A-site (Aminoacyl site) - This is the second of two ribosomal binding sites for tRNA molecules.

Adenine - A nitrogenous base found in both DNA and RNA . Adenine base-pairs with  Thymine in DNA and with Uracil in RNA.

Allele -  Alternative form of a genetic locus; a single allele for each locus is inherited separately from each parent (e.g., at a locus for eye color the allele might result in blue or brown eyes).

Amino Acids - Any of a class of 20 molecules that are combined to form proteins in living things. Amino acids are the building blocks of proteins. The sequence of amino acids in a protein and hence protein function are determined by the genetic code.

Anaphase - The stage in mitosis in which the connection of the sister chromosomes is severed, allowing each chromatid to be pulled towards the spindle pole to which it is connected by it's kinetochore microtubule.

Anticodon - A triplet of nucleotide bases in transfer RNA that identifies the amino
acid carried and binds to a complementary codon in messenger RNA
during protein synthesis at the ribosome.
 
ATP - (Adenosine triphosphate) A phosphorylated nucleoside of adenine
that supplies energy for many biochemical cellular processes by undergoing enzymatic
hydrolysis especially to ADP (Adenosine diphosphate).

Autosome - A chromosome not involved in sex determination. The diploid human genome consists of 46 chromosomes, i.e. 22 pairs of autosomes and 1 pair of sex chromosomes (X and Y).
 
Autosomal dominant disease - A disease caused by a dominant mutant gene on an autosome.

Autosomal recessive disease - A disease caused by the presence of two recessive mutant genes on an
 autosome.