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Cell division

What you need to know

Reflections and Exam tips


Cell division

New cells come from pre-existing cells by a process of cell division- so says the Cell Theory.

But then- if this is so, where did the first cell come from? This is more like a chicken and egg relationship!

The origin of cells can be traced back to evolution.

  • Life began in an aqeous solution (water)
  • This solution called a primeval soup (also called primordal soup- a nice soup indeed) had all the ingredients, organic and inorganic compounds to form life.
  • These came together to form the first cells, more like very simple bacteria cells.
  • From then cells developed complexity and they continue to form by cell divison.

Aghhh, the chicken and egg puzzle solved!

Types of cell division:

Before we talk about these, it is important to know these basic facts:

  • Cell division is mainly about the genetic material (DNA)
  • There are 2 types of cells in organisms: reproductive/sex cells (also called gametes), and the rest of the cells around the body (body cells)
  • Body cells have double the DNA content ( in humans 46 chromosomes)
  • Sex cells/gametes have half the DNA (in humans 23 Chromosomes in sperm and eggs)
  • During fertilisation, the gametes join up to return the number to double (2N).



This is duplication division, as this suggets, it just copies a cell and makes another copy (clone) of the cell.

So mitosis:

  • Produces clones of cells- produces genetically identical cells and organisms
  • Occurs in body cells
  • Also occurs during asexual reproduction
  • Has one division



This is reduction division

This reduces the DNA content of the cell by half. Meiosis happens in reprodudctive organs that produce sex cells, in animals, testes (sperm) and ovaries for eggs (not penis and vagina), in plants in anthers (pollen) and ovaries (ovules/eggs).

So meiosis:

  • Produces cells with half the DNA content of the original cells
  • Occurs in reproductive organs
  • Is useful for sexual reproduction
  • Mixes up chromosomes in the resultant cells so:
  • Produces genetic variation in sexually produced organisms which is essential for survival and evolution.


Genetics is the study of inheritance, how characteristics are passed on from parent to offspring.

Genes: genes are units of inheritance, parts of chromosomes which are responsible for passing characteristics.

Gregor Mendel:

The guy who developed our understanding of inheritance. He is sometimes called the father of genetics.

Mendel's work demonstrates the patience and virtues of being a scientist. He carried out long, time consuming experiments with peas, looking at how features like height, seed shape and flower colour are passed on.

Before we explore his experiments, we can look at a few key definitions:

  • Gene- a section of a chromosome that determines a particular feature/characteristic
  • Alleles- alternative forms of the same gene which code for contrasting features e.g. tall v short
  • Dominant gene/allele- the gene which always shows out in the presence of another (overides another)
  • Recessive gene/allele- the gene which does not show out when present with another (is supressed)
  • Genotype- the particular combination of alleles present, e.g. TT or Tt
  • Phenotype- how the alleles express themselves on the organism, e.g. tall or short etc
  • Homozygous- when the 2 alleles present are the same, e.g. TT, tt, CC, PP, DD, BB, bb.
  • Heterozygous- when the 2 allels are different e.g. Tt, Cc, Bb, Pp etc.

For each characteristic, we inherit 2 alleles (so we gonna have 2 letters), one from mama (martenal), one from papa (partenal), which ever comes out in our phenotype depends on which one is dominant/recessive.

Let us look at some of Mendel's works:

Mendel crossed pure breeding (homozygous) tall plants with pure breeding (homozygous) Short plants

Tall x Short

The resulting first generation (also called F1 generation) were all tall!

Tall x Short

All Tall

Why were they all tall? mendel asked!

Mendel then crossed these tall F1 plants together to get an F2 Generation

Tall xTall

The result 3 Tall 1 Short

But why?

  • It seemed that the 2 features did not blend/ mix in the F1 generation, they remained seperate
  • Because the Short feature reappeared in the F2 generation.

These can be worked out using a punnet square, more or less like a Maths multiplication table:

F1 Tall (HH) x Short (hh)

All Hh


F2 Tall (Hh) x Tall (Hh)

F2 generation 3 Tall: 1 Short

Most features are Inherited this way.


Inheritance of gender/sex

We have 2 sex chromosomes numbers 23,

  • Males have 1 X chromosome and 1 Y chromosome, XY
  • Females have 2 X chromosomes XX

Genetic disorders

Cystic Fibrosis (CF)*

Cystic Fibrosis is a disease that affects inner membranes of our body.

Its caused by a recessive gene (allele) which results in the shortage of a key membrane protein. This results in excessive production of mucus in the Breathing and digestive systems. The mucus affects gaseous exchange, absorption of food and may encourage the growth of infectious bacteria.

As with all conditions caused by recessive genes, you need 2 recessive Alleles to be affected i.e. cc. If you are heterozygous Cc, you are a carrier of the condition. You have the allele but you are not affected. You may pass the condition to your kids.

Normal people have CC.

Try doing a punnet square to check the probability of a child inheriting it from 2 carrier parents.



Polydactyly is having more than 5 fingers and toes on each limb.

It is caused by a Dominant allele, so you just need to inherit 1 of them to be affected. (both PP and Pp are affected)

Normal people are recessive pp


Again, practise genetic crosses to show how this is inherited.




Key Ideas

Cell Theory


Body cell






Genetic variation

Asexual reproduction

Sexual reproduction