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CELL DIVISION and CHROMOSOMES


Use appropriate words to fill in the spaces in the text below.


The contents of the nucleus

The nucleus contains some coiled strand-like bodies known as chromosomes , which are only really visible in dividing cells, when they will absorb stain. These contain the genetic information, encoded in the structure of the DNA (Deoxyribonucleic acid) molecule, which enables the cell to carry out all its functions. Genes - the units of inherited information - are parts of chromosomes, and these are passed out equally amongst the cells of the body and in even shares from one generation to the next. In other words, chromosomes carry genes from one cell to the next. DNA has a very special structure: it can split into two halves, each of which can then be built up to reform the missing section. It is effectively a molecule which is capable of reproduction, the basis of life. It can also pass instructions out to the cytoplasm, to produce proteins.

Mitosis - nuclear division

Before a cell divides, its nucleus must divide, in order to pass on this information to both daughter cells. In higher plants and animals a process known as mitosis ensures that chromosomes, after dividing, are distributed evenly between the two cells. In other words, nuclear division comes before cell division.
As a result of mitosis, each dividing cell produces two cells, each of which contains identical sets of chromosomes. So the main function of mitosis is for growth or replacement of cells.

Cell division

vertical section through a root tip Growth occurs by an increase in the number of cells and/or by a (limited) increase in cell size. Increase in cell number takes place as a result of division of cells. This is the basis of asexual reproduction.

This cell division process frequently occurs during the development of a plant or animal. A plant continues to grow throughout its life, so that cell division is a perpetual process in some regions, such as the tips of roots and shoots. These regions of active cell division in plants are known as meristems.

See the diagram alongside


In all organisms that reproduce sexually, cell division is an important phase in the life-cycle because it results in the production of many copies of cells by duplication.
In developing from a fertilised egg or zygote, animal cells also undergo division to make a ball of cells which eventually become the embryo (and foetus in mammals).
Cells from an early embryo cannot be distinguished from one another, but as the process progresses, the cells become specialised to perform different functions (differentiation).
Once they have reached their normal size, animals do not continue to grow but cell division still takes place, in order to repair damage and replace worn-out tissues, e.g. there is a continual turnover of red blood cells (produced in the bone marrow) and skin is replaced by division of cells from an underlying layer (the Malpighian layer).

Each cell normally divides into two halves , each of which then usually enlarges to the size of the original cell. Usually, one cell is able to divide again. See the diagrams below.
cell division in Amoeba cell division in plants
At cell division, animal cells pinch into two because they only have a flexible membrane on the outside of the cells, but plant cells form a thin structure (middle lamella) across the centre of the old cell, which then thickens up to form a cell wall.

In a complex organism made of many cells, all cells are derived by division from the fertilised egg. Mitosis ensures that each cell receives the same genetic information, but different groups of cells become developed to perform different functions - differentiation, e.g. liver cells, blood cells, muscle cells, nerve cells. Such cells do not usually divide again. There is great current interest in stem cells (present in small numbers in some tissues) which remain undifferentiated and can therefore develop into various cell types, either in normal development or the repair of damage.

It is not fully understood how, but in differentiation only a part of the genetic information is decoded to control the development of the cell and the biochemical activity specific to it, such as production of proteins like haemoglobin in red blood cells, and digestive enzymes in cells of the various glands of the gut.

Can you think of any other examples of different cell types, and their own (protein) products?
> digestive gland cells,
> digestive enzymes
e.g. salivary e.g. amylase

Meiosis

Certain organs (sex organs) produce gametes (sex cells). In animals, testes produce the male gametes: sperms, and ovaries produce female gametes: eggs (ova).
In flowering plants, anthers produce pollen which contains male gametes, and the embryo sac within the ovary contains a female gamete.
Here, a different version of nuclear division takes place: meiosis (also called reduction division).
Chromosomes pair up with their partners and then divide, and are distributed into cells after 2 nuclear divisions so that 4 cells are formed, which then develop into gametes. The result is that each gamete contains half the normal number of chromosomes possessed by any ordinary body cell - the haploid number - n. For instance, in Man sperms and eggs each contain 23 chromosomes.

Fertilisation

At fertilisation, two gametes join together. Nuclear fusion takes place, restoring the normal double number of chromosomes in the (single) cell, the zygote. This (diploid - 2n) number is then retained during cell division by mitosis to provide all the cells of the developing embryo with 2n chromosomes.
A similar process occurs in higher plants, resulting in the formation of seeds.

NOTE: Asexual reproduction (vegetative propagation) relies on mitosis, whereas sexual reproduction relies on meiosis.

Give 2 examples each of types of tissues (not organs) in both higher plants and higher animals, in which mitosis regularly occurs.
(animals) > bone marrow > skin (Malpighian layer)
making red blood cells
(plants) >root tips > shoot tips

Towards the end of the cell division process, dividing animal and plant cells do something different. What is the difference?
> Animal cells pinch in, whereas plant cells form middle lamella across middle of cell.
Explain why this difference occurs.

> Animal cell outer layer (cell membrane) is flexible, whereas plant cell outer layer (cell wall) is inflexible.
What do you consider to be the biological significance of mitosis to a multicellular organism, and why should it be especially useful to the gardener?
> Asexual reproduction following cell division and mitosis results in identical cells and eventually identical plants (clone) so cuttings etc have the same characteristics.

CELL DIVISION and CANCER


There are many forms of cancer, but their main feature is that cells of the body divide in a way that is out of control. Biologists believe that DNA in their nuclei has been damaged by something in the environment of the cells. This damage may be caused by a variety of factors, which may be placed under 3 headings:
Complete the table below

physical e.g. radiation from >rocks,> medical X-rays, > nuclear reactors, waste dumps
chemical > chemicals from tobacco, industrial chemicals, food additives, alcohol

biological > sexual intercourse, dietary fat and salt, etc

If the cells divide to produce a lump of cells it is called a "growth" or tumour; if it stops growing or grows only slowly, and does not spread then it is said to be benign; if it shows no sign of slowing down, breaks up and spreads to other parts of the body it is called malignant.

Some types of cancer may be treated by direct means, such as surgery, or by laser treatment. Give some examples of cancers which may be treated in this way.

>skin cancer

> cervical cancer

>breast cancer


Chemotherapy and radiotherapy (treatment with chemicals and penetrating radiation) are sometimes used against cancer which is difficult to reach directly. These are often accompanied by unpleasant side-effects such as loss of hair.
Try to explain these side-effects.

> Hair growth also depends on cell division and this would be prevented by the treatments aimed at stopping cancer cell division.

TISSUE CULTURE and MEDICINE


Some tumour cells can be grown outside the body, even after the human (or animal) donor has died. These cells may produce chemicals such as hormones, required by the body to regulate its normal growth and development, or antibodies to fight disease. In some cases these cells can accept DNA containing genes from a completely different source, and there are a wide number of possible benefits in medicine as well as industrial manufacturing. Stem cells - which divide in the body and may develop into many different cell types - may conceivably be used in cloning. Other recent lines of research have centred on attempts to understand the mechanism of cell differentiation so as to allow the production of replacement tissues. This is a form of biotechnology - genetic engineering - which may have much promise in the future.


This topic has connections with other units on:-

CELL STRUCTURE
THE HUMAN REPRODUCTIVE SYSTEM
Birth control
PREGNANCY AND BIRTH
TRANSPORT & SUPPORT IN PLANTS
DNA and Nuclear Division
ANTIBODIES, ANTIGENS, MONOCLONAL ANTIBODIES
Asexual reproduction and artificial propagation


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