What is DNA?


by Zoë Gamble

DNA stands for deoxyribonucleic acid. It’s the genetic code that determines all the characteristics of a living thing.  Basically, your DNA is what makes you, you!

You got your DNA from your parents, we call it ‘hereditary material’ (information that is passed on to the next generation). Nobody else in the world will have DNA the same as you, unless you have an identical twin.

Deoxyribonucleic acid is a large molecule in the shape of a double helix. That’s a bit like a ladder that’s been twisted many times.

The structure of DNA - a double helix

The structure of DNA – a double helix.  Image: Caroline Davis2010 (CC-BY)

 

It’s made up of repeating units called nucleotides. Each nucleotide contains a sugar and a phosphate molecule, which make up the ‘backbone’ of DNA, and, one of four organic bases. The bases are adenine (A), guanine (G), cytosine (C) and thymine (T). It’s the specific order of A, G, C and T within a DNA molecule that is unique to you, and gives you your characteristics.

 


Make your own DNA model out of sweets

What you will need:

  • Two long bendy sweets such as strawberry pencils or
  • About 20 smaller brightly coloured, soft sweets such as jelly babies, wine gums or assorted liquorice. (you need at least 4 different colour sweets.)
  • Cocktail sticks

Make your DNA model. Image: science made simple (CC-BY-NC)

What to do:

1. Arrange your sweets into groups of 4 different colours, and eat any spares.

These represent the 4 organic bases, A, C, T and G

2. Pair up your sweets so that one colour always goes with another. Here we’ve paired red with black, and yellow with orange.

In base pairing, C will always pair with G, and T will pair with A.

3. Skewer the pairs of sweets end-to-end with the cocktail sticks in any order you want.

This represents the hydrogen bonding between base pairs. In real DNA, the precise order of the base pairs is unique to you and gives you your characteristics – a bit like a bar code.

4. Attach your skewered sweets to the long flexible sweets, to make a ladder (we used cocktail sticks to attach two strawberry pencils to make them longer)

This represents how the nucleotides are attached to the sugar-phosphate backbone

5. Pick up your DNA molecule, and twist it gently to form the double helix

And voilà! Your very own DNA model. Good enough to eat!

Chromosomes and Genes

Within the nucleus of cells, DNA is arranged into structures called chromosomes. Humans cells contain 23 pairs of chromosomes, so 46 per cell. 22 pairs are autosomes, which contain most of the hereditary information, and one pair are sex chromosomes, which can be ‘X’ or ‘Y’ and determine gender.

The karyotype of a human male – Image: National Human Genome Research Institute (Public Domain)

 

Females have two X chromosomes, their sex chromosomes are ‘XX’. Males have one X and one Y, so are XY. As females are XX, they can only pass on the X chromosome to their offspring. It’s actually the male’s chromosome that decides gender. If a male passes on his X chromosome, the baby will be a girl, if he passes on his Y chromosome, the baby will be a boy.

The number, type and appearance of a person’s chromosomes is called their karyotype.

 

The Royal Institution have some great clips from the 2013 Christmas Lectures ‘The Life Fantastic’  about  DNA- The Stuff of Life, and suggestions of how they could be used in lessons here

 

 

Curriculum links:

Key Stage 3:  Biology (genetics and evolution)

  • heredity as the process by which genetic information is transmitted from one generation to the next.
  • a simple model of chromosomes, genes and DNA in heredity.

GCSE: Biology

  • The nucleus of a cell contains chromosomes.
  • Chromosomes carry genes that control the characteristics of the body.
  • Different genes control the development of different characteristics of an organism.

A Level: Biology

Structure of DNA The double-helix structure of DNA, enabling it to act as a stable information-carrying molecule, in terms of
• the components of DNA nucleotides: deoxyribose, phosphate and the bases
adenine, cytosine, guanine and thymine.
• two sugar-phosphate backbones held together by hydrogen bonds between
base pairs.
• specific base pairing.

 

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Posted in Activity, Biology