Download Cell Division and Cell Continuity

Cell Division and Cell Continuity
Mitosis
Meiosis
One parent cell
Two parent cells
Replication (chromosome number stays Reduction (chromosome number halved
the same).
i.e. 2n to n)
No crossing over.
Crossing over.
Genetic stability (chromosome structure Genetic variation (chromosome
stays the same).
structure changes).
Occurs in somatic (non-sex) cells.
Reproductive (sex) cells.
Two identical daughter cells made.
Four daughter (unique offspring) cells.
Occurs in haploid or diploid cells.
Occurs in diploid only
One cell divides to form 2 new cells
One cell divides to form 4 new cells.
Chromosomes are copied; original and
Chromosomes are copied; homologous
copy are separated
pairs are separated first, the original and
copy (of each) are separated
 Cell continuity means that cells can only be formed from pre-existing cells.
 It is the ability of cells to divide and survive from one generation to the next.
When a new cell is formed, it goes through three stages
 Producing (synthesising) all the materials required
 Growing larger
 Reproducing
Chromosomes
 A chromosome is a long, collided thread of DNA embedded in protein and found in the nucleus in
definite pairs.
 Humans have 23 pairs of chromosomes (46 individual chromosomes).
 They contain genes which control the development of individual characteristics.
 When cells are not dividing, the chromosomes are elongated into a long, thread like material called
chromatin
Haploid
 Means a single set of chromosomes.
 It is the number of chromosomes in a gamete.
 It is represented as ‘n’.
Diploid
 Means a double set of chromosomes.
 It is the number of chromosomes in a zygote (fertilised egg) and in the rest of the body.
 It is represented as ‘2n’.
Human haploid number = 23
Human diploid number = 46
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The necessity for a cell to divide becomes apparent when it is considered that every cell must grow,
repair and reproduce.
The cell cycle consists of three phases:
1. Interphase (longest stage)
2. Mitosis (nuclear division)
3. Cytokinesis (cell division).
Interphase:
 When cells are not actively dividing, they are said to be in interphase.
 During this phase the cell grows, proteins and enzymes are made and the number of cytoplasmic
organelles increases.
 Near the end of interphase chromosome duplication occurs so that at the onset of mitosis there is
double the normal amount present.
a) DNA replication:
The single stranded chromosomes duplicate i.e. sister chromatids are made, held together by the
centromere. DNA replication is important to pass DNA(genes) on to new generations of cells
unchanged.
b) Cell organelles e.g. mitochondria and centrioles are replicated.
Energy is build up to carry the process through.
1. 1 parent cell produces 2 daughter cells
2. There are the same no. of chromosomes in parent/daughter cells (human cells have 46
chromosomes)
3. Cells are genetically identical.
Mitosis:
 Is the division of a nucleus into two genetically identical copies of itself (one nucleus to two nuclei)
 Mitosis has four stages:
1. Prophase
2. Metaphase
“I party Monday
3. Anaphase
and Tuesday”.
4. Telophase
Mitosis has four stages: Prophase, Metaphase, Anaphase and
Telophase.
1. Prophase:
 Occurs in late interphase – chromosomes contract (thicken
and shorten)
 The chromatin shortens so that individual chromosomes
become visible.
 Nucleolus and nuclear membrane dissolve
 Centrioles separate to opposite sides of cell. (N/S Pole)
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2. Metaphase:
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Nuclear membrane fully broken down
The centrioles send out protein fibres from pole to pole
forming a structure called the spindle
Chromosomes line up along the equator and attach
themselves to spindle fibres at their centromeres.
 (Note: homologous chromosomes behave independently)
3. Anaphase:
 Spindle fibres contract, each chromosome is pulled apart
 Chromosomes split at centromere, sister chromatids separate
and are pulled to opposite poles.
 The chromatids are now called daughter chromosomes.
4. Telophase:
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The chromosomes elongate reverting to chromatin.
Two nuclear membranes form.
Nucleoli reform.
Centrioles replicate.
Cell division or cytokinesis occurs immediately after
mitosis
Animal Cell Cytokinesis
 Occurs by a process called cleavage.
 A shallow groove (cleavage furrow)
appears at the equator.
 It becomes deeper and deeper until
eventually it divides the cytoplasm and
the cell splits in two.
Plant Cell Cytokinesis
1. The rigid cell wall prevents a cleavage furrow
from forming.
2. Instead, membrane enclosed sacs called vesicles
gather along the equator.
3. These vesicles contain cellulose to form the new
cell walls.
4. They form a structure called the cell plate.
5. The cell plate enlarges and its membranes join
with the cell membrane of the original cell wall.
6. Two cell walls form within the cell, one for
each daughter cell.
7. The region between is the middle lamella.
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Functions of mitosis:
Multi-cellular organisms – for growth.
Single-celled organisms – to allow organisms to reproduce.
 Growth - increasing the number of cells in an organism e.g. fertilised egg → adult
 Cell replacement - repair of damaged or inefficient cells e.g. skin, blood corpuscles.
 Asexual reproduction e.g. budding in yeast, binary fission in bacteria/Amoeba, vegetative
reproduction e.g. cuttings in flowering plants, spore formation in Rhizopus.
 Formation of gametes e.g. in flowering plants.
Significance of mitosis:
 Exact copies produced – important in growth and repair of tissues in that all cells are genetically
identical
 Asexual reproduction – mitosis produces offspring (a clone) that are identical to parent e.g. Amoeba
or yeast. They all have the same advantages of the parent in surviving in the same habitat.
Meiosis
 Is a form of cell (nuclear)0 division in which a diploid cell divides to produce four new cells each of
which is haploid.
Occurrence of meiosis:
 In mammals, in the testes of the male and ovaries of the female.
 In angiosperms, the anther and ovule.
Significance of Meiosis
Sexual reproduction/gamete formation
 As the daughter cells have half the number of chromosomes they can combine with similar cells at
fertilisation to restore the diploid number.
Genetic variation:
 Crossing over changes the arrangement of genes.
 Independent assortment.
 A cell with the genes AaBb will produce equal numbers of gametes with the genes AB, Ab, aB, ab.
Therefore, at fertilisation new genetic combinations may arise.
 In sexual reproduction, variety in offspring is got by mixing the different genes of the two parents.
Formation of haploid spores e.g. Flowering plants
 i.e. Pollen mother cells (male sporophyte) in anther of stamen undergo meiosis forming haploid
microspores (pollen grains = gametophyte) from which the sperm/gametes develop by mitosis.
Embryo sac mother cell (female sporophyte) in the ovule of the carpel undergoes meiosis to from
the haploid megaspores (the embryo sac = gametophyte) which develop into the egg and polar
nuclei (= gametes) by mitosis.
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Zygospore germination
 The development of an active haploid vegetative phase in Rhizopus from the diploid dormant
zygospore.
Note: Meiosis in animals produces gametes and in plants produces spores (which later produce
gametes).
There are two types of cell division
 Mitosis
 Meiosis
Mitosis
Meiosis
 Is a form of cell replication
 The chromosome number remains constant
in each of the two identical cells generated
from one.
 It occurs in human somatic cells (non-sex
cells).
 It is used for:
 Growth and repair
 Replacement of dead cells
 Asexual reproduction in single celled
organisms e.g. Amoeba.
 Is a form of nuclear division
 The four daughter nuclei produced contain
half the chromosome number of the parent
nucleus.
 Also, the chromosomes in the daughter
cells may be different due to crossing over.
 Functions of meiosis:
 To maintain the parental chromosome
number by gamete or haploid cell
production in sexual reproduction.
 To introduce variation into the species by
exchange of genetic material between
homologous chromosomes.
Mitosis
Meiosis
One parent cell
Two parent cells
Replication (chromosome number stays Reduction (chromosome number halved
the same).
i.e. 2n to n)
No crossing over.
Crossing over.
Genetic stability (chromosome structure Genetic variation (chromosome
stays the same).
structure changes).
Occurs in somatic (non-sex) cells.
Reproductive (sex) cells.
Two identical daughter cells made.
Four daughter (unique offspring) cells.
Occurs in haploid or diploid cells.
Occurs in diploid only
One cell divides to form 2 new cells
One cell divides to form 4 new cells.
Chromosomes are copied; original and
Chromosomes are copied; homologous
copy are separated
pairs are separated first, the original and
copy (of each) are separated
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Cancer is the uncontrolled mitotic division of normal cells.
Define and give any two causes of cancer.
Normally, cells divide to produce more cells only when the body needs them. If cells divide when
new ones are not needed, they form a mass of excess tissue, called a tumour. It these cells stay
together they are called benign tumours e.g. warts (caused by a virus), ‘skin tags’ (small blobs of
raised skin) and most breast tumours. These tumours can be removed surgically. A benign tumour is
a non-life threatening form of tumour where the cells stop dividing after some time. e.g. warts, skin
tags etc.
If they invade nearby organs or travel through the lymphatic or bloodstream to new body sites they
are called malignant tumours or cancer. Cancer cells can also break away to form new tumours.
The spread of cancer is called metastasis. A malignant tumour may be life-threatening because they
continue to divide indefinitely and can invade other cells and move around the body. This migration
is called metastasis.
A carcinogen is any chemical, biological or physical agent that could possibly be a cause of cancer
e.g. tobacco smoke, asbestos, hydrocarbons in tars, radiation such as UV light, X-rays and uranium,
as well as certain viruses. Viruses work by taking over the nucleus of the cell they are attacking.
Some viruses appear to damage the oncogenes (cancer-causing genes) of cells and hence may cause
cancer.
Carcinogens cause mutations (changes) in the DNA that controls cell division. Cancer is caused
when normal genes are altered to from called oncogenes. The risk of cancer increases with age as
the number of mutations builds up.
Symptoms of cancer: (CAUTION)
Change in bowel or bladder habits;
A sore that does not heal;
Unusual bleeding or discharge;
Thickening or a lump in the breast or any other part of the body;
Indigestion or difficulty;
Obvious changes in a wart or mole;
Nagging cough or hoarseness.
A biopsy, where a tiny sample of tissue is examined under the microscope checks for cancer cells.
To reduce your risk of cancer:
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Do not smoke.
Avoid sunburn. Us sunscreen to block UV rays.
Take cancer screening tests. All women should learn to examine their breast for lumps. They
should have a mammogram (X-ray of breast tissue) and a cervical smear test done every 3
years. All men should learn to examine their testes for lumps. Prostate cancer is the most
common cancer in men, especially older men.
Eat a healthy diet - less fat and more fibre, fruit and veg
Drink only moderate amounts of alcohol.
Observe safety rules in jobs where exposure to chemicals, radiation and other hazards increase
your risk.
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