· The cell cycle is the entire sequence of events that take place in a cell between one cell division and the next. It describes the events that take place as one parent cell divides to produce 2 new daughter cells which then grow to full size. (In some organisms the cell cycle is also the life cycle and each daughter is a new single celled organism.)

· Cells do not divide continuously, but undergo a regular cycle of division separated by periods of cell growth and this is known as the cell cycle.

· Interphase – occupies most of the cell cycle, and is sometimes known as the resting phase, because no division takes place. In one sense, this could hardly be further from the truth, as interphase is a period of intense chemical activity, divided into three parts:

o First growth phase, when the proteins from which cell organelles are synthesised are produced.

o Synthesis phase, when DNA is replicated.

o Second growth phase, when organelles grow and divide and energy stores are increased.

Chromosomes

· Chromosomes have a characteristic shape, occur in pairs, and carry the hereditary material of the cell.

· Chromosomes are only visible as discrete structures when a cell is dividing. The rest of the time, they consist of widely spread areas of darkly staining material called chromatin.

· When they are visible, chromosomes appear as long, thin threads around 50 Îm long. They are made up of two strands called chromatids, joined at a point called the centromere.

· Chromosomes are made up mainly of three materials:

o proteins (70%), mostly in the form of histones, scaffold proteins and polymerases

o deoxyribonucleic acid - DNA (15%)

o ribonucleic acid – RNA ( 10%).

· To fit in, the considerable length of DNA found in each cell is highly coiled and folded. This DNA is held in position by proteins called histones, which together form a complex known as chromatin.

Chromosome number

Although the number of chromosomes is always the same for normal individuals of a species, it varies from one species to another. The number of chromosomes is no indication of the level of organisation, complexity or evolutionary status of a species.

A karyotype (p91 TT) shows all the chromosomes in the cell where they have been placed in pairs and given numbers to identify them.

Homologous pair of chromosomes – consists of one maternal and one paternal chromosome that have the same genes and therefore determine the same features. They are not identical as individual alleles of the same gene may vary e.g. one chromosome may carry the allele for blue eyes and the other the allele for brown eyes.

Mitosis is the type of nuclear division that produces 2 daughter cells that are genetically identical to the parent cell.

Mitosis can be divided into 4 stages: prophase, metaphase, anaphase and telophase.

Prophase

· Chromosomes become visible as long thin tangled threads which gradually shorten and thicken as they coil up.

· Each chromosome can be seen to consist of two chromatids held together by a centromere.

· Animal cells contain organelles called centrioles and these move to opposite poles (ends) of the cell.

· Short microtubules may be seen radiating from the centrioles these are called asters. These microtubules develop so that they stretch from pole to pole and form a structure called a spindle. (Plant cells do not have centrioles but still form a spindle).

· The nucleoli disappear.

· The nuclear envelope disintegrates.

Metaphase

· Chromosomes are attached to the equator of the spindle by their centromeres.

· The spindle fibres are microtubules.

Anaphase

· This stage is very rapid.

· Centromeres split into two and the spindle fibres pull the sister chromatids towards opposite poles.

Telophase

· Chromatids reach the poles of the cell.

· Chromatids uncoil and lengthen again.

· Spindle fibres disintegrate.

· A nuclear envelope reforms around the chromosomes at each pole and the nucleoli reappear.

· Telophase may lead straight into cytokinesis

Cytokinesis (Cell Division)

Animal Cells

· Cell membrane begins to invaginate at the equator of the cell (constriction of centre of parent cell from the outside to the inside).

· A furrow forms which deepens until the cell is divided into two.

Plant cells

· Cell plate forms across the equator of the cell

· Cell plate is formed from the fusion of vesicles.

· Cellulose is laid down on the cell plate to from the cell wall.

Differences between mitosis in animal and plant cells

Animals

• Centrioles present
• Asters form
• cell division involves furrowing and cleavage of cytoplasm.
• Occurs in tissues throughout the body.

Plants

• No centrioles present
• No aster forms
• Cell division involves formation of cell plate.
• Occurs mainly at meristems.

Biological significance of mitosis

Genetic stability

· Nuclei of daughter cells have the same number of chromosomes as the parent nucleus.

· Nuclei of daughter cells are genetically identical to each other and the parent cell

Growth, repair and asexual reproduction can be brought about by cell division by mitosis. They rely on the above factors.

Growth

· Number of cells within an organism increases by mitosis.

· New cells are genetically identical to the parent cell.

Repair/Replacement of cells

· Damaged cells are replaced by genetically identical copies of the original by mitosis.

· Cells are constantly dying and are being replaced by identical cells e.g. skin

Asexual reproduction

· Production of new individuals of a species by one parent organism

· Very common in plants.

· The ability to generate whole organisms from single cells or small groups of cells is important in biotechnology and genetic engineering.

· Asexual reproduction, does not involve gametes or fertilisation.

· The single-celled fungus Saccharomyces cerevisiae, yeast, reproduces by budding. A new cell is formed from the old one by mitosis and then breaks away.

· Your body contains about 10¹³ cells. They have all developed from the single cell with which you began your life - the zygote ( formed by the fusion of an egg cell and a sperm cell).

· The zygote divided to form a tiny ball of cells called a blastocyst which continued to divide to form an embryo.

· In multicellular organisms, there are usually many different kinds of cells which have become specialised to carry out different functions.

· The body is made up of 'teams' of cells, usually grouped together into tissues, which work together closely while each performing their own specialist functions. The specialisation of a cell to carry out a particular function is called differentiation.

· Once a human cell has differentiated, it usually cannot change into another kind of cell.

· This is very different from the abilities of the cells in the blastocyst. These cells have the potential to become any of the many different kinds of cells within a human. They are stem cells. Stem cells differ from most human cells because:

· they are unspecialised

· they can divide repeatedly to make new cells

· they can differentiate into several kinds of specialised cells.

· All the cells in a blastocyst are stem cells, and they can differentiate into any kind of specialised cell. They are therefore said to be 'totipotent'.

· Stem cells occur in small numbers in adults.

· There are stem cells in bone marrow that can form white and red blood cells. But they cannot differentiate into neurones, or any other kind of cell.

· There is much interest in stem cells, as they could cure many diseases. For example, Parkinson's disease is caused by the death of a particular group of cells in the brain. One day, it may be possible to use stem cells to replace these brain cells.

· Erythrocytes (red blood cells) They transport oxygen in the blood. They have a very short life span. Every second, around 10 million old erythrocytes are destroyed in your spleen and 10 million new ones are made. They are made from stem cells in the bone marrow, especially in the ribs, vertebrae, pelvic bones and skull.

· Neutrophils These are cells that attack and destroy invading microorganisms by phagocytosis. They are made by the same stem cells. Neutrophils are a type of leucocyte (white blood cell).

· Spermatozoa Sperm cells are the male gametes. They are made in the testes throughout a man's life. They are adapted to find and fertilise a female gamete.

Plants do not have stem cells - most of their cells retain the ability to differentiate into other kinds of cells throughout their lives. However, there are several parts of a plant where cells are able to divide, and places where this occurs at a high rate are called meristems.

· A meristem that forms a ring of tissue in the stem is called the cambium. These cells can divide to form xylem vessels on the inside of the ring and phloem sieve tubes on its outside, which help to form these two transport tissues.

· The cells that are often considered to be 'typical' plant cells are the palisade cells - the main type of photosynthetic cell found in plant leaves. They are highly specialised containing many chloroplasts in which photosynthesis takes place.

· Root hair cells are found near the tips of roots. They are specialised epidermal cells - cells that cover the outside of a plant organ. They have long thin extensions that grow between the soil particles, providing a large surface area that is in contact with the layer of water that is usually present on and between these particles. The water contains various mineral ions in solution, which root hairs absorb. They have a short life, being easily broken as the root grows through the soil. Recently divided cells near the root tip differentiate to form new root hair cells.

Guard cells

· Guard cells are found in pairs in the epidermis of leaves and control the opening and closing of stomata (sing. stoma).

· Epidermal cells do not contain chloroplasts but they are found in guard cells.

· Water moving into the guard cells causes them to change shape and the stoma to open.

· Inside a multicellular organism, cells that carry out the same function are usually grouped together forming a tissue.

· Tissues may be further grouped into organs and organs into systems.

· Tissue

o A tissue is a group of specialised cells that is specialised to perform a one or more particular functions together. This includes any intercellular ('between cells') secretion produced by them.

o The cells are often of the same type, such as palisade tissue in a plant leaf or squamous epithelium and nervous tissues in animals.

o Other examples of animal tissues are blood (for transport) and muscle (for movement) and plant tissue are xylem tissue which transports water and phloem tissue which transports sugars (mainly sucrose).

· Organ

o An organ is a collection of tissues working together e.g. leaf, stem and root are all - plant organs; Brain, lungs, heart and kidneys are examples of animal organs.

· System

A system is a collection of organs with particular functions, such as the excretory, reproductive, cardiovascular (heart and blood vessels) and digestive systems

You have to know how cells are organised into tissues using the following examples.

Examples of animal tissue:

· Tissues that cover a surface in an animal are called epithelial tissues.

· The following two examples are only one cell thick and so they are simple epithelia.

· The cells rest on a basement membrane. This is not a cell membrane. It is a network of collagen and glycoproteins secreted by the epithelial cells and holds the epithelial cells in position.

· Squamous epithelium

o This covers many surfaces in the human body, including the inner lining of the cheeks, the inner surfaces of blood vessels, and the inner surfaces of the atria and ventricles in the heart. It also forms the thin walls of the alveoli in the lungs.

o The individual cells are smooth, flat and very thin. They fit closely together, providing a smooth, low-friction surface over which fluids can move easily – found lining blood vessels.

o In the alveoli, the thinness of the cells forming the walls of the alveoli allows rapid diffusion of gases between the alveoli and the blood.

· Ciliated epithelium

o This is made up of column shaped cells that possess cilia.

o This tissue is often found on the inner surfaces of tubes e.g. trachea, bronchi and bronchioles in the lungs. Some cells called goblet cells produce mucus which can trap small particles and microorganisms.

o The cilia move in a synchronised wave and move the mucus up and out of the lungs.

o The cells are ciliated epithelium is also found in the oviducts

o The cilia move the egg cells from the ovary along the oviduct.

Some examples of plant tissues

· Xylem and phloem are the 2 types of conducting tissue in plants. They each contain more than one type of cell.

· Xylem and phloem come from dividing meristem cells in this case cambium. The meristem cells undergo differentiation to form the different types of cells in the transport tissues.

Xylem Tissue

o 2 major functions:

1. Conduction of water and mineral salts.

2. Support

o Consists of parenchyma, fibres and vessels.

o The vessels are dead cells with no end walls. They transport water and mineral salts. All vessels are made up of cells whose cross walls have broken down resulting in the formation of long tubes ideal for carrying water.

o The meristem cells produce small cells that elongate and the normal cellulose cell walls have lignin deposited in them. The lignin reinforces the cells and makes them waterproof.

Phloem Tissue

o Function – food conducting tissue transporting sugars (produced by photosynthesis), amino acids, some mineral ions and growth substances (plant hormones).

o Consists of sieve tube elements, companion cells, parenchyma cells and fibres

o The sieve tube element cells were formed by the meristem tissue producing cells that elongate and line up end-to-end to form a long tube. They have cellulose cell walls and the ends of the cells do not break down completely but form sieve plates between the cells. These allow the movement of the sugars and other substances up and down the tubes.

o The cells are still living (even when there is no nucleus), but are dependent on companion cells which develop at the same time from the same meristematic cell.

o The companion cells are very active metabolically and play an important role in moving the substances up and down the sieve tubes.

o These metabolically active cells communicate with their sieve tube element via plasmodesmata (cytoplasmic strands which link the cytoplasm of one cell with another cell).