MYP Integrated Sciences DNA Notes

What Is DNA?

1. DNA is the molecule that stores the instructions for building and maintaining an organism.
2. These instructions determine traits such as eye color, enzyme shape, and blood type.
3. All living things use DNA, which shows that life shares a universal genetic code.

Why Is DNA Important?

1. DNA contains genes that give cells instructions for building proteins.
2. Proteins carry out nearly every function in the body, so differences in DNA lead to differences in traits.
3. Because DNA instructions are passed from parent to offspring, DNA is the basis of inheritance.

What Is the Structure of DNA?

1. DNA is made of two strands twisted into a double helix.
2. Each strand is built from repeating units called nucleotides.
3. There are four bases: A (adenine), T (thymine), C (cytosine), G (guanine).
4. Bases pair using complementary base pairing:
   1. A pairs with T
   2. C pairs with G
5. Hydrogen bonds hold these base pairs together, stabilizing the double helix.

How Did Patterns Help Crick and Watson Discover DNA’s Structure?

1. Rosalind Franklin used X-ray diffraction to reveal repeating patterns in DNA fibres.
2. The pattern showed a regular spiral shape with a consistent width.
3. Crick and Watson realised this constant width only made sense if:
   1. A paired with T
   2. C paired with G
4. These insights allowed them to propose the correct double-helix model.

How Are DNA, Genes, and Chromosomes Related?

1. DNA is packaged into long structures called chromosomes.
2. Each chromosome contains many genes.

How Does a Cell Use DNA Instructions?

1. Cells use DNA in two different ways:
   1. To build proteins: uses transcription then translation
   2. To make more DNA when preparing to divide: uses DNA replication

1. Transcription: Copying a Gene Into mRNA

1. Transcription happens in the nucleus.
2. The cell selects the gene it needs.
3. The DNA strands separate at that gene.
4. The cell builds a single-stranded mRNA copy of the gene.
5. mRNA leaves the nucleus and travels to a ribosome.

2. Translation: Turning mRNA Into a Protein

1. Translation happens at the ribosome.
2. The ribosome reads the mRNA instructions.
3. Every three bases code for one amino acid.
4. Amino acids join to form a polypeptide chain.
5. The chain folds into a protein with a specific shape and function.

Analogy

• DNA is a cookbook.
• mRNA is a copied recipe.
• The ribosome is the kitchen assembling the final dish.
• This flow is also referred to as the "central dogma"

3. DNA Replication: Making a Copy of DNA

1. Replication prepares the cell for mitosis.
2. The DNA double helix unzips, separating into two strands.
3. Each original strand acts as a template, meaning it guides which new bases are added.
4. Free nucleotides pair with exposed bases using A–T and C–G pairing.
5. Two new DNA molecules form, each made of:
   1. one original (old) strand
   2. one newly built strand