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School Biology Notes: CELL DIVISION - mitosis, meiosis, binary fission and growth

CELL DIVISION by mitosis, meiosis and binary fission

How do eukaryotic and prokaryotic cells multiply?

Implications for growth, percentile charts, cancer, sexual or asexual reproduction (maybe both!)

 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

 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'



(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.

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.

phases of the cell cycle interphase gap phase synthesis phase

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.

 


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(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

mitosis cell division Interphase Prophase Metaphase Anaphase Telophase Cytokinesis gcse biology igcse

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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(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.

the maths of cell division mitosis growth binary fission of prokaryotes gcse biology igcse 

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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(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


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(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.

diagram explanation of cell division by meiosis stages DNA chromosomes

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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(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!


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(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.

diagram explanation of cell division by binary fission stages DNA chromosomes

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 mitosis growth binary fission of prokaryotes gcse biology igcse

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

Microorganism binary cell division bacterial growth curve lag phase exponential growth phase stationary phase death phase

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


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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

Introduction to the inheritance of characteristics and genetic diagrams (including Punnett squares) 


Doc Brown's School Biology Revision Notes

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