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Dr. Anand Saggar

Dr. Anand Saggar - MBBS, CCST, FRCP

Medical Genetic Specialist

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07970 886663

Glossary

E

 

A

Adenine
A nitrogenous base, one member of the base pair A-T (adenine-thymine).

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

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

Amplification
An increase in the number of copies of a specific DNA fragment; can be in vivo or in vitro. See cloning, polymerase chain reaction

Autosome
A chromosome not involved in sex determination. The diploid human genome consists of 46 chromosomes, 22 pairs of autosomes, and 1 pair of sex chromosomes (the X and Y chromosomes).

B

Balanced Translocation
Occurs when two chromosomes break and exchange places leaving the same amount of genetic material. An individual with a balanced translocation will be unaffected, but children may be affected in a variety of ways.

Base pair (bp)
Two nitrogenous bases (adenine and thymine or guanine and cytosine) held together by weak bonds. Two strands of DNA are held together in the shape of a double helix by the bonds between base pairs.

Base sequence
The order of nucleotide bases in a DNA molecule.

Biotechnology
A set of biological techniques developed through basic research and now applied to research and product development. In particular, the use by industry of recombinant DNA, cell fusion, and new bioprocessing techniques. bp: See base pair.

C

Carrier (heterozygote)
An individual who carries one copy of a recessive gene.

Cell
The basic structural unit of all living organisms. A human cell is made up a central nucleus (containing DNA) a cytoplasm and a outer cell membrane.

Centimorgan (cM)
A unit of measure of recombination frequency. One centimorgan is equal to a 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossing over in a single generation. In human beings, 1 centimorgan is equivalent, on average, to 1 million base pairs.
Centromere A specialised chromosome region to which spindle fibers attach during cell division.

Chromatid

The two identical halves of a chromosome produced for cell division and meiosis.

Chromosome
Structures found in the nucleus of cells composed of DNA and proteins. Normally humans have 46 chromosomes in each cell, 23 from each parent. Of these, 22 are autosomes and 1 is a sex chromosome.

Clones
A group of cells derived from a single ancestor.

Cloning
The process of asexually producing a group of cells (clones), all genetically identical, from a single ancestor. In recombinant DNA technology, the use of DNA manipulation procedures to produce multiple copies of a single gene or segment of DNA is referred to as cloning DNA.

Conserved sequence

A base sequence in a DNA molecule (or an amino acid sequence in a protein) that has remained essentially unchanged throughout evolution.

Crossing over

The breaking during meiosis of one maternal and one paternal chromosome, the exchange of corresponding sections of DNA, and the rejoining of the chromosomes. This process can result in an exchange of alleles between chromosomes. Compare recombination.

Cytosine (C)
A nitrogenous base, one member of the base pair G-C (guanine and cytosine).

D

Diploid
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. Compare haploid.

Dizygotic
Two cells having been fertilised at the same time (often resulting in twins.)

DNA (deoxyribonucleic acid)
The molecule that encodes genetic information. DNA is a double-stranded molecule held together by weak bonds between base pairs of nucleotides. the four nucleotides in dna contain the bases stranded 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.

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. See base sequence analysis.

Double helix
The shape that two linear strands of DNA assume when bonded together.

Duplication
Duplication of a sequnece of DNA or section of chromosome.

E

Electrophoresis
A method of separating large molecules (such as DNA fragments or proteins) from a mixture of similar molecules. An electric current is passed through a medium containing the mixture, and each kind of molecule travels through the medium at a different rate, depending on its electrical charge and size. Separation is based on these differences. Agarose and acrylamide gels are the media commonly used for electrophoresis of proteins and nucleic acids.

Enzyme
A protein catalyst which is essential to the correct functioning of biochemical reactions.

Eukaryote
Cell or organism with membrane-bound, structurally discrete nucleus and other well-developed subcellular compartments. Eukaryotes include all organisms except viruses, bacteria, and blue-green algae. Compare prokaryote.

Exogenous DNA
DNA originating outside an organism.

Exons
The protein-coding DNA sequences of a gene. Compare introns.

F

Familial
Pertaining to the family group.

FISH (fluorescence in situ hybridization)
A physical mapping approach that uses fluorescein tags to detect hybridization of probes with metaphase chromosomes and with the less-condensed somatic interphase chromatin.

Fragile Site
A site on a chromosome vulnerable to mutation to breakage .

G

Gamete
Mature male or female reproductive cell (sperm or ovum) with a haploid set of chromosomes (23 for humans).

Gene
The fundamental physical and functional unit of heredity. A gene is an ordered sequence of nucleotides located in a particular position on a particular chromosome that encodes a specific functional product (i.e., a protein or RNA molecule). See gene expression .

Gene expression
The process by which a genes coded information is converted into the structures present and operating in the cell. Expressed genes include those that are transcribed into mRNA and then translated into protein and those that are transcribed into RNA but not translated into protein (e.g., transfer and ribosomal RNAs).

Gene families
Groups of closely related genes that make similar products.

Gene mapping
Determination of the relative positions of genes on a DNA molecule (chromosome or plasmid) and of the distance, in linkage units or physical units, between them.

Gene product
The biochemical material, either RNA or protein, resulting from expression of a gene. The amount of gene product is used to measure how active a gene is; abnormal amounts can be correlated with disease-causing alleles.

Gene therapy

Insertion of normal DNA directly into cells to correct a genetic defect.

Genetic code
The sequence of nucleotides, coded in triplets (codons) along the mRNA, that determines the sequence of amino acids in protein synthesis. The DNA sequence of a gene can be used to predict the mRNA sequence, and the genetic code can in turn be used to predict the amino acid sequence.

Genetic counselling
Information and support provided by a specialist doctor, usually a geneticist, to parents who have known conditions in their families or who are concerned about the future possibility of genetically transmitted conditions.

Genetic engineering
technologies See recombinant DNA technologies.

Genetics
The study of the patterns of inheritance of specific traits.

Genome
All the genetic material in the chromosomes of a particular organism; its size is generally given as its total number of base pairs.

Genome projects
Research and technology development efforts aimed at mapping and sequencing some or all of the genome of human beings and other organisms.

Genomic library

A collection of clones made from a set of randomly generated overlapping DNA fragments representing the entire genome of an organism. Compare library, arrayed library.

Guanine (G)
A nitrogenous base, one member of the base pair G-C (guanine and cytosine).

H

Haploid
A single set of chromosomes (half the full set of genetic material), present in the egg and sperm cells of animals and in the egg and pollen cells of plants. Human beings have 23 chromosomes in their reproductive cells. Compare diploid.

Heterozygosity
The presence of different alleles at one or more loci on homologous chromosomes.

Homologous chromosomes

A pair of chromosomes containing the same linear gene sequences, each derived from one parent.

Homozygote
An individual with both identical alleles (versions of a single gene) at one locus (position).

Hybridisation

The process of joining two complementary DNA strands

I

Imprinting
Patterns of inheritance affected by whether the inheritance was from the mother or father.

In situ hybridisation

Use of a DNA or RNA probe to detect the presence of the complementary DNA sequence in cloned bacterial or cultured eukaryotic cells.

In vitro
Outside a living organism.

Inheritance

The passing of familial elements from one generation to the next.

Introns
The DNA base sequences interrupting the protein-coding sequences of a gene; these sequences are transcribed into RNA but are cut out of the message before it is translated into protein. Compare exons.

Inversion
Occurs where a chromosome breaks in two and becomes reattached in reverse orientation. This may or may not affect gene function.

K

Karyotype
A photomicrograph of an individual's chromosomes arranged in a standard format showing the number, size, and shape of each chromosome type; used in low-resolution physical mapping to correlate gross chromosomal abnormalities with the characteristics of specific diseases.

Kilobase (kb)
Unit of length for DNA fragments equal to 1000 nucleotides.

L

Linkage
The proximity of two or more markers (e.g., genes, RFLP markers) on a chromosome; the closer together the markers are, the lower the probability that they will be separated during DNA repair or replication processes (binary fission in prokaryotes, mitosis or meiosis in eukaryotes), and hence the greater the probability that they will be inherited together.

Linkage map
A map of the relative positions of genetic loci on a chromosome, determined on the basis of how often the loci are inherited together. Distance is measured in centimorgans (cM).

Localize
Determination of the original position (locus) of a gene or other marker on a chromosome.

Locus (pl. loci)
The position on a chromosome of a gene or other chromosome marker; also, the DNA at that position. The use of locus is sometimes restricted to mean regions of DNA that are expressed. See gene expression.

M

Marker
An identifiable physical location on a chromosome (e.g., restriction enzyme cutting site, gene) whose inheritance can be monitored. Markers can be expressed regions of DNA (genes) or some segment of DNA with no known coding function but whose pattern of inheritance can be determined. See RFLP, restriction fragment length polymorphism.

Megabase (Mb)
Unit of length for DNA fragments equal to 1 million nucleotides and roughly equal to 1 cM.

Metaphase
A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plane of the cell.

Mitochondria 
Mitochondria are the cell’s power sources, compartments within the cell that provide its energy. They contain their own DNA, called mitochondrial DNA, a very small amount of DNA inherited only from your mother.

Mitosis
Cell division producing two genetically identical cells.

Monosomy
The total loss of one of a pair of chromosomes. This occurs, for example, in Turner Syndrome where one X chromosome is lost leaving a total of 45 chromosomes.

Mosaicism

Where a genetic or chromosomal abnormality does not occur in all body cells. Often related to X chromosome innactivation.

Mutant
A gene having undergone genetic change or mutation.

Mutation

Any heritable change in DNA sequence. Compare polymorphism.

N

Nucleic acid
A large molecule composed of nucleotide subunits.

Nucleotide
A subunit of DNA or RNA consisting of a nitrogenous base (adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a phosphate molecule, and a sugar molecule (deoxyribose in DNA and ribose in RNA). Thousands of nucleotides are linked to form a DNA or RNA molecule.

Nucleus
The cellular organelle in eukaryotes that contains the genetic material.

O

Oncogene
A gene, one or more forms of which is associated with cancer. Many oncogenes are involved, directly or indirectly, in controlling the rate of cell growth.

Ovum
Female egg

P

Penetrance
An "all or none" reference to clinical expression of a mutant gene.

Polygenic disorders
Genetic disorders resulting from the combined action of alleles of more than one gene (e.g., heart disease, diabetes, and some cancers). Although such disorders are inherited, they depend on the simultaneous presence of several alleles; thus the hereditary patterns are usually more complex than those of single-gene disorders. Compare single-gene disorders.

Polymerase chain reaction (PCR)
A method for amplifying a DNA base sequence using a heat-stable polymerase and two 20-base primers, one complementary to the (+)-strand at one end of the sequence to be amplified and the other complementary to the (-)-strand at the other end. Because the newly synthesized DNA strands can subsequently serve as additional templates for the same primer sequences, successive rounds of primer annealing, strand elongation, and dissociation produce rapid and highly specific amplification of the desired sequence. PCR also can be used to detect the existence of the defined sequence in a DNA sample.

Polymerase, DNA or RNA
Enzymes that catalyse the synthesis of nucleic acids on preexisting nucleic acid templates, assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.

Polymorphism

A Difference in DNA sequence among individuals. Genetic variations occurring in more than 1% of a population would be considered useful polymorphisms for genetic linkage analysis. Compare mutation.

Predisposition
Intrinsic likelyhood of developing a particular disorder.

Primer
Short preexisting polynucleotide chain to which new deoxyribonucleotides can be added by DNA polymerase.

Probe

Single-stranded DNA or RNA molecules of specific base sequence, labeled either radioactively or immunologically, that are used to detect the complementary base sequence by hybridisation.

Promoter
A site on DNA to which RNA polymerase will bind and initiate transcription.

Protein

A large molecule composed of one or more chains of amino acids in a specific order; the order is determined by the base sequence of nucleotides in the gene coding for the protein. Proteins are required for the structure, function, and regulation of the bodys cells, tissues, and organs, and each protein has unique functions. Examples are hormones, enzymes, and antibodies.

Purine

A nitrogen-containing, single-ring, basic compound that occurs in nucleic acids. The purines in DNA and RNA are adenine and guanine.

Pyrimidine
A nitrogen-containing, double-ring, basic compound that occurs in nucleic acids. The pyrimidines in DNA are cytosine and thymine; in RNA, cytosine and uracil.

R

Recombinant
DNA molecules A combination of DNA molecules of different origin that are joined using recombinant DNA technologies.

Recombinant

DNA technologies Procedures used to join together DNA segments in a cell-free system (an environment outside a cell or organism). Under appropriate conditions, a recombinant DNA molecule can enter a cell and replicate there, either autonomously or after it has become integrated into a cellular chromosome.

Recombination
The process by which progeny derive a combination of genes different from that of either parent. In higher organisms, this can occur by crossing over.

Regulatory regions or sequences
A DNA base sequence that controls gene expression.

Restriction enzyme cutting site

A specific nucleotide sequence of DNA at which a particular restriction enzyme cuts the DNA. Some sites occur frequently in DNA (e.g., every several hundred base pairs), others much less frequently (rare-cutter; e.g., every 10,000 base pairs).

Ribonucleic acid (RNA)
A chemical found in the nucleus and cytoplasm of cells; it plays an important role in protein synthesis and other chemical activities of the cell. The structure of RNA is similar to that of DNA. There are several classes of RNA molecules, including messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs, each serving a different purpose.

Ribosomal RNA (rRNA)

A class of RNA found in the ribosomes of cells.

Ring Formation

Occurs where the ends of a chromosome bend over and fuse together with a loss of genetic material.

S

Sequencing
Determination of the order of nucleotides (base sequences) in a DNA or RNA molecule or the order of amino acids in a protein.

Sex chromosomes

The X and Y chromosomes in human beings that determine the sex of an individual. Females have two X chromosomes in diploid cells; males have an X and a Y chromosome. The sex chromosomes comprise the 23rd chromosome pair in a karyotype. Compare autosome.

Single-gene disorder

Hereditary disorder caused by a mutant allele of a single gene (e.g., Duchenne muscular dystrophy, retinoblastoma, sickle cell disease). Compare polygenic disorders.

Somatic cells
Any cell in the body except gametes and their precursors.

Status Mode (Pattern of Inheritance)

Describes how the gene is inherited

T

Telomere
The ends of chromosomes. These specialized structures are involved in the replication and stability of linear DNA molecules. See DNA replication.

Thymine (T)
A nitrogenous base, one member of the base pair A-T (adenine-thymine).

Transcription
The synthesis of an RNA copy from a sequence of DNA (a gene); the first step in gene expression. Compare translation.

Transformation
A process by which the genetic material carried by an individual cell is altered by incorporation of exogenous DNA into its genome.

Translation
The process in which the genetic code carried by mRNA directs the synthesis of proteins from amino acids. Compare transcription.

Translocation
Chromosomal rearrangement in which a piece of one chromosome is transferred to another one.

Triploidy
The presence of a full extra set of chromosomes. Often lethal.

Trisomy
3 copies of a particular chromosome (normally we have only 2.)

Twins
May be genetically identical (monozygous) when they arise from a single fertilised egg or non-identical (dizygous) when they arise from two separate eggs.

U

Uracil
A nitrogenous base normally found in RNA but not DNA; uracil is capable of forming a base pair with adenine.

V

Virus
A noncellular biological entity that can reproduce only within a host cell. Viruses consist of nucleic acid covered by protein; some animal viruses are also surrounded by membrane. Inside the infected cell, the virus uses the synthetic capability of the host to produce progeny virus.

Y

Yeast artificial chromosome (YAC)
A vector used to clone DNA fragments (up to 400 kb); it is constructed from the telomeric, centromeric, and replication origin sequences needed for replication in yeast cells. Compare cloning vector, cosmid.