Science

Mitochondrial DNA

by Amy Collins March 29, 2019

Mitochondrial DNA is a unique link between mother and child. While most of our DNA is found in the nuclei of our cells, mitochondrial DNA is different. It is passed down exclusively from mother to child and, unlike the rest of our DNA, it is shaped like a ring! Make a mitochondrial DNA (mtDNA) model to give to your mum on Mother’s Day or take a picture of your creation and make it into a card, either way, your creation will be as unique as your DNA link.

You will need:

  • 1 x Shallow Gratnells tray with 6-section insert
  • 1 x Art tray to work in
  • Big bag of small, chewy, soft, gummy, coloured fruit sweet e.g. mini jelly babies, large jelly tots, gummie bears or a few packets of fruit pastilles. Or for a healthier option, use fruits, e.g. raspberries, red grapes, green grapes, black grapes, blueberries.
  • Big bag of long strawberry/red liquorice sticks/laces or Twizzlers
  • Cocktail sticks or tooth picks
  • Scissors

Tip: Use small sweets or fruits to make it easier to turn your model into a helix and then a ring. It is possible to use larger sweets (such as marshmallows or full-size jelly babies) but the final structure will be bulkier, and it will be more difficult to turn it into a circle.

Preparation:

  • Wash your hands and ensure the trays and inserts are cleaned and sanitized before use, especially if you would like to eat your creation afterwards or if you are giving it as a gift to be eaten.
  • Use the shallow tray with 6- section insert to sort and group your sweets. You will need four sweet colours for your model (you can eat the spares!), use one insert section for each colour. Use the remaining two sections to hold the cocktail sticks and long liquorice separately.

What to do:

  • Decide which coloured sweet you will use for each chemical (base) that makes up mtDNA, write this down and colour code it. For example:
    • Adenine (A) = Green
    • Thymine (T) = Yellow
    • Guanine (G) = Orange
    • Cytosine (C) = Purple
  • Work in the art tray to keep your work in a clean area and to contain any rolling, escaping sweets.
  • Start building. You may only pair A with T and G with C as this is how DNA pairs up in mtDNA. Use the cocktail sticks to connect the sweets into base pairs, poking them all the way through each sweet. The pair of sweets should sit in the centre of the stick.
  • Build six to ten base pairs to start with and keep checking your colour key to make sure you do not break A-T and G-C the pattern.
  • Connect your base pairs to the sugar phosphate backbone (red liquorice), to create a ladder of parallel base pairs. Push each end of the cocktail stick into one strand of liquorice (ladder side rail). The order of the base pairs is not important, but it is good to mix up the order and orientation (left to right or right to left) to be more representative of real-life mtDNA. Tip: For a stronger structure, you may want to offset each backbone piece (ladder side rail) by two base pairs so, for the later step, the joins are separated on each side.
  • Make three to five ladders and join them together using cocktail sticks to link the top and bottom of each ladder (push the cocktail stick into the centre of the red liquorice so it is not visible when the pieces are joined. The number of ladders you need will depend on the size and flexibility of your sweets.
  • Twist the long ladder 3 to 4 times to create a double helix.
  • Bring the two ends of the helix around to meet each other and use a final pair of cocktail sticks to join them into a ring shape.
  • Place your finished mtDNA model in the middle of the art tray and take a photograph to evidence your work.
  • Give your model to your mum or maternal grandmother for Mother’s Day.

What is happening:

DNA is usually found in the in chromosomes and contained within the nucleus. However, mitochondria contain small amounts of their own DNA, known as mitochondrial DNA (mtDNA). mtDNA is a double-stranded circular molecule, unlike normal cellular DNA which is linear. mtDNA is inherited from the mother in all multicellular organisms as it originates from the oocyte (egg). There are between 2 to 10 transcripts of mtDNA in each mitochondrion. Compared to chromosomal DNA, mtDNA is small and contains far fewer genes. Changes in mtDNA can cause maternally inherited genetic diseases.

Mitochondrial DNA contains 16,569 building blocks (base pairs), a very small number compared to the human nuclear genome (3.3 billion base pairs) present in cells. Of the 37 mitochondrial genes, 13 are responsible for making proteins (enzymes) involved in oxidative phosphorylation. 22 genes are responsible for making transfer RNA (tRNA) and 2 for ribosomal RNA (rRNA), which help to turn amino acids into proteins.

DNA can only be built in a certain way. Base pairs are not interchangeable, Adenine can only be paired with Thymine and Guanine can only be paired with Cytosine. The sugar phosphate backbone holds the base pairs in order and provides the double helix and circular structure.

Because mtDNA is only inherited from your mother, your mtDNA will be the same as your mothers, your grandmothers, any maternal siblings you have and anyone else in your maternal family line. Isn’t DNA amazing!

Other things to try:

  • Once you have built your DNA model, draw it as a flat ladder and write down the DNA sequence that you have created with your base pairs. Label the ladder with arrows to show what each part of the model represents. Colour code your bases using highlighter pens.
  • How could the model be improved? Think about what is missing. What alternative products could you use or add to make the model more accurate?
  • Share your mtDNA model photographs on social media using #WhatsInMyTray #MothersDay, tag your mum and @Gratnells

Health & Safety

As with all Gratnells Learning Rooms What’s In My Tray activities, you should carry out your own risk assessment prior to undertaking any of the activities or demonstrations. In particular, cocktail sticks can be sharp.