CELL DIVISION by mitosis, meiosis and binary fission
IGCSE AQA GCSE Biology Edexcel
GCSE Biology OCR Gateway Science Biology OCR 21st Century Science
Biology Doc Brown's
school biology revision notes: GCSE biology, IGCSE biology, O level
biology, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of biology
How do eukaryotic and prokaryotic cells multiply?
Implications for growth, percentile charts, cancer, sexual or asexual reproduction (maybe both!)
What is the cell cycle? Be able to describe the stages of cell
division by mitosis. Be able to describe the stages of cell
division by meiosis. What are the similarities and differences
between cell division by mitosis and meiosis? Comparing sexual and asexual
reproduction. Rates of cell division and cancer.
Sub-index for this
page
(a)
Introduction
- the cell cycle, cell division, DNA and chromosomes
(b)
Detailed description
of cell division by mitosis
(c)
Cell division -
number crunching replication, control and cancer
(d)
Cell division, growth,
development, percentile charts and cancer
(e)
Sexual reproduction and cell division by meiosis
(f)
Comparing sexual and
asexual reproduction
(g)
Reproduction in prokaryotes -
binary fission to replicate cells and more maths
See also
The human GENOME project - gene expression, chromosomes, alleles, genotype, phenotype, variations,
uses of genetic testing including 'pros and cons'
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(a) Introduction
- the cell cycle, cell division, DNA and chromosomes
The primary purpose of cell division is to replicate each parent cell, by
dividing into two cells by mitosis and to maintain the original cell's genome.
In order for an
multi-cellular organism to grow, its cells
must divide in two to produce new cells.
The cell cycle is divided into two
main phases (mitosis and interphase), which alternate with each other. This
actual cell division is called mitosis
and is part of the cell cycle - simplified diagram..
Mitosis occurs in stages and the
rest of the life of the cell is described as the interphase.
Most cells have a nucleus, which must go
through a series of changes so that each new cell has its own nucleus containing
all the necessary genetic material (chromosomes-genes-DNA). Without a nucleus, a cell cannot survive.
Growth of multicellular organisms
must involve an
increase in the number of body cells, which must be able to make copies of
themselves ....
... since new cells
from mitosis are not only needed for growth of new tissue, but also
replace damaged or dead cells and mitosis is also important
in asexual reproduction.
All new cells can only be created from existing
cells when they divide.
New body cells are created as part of the cell cycle
(diagram above, more detailed description below).
At the end of the cell cycle two new
cells, identical to the original cell, are created and each with the
same, and correct, number of chromosomes.
During interphase the cell grows larger, the numbers of organelles increase, and
each chromosome is copied.
Then during mitosis the chromosome copies separate,
the nucleus divides, and the cell divides to produce two new cells that are
genetically identical to one another (diagram and details below).
A more detailed diagram and description of the cell cycle.

The cell cycle consists of the
interphase, (divided into three phases) and the actual cell division by
mitosis.
The cell must grow and prepare for
mitosis - e.g. duplicating DNA and making more protein.
In gap phase 1 (G1) the
cell grows, and in the process making more sub-cellular structures -
organelles including mitochondria and ribosomes.
In the synthesis phase (S)
the cell replicates the DNA so when it splits by mitosis the two daughter
cells will have identical DNA. Further growth occurs and the DNA is
checked for errors and repairs made if necessary.
In the gap phase 2 (G2) the
cell continues to grow and the proteins needed for cell division are made.
In the mitosis phase (M) the cytoplasm divides in two, hence
the cell itself can now divide into two identical daughter cells
(cytokinesis - division of the cell's cytoplasm) - therefore have the same
genetic make-up - same genome.
A cell might spend about 1/6th of its
life undergoing the actual cell-dividing mitosis.
Typically in human cells actively
dividing, the whole cell cycle takes about an hour.
Diagram of the replication of DNA in the
synthesis phase S of the cell cycle
1. The DNA double helix molecules splits
in two, and the two strands then act as templates.
2. Freely moving nucleotides can be
matched up to form the weak bonds between the complimentary base pairs.
3. Two identical strands of DNA
produced, both identical in their original sequence of bases.
For more on DNA see
DNA and RNA structure and Protein Synthesis
for more details gcse
biology notes
Reminders:
DNA
DNA is the acronym for
deoxyribonucleic acid and these giant molecules have all the coded
instructions for reproduction and developing an organism and keeping the
organism alive!
In the nucleus of a cell the DNA
is collected together in huge sections called chromosomes.
Shorter sections of chromosomal
DNA are called genes contain the code instructions to make specific proteins
or differentiate the functions of specific cells etc. (etc. meaning
everything!).
See
DNA and RNA structure and Protein Synthesis gcse
biology revision notes
Chromosomes and genetic
information
The majority of cells in your
body have a nucleus containing all the genetic information
required for growth and development.
The genetic 'instructions
codes' are in
coiled up bundles of DNA molecules known as chromosomes.
Every chromosome has a large
number of genes which determine all your different
characteristics.
Body cells usually have
two
copies of each chromosome (diploid cells) one set from the organism's 'mother'
(from female) and a 2nd set from the organism's 'father' (from
male).
The human body cell has 23
pairs of chromosomes (illustrated above).
When a cell divides by mitosis (details below)
two identical copies of the original
cell are made and each of the nuclei of the new diploid cells contains
the same number of chromosomes as the original cell.
TOP OF PAGE
and sub-index
(b) Cell division by mitosis - the
details
In
the cell cycle
mitosis occurs during growth of new cells, replace damaged cells and asexual reproduction
in multicellular organisms.
In body cells the chromosomes are normally found in pairs and
that the chromosomes contain the genetic information.
Some organisms e.g. certain plants,
use mitosis to reproduce and is called asexual reproduction.
Cell division by mitosis
(diagram and notes below)
You can actually observe mitosis
in the school laboratory with a powerful
optical microscope.
In a fertilised egg, multiple
cell divisions occur by mitosis to produce all the huge number of
cells a complex living organism like ourselves needs to grow and
develop.
As an organism grows develops
the cells produced by mitosis must all contain the same genetic
information.
The genetic information is
found in the nucleus where the many long strands of DNA
aggregate together forming the chromosomes.
Most prokaryotes
like bacteria have a single circular chromosome, and
thus only a single copy of their genetic material.
Eukaryotes like us
humans, in contrast, tend to have multiple rod-shaped
chromosomes and two copies of their genetic material -
diagrams below.
There are 23 pairs of
chromosomes in human cells (diagram below) and they all have
characteristic shapes.
Each chromosome in a pair has
the same type of genes along its length and there are 46
chromosomes in total.
Diagrams (i-ii) of chromosomes from micrographs
(i) In the above diagram the
pairs of chromosomes are shown joined together by a centromere
during duplication to give the X shape.
(image adapted from shutterstock.com 701025034) e.g. see the
details of cell division by
meiosis in the diagrams and notes below. This profile
of a set of chromosomes is an example of a karyotype.
(ii) In this diagram the pairs of chromosomes are shown as
separate chromatids.
(image adapted from the US National Library of Medicine)
The 23rd pair are the sex
chromosomes, XY for male and XX for female (for more on this see
Genetics of Human Reproduction)
As already mentioned, new cells are
needed for growth and development and to replace worn out or damaged body
cells.
When new cells are formed they must be
identical to the parent cell.
When a parent cells split in two by
mitosis, two daughter cells are made.
The full sequence description
Interphase
==> Prophase
==> Metaphase
==> Anaphase
==> Telophase
==> Cytokinesis
Acronym:
IPMATC =
I
Perform
Magic
And
Teach
Chemistry
(sorry about the latter
heresy on a biology page!, but it is all biochemistry really!,
however I'll leave it to you to decide on your own memory aid)
MITOSIS diagram shown below plus
text explanations
1.
The interphase.
Starting with the parent cell:
Prior to cell division,
in a cell that's not dividing, the
DNA is spread out in long strings within the very thin membrane of the nucleus.
Before the cell
can divide it has to grow and increase the number of sub-cellular
structures such as the mitochondria (respiration - energy source)
and ribosomes (from DNA to protein synthesis).
When the cell gets the
signal to divide, the DNA must be copied (duplicated,
exactly to provide for 2 cells), and the result is double
chromosomes, mostly roughly X shaped.
Remember, to make two identical
cells, you need two lots of identical DNA and both V-sections of the
X-shaped arms of chromosome are identical (diagram 2.) - the right and left arms (each
half) of
the chromosomes are identical - an identical set of
genes.
At this point the original,
now duplicated DNA, is only about 1/50,000 thousandth of its
original length in the highly compacted chromosome of genes.
When the cell contents have been
copied the cell is ready to divide by mitosis.
Parts 2.
to 6. illustrate the actual cell division by mitosis
The actual mitosis can be described
in 4 stages - prophase, metaphase, anaphase and telophase.
2.
Prophase
The X shaped chromosomes condense, getting
shorter and fatter.
The nucleus membrane is
temporarily removed (breaks down) and the X-shaped chromosomes are free to move in the
cytoplasm.
3. Metaphase
The X chromosomes line up across the
centre of the cell.
(Note that the chromosomes are
only X shaped just before the cell divides.)
4. Anaphase
Simultaneously, very fine
cell fibres pull each X-shaped
chromosome apart into two identical sections (both V-shaped) which
are pulled to each end of the cell - these two clumps of chromosomes
will eventually form the nuclei of the two new cells.
5.
Telophase
The two sets of chromosomes collect together on
opposite sides of the cell and a nuclear membrane
forms around each set of chromosomes to form the nuclei of the two new
cells.
In other words the nucleus has split into two nuclei, but each
has complete copies of the DNA.
6. Cytokinesis
Finally, before the telophase
finishes, the cytoplasm divides in two
(process called cytokinesis) with both sections surrounded by
its own cell membrane to give two
genetically identical diploid cells - sometimes referred to as
'daughter cells' - which are genetically identical to the parent cell
- in humans, all will have a full set of 46 chromosomes (23 pairs).
Summary of the overall change and comment
Human body cells are diploid
because have two versions of each chromosome, one from the individual's
father and one from the individual's mother (23 pairs of chromosomes in
total).
On cell division, two identical
cells are formed in mitosis, and both nuclei will contain the same number of
chromosomes as the original cell (i.e. both cells are once again diploid).
Mitosis creates new cells for growth,
replacing damaged cells or tissue, and many organisms (both plant and
animal) use mitosis for asexual reproduction.
It should be noted that in asexual
reproduction, there is no genetic variation.
Mitosis is crucial to replacing damaged cells,
growth of new tissue and asexual reproduction.
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and sub-index
(c) Cell division -
number crunching replication, control factors
How many cells are formed after so many cell divisions?
You can quite easily estimate the number of cells
produced by mitosis from the simple formula 2n,
where n = the number of cell divisions by mitosis.
Starting with a single cell, if you then have two cell
divisions, you end up with 4 cells (22 = 2 x 2 = 4).
By varying n you get a simple arithmetical progression
of numbers
Number of cell divisions n |
0 |
1 |
2 |
3 |
4 |
5 etc. |
Number of cells resulting n2 |
1 |
2 |
4 |
8 |
16 |
32 etc. |
You can also do some estimates of cell count from the rate of cell
division e.g.
Suppose in a culture medium a cell is observed to divide every
10 minutes.
How many cells would be produced from each initial cell in one
hour?
1 hour = 60 mins, so number of cell divisions in 1
hour = 60/10 = 6
Therefore each initial cell (from a given point in time) will
produce:
26 =
64 cells
after 1 hour
What controls the rate of cell division?
Although the estimate calculations of cell numbers is quite simple,
in reality, things are never that simple.
The rate at which cells divide by mitosis is controlled
by the cell's genes - but how effectively they operate depends on other
factors e.g. the environment in which the cells are living.
You can't be sure the rate of cell division is constant -
the above calculations assumes this.
Different environmental conditions will produce different rates
of cell division e.g.
availability of food - nutrition, lack of nutrition will
reduce the cell division rate,
cells might die reducing the rate of population increase
since less cells to divide,
temperature - warmer conditions may increase the cell
division rate,
storing food in a refrigerator decreases the rate of
bacteria growth,
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and sub-index
(d) Cell division, growth, development, percentile charts and
cancer
Growth of any multicellular organism
involves increase in size and mass.
Plants and animals grow and develop by
various processes:
Cell differentiation is the
process in which a cell becomes specialised for a particular function -
this increases the efficiency and viability of multi-cellular organisms
to survive.
Cell division by mitosis
(previously described on this page)
The two points above apply to any
multicellular organisms, but plants can also grow by cell elongation
- plant cells can expand to grow bigger so the whole plant increases in
size.
Animal growth and development
All growth in animals occurs by cell
division, and most growth happens when the animal is young.
After full growth to be an adult, the
animal stops growing.
Therefore when an animal is young you
get the fastest rates of cell division - fastest growth rates.
In adulthood most cell division is
for 'body repairs', that is growing new cells to replace damaged or dead
cells.
In most animals cell differentiation
is lost at a relatively early age of development.
Percentile charts and monitoring
growth of young children
Growth charts are used to
monitor a child's development in size or weight to see if there appears
to be any problem.
Example 1. Babies and young children
Its normal to monitor a baby's growth
after birth to see if it is growing normally - three common measurements
are used - length, mass ('weight') and head circumference.
You should immediately bear in mind
the wide variety of 'sizes' in young children - we are talking
'statistics'.
Doctors will be called in to
investigate if the baby's/child's size or weight is above the top percentile or below the
bottom percentile - in other words - is there abnormal growth in some
way - too much or too little?
Also, if a baby's growth increases or
decreases by two or more percentile lines, or the growth pattern is
inconsistent, then a medical investigation may be needed.
Using thousands of data sets from
many babies you can plot growth charts.
Growth charts are plotted as a series
of 'percentile' graph line e.g.
a 50th percentile line will show that 50% of babies/children will have
reached a particular value or less of height or weight etc.
Statistically, the 50th percentile
line is the median (middle value) of the data set.
The data for boys and girls weight aged 1 to 4 years are shown in the
percentile charts above.
Example of percentile chart
interpretations:
At age 3, 50% of boys have
reached a weight of up to 14.5 kg, so 50% will have a weight of over
14.5 kg..
At age 2, 50% of girls have
reached a weight of up to 11.5 kg, so 50% will have a weight
of over 11.5 kg..
Example 2. Young children and
teenagers
The percentile data on height in cm for
boys aged 9 to 18 years (note the 'acceleration' due to puberty, and then a
levelling off of the graph into adulthood).
Example of percentile chart
interpretation: 75th percentile line
At age 14, 75 % of boys have grown to a
height of 168 cm or less, so 25 % of boys are taller than 168 cm aged 14
years.
The percentile data on height in cm for
girls aged 8 to 18 years (note the 'acceleration' due to puberty, and then a
levelling off of the graph into adulthood).
Example of percentile chart
interpretation: 25th percentile line
At age 14, 25 % of girls have grown to a
height of 157 cm or less, so 25 % of girls are taller than 157 cm aged 14
years.
The percentile data on weight in kg for
boys aged 9 to 18 years (note the 'acceleration' due to puberty, and then a
levelling off of the graph into adulthood).
Example of percentile chart
interpretation:
At age 13, 50 % of boys have grown to a
weight of 43 kg or less, so 50 % of boys have a weight greater of 43 kg aged
13 years.
The percentile data on weight in kg for
girls aged 8 to 18 years (note the 'acceleration' due to puberty, and then a
levelling off of the graph into adulthood).
Example of percentile chart
interpretation:
At age 16, 75 % of girls have grown to a
weight of 62 kg or less, so 25 % of girls have a weight greater of 62 kg
aged 16 years.
Example 3. Comparing percentile
trends with each other
You can also do comparison charts of
one percentile versus another.
This is another diagnostic approach to looking for
overweight (obese) boys and girls aged 2 to 18
For lots of examples of growth charts
from the UK see
and
https://www.rcpch.ac.uk/resources/uk-who-growth-charts-0-4-years
https://www.rcpch.ac.uk/sites/default/files/Boys_2-18_years_growth_chart.pdf
https://www.rcpch.ac.uk/sites/default/files/Girls_2-18_years_growth_chart.pdf
for downloads to study and from which I prepared the chart
diagrams in this section.
Plant growth and development
With plants most growth in height is
due to cell elongation.
Most plant cell division occurs in
the tips of the roots and shoots in parts called meristems.
Most plants grow continuously
producing new branches i.e. plants continue to differentiate producing
new leaves, flowers or roots.
For more details see
hormone
control of plant growth
Cancer and cell division
Cells normally grow and divide by mitosis
when the body needs new cells to replace old or damaged cells.
A random change in a gene can produce a
mutation - though it takes an accumulation of several mutations
to create a rogue cancer cell.
The rate at which cells divide by mitosis is
controlled by the cell's genes - source of DNA instructions.
Therefore, if mutations in
one or more of the genes
that controls cell division occurs, a cell may start to divide in an uncontrolled manner
- these are known as cancerous cells and the body does NOT need them!
This uncontrolled cell division can result in a mass of
abnormal cells to form a tumour.
If the tumour invades and kills surrounding tissue it is
called a cancer.
The cancer can only affect the individual, its a
non-communicable disease - cannot spread to someone else.
For more details on cancer see notes on
non-communicable diseases -
cancer
TOP OF PAGE
and sub-index
(e)
Sexual reproduction and cell division by meiosis
Know the cells in reproductive
organs: testes and ovaries in humans, divide to form gametes (see diagrams
below).
Sexual reproduction is when genetic
information is obtained from two organisms - a 'father' and a 'mother' - two
sources of DNA combining to produce offspring which
genetically different from either parent - though individual
characteristics are passed on.
Sexual reproduction means the
offspring inherit features from both the mother and father and the mix
of chromosomes determines the outcome!
It therefore means that you get
variation in the offspring as well as similarities in the
phenotypes.
In sexual reproduction involves sex cells
called gametes which are produced by meiosis, a
different type of cell division than mitosis described in the previous
section.
Gametes are haploid cells because they only have
one
copy of each chromosome - half the number of chromosomes of a normal
cell (full compliment in a diploid cell) - remember all the DNA genetic
information is bundled together in genes which make up the structure of
chromosomes.
Simplified diagram of sperm cell
The male gamete is the sperm produced
in the testes.
Note: In the sexual reproduction in
plants, the male gametes are called pollen.
Simple diagram of egg cell
The female gamete is the egg cell
(ova/ovum) produced in the ovaries.
Note: In the sexual reproduction in
plants, the female gametes are called ovules.
For more details of egg and
sperm structure see
Stem cells and an introduction to cell specialisation notes.
When gametes fuse together (e.g. egg plus
sperm) you then get the correct number of chromosomes in the fertilised cell
from which the organism develops.
After interphase (during which the
chromosome number has doubled), two meiotic divisions occur.
Body cells have two
sets of chromosomes but sex cells (gametes) have only one set.
Gametes only contain
half the number of chromosomes found in body cells (one chromosome from each
pair).
At
fertilisation, maternal and paternal chromosomes pair up, so the
zygote has the normal chromosome number.
A male gamete fuses with an egg cell
and the fertilised egg is called the zygote.
The zygote cell contains the
full set of chromosomes and is therefore a diploid cell.
Chromosomes from the mother pair up
with chromosomes from the father - that's why you end up with the zygote
having a full set of chromosomes.
So in each pair of chromosomes,
one is from the farther, and one is from the mother.
The zygote undergoes multiple cell
division by mitosis as it develops into the
embryo.
(see the summary diagram above -
details of meiosis further down the page).
Zygote cells are diploid because they have
two copies of each chromosome e.g. human cells have 23 pairs of chromosomes
(46 chromosomes in total).
Gamete cells contain one copy of
each chromosome (23 in human haploid cells).
The embryo inherits
characteristics from both parents ('father' and 'mother') because it
is derived from a mixture of two sets of chromosomes - therefore
inheritance from two sets of genes, half from the mother and
half from the father.
This mixture of genetic coding
(genetic information of the DNA) produces the variation of
phenotypes in the offspring - just look around at your own family!
The formation of
gametes (sex cells) by meiosis
Unlike mitosis, where two
identical daughter cells are produced with the full compliment of
chromosomes (diploid), meiotic cell division reproduces four
non-identical cells with only half the normal number of chromosomes
(haploid).
Know and understand the type of cell division in
which a cell divides to form gametes is called meiosis
(details below).
In order to make gametes with half the
original chromosomes, cells divide by meiosis, a process which involve two
cell divisions. In humans, this can only happen in the reproductive organs -
sperm cells in the testes of males and egg cells in the female ovaries.
Meiosis is a type of cell division
that produces genetically different cells with half the chromosomes of the
original parent cells.
In humans meiosis only happens in the
reproductive organs - the female ovaries and male testes.
1. Before meiosis can
happen the cell goes through an interphase period in which the
DNA is duplicated - duplicating the genetic information.
The process then starts with a diploid cell in
which the DNA has been replicated to form X-shaped chromosomes (so has two
'two armed' copies of each chromosome).
Each arm of the X-shaped
chromosomes is an exact copy of the other arm.
Note that in the 'starter' parent cell,
half of the chromosomes came from the organism's father and half
from the organism's mother.
Note the numbers of
chromosomes are given in (brackets) in terms of human
organisms.
In the case of human organisms,
the initial number of chromosomes is 46 in the diploid cell, BUT,
temporarily, is doubled to 92 in the interphase prior to the first
meiotic cell division.
2. For
the first meiotic cell division, the chromosomes line up in pairs, held by very
fine fibres in the centre of the cell.
One chromosome in each pair comes
from the mother organism and the other from the father organism.
The first cell division has
involved the prior duplication of the DNA - doubling the
chromosomes.
3. The pairs of chromosomes are then pulled
apart to form two groups, each encased in a nuclear membrane, so forming two
separate nuclei (temporarily within the same cellular membrane).
Each new cell has only one copy
of each chromosome, but, some of the
father's chromosomes and some of the mother's chromosomes go into
each new gamete cell.
This process is much more complicated than shown in the
diagram and the alleles can get quite mixed up creating considerable
genetic variation in the offspring (see also
5.).
4. The cytoplasm divides in two, completing
the first cell division, again, noting that some of the male's chromosomes and some
of the female's chromosomes go into each new cell - this very important
because this creates genetic variation.
This also means that each new
cell only has half the chromosomes of the original parent
cell.
5. The
2nd cell meiotic division is a bit like
mitosis
(full details not shown on the above diagram,
and the daughter cells are NOT identical).
The chromosomes will line up
again in the centre of the cell and the arms of the chromosomes
pulled part.
From each of the two cells from
stage 4., two new nuclei are formed and the cytoplasm divides to
give 4 haploid cells with their own surrounding membranes.
Note that this 2nd cell division
does NOT involve the replication of DNA.
This 2nd cell division produces four haploid gamete cells, each with their own
unique single set of chromosomes.
Each of the gametes is
genetically different from the three others because the
chromosomes get 'shuffled around' in the process of
meiosis as each gamete gets half the chromosomes in a randomised
way.
In the case of human organisms,
each haploid sperm/egg cell has 23 chromosomes - a single set of
chromosomes.
This is a further source of genetic variation when gamete cells
combine in sexual reproduction because each of the four haploid gamete cells is
genetically different
from the others.
So the division of a cell by meiosis
as the production of four daughter cells, each with half the number of
chromosomes, and that this results in the formation of genetically different
haploid gametes.
This double cell division process is called
meiosis and only occurs in the reproductive organs.
Because these haploid gamete cells have
different single sets of chromosomes, it explains why sexual reproduction
produces genetic variation.
Gamete cells contain one copy of
each chromosome (23 in human haploid cells).
In human sexual reproduction two
gametes (sex cells) combine to form a new individual with the full
compliment of chromosomes (46 in human diploid cells, 23 from mother's egg -
female DNA,
23 from father's sperm - male DNA) and, because the offspring cells have a mixture of
the two sets of male and female chromosomes, each new individual is unique
in genetic and phenotype character.
A
new individual then grows and develops by this cell repeatedly dividing by
mitosis.
The fertilised cell has 23 + 23
= 46 chromosomes and so inherits characteristics from both parents (male +
female).
Diagram reminder of the overall
process from gametes to offspring
After the gametes have fused together
in the fertilisation process, the resulting fertilised cell divides by
mitosis i.e. it makes a copy of itself.
The mitosis cell division is
repeated many times and all these new cells develop the embryo.
Mitosis occurs rapidly in a newly
fertilised egg.
As an embryo develops, these new
cells begin to differentiate into all the different types of
specialised cell that an organism needs to develop and mature.
Extra notes:
Identical twins from
sexual reproduction are
genetically identical because they are derived from a single zygote cell that
splits in two by mitosis and then two separate embryos develop.
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and sub-index
(f) Comparing sexual
and asexual reproduction
Sexual reproduction involves two
parents e.g. human reproduction described above.
Cells of the offspring
produced by asexual reproduction are produced by
mitosis from the parental
cells, but there is only one parent!
In other words the parent cell
makes a new cell by dividing in two.
There is NO fusion of gametes,
so NO mixing of chromosomes, so NO genetic variation
from parents to offspring.
So, in
asexual reproduction the offspring contain the same alleles as the parents,
so they are genetically identical.
Therefore asexual reproduction
produces clones.
Some plants reproduce by
mitosis, so all new plants have identical genes and so are identical plants.
Bacteria reproduce asexually (and
fast, to our cost at times!) and some simple animal organisms like
amoeba which like to live in warm lakes and rivers.
Many fungi like moulds reproduce
asexually much of the time, but can reproduce sexually if they meet a
different strain.
Flowering plants
Flowering plants can reproduce by
sexual reproduction.
Such plants have female egg cells called
ovules,
but they interact with pollen - the equivalent of male sperm.
Pollination is the transfer of pollen
from a male part of a plant to a female part of a plant, so enabling
fertilisation and the production of seeds, most often by an animal (e.g.
insects like bees) or by blown onto a plant by wind.
Comparison of asexual reproduction and sexual reproduction - advantages and
disadvantages
Obviously, reproduction is important for
all organisms to pass on their genes and maintain the species line.
We reproduce sexually as do most other
animals and many plants.
Some organisms e.g. some species of
plant, use asexual reproduction by mitosis.
Some organisms can reproduce both sexually and
asexually depending on conditions.
Cells that reproduce asexually, divide
by mitosis
to give two diploid daughter cells which are genetically identical to
each other and their parent cell.
Sexual reproduction involves
meiosis
and the production of genetically different haploid gametes which
fuse together to form a diploid zygote cell after fertilisation.
|
ASEXUAL REPRODUCTION |
SEXUAL REPRODUCTION |
Parental
genes |
One parent,
genetically identical diploid daughter cells formed by mitosis -
giving limited variations in genes. |
Two parents,
genetically non-identical daughter haploid cells formed by meiosis -
much more diverse gene pool. |
Advantages |
1. If conditions are
favourable, with asexual reproduction,
lots of offspring are produced quickly because the reproductive life
cycle is fast - e.g. E-coli bacteria can divide every 30 mins and
rapidly colonise a new area. 2. Only one
parent is needed, so the organism can reproduce whenever conditions
are favourable e.g. abundance of food - they don't have to wait for
a mate of the opposite sex!
3. Asexual reproduction requires less
resources e.g. energy - also helps towards an increased rate of
reproduction producing identical offspring.
Dandelions flourish on our lawns with a drop
of rain and sunshine!
|
1. Offspring from sexual
reproduction have a mix of two sets of chromosomes from both
parents. This creates
genetic variation within a given population - giving different
individuals of differing characteristics = phenotypes.
If environmental
conditions change, its more likely that some individuals in the
population will have the right characteristics to survive - and
inherited - survival advantage.
In time this can lead to natural selection and
evolution as the species becomes
better adapted to their new environment and breed more successfully
- the process of natural selection.
We
can do this artificially with selective breeding of plants or
animals for details see
Evolution - theories and evidence, speciation -
new/old species & extinctions, selective breeding notes. |
Disadvantages |
1. There is no genetic variation
within the offspring of the population - more likely to be affected
by disease despite a fast rate of reproduction. Therefore, if environmental
conditions change to become more unfavourable, the whole population
can be affected because there is less chance of genetic variants in
the genome that could
cope with the change
e.g. if a disease infects a population of a
specific plant species - much of which can be wiped out due to
lack of a disease resistant plants. |
1. Sexual reproduction takes more
time and energy than asexual reproduction and the organisms produce
fewer offspring in their longer life-cycle.
2. Such organisms need to
find and attract a mate of the opposite sex. Two parents are needed for sexual
reproduction - a problem if a potential mate is isolated in the
habitat - 'mates' could be few and far between! |
Organisms
can reproduce by both sexual and asexual reproduction
Examples
Plants
Despite the fact that lots of plants
reproduce sexually - producing seeds sexually, they can also
produce asexually.
Asexual reproduction in plants can take
place in a variety of ways e.g.
(i) strawberry plants produce 'runners' -
stems that grow horizontally away from the plant on the surface
of soil.
At various points on the runner a new
strawberry plant can form which is identical to the original
plant.
(ii) Plants that grow from bulbs like the
familiar daffodil, also reproduce asexually, because new bulbs
can form from the main bulb.
The new bulbs can divide off from the
original bulb to grow into daffodils identical to the parent
plant.
Fungi
The various and numerous species of fungi
are pretty versatile in the reproduction - a specific species
might reproduce by both sexual and asexual reproduction.
These type of species produce and release
spores into the air, these develop into the fungus if they land
on a suitable place.
Spores can be produced sexually or
asexually,.
The spores from asexual reproduction are
genetically identical to the parent fungus and do NOT introduce
genetic variants into the population and are more susceptible to
changes in the environment - narrower gene pool - less chance of
survival.
However, spores from sexual reproduction
produce genetic variants from the random chromosome 'shuffling'
in the process. This increase in variation in the gene pool
increases the chance of individuals surviving if environmental
conditions become less favourable.
Single celled organisms
The parasite that causes malaria is a
microscopic, single-celled organism called a plasmodium.
Malaria is a mosquito-borne infectious disease
that affects humans and other animals.
If a mosquito carrying the parasite bites a
human being the parasite can be transferred to the bloodstream of
bitten person.
The parasite reproduces sexually inside the
mosquito carrier, but reproduces asexually and rapidly in your blood
- giving you an attack of malaria!
TOP OF PAGE
and sub-index
(g) Reproduction in prokaryotes -
binary fission to replicate cells and more maths!
Bacterial reproduction - bacteria usually reproduce by
a simple form of asexual reproduction called binary fission (splitting in
two).
This differs from the normal process of cell division in higher plants
and animals which starts with mitosis.
Prokaryotes like bacteria can replicate themselves by
this simple cell division process of binary fission.
Binary fission involves prokaryotes with a single
chromosome (its not the same as mitosis in eukaryotic cells.
Step 1.
In the parent cell, the large
jumbled rings of DNA and the smaller plasmid rings are replicated to provide
enough genetic material for two cells.
Step 2. The parent
cell becomes enlarged with a greater volume of cytoplasm and the two bundles
of DNA separate and move to opposite ends ('poles') of the enlarged cell.
Step 3.
The cytoplasm begins to divide and new separate cell walls begin to form.
Step 4. The
cytoplasm divides in two, so each of the two 'daughter' cells has its own
cell wall AND its own single copy of the jumbled ring of DNA. The
copies of the plasmids can be variable.
The arithmetic of cell division by binary fission
The mean cell division time is the average time it takes for one
bacteria cell to divide in two (by binary fission).
From the mean division time you can work out how many times a cell
will divide in a given time and therefore how many cells will be
produced in that time.
The maths of cell division is illustrated
above.
Starting with one cell, the number of cells
produced = 2n,
where n = the number of
cell divisions.
This produces the arithmetical series 1, 2,
4, 8, 16, 32 etc. for n = 0 to 5 etc.
For example:
Suppose a bacterial cell has a mean division time of 15
minutes.
How many daughter cells will be produced in 1.5 hours
by binary fission?
1.5 hours = 1.5 x 60 = 90 minutes
since each cell on dividing, makes two cells, the number
of cells increases by a factor of 2 for each cell division
cell divisions per cell in 1.5 hours = 90 / 15 = 6
number of cells produced = 2cell divisions = 26
=
64 cells
After 20 divisions over a million bacterial
cells can be produced from just one original cell.
The maths is simple on your scientific
calculator: 220 = 1 048 576 or
1.05 x 106
(to 3sf but still
scary!)
Using the above example, this will take 20 x
15 mins = 300 mins =
5 hours, if this is a bacterial pathogen in
your body, then this becomes very scary indeed !!!
Cell
division n microorganisms - bacterial growth curves
You can estimate the quantity of bacteria in a
colony over time and when you plot the results over a long period of
time e.g. many hours, you can derive and bacterial growth curve
graph like the one shown above.
The x axis is time, the y axis is the
logarithm of the number of bacteria - logarithms are used because
the range of numbers is to great to fit on an appropriate scale.
1. The lag phase:
During the initial lag phase there is no
cell division i.e. no reproduction of the bacteria.
In this lag phase the bacteria are copying
their DNA and synthesising the necessary proteins in order to
facilitate the binary fission - this mode of cell division.
2. The exponential growth phase:
In this exponential growth phase, lots of
food available, so cell division by binary fission rapidly takes
place.
The number of bacteria can double in a
relatively short time e.g. doubling in number every 10-20 mins,
hence the 'acceleration' in the graph line.
3. The stationary phase
However, growth of the bacterial colony
cannot be continuously accelerating because the nutrient
resources are becoming depleted.
In time, the rate of bacterial growth is
matched by the rate of bacterial death, so the graph line
becomes horizontal.
However, if you were to introduce more
nutrients (food), the colony can grow in number again, but,
otherwise .... read on ... !
4. The death phase:
Finally, in the growth of the colony, not
only are the food resources being diminished, but bacteria
produce toxins as a waste product.
So, they become poisoned by the build-up
of these toxins and the live bacteria in the colony begin to
steadily diminish in number.
For more on prokaryotic cell structure see
Introduction to plant and animal cell structure and
function gcse biology notes
See also
Culturing microorganisms like bacteria - testing
antibiotics and antiseptics gcse
biology revision notes
TOP OF PAGE
and sub-index
GCSE Cell biology revision notes index
Stem cells and an introduction to cell specialisation gcse biology revision notes
Stem cells and an introduction to cell specialisation gcse biology revision notes
Microscopy - the development and use of microscopes in biology gcse biology revision notes
Diffusion, osmosis, active transport, exchange of
substances - examples fully explained
RESPIRATION - aerobic and anaerobic in plants, fungi and animals,
conditions, substrates etc. gcse
biology
ENZYMES - structure, function, optimum conditions,
investigation experiments gcse
biology revision notes
CELL DIVISION - cell cycle - mitosis and meiosis
in sexual reproduction gcse
biology revision notes
Genetics - from DNA to GM and lots
in between!
DNA and RNA structure and Protein Synthesis gcse
biology revision notes
GENOME - gene expression -
considering chromosomes, alleles, genotype, phenotype, variations
An introduction to genetic
variation and the formation and consequence of mutations
gcse biology
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