell Division, Diversity and Organisation

1.1.3 Cell Division and Organisation

Module 1.1.3 Cell Division, Cell Diversity and Cellular Organisation Le

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s)After studying this section, Student should be able to:

state that mitosis occupies only a small percentage of the cell cycle and that the remaining percentage includes the copying and checking of genetic information;

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 the 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; define the term differentiation, with reference to the production of

erythrocytes (red blood cells) and neutrophils derived from stem cells in bone marrow, and the production of xylem vessels and phloem sieve tubes from cambium;

describe and explain, with the aid of diagrams and photographs, how cells of multicellular organisms are specialised for particular functions, with reference to erythrocytes (red blood cells), neutrophils, epithelial cells, sperm cells, palisade cells, root hair cells and guard cells;

explain the meaning of the terms tissue, organ and organ system; explain, with the aid of diagrams and photographs, how cells are

organised into tissues, using squamous and ciliated epithelia, xylem and phloem as examples;

discuss the importance of cooperation between cells, tissues, organs and organ systems (HSW4)

Cells are produced from other cells by cell division. Every cell that is capable of undergoing division passes through a cyclic sequence of events involving growth and division. It is called the Cell Cycle. It involves the entire sequence of events that occurs in a cell from the time it is formed from its parent cell till the time of its own division into daughter cells.

During the cell cycle, genetic information is copied and passed to daughter cells. Microscopes can be used to view the different stages of the cycle.

The cell cycle and mitosis serve four functions:


1. It allows for asexual reproduction in unicellular organisms like bacteria and yeast cells2. It enables multicellular organisms to grow3. It allows for replacement of old cells4. It allows for repair of tissues

Mitosis is only a small part of the cell cycle. The cell cycle has three main stages namely:

Interphase - This is a period of intense synthesis and growth in the cell. Newly formed cells grow and carry out normal function. The cell produces many materials required for division. Some organelles increase in number. The genetic material DNA replicates during interphase. Interphase can be further subdivided into G1 (cell organelles replicated), S1 (DNA replicates) and G2 (DNA is checked for errors).

Mitosis (nuclear division or Karyokinesis) - It is the process of nuclear division, which involves separation of chromatids and their redistribution as chromosomes into daughter cells. Mitosis is divided into four phases; prophase, metaphase, anaphase and telophase.

Cytokinesis (cell division) – Organelles become arranged at opposite ends of the around the two newly separated nuclei and the cytoplasm divides resulting in the formation of daughter cells.

Interphase is divided into two growth phases (G1 & G2) separated by a synthesis (S) phase:

G1 phase. Gap phase 1 or Growth phase 1. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and


moves into the S phase. S phase. DNA synthesis replicates the genetic material. Each chromosome now consists

of two sister chromatids. G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis

and cytokinesis. DNA is checked for errors.

The events in mitosis (also known as karyokinesis) occur continuously but are divided for convenience into the following subphases: prophase, metaphase, anaphase and telophase.

Mitosis is usually followed by cytokinesis.

How To Observe MitosisMitosis can be observed by looking at actively dividing cells under the microscope.

In a growing plant root, the cells at the tip of the root are constantly dividing to allow the root to grow. Because each cell divides independently of the others, a root tip contains cells at different stages of the cell cycle. This makes a root tip an excellent tissue to study the stages of cell division.

Onion or garlic root tips are often used to observe mitosis - easy to squash between microscope slide and cover slip. We use a dye e.g. acetic ocean to stain the chromatin threads.

Preparing the slide1. Cut the final 5 mm tip from a growing root (e.g. garlic or onion). This is where the

most division (mitosis) occurs. What safety precautions do you take?2. Place the root tip on a watch glass and add a few drops of hydrochloric acid for 5 mins3. Put in 5 cm³ water for 5 mins and dry with filter paper4. Add one drop of Toluidine Blue for 2 mins – it stains chromosomes dark. What safety

precautions do you take with the acid and the stain?5. Place the root tip on a microscope slide and cover with cover slip and blot dry6. Macerate with mounted needle. What safety precautions do you take?

http://www.phschool.com/science/biology_place/glossary/jk.html#karyokinesis


7. View under microscope

USES OF MITOSIS:

Growth of multi-cellular organisms. Mitosis produces cells identical to the parent cell. Repairing damaged tissues Replacement of old cells In some organisms, mitosis is important for reproducing asexually producing offspring

that is genetically identical to parent offspring.

How the stages may actually look under the microscope.


Prophase

Metaphase

Anaphase

Telophase

Asexual reproduction and natural cloning

Asexual reproduction is reproduction involving only one individual. The individual may divide by mitosis to produce two identical daughters or clones. The main disadvantage is that the progeny are all identical and there is no genetic diversity. If there is a change in the environment for the worse, e.g. a new disease, the individuals will not be adaptable and may all die out.

Asexual reproduction takes several forms


1. Budding – this is how yeast cells (Saccharomyces cerevisiae) reproduce. The nucleus undergoes mitosis to produce two nuclei. One of the two nuclei migrates to one side of the cell. The cell bulges and the nucleus moves into the bulge. The bulge increases in size and eventually the cytoplasm and cell wall divides (cytokinesis) to separate the two nuclei.

2. Binary fission – occurs in simple unicellular organisms like bacteria and amoeba. It is also used for growth and repair of tissues in complex multicellular organisms. It involves mitosis (nuclear division) followed by cytokinesis (splitting of cell into two).

3. Sporulation – spores may be formed and they may be dormant for some time but can grow into a new individual when conditions are right. Restricted to fungi like yeasts and some plants.

DNA, Genes and Chromosomes

Deoxyribonucleic Acid (DNA)

Polymer - made of monomers called nucleotides, inside the nucleus of all cells, carries the genetic instructions for making living organisms. It is responsible for inheritance of characters

Genes

A gene is a length of DNA that codes for a specific protein. So, for example, one gene will code for the protein insulin, which has an important role in helping your body to control the amount of sugar in your blood. Genes are the basic unit of genetics.

Genes control protein synthesis by passing the triplet code first to mRNA and then from mRNA into a series of amino acids.

Chromosomes

In Eukaryotes, DNA molecules are tightly packed around proteins called histones to make structures called chromosomes. The cells of different organsisms have a fixed number of chromosomes. Humans have 46 (23 homologous pairs).


Homologous pairs of Chromosomes are two chromosomes in which:

1. One is inherited from the father and one from the mother2. They have the same shape and size3. They have the same genes at the same position

(loci) on the two chromosomes (they may have different alleles of the same gene)

4. They have the centromere at the same position

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

State 2 processes that take place in Meiosis that allow the cells to be genetically non-identical.

1 _____________________________________________________________

______________________________________________________________

2 _____________________________________________________________

______________________________________________________________

Answer:

1. Crossing over – In prophase 1 homologous chromosomes come together and pair up to form a bivalent. The bivalent has four chromatids (two from each chromosome). The chromatids can intertwine and exchange segments in a process called crossing over

2. Random assortment – In meiosis 1 the orientations of the homologous chromosomes at the equatorial plate of the spindle is random. Meaning, each time they line up at the equatorial plate, the two pairs of chromosomes in a homologue may appear on different sides of the plate. As you already know, the alleles on the two homologous chromosomes are different. Thus, when the cell separates during meiosis, the resulting cells contain different genetic codes


Cellular Organisation

To understand how a whole organism functions, it is essential to understand the importance of cooperation between cells, tissues, organs and organ systems.

Multicellular organisms have many cells but these cells are not randomly arranged. They are specialised or differentiated to carry out specific functions and grouped into tissues. Specialisation means cells having different shapes or different sizes or even different numbers of organelles so that they are best adapted to carry out their function. Once a cell becomes specialised to carry out one function, i.e. once it becomes differentiated into one type of tissue, it cannot change into another type of cell.

Structurally, cells of multicellular organisms are organized into four levels:

Cells → Tissues → Organs → Organ systems

A tissue is a group of similar cells, that develop from the same kind of cells and which work together to perform a common function. There are four main types of tissues in animals:

i) connective tissue, ii) Muscle tissue, iii) nervous tissue and vi) epithelial tissue such as muscle tissue. These are seen in the diagram on the below.

1. Connective tissue – This is the main supporting tissue in the body. Modified versions of connective tissue include bone, blood and cartilage.

2. Epithelial tissue – lines surfaces both inside and outside the body. Different types of epithelial tissue are classified according to their shape (e.g. columnar or cuboidal) or according to the number of layers (simple or compound stratified). The skin is made mainly of stratified epithelium whilst ciliated epithelium lines the tracheae

3. Muscle tissue – these are made of elongated cells called fibres which contract to aid movement. Different types include smooth muscle, striated muscle and cardiac muscle

4. Nervous tissue – in brain, spinal cord and nerves. They contain nerve cells or neurones and these transmit electrical impulses around the body.


Some examples of plant tissues are collenchyma and schlerenchyma tissues which are for support as well as vascular/conductive tissue (xylem & phloem) which are for transporting water and dissolved minerals/food.

Tissues are organised into organs so they can work effectively together – the heart, liver and small intestine are examples of organs in humans. In plants the root and leaf in plants are examples of organs.

In animals, many organs are then grouped together into systems to carry out a large scale function. A good example of an organ system is the digestive system which consists of many organs (stomach, liver, oesophagus, small intestine, pancreas e.t.c.) which in turn consist of many combinations of tissues.


The diagram shows a section through the plant organ, the Leaf. Label it to show the different tissues it has.


Stem Cells and Specialisation

In multicellular organisms, some cells are undifferentiated or unspecialised. They have the ability to differentiate into other specialised cells or tissues and they are called stem cells. Stem cells divide to form other cell types, either for growth or to replace damaged or old tissues.

In plants, Stem Cells are found in meristematic tissue inside the vascular tissue (cambium – differentiate into xylem and phloem) and at the growing tips (apical meristem). New cells are continuously being produced since the plant continues to grow throughout its lifetime.

In humans, the fertilized cell or zygote is made of stem Cells that can differentiate into all the different specialized cells of the body. These cells are described as Totipotent. Stem Cells are also found in a few places in adults, but these can only differentiate into a limited number of types of cells and are described as pluripotent, if they can differentiate into most but not all cell


types or multipotent if they can differentiate into a few related specialized cells. For example, blood stem cells in bone marrow are multipotent because they can differentiate into all types of blood cells (red blood cells, white blood cells, platelets etc) but they cannot differentiate into brain cells.

Understanding how stems cells can be modified has huge potential in science generally and in medicine in particular.

It is for example possible to freeze/store the blood from the umbilical cord after birth (contains stem cells) and if the child grows into an adult and later has a problem with one of her/his organs, the stem cells can be activated to form the cell types needed.

Even adult stem cells like bone marrow stem cells can be extracted from a patient and used to produce cell types that may be needed by that patient.

In cattle farming, it is a common practice to remove embryo (made of stem cells) from a high top breed animal, divide them up and then implant them into surrogate mothers who will then deliver clones of the original top breed cattle.

CELL SPECIALISATION

The examples of specialised cells in the diagram are from Dr parry’s website: http://www.livingscience.co.uk/year7/cells/cells.htm

Have a look at diagrams of some specialised cells and fill out the table

http://www.livingscience.co.uk/year7/cells/cells.htm


Cell Type and Function Specialisation How the specialisation helps the function

Red Blood CellsCarry O2 in the blood

Small and flexible To fit through tiny capillariesFull of haemoglobin To bind to oxygenNo nucleus To allow more space for haemoglobinBiconcave shape To provide large surface are for diffusion of O2

Sperm cells carry DNA from the father to the egg of the mother

smallAcrosome To help digest the egg surface membrane

To enable rapid movementMany mitochondria Provides energy for swimming to egg

Epithelial cell form the lining of some structures in the body

They may have ciliaThey may be cuboidal (shape) To provide a barrierThey are often thin

Neutrophils (white blood cells) engulf and digest foreign material

Flexible shapeLobed nucleiMany ribosomes/lysosomes

Root hair cells absorb water and ions

1.3 Cell Division, Diversity and Organisation

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