The genome

1. The genome of an organism is the entire DNA that the organism contains.
2. That total includes every chromosome, every gene carried on those chromosomes, and the DNA that lies between and around the genes.
3. The human genome is about three billion base pairs long, which is written as $3\times 10^9$ base pairs.
4. Spread through that length are roughly $20000$ genes.
5. A complete copy of the genome sits in the nucleus of almost every body cell, so a liver cell and a skin cell hold the same instructions as each other.
6. A large part of the genome does not code for a protein at all, so the genome is far more than a list of genes.
   1. This non-coding DNA still has effects on how the genes are used, and a later article deals with what it does.

Organising genetic material

1. In a body cell the DNA is held inside the nucleus.
2. Inside the nucleus the DNA is packaged into chromosomes, and a human body cell contains $46$ chromosomes arranged as $23$ pairs.
3. Each chromosome is one very long DNA molecule, coiled up tightly so that its full length fits into the nucleus.
4. A gene is one section of that molecule, picked out by where it begins and ends along the strand rather than by being a separate molecule of its own.
5. Within that section the bases lie in a particular order, and this base sequence is what carries the coded instruction.
   1. The levels therefore run in this order: nucleus, then chromosome, then the DNA molecule the chromosome is made of, then a gene as one section of that molecule, then the sequence of bases inside the gene.
6. One chromosome carries many genes, set out one after another along its length.
7. Every gene sits at a fixed position on one particular chromosome.
8. Because that position is fixed, the same gene is found in the same place on the same chromosome in every member of a species.

Genes code for proteins

1. A gene is a section of a DNA molecule that codes for a specific protein.
2. Proteins are built from amino acids, small molecules joined end to end into a long chain.
3. The phrase codes for has an exact meaning here: the order of the bases along the gene determines the order of the amino acids in the protein made from it.
4. The order in which the amino acids are joined decides how the finished chain folds, and therefore which protein it becomes.
5. A different gene has a different base sequence, so it sets a different order of amino acids and produces a different protein.
6. The protein itself is assembled outside the nucleus, and later articles set out the steps by which the coded instruction is read and used.

Functions of proteins

1. Genes matter because proteins do the work of the body, and different genes code for different proteins.
2. Amylase is an enzyme that digests starch, breaking it down into sugars small enough to be absorbed.
3. Haemoglobin is the protein inside red blood cells that binds oxygen in the lungs and carries it to the tissues.
4. Keratin is a structural protein that makes hair and nails tough and hard-wearing.
5. Antibodies are proteins made by white blood cells, and they bind to pathogens as part of the immune response.
6. Each of these four proteins is coded for by its own gene, with its own base sequence, in a fixed position on a chromosome.
7. Because the genome carries the code for every protein an organism can make, it controls how that organism is built and how its body works.

Chromosomes, genes and genomes

1. A chromosome is a whole DNA molecule, coiled up, carrying many genes along its length.
2. A gene is one section of that molecule, and it codes for one specific protein.
3. The genome is all of an organism's DNA taken together, which means every chromosome and every gene along with the DNA between them.
4. The three terms name three different scales of the same material rather than three different substances.
5. Different versions of the same gene are called alleles, and they are covered in the inheritance articles.