So if you know the sequence of bases on one side of the 'ladder,' you can automatically determine the sequence of bases on the other side. Each side of the ladder is called a strand. One strand is complementary to the other, and sometimes the two strands are referred to as the plus and minus strand, or sense and antisense. DNA is 'read' from left to right. The 5' and 3' designations refer to the number of carbon atom in a deoxyribose sugar molecule to which a phosphate group bonds.
Some organisms contain a single chromosome, and some contain multiple chromosomes.
The number of chromosomes generally increases with the complexity of the organism. The size of genomes vary, as noted in the section on chromosomes. The human genome is estimated to contain three billion base pairs. A karyotype is an image of a cell showing all the chromosomes within that cell. The region of a chromosome that codes for a specific protein is called a gene.