Genetics Glossary: From DNA to Phenotype
DNA
One of the two nucleic acids found in cells. The sequence of nitrogenous bases in the DNA molecule contains the genetic instructions used to build living organisms. In eukaryotic cells, DNA is found in the cell nucleus, supercoiled around proteins called histones to form chromosomes. DNA is the material that contains the genetic information of living things. Scientists once believed that proteins were the carriers of this information.
DNA Polymerase
An enzyme that catalyzes the replication of DNA. It can direct the incorporation of nucleotides into a complementary DNA strand from a template DNA strand.
RNA
A chemical similar to DNA (also a nucleic acid). It differs from DNA in that its molecule is composed of a single chain of nucleotides, and the base uracil (U) replaces thymine (T) found in DNA. There are three main types of RNA: messenger RNA, transfer RNA, and ribosomal RNA.
- Messenger RNA (mRNA) carries the genetic information from DNA to the cytoplasm.
- Ribosomal RNA (rRNA) is part of the ribosomes.
- Transfer RNA (tRNA) carries the amino acids to the ribosomes to assemble proteins.
Alleles
One of the possible forms of a gene that can occur at a single gene locus. Different alleles of a gene produce different manifestations of a hereditary characteristic.
Amino Acids
There are twenty different natural amino acids, which are joined together in long chains to form different proteins. Each protein has a specific amino acid sequence.
Gene Amplification
A mutation that occurs when the cell makes a mistake in its DNA replication and copies portions of the DNA more than once. In some cases, such as tumors, genes are amplified (re-copied) thousands of times. These extra copies of genes can cause the tumor to be more aggressive or more likely to spread.
Dominant Allele
The gene that prevents the expression of its recessive counterpart. It is symbolized by a capital letter.
Recessive Allele
The gene that is not expressed if its partner is dominant. It is symbolized by a lowercase letter.
Messenger RNA (mRNA)
One of the types of RNA molecules. It is responsible for reading a specific DNA sequence and transporting it from the nucleus to the cytoplasm, where it is translated on ribosomes into a polypeptide or protein. The base sequence of an mRNA strand is complementary to the transcribed DNA sequence. Each set of three bases of mRNA, called a codon, specifies an amino acid in the final protein.
Nitrogenous Bases
Essential parts of nucleotides. These ring-shaped compounds can be purines or pyrimidines.
- Pyrimidine bases are derivatives of pyrimidine and include cytosine, thymine, and uracil.
- Purine bases are derived from purine and include adenine and guanine.
Codon
Each group of three bases in a DNA or RNA sequence that specifies an amino acid. Codons are the “words” of the genetic code that specify either the beginning of a protein, the end of a protein, or one of the amino acids it contains.
Character or Hereditary Factors
One of the characteristics that are inherited from parents to offspring.
Zygote
The first cell of a new individual, resulting from the union of two gametes. A zygote has a pair of alleles for each character.
Genotype
The set of genes carried by a living being in each of its cells.
Double Helix
The spatial structure of DNA, which can be represented as a ladder twisted into a helix or spiral around a central axis. The structures forming the helix are molecules called nucleotides, linked together by hydrogen bonds.
The discovery of the double helix structure of DNA is attributed to James Watson and Francis Crick in 1953. By studying X-ray photographs of DNA taken by Rosalind Franklin, they deduced the actual chemical structure of this molecule, which contains all the information living beings need to carry out their biological functions.
Example: DNA is like a ladder, and each rung of the ladder comprises a pair of bases (represented by the letters A, T, C, and G), in which A always pairs with T and C with G. This sequence of A, C, G, and T determines the genetic instructions for all organisms. Humans have 3 billion of these base pairs in their DNA molecule.
All mammals generally have DNA molecules of similar size. However, simpler organisms tend to have less DNA, though not always. For example, the fruit fly has only 150 million base pairs, the nematode worm about 100 million, yeast approximately 15 million, and bacteria from one to five million base pairs, depending on the type.
Enzymes
Proteins that catalyze chemical reactions that occur in living things. Many reactions that occur during normal metabolism and bodily functions could not occur without enzymes, which generally accelerate the rate at which these reactions occur spontaneously.
Restriction Enzymes
Proteins that can recognize specific sequences of nitrogenous bases in DNA and cut the molecule at that site. Restriction enzymes are frequently used in the laboratory as the cornerstone of recombinant DNA technology.
Phenotype
The observable traits or characteristics of an organism (e.g., hair color, weight, presence of disease). Phenotypic traits are not necessarily genetic. The phenotype is the expression or result of the genetic constitution in a particular environment in which the organism grows and develops.
Gene
Each piece of DNA in the cell nucleus where the information for one character is located. Genes are symbolized by letters.
Gametes
Sex cells (sperm and egg) that unite during fertilization to give rise to a new individual. Each gamete carries half the genetic information of a cell; therefore, it carries a single allele for each character.
Genome
The complete genetic material of an organism, which includes both the chromosomes (in the cell nucleus) and mitochondrial DNA. Each cell of an organism has a complete copy of that organism’s genome.
Genetics
The branch of biology that studies the transmission of characteristics from one generation to the next.
Homozygous or Purebred
An individual having two identical alleles for a particular character.
Heterozygous or Hybrid
An individual having two different alleles for a particular character.
Helicase
An enzyme that breaks the hydrogen bonds holding together the nitrogenous bases of double-stranded DNA.
Locus
The specific location on a chromosome where a particular gene is located. In other words, it is the gene’s physical address. Our chromosomes contain between 30,000 and 40,000 genes, distributed among 23 pairs of chromosomes.