Density-dependent factor - Any factor influencing population regulation that has a greater impact as population density increases.

Dipeptide - A pair of amino acids connected by a peptide bond.

Diploid - Containing a full set of genetic material, consisting of paired chromosomes one chromosome from each parental set. Most animal cells except the gametes have a diploid set of chromosomes. The diploid human genome has 46 chromosomes. Zygotes and other cells carrying two matching sets of chromosomes are described as diploid.

Disjunction - The separation of homologous chromosomes during meiosis.

DNA - Deoxyribonucleic acid; the molecule of heredity that encodes genetic information.

DNA replication - The use of existing DNA as a template for the synthesis of new DNA strands. In humans and other eukaryotes, replication occurs in the cell nucleus.

DNA sequence - The relative order of base-pairs, whether in a fragment of DNA, a gene, a chromosome, or an entire genome. DNA is a doublestranded molecule held together by weak bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases: adenine (A), guanine (G), cytosine (C), and thymine (T). In nature, base pairs form only between A and T and between G and C; thus the base sequence of each single strand can be deduced from that of its partner.

Domain - A discrete portion of a protein with its own function. The combination of domains in a single protein determines its overall function.

Dominant allele - An allele that expresses it's phenotype even when heterozygous with a recessive allele.

Double-Helix - The shape that 2 linear strands of DNA assume when bonded together.

Downstream - This indicates direction towards the 3' end of a DNA strand.

Dyad Symmetry - Symmetry in which letters of opposite ends (of the word) are symmetric (the same). e.g. the word NAVAN shows dyad symmetry.