CELL DIVISION by mitosis, meiosis and binary fission

How do eukaryotic and prokaryotic cells multiply?

Cell division and sexual reproduction

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Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent


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.


Introduction - the cell cycle, cell division and chromosomes

In order for an multi-cellular organism to grow, its cells must divide in two to produce new cells.

This 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.

New cells are not only needed for growth, but also replace damaged or dead cells.

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).

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).

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

The majority of cells in your body have a nucleus containing your genetic information in coiled up bundles of DNA. 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 - one set from the organism's 'mother' and a 2nd set from the organism's 'father'.

The human body cell has 23 pairs of chromosomes.

 


Cell division by mitosis - the details

Know that mitosis occurs during growth, repair and asexual reproduction

In body cells the chromosomes are normally found in pairs and that the chromosomes contain the genetic information.

Cell division by mitosis (diagram and notes below)

1. Prior to cell division, the DNA is spread out in long strings within the very thin membrane of the nucleus.

2. 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 pairs of identical X shaped chromosomes.

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.

The right and left arms of the chromosomes are the same.

When the cell contents have been copied the cell is ready to divide by mitosis.

3. to 5. describe the second stage - the actual cell division by mitosis

3. The nucleus membrane is temporarily removed and the X-shaped chromosomes then line up across the centre of the cell. Simultaneously, very fine fibres pull each X-shaped chromosome apart into two identical sections (e.g. both V-shaped) which are pulled to each end of the cell.

4. The two sets of chromosomes collect together on opposite sides of the cell and two nuclear membranes form 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.

5. Finally, the cytoplasm divides in two with both sections surrounded by a cell membrane to give two genetically identical diploid cells - sometimes referred to as 'daughter cells' - genetically identical to the parent cell.

 

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.

 


Cell division - number crunching, control and cancer

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 etc.
Number of cells resulting n2 1 2 4 8 16 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 in 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,

-

 

Cancer and cell division

A random change in a gene can produce a mutation.

The rate at which cells divide by mitosis is controlled by the cell's genes.

Therefore, if there is a mutation in the genes that control cell division a cell may divide in an uncontrolled manner.

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.


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.

This produces offspring which genetically different from either parent - though individual characteristics are passed on.

In sexual reproduction, gametes 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.

The male gamete is the sperm produced in the testes.

The female gamete is the egg cell (ova/ovum) produced in the ovaries.

For more details of their structure see Stem cells and an introduction to cell specialisation notes

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.

Chromosomes from the mother par up with chromosomes from the father - that's why you end up with the zygote having a full set of chromosomes.

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 - genes.

 

Know and understand the type of cell division in which a cell divides to form gametes is called meiosis (details below).

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.

The process then starts with a diploid cell in which the DNA has been replicated to form X-shaped chromosomes (so has two 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 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).

This means that 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 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 (details not shown on the above diagram).

The chromosomes will line up again in the centre of the cell and the arms of the chromosomes pulled part.

Note that this 2nd cell division does NOT involve the replication of DNA.

This produces four haploid gamete cells, each with their own unique single set of chromosomes.

In the case of human organisms, each haploid sperm/egg cell has 23 chromosomes.

This is a further source of genetic variation when gamete cells combine in sexual reproduction because each gamete 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).

 

Extra notes:

Identical twins are genetically identical because they are derived from a single zygote cell that splits in two by mitosis and then two separate embryos develop.

Asexual reproduction

Cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells.

In asexual reproduction the offspring contain the same alleles as the parents.

Some plants reproduce by mitosis, so all new plants have identical genes and so are identical plants.

 


Reproduction in prokaryotes - binary fission to replicate cells.

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.

Step 1. 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 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.

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?

1.5 hours = 1.5 x 60 = 90 minutes

since each cell on dividing, makes two cells, the number of cells increases by a power of 2 for each cell division

cell divisions per cell = 90 / 15 = 6

number of cells produced = 2cell divisions = 26 = 64 cells

 

For more on prokaryotic cell structure see Introduction to plant and animal cell structure and function gcse biology notes


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) 

 


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