DNA Structure and the Genetic Code

The Double Helix and Backbone

• DNA consists of two separate strands of nucleotides that twist together to form a shape called a double helix.
• The long strands form a backbone on the outside made of an alternating sugar and phosphate section.
• The bases are attached to the sugar molecules and point inwards toward each other.
• It is important to note that parts of the DNA strand, known as non-coding DNA, do NOT code for proteins; instead, they act like switches to turn genes on and off.

Complementary Base Pairing

• The two separate strands of the double helix are held together by weak hydrogen bonds between the bases.
• These bases always pair up in a very specific way, a rule known as complementary base pairing.
• Adenine (A) always pairs with Thymine (T).
• Cytosine (C) always pairs with Guanine (G).

Worked Example: Base Calculation

If a DNA sample contains 20% Adenine (A), you can calculate the percentages of the other three bases.

Step 1: State the pairing rule.

• Adenine pairs with Thymine, so Thymine (T) must also be 20%.

Step 2: Calculate the remaining percentage.

• $100\% - (20\% + 20\%) = 60\%$

Step 3: Divide the remainder equally between the other two bases.

• Cytosine (C) = 30% and Guanine (G) = 30%.

Worked Example: DNA Length Calculation

A DNA molecule contains 1000 nucleotides. The distance between each base pair is $0.34$ nm. Calculate the total length of this DNA section.

Step 1: Write down the formula.

• $(\text{Total nucleotides} \div 2) \times 0.34$ nm $= \text{Total length}$

Step 2: Substitute the known values.

• $(1000 \div 2) \times 0.34$ nm

Step 3: Calculate the number of pairs.

• $500 \times 0.34$ nm

Step 4: State the final answer with units.

• Total length = $170$ nm

The Genetic Code

• How does a simple sequence of four letters create the immense complexity of the human body?
• A gene is a small section of DNA on a chromosome that codes for a specific protein.
• The DNA code is read in groups of three bases, known as a triplet.
• Every triplet codes for one specific amino acid.

Protein Synthesis Mechanism

• The specific order of the bases in the DNA determines the exact order in which amino acids are joined together.
• Proteins are synthesised (built) on structures called ribosomes in the cytoplasm of the cell.
• Because DNA is too large to leave the nucleus, the cell copies the gene into a smaller template molecule.
• This template leaves the nucleus and binds to a ribosome.
• Finally, carrier molecules read the triplets on the template and bring the specific amino acids to the ribosome in the correct order.

Worked Example: Sequence Mapping

If a protein chain consists of 120 amino acids, what is the minimum number of bases required in the gene to code for it?

Step 1: Identify the relationship.

• 1 amino acid requires 3 bases (a triplet).

Step 2: Substitute into the calculation.

• $120 \times 3$

Step 3: Calculate the final answer.

• $360$ bases.

Protein Shape and Function

• A protein's shape is like a precise key; if the teeth of the key are bent, it will no longer open the lock.
• Once the chain of amino acids is fully assembled at the ribosome, it folds up into a unique 3D shape.
• This unique shape is absolutely essential for the protein to perform its specific function.
• Enzymes are biological catalysts that fold to create an active site specifically shaped to fit a certain substrate.
• Hormones are chemical messengers (like insulin) that rely on their shape to bind to target receptors.
• Structural proteins like collagen form physically strong fibers to provide support.

The Impact of Mutations

• A mutation is a random change in the DNA base sequence.
• If the base sequence changes, the triplet code changes, which can result in a different amino acid being added to the chain.
• This alters the way the protein folds, changing its final 3D shape.
• If an enzyme's shape changes, its active site will no longer fit the substrate, meaning the protein loses its function.