6.1.5a DNA structure

The double helix

1. DNA is a polymer, a long molecule built from many small repeating units joined together.
2. A DNA molecule is made of two strands lying alongside each other.
3. The two strands are twisted around each other into a spiral shape called a double helix.

Nucleotides make up DNA

1. DNA is built from repeating units called nucleotides.
2. Every nucleotide is built from three parts: a sugar, a phosphate group, and one base chosen from four.
3. The sugars and phosphates join up to form a sugar-phosphate backbone that runs along the outside of each strand.
4. The bases are attached to the sugars and point inwards, towards the other strand.
5. A base on one strand pairs with a base on the other, and these base pairs form the rungs that hold the two strands together.

How DNA codes for a protein

1. The four bases are known by their initials: A, C, G and T.
2. The order of the bases along a gene acts as a code.
3. Read in groups of three, each triplet of bases stands for one amino acid.
4. So the order of the bases sets the order of the amino acids, and that decides which protein is built.

6.1.5b DNA structure (Higher tier)

Complementary base pairing

1. The two strands of DNA are held together by pairs of bases.
2. The pairing always follows the same rule: A always pairs with T, and C always pairs with G.
3. These are called complementary base pairs.
4. Because the pairing is fixed, the base sequence on one strand sets the sequence on the other.

Transcription: copying the gene into mRNA

1. A gene's base sequence is the code for a protein, but proteins are built in the cytoplasm, not in the nucleus.
2. So a copy of the gene is first made inside the nucleus, in a step called transcription.
3. The two DNA strands unzip over the gene, and a matching copy is built against one strand using complementary base pairing.
4. This copy is a molecule of mRNA (messenger RNA), which is a single strand, unlike the double-stranded DNA.
5. The mRNA carries the same coded message as the gene, but in a form small enough to leave the nucleus.
6. The mRNA then travels out into the cytoplasm and attaches to a ribosome.

Translation: building the protein

1. At the ribosome, the mRNA code is read in order, in groups of three bases, and each group codes for one amino acid.
2. Carrier molecules bring the matching amino acids to the ribosome in the order set by the code.
3. The ribosome joins the amino acids together, one after another, into a chain; this stage is called translation.
4. This builds a chain of amino acids in the exact order set by the gene.
5. When the protein chain is complete, it folds up into a unique three-dimensional shape.
6. The way it folds depends on the order of the amino acids, so the gene ultimately decides the protein's shape.
7. This unique shape lets the protein do its job as an enzyme, a hormone, or a structural protein such as collagen.

Mutations

1. A mutation is a change in the base sequence of DNA, and mutations happen continuously.
2. A point mutation is a change to a single base, and it can happen in three ways.
   1. Substitution: one base is swapped for a different base.
   2. Insertion: an extra base is added into the sequence.
   3. Deletion: a base is removed from the sequence.
3. An insertion or a deletion shifts how all the bases after it are read in threes, so it can change many of the amino acids that follow.
4. Most mutations do not change the protein, or change it so slightly that its function is unaffected.
5. A few change the protein's shape, so an enzyme may no longer fit its substrate, or a structural protein may lose its strength.

Non-coding DNA

1. Not all of the DNA codes for proteins.
2. These non-coding parts of DNA can switch genes on and off.
3. A variation in one of these regions can change how a gene is expressed, meaning whether a protein is made and how much.
4. This is one reason cells with the same DNA can develop into different specialised types.