Genes and Chromosomes

• DNA contains genes which is a sequence of DNA bases that codes for either a polypeptide or functional RNA.
• The sequence of amino acids in a polypeptide forms the primary structure of a protein.
• Different polypeptides have a different number and order of amino acids.

  Its the order of bases in a gene that determines the order of amino acids in a particular polypeptide.
• Each amino acid is coded for by a sequence of three bases in a gene called a triplet or codon.
• Genes that don't code for a polypeptide code for functional RNA instead.

  Functional RNA is RNA molecules other than mRNA, which perform special tasks during protein synthesis
• The complete set of genes in a cell is known as the cells genome and the full range of proteins that the cell is able to produce is known as the proteome.
• In eukaryotes, a lot of the nuclear DNA (DNA stored in the nucleus) doesn't code for polypeptides.
• Some genes don't code for polypeptides at all - they code for functional RNA
• Even genes that do code for polypeptides contain sections that don't code for amino acids. These sections are called introns.
• Introns in eukaryotes are removed during protein synthesis - so they don't affect the amino acid order.
• Prokaryotic DNA doesn't have introns. All the bits of a gene that do code for amino acids are called exons.
• Eukaryotic DNA also contains regions of multiple repeats outside of genes. These are DNA sequences that repeat over and over.
• A gene can exist in more than one form. These forms are called alleles. The order of bases in each allele is slightly different, so they code for slightly different versions of the same polypeptide.
• In a eukaryotic cell nucleus, DNA is stored as chromosomes. Humans have 23 pairs of chromosomes, 46 in total.
• Pairs of matching chromosomes are called homologous pairs.
• In a homologous pair both the chromosomes are the same size and have the same genes, although they could have different alleles.

  Alleles coding for the same characteristic will be found at the same fixed position (locus) on each chromosome in a homologous pair.