Basic Definitions

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

A

Adenine - one of the bases of the nucleotides that make up DNA and RNA. Adenine pairs with thymine in a DNA molecule and with uracil in an RNA molecule. Adenine is often represented by the letter “A” in DNA/RNA diagrams.

Allele - one of two or more different forms of a gene . For example, the allele for blue eyes and the allele for brown eyes are different versions of the gene for eye color. Alleles are located at the same genetic locus .

Amino acid - An amino acid is one of 20 different molecules commonly referred to as the “building blocks” of the human body due to the fact that amino acids are used by the cells to create proteins .

Anaphase - a stage in cell division. Anaphase occurs once in mitosis and twice during two separate divisions in meiosis . Sister chromatids or homologous chromosomes are pulled apart and towards the centrioles during anaphase.

Autosome - an autosome is a specific type of chromosome , and could also be called a “non-sex chromosome.” The first 22 of the 23 pairs of human chromosomes are autosomes.

B

Base pairs - pairs of bases of nucleotides on opposite strands of DNA . The base adenine always pairs with the base thymine , and the base cytosine always pairs with the base guanine .

C

Carrier - An individual who can pass a disease-causing mutation on to their children, but does not show any signs or symptoms of the disease themselves.

Cell - the smallest unit of living matter, also referred to as the “building block” of life. Trillions of cells make up the human body. Nearly every human cell contains a full copy of the 46 chromosomes in its nucleus . Two major exceptions are the eggs and sperm, which each contain only 23 chromosomes. A cell also contains organelles , including ribosomes and mitochondria . See The Human Cell under Basic Concepts

Centriole - Centrioles are found at either pole of a cell during cell division. Spindle fibers attach to the centrioles at one end and chromosomes at the other end. When homologous chromosomes or sister chromatids are separated during cell division, they move towards the centrioles.

Chromatid - Chromatids are the two long subunits of a chromosome that has been replicated in preparation for cell division. When together, the two chromatids are called sister chromatids. When they have been separated during division, they are called daughter chromatids.

Chromosome - a long, thread-like structure on which the genes can be found. Humans generally have 46 chromosomes in each cell of the body. The 46 chromosomes come in 23 pairs. The first 22 pairs are the same in males and females and are called autosomes. The 23 rd pair is called the sex chromosomes . Males typically have one X sex chromosome, and one Y sex chromosome, while females have two X sex chromosomes.

Complementary - complementary simply means “matching” or “corresponding.” A complementary strand of DNA matches up with another strand of DNA via base pairs . For example, if “C” (cytosine) was found at a particular point on one strand, the complementary strand would have a “G” (guanine) at that particular point to match it.

Cytosine - one of the bases of the nucleotides that make up DNA and RNA . Cytosine pairs with guanine in both DNA and RNA molecules. Cytosine is often represented by the letter “C” in DNA/RNA diagrams.

D

Daughter cells - These cells are the products of cell division. Mitosis produces two daughter cells from one parent cell; the daughter cells are identical to the parent cell. Meiosis produces four daughter cells from one parent cell; the daughter cells have half the number of chromosomes as the parent cell.

Diploid - A cell that is diploid has double the number of the basic chromosome number. The basic chromosome number refers to the number of chromosomes found in a sex cell (i.e. egg or sperm). A cell with the basic chromosome number is a haploid cell. Human sex cells have 23 chromosomes, so the human basic chromosome number is 23. A diploid number of chromosomes would be 46. Most cells in the human body are diploid.

DNA - short for deoxyribonucleic acid. DNA is the material that carries all of our genetic information. It is made up of nucleotides , and is usually found in the form of a double-stranded helix. DNA is found on the chromosomes and makes up our genes .

Dominant - an allele of a gene is dominant when it is nearly always expressed, even if only one copy is present. A mutation in a dominant allele can lead to a genetic disorder that is inherited in a dominant manner, meaning that the mutation in only one copy of the gene will lead to a lost or changed gene function, and the characteristics of a genetic disorder will be seen. See Autosomal Dominant Inheritance under Basic Concepts .

E

F

G

Gene - a gene is a segment of DNA that directs a particular function of the body. A gene can be thought of as an instruction for the body to follow. There are genes for eye color and hair color. There are genes that tell heart how to beat and the stomach how to digest. It is estimated that there are approximately 30,000 genes in the human genome . Genes are found on chromosomes , and are thus in pairs just as chromosomes are in pairs. Each gene is located at a specific point on a chromosome known as a locus . At a locus, there can be different versions of a gene, called alleles . The alleles of a gene determine the different characteristics seen in individuals.

Genetics - the study of inheritance

Genome - the genome is the entire genetic composition. It contains all of the hereditary information of an individual, and includes all genes .

Genotype - the genetic makeup of an individual. A person's genotype determines their phenotype .

Guanine - one of the bases of the nucleotides that make up DNA and RNA . Guanine pairs with cytosine in both DNA and RNA molecules. Guanine is often represented by the letter “G” in DNA/RNA diagrams.

H

Haploid - The basic number of chromosomes is found in a haploid cell . The basic chromosome number refers to the number of chromosomes found in a sex cell (i.e. egg or sperm). Human sex cells have 23 chromosomes, so the human basic chromosome number is 23. A human haploid cell thus contains 23 chromosomes. Most cells in the human body have double the basic chromosome number and are diploid .

Hereditary - referring to characteristics that are inherited, or passed down from parent to offspring. Those characteristics are passed down through the genes .

Heterozygous - A person is heterozygous (a heterozygote) for a certain trait if they have two non-identical alleles at the same locus.

Homologous - Homologous chromosomes are identical to one another in terms of the genetic loci they contain. Homologous chromosomes usually come paired together. For example, a human female has 23 pairs of homologous chromosomes. A human male has 22 pairs of homologous chromosomes; his X and Y chromosomes are not homologous, as they are not identical.

Homozygous - A person is homozygous (a homozygote) for a certain trait if they have identical alleles at the same locus .

I   

J

K

Karyotype - a picture of the chromosomes arranged for easy analysis. In a karytope, the chromosomes are arranged according to size and numbered accordingly. The arrangement of chromosomes is always the same and allows scientists to spot certain abnormalities. An example of a karyotype can be seen in the What Causes Genetic Disease section of Basic Concepts .

L

Locus - the specific position or location of a gene on a chromosome . There can be several genes at one locus. The plural of locus is loci.

M

Meiosis - process of two consecutive cell divisions by which cells produce four daughter cells with half the number of chromosomes as the parent cell. Haploid egg and sperm cells are produced from regular diploid cells by this process. See Mitosis and Meiosis under Basic Concepts

Metaphase - a stage in cell division. Metaphase occurs once in mitosis and twice during two separate divisions in meiosis . Sister chromatids or homologous chromosomes line up at the center of the cell between the centrioles during metaphase.

Mitochondria - the organelle in a cell that is responsible for creating energy the cell can use. There are usually several mitochondria in each cell. Mitochondria have their own genes and DNA , separate from those found on chromosomes .

Mitosis - process of cell division by which cells replicate themselves to produce two daughter cells with the exact same number of chromosomes . See Mitosis and Meiosis under Basic Concepts

Molecule - the smallest particle of a substance or compound which still maintains all of its properties

mRNA - stands for messenger RNA . mRNA is one type of RNA molecule and its main function is to encode and carry genetic information from the nucleus to the ribosomes in a cell during transcription .

Mutation - any change in the genetic material. A mutation alters the structure and/or function of one or more genes and can cause changes in the expression of those genes. Mutations are permanent and heritable and can be passed down from a parent to its offspring. Usually, mutations are harmful or have no effect, but occasionally a mutation can be beneficial.

N

Nucleus - an enclosed structure in the cell in which chromosomes can be found; sometimes called the “brain” of the cell because it contains most of the genetic information

Nucleotide - a molecule consisting of sugar, phosphate, and a nitrogen base. Nucleotides make up DNA and RNA . There are five nitrogen bases that could potentially be part of a nucleotide: adenine, cytosine, guanine, thymine, and uracil.

O

Organelle - An organelle is a small structure in a cell with a specific purpose. The overall function of organelles is to keep the cell healthy and to help it maintain its function. Examples of organelles include ribosomes and mitochondria .

P

Pedigree - a drawing of a family tree. Medical professionals draw family trees using specific techniques and symbols in order to spot patterns, trends, and significant items quickly and easily. An example of a pedigree can be found in the Genetics and Family History section.

Phenotype - the characteristics of an individual determined by the interaction of their genes with the environment. A person's phenotype is determined by their genotype. Physical traits (e.g. eye color, hair color) are examples of phenotypes.

Prophase - a stage in cell division. Prophase occurs once in mitosis and twice during two separate divisions in meiosis . Sister chromatids or homologous chromosomes condense and become visible during prophase. Late in prophase during meiosis I, crossover between homologous chromosomes occurs. See Mitosis and Meiosis under Basic Concepts

Protein - genes have their effect on the body by coding for different proteins. Proteins each have a unique function and are required for the structure, function, and regulation of the body's cells , tissues, and organs.

Q

R

Recessive - an allele of a gene is recessive when it can only be expressed if two copies are present. A mutation in a recessive allele can lead to a genetic disorder that is inherited in a recessive manner, meaning that two mutations must be inherited in order for signs and symptoms to appear. A child born with a recessive condition must have inherited two mutated alleles from his or her carrier parents. See Autosomal Recessive Inheritance under Basic Concepts .

Replication - Replication of chromosomes takes place before cell division. Replication involves a chromosome making an exact copy of itself to form a sister chromatid .

Ribosome - A ribosome is an organelle found in a cell , and can be found free-floating or attached to the endoplasmic reticulum. Ribosomes are crucial in the creation of proteins from genes

RNA - short for ribonucleic acid. RNA is a molecule similar to DNA that is necessary in the process of creating the proteins coded for by the genes . The RNAs that aid in the creation of the proteins are known specifically as messenger RNA ( mRNA ) and transfer RNA ( tRNA ).

S

Sex chromosomes - the 23 rd pair of chromosomes in the human body, which determine the sex of an individual. Females typically have two X chromosomes in a diploid cell , and males typically have an X and a Y chromosome. When passing chromosomes on to offspring, mothers will always pass on an X chromosome. Fathers may pass on an X or a Y chromosome. If he passes on an X, the child will be female. If he passes on a Y, the child will be male.

Spindle fibers - long, threadlike structures in a cell . The spindle fibers appear during cell division and are responsible for separating sister chromatids and/or homologous chromosomes from each other.

Syndrome - the combination of specific signs and symptoms found together in a particular disorder

T

Telophase - a stage in cell division. Telophase occurs once in mitosis and twice during two separate divisions in meiosis . Once the sister chromatids and homologous chromosomes have been separated, the rest of the cell begins to divide and separate during telophase. Cell division is complete after telophase and the creation of two daughter cells .

Thymine - one of the bases of the nucleotides that make up DNA . Thymine pairs with adenine in DNA molecules. Thymine is often represented by the letter “T” in DNA diagrams. Thymine is not found in RNA . The base uracil pairs with adenine in RNA.

Transcription - Transcription is the first step in the process of gene expression. mRNA is created from the DNA (the genes ) of a cell . The process of translation follows transcription. See Transcription and Translation under Basic Concepts .

Translation - Once mRNA has been generated from the transcription process, it is used to synthesize proteins using building blocks called amino acids ; this process is known as translation. The end result of the transcription and translation processes is a protein that carries out a function designated by the original gene . See Transcription and Translation under Basic Concepts .

tRNA - stands for transfer RNA. tRNA is one type of RNA molecule and its main function is to and bring the amino acid coded for by an mRNA strand to the ribosome during protein synthesis.

U

Uracil - one of the bases of the nucleotides that make up RNA . Uracil pairs with adenine in RNA molecules. Uracil is often represented by the letter “U” in RNA diagrams. Uracil is not found in DNA . The base thymine pairs with adenine in DNA.

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Y

Z