Download Cell and Nuclear Division

Cell and nuclear division
 Replication and division of nuclei and cells
 Understanding of chromosome behaviour in mitosis
Learning objective:
a. Explain the importance of mitosis in the production of
genetically identical cells, growth, repair and asexual
reproduction;
Mitosis – the division of a nucleus into two so that the two daughter cells have exactly the
same number and type of chromosomes as the parent cell.
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Growth. The two daughter cells formed have the same number of chromosomes as
the parent cell and are genetically identical (clones). This allows growth of
multicellular organisms from unicellular zygotes. Growth may occur over the entire
body, as in animals, or be confined to certain regions, as in meristems (growing
points) of plants.
Replacement of cells and repair of tissues is possible using mitosis followed by cell
division. Cells are constantly dying and being replaced by identical cells. In the
human body, for example, cell replacement is particularly rapid in the skin and in the
lining of the gut. Some animals are able to regenerate whole parts of the body; for
example, starfish can regenerate new arms.
Asexual reproduction. Mitosis is the basis of asexual reproduction, the production of
new individuals of a species by a single parent organism. This can take many forms.
For a unicellular organism such as Amoeba, cell division inevitably results in
reproduction. For multicellular organisms, new individuals may be produced which
bud off from the parent in various ways. Budding is particularly common in plants; it
is most commonly a form of vegetative propagation in which a bud on part of the
stem simply grows a new plant. The new plant eventually becomes detached from
the parent and lives independently. The bud may be part of the stem of an
overwintering structure such as a bulb or tuber. The ability to generate whole
organisms from single cells or small groups of cells is important in biotechnology and
genetic engineering.
Identical cells. Of B- and T – lymphocytes during the immune response is dependent
on mitosis.
b. Describe, with the aid of diagrams, the behaviour of
chromosomes during the mitotic cell cycle and the associated
behaviour of the nuclear envelope, cell membrane, centrioles
and spindle (names of the main stages are expected);
The cell cycle
The cell cycle is regularly sequence of events that takes place between one cell division and
the next. It has three phases, namely interphase, nuclear division and cell division.
During interphase, the cell grows to its normal size after cell division and carries out its
normal functions, synthesising many substances, especially proteins, in the process. At some
point during interphase, a single may be received that the cell should divide again. If this
happens, the DNA in the nucleus replicates so that each chromosome consists of two
identical chromatids.
This phase of the cell cycle is described as the S phase – S stands for synthesis (of DNA).
Interphase can therefore be divided into two growth phases, known as the G1 phase and the
G2 phase, separated by the S phase. During G2 the new DNA is checked and any errors are
usually repaired.
Nuclear division follows interphase. This may be referred to as the M phase (M for mitosis).
Growth stops temporarily during mitosis. After the M phase, when the nucleus has divided
into two, the whole cell divides.
The length of the cell cycle is very variable, depending on environmental conditions and cell
type. On average, root tip cells of onion divide once every 20 hours; epithelial cells in the
human intestine every 10 hours.
In animal cells, cell division involves constriction of the cytoplasm between the two new
nuclei, a process called cytokinesis. In plant cells, it involves the formation of the new cell
wall between the two new nuclei.
c. Explain how uncontrolled cell division can result in cancer and
identify factors that can increase the chances of cancerous
growth;
Control of cell division
Each cell contains genes that help to control when it divides. It is important that cells divide
by osmosis only when they are required to do so. This usually involves signals from
neighbouring cell, to which the cell responds by either dividing or not dividing. If this control
goes wrong, then cells mat not divide when they should (so growth does not take place, or
wounds do not heal) or they mat divide when they should not (so that a tumour mat form).
Cancer is a disease that can result when genes that normally control cell division mutate.
The cell may divide over and over again, forming an irregular mass of cells.
A small group of tumour cell is called a primary growth. There are two types of tumours:
1. Benign tumours, which do not spread from their site of origin, but can compares and
displace surrounding tissues – for example warts, ovarian cysts and some brain
tumours.
2. Malignant (cancerous) tumours, which are far more dangerous since they spread
throughout the body, invade other tissues and eventually destroy them.
Malignant tumours interfere with the normal functioning of the area where they have
started to grow. They may block the intestines, lungs or blood vessels. Cells can break off
and spread through the blood and lymphatic system to other parts of the body to form
secondary growths.
It is thought that several different control genes must mutate before a cell becomes
cancerous. This can happen just by chance. The risk is increased by any factor that can cause
mutation, including:
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Ionising radiation
Ultraviolet radiation
Various chemicals, including several contained in tar from tobacco smoke
Some viruses, for example the human papilloma virus (HPV), which can cause
cervical cancer.
d. Explain the meanings of the terms haploid and diploid and the
need for a reduction division (meiosis) prior to fertilisation in
sexual reproduction;
Haploid – one that possesses one complete set of chromosomes; the abbreviation for
haploid is n
Diploid – one that possesses two complete sets of chromosomes; the abbreviation for
diploid is 2n.
Meiosis – the type of division that results in a halving of chromosomes number and a
reshuffling of alleles; it occurs in the formation of gametes
Most of the cells in your body are diploid cells. This means that they contain two complete
sets of chromosomes. Each cell has 2 sets of 23 chromosomes, making 46 chromosomes
altogether.
In the original cell from which you began, one of these sets came from your father and one
from your mother, as this cell divided by mitosis, each daughter cell obtained a complete
copy of each set.
The sperm and egg that fused at fertilisation to produce that original cell each contained
only one set of chromosomes. They were haploid cells. They each contained 23
chromosomes. When they fused together, this produced a diploid zygote with two sets of
chromosomes.
Sperm and egg cells are produced from diploid cells by a special type of nuclear division
called meiosis. In meiosis, the chromosomes are shared out so that each daughter cell gets
only half of the original number of chromosomes. Meiosis produced haploid cells from
diploid cells. In humans, meiosis only happens in testes and ovaries. Meiosis is sometimes
known as reduction division, because it reduces the number of chromosomes in a cell by
half.