Genetics Glossary
From SkepticWiki
This is a glossary for some of the technical terms used in our textbook on Genetics and in related articles.
It is also to some extent an index, in that for each word defined, it gives a reference to the main article in which the concept is introduced and discussed (if there is one). Obviously anyone who wants to find all instances of a word or phrase in the SkepticWiki can simply use the Search function in the toolbar on the left.
[edit] A
A is the abbreviation for adenine.
Main article: DNA: chemical structure of DNA
[edit] Adenine
Adenine (A) is one of five bases that can occur as side chains of nucleotides. It base-pairs with thymine in DNA and with uracil in RNA.
Main article: DNA: chemical structure of DNA
[edit] Allele
An allele is one of the possible variant forms of a gene.
[edit] Alternative gene splicing
Alternative gene splicing is the situation where a piece of messenger RNA may be spliced in a number of different ways, with different bits of it being treated as introns and exons, conditional on such factors as the sex of the organism, the age of the organism, the type of body cell that the gene is in, etc.
Main article: Splicing: Alternative splicing
[edit] Amino acids
Amino acids are the monomers which polymerize (link together in a chain) to form polypeptides. There are twenty standard amino acids found in life; their order in a polypeptide determines its shape and chemical properties, and hence its function in the cell.
Main articles: DNA: Translation; Genetic Code.
[edit] Anticoding strand
The anticoding strand is the strand in a piece of double-stranded DNA opposite the strand to be transcribed into RNA. transcription is achieved by complementing the anticoding strand with ribonucleotides: because of the base-pairing rules, this results in a copy of the coding strand, except that the resulting RNA has uracil as a base where the coding strand had thymine. Note that the same strand of DNA will not be the anticoding strand throughout the length of a chromosome: a strand may be the coding strand for some genes and the anticoding strand for others.
Main article: DNA: Transcription
[edit] Antiparallelism
Because of the way that base-pairing works, double-stranded DNA is antiparallel. That is to say, if you work your way along a double strand of DNA, you will be following one strand from the 5' end to the 3' end, and the other strand from the 3' end to the 5' end: the two strands lie in opposite orientations, like two people sharing a bed sleeping head to foot.
Main article: DNA: chemical structure of DNA
[edit] Artificial selection
Artificial selection is the intelligent selection of breeding stock from a population, on the basis of heritable traits favored by the stockbreeder, with the effect of encouraging the spread of those traits through the breeding population.
Main article: Artificial Selection
[edit] Base
The bases of DNA and RNA are the side chains of their nucleotides: adenine, guanine, cytosine, and thymine in the case of DNA, adenine, guanine, cytosine and uracil in the case of RNA. They are customarily abbreviated to A, C, G, T and U.
Main article: DNA: chemical structure of DNA
[edit] Base-pairing
The bases of DNA and RNA can only join together in certain ways to form double strands: G and C can join together, as can A and T; and A and U. This is known as base-pairing.
Main article: DNA: chemical structure of DNA
[edit] C
C is the abbreviation for cytosine
Main article: DNA: chemical structure of DNA
[edit] Catalyst
A catalyst is a chemical that encourages some chemical reaction to take place frequently which would take place only rarely (though occasionally) in the absence of the catalyst. The catalyst is not used up in the reaction that it catalyses, and so can cause many such reactions.
[edit] Centromere
The centromere is the area in a eukaryotic chromosome where two sister chromatids attach. The DNA in a centromere does not code for genes, but performs an important functional role in mitosis and meiosis.
Main article: Chromosomes
[edit] Character
A character is a feature that can vary from organism to organism, the different varieties being known as traits.
Main article: Mendelian Genetics: characters and traits
[edit] Chromatids
A chromatid is one of two identical pieces of double-stranded DNA produced during DNA replication in eukaryotic cells, attached together at their centromeres. These remain together for much of the cell cycle until they are separated by mitosis. This pairing should not be confused with the fact that DNA is double-stranded, nor with the pairs of sister chromosomes found in diploid organisms.
Main article: Chromosomes
[edit] Chromosome
An entire single piece of DNA. Eukaryotes tend to have several linear chromosomes; prokaryotes tend to have a single circular chromosome.
Main article: Chromosomes
[edit] Coding strand
The coding strand is the strand in a piece of double-stranded DNA to be transcribed into RNA. This is achieved by complementing the other strand of DNA (the "anticoding strand") with ribonucleotides: because of the base-pairing rules, this results in a copy of the coding strand, except that the resulting RNA have uracil as a base where the coding strand had thymine. Note that the same strand of DNA will not be the coding strand throughout the length of a chromosome: a strand may be the coding strand for some genes and the anticoding strand for others.
Main article: DNA: transcription
[edit] Codon
A codon is three bases of DNA, or the three equivalent bases in a strand of messenger RNA, coding for a single amino acid in a polypeptide.
Main articles: Genetic Code; DNA: transcription
[edit] Complementary
Two strands of DNA or RNA are said to be complementary if the sequences of their bases obey the base-pairing rules, so that they could be joined as a double strand by connecting their bases.
Main article: DNA: chemical structure of DNA
[edit] Complementation
Complementation is the formation of a strand of DNA or RNA which is complementary to some existing strand of DNA or RNA, by linking together nucleotides in such a way that their bases form base-pairs with the bases of the pre-existing strand.
Main articles: DNA: chemical structure of DNA; DNA: replication; DNA: transcription
[edit] Conjugation
Conjugation is a form of lateral gene transfer in which one bacterium donates genes to another.
Main article: Lateral gene transfer: examples
[edit] Crossing over
Crossing over is another term for recombination.
Main article: Recombination
[edit] Cytoplasm
Cytoplasm is the fluid with which cells are filled.
[edit] Cytosine
Cytosine (C) is one of five bases that can occur as side chains of nucleotides. It base-pairs with guanine.
Main article: DNA: Chemical structure of DNA
[edit] Degenerate code
A code is said to be degenerate if it has more than one symbol with the same meaning. Hence the genetic code is said to be degenerate because more than one codon can code for the same amino acid.
Main article: Genetic Code
[edit] Deletion
A deletion is a mutation in which a section is deleted from a piece of DNA.
Main article: Mutation
[edit] Diploid
An organism is said to be diploid if (with the exception of its sex chromosomes) its chromosomes come in homologous pairs of sister chromosomes.
Main article: Chromosomes: diploidy
[edit] DNA
Deoxyribonucleic acid: a polymer of nucleotides. The sequence of bases in DNA controls the production of functional RNA and messenger RNA, and so ultimately controls the synthesis of proteins and hence the workings of the cell.
Main article: DNA
[edit] DNA polymerase
DNA polymerase is the enzyme that directs the synthesis of DNA during DNA replication.
Main article: DNA: replication
[edit] DNA replication
DNA replication is the process whereby one piece of double-stranded DNA is copied to produce two identical pieces of double-stranded DNA, by complementation of both of the two original strands of the DNA.
Main article: DNA: replication
[edit] Dominant
A dominant allele is one that will cause an organism possessing it to express the trait associated with that allele even if the organism has only one copy of the allele.
Main article: Mendelian genetics: Dominant and recessive alleles
[edit] Double-stranded
A piece of DNA or RNA is said to be double-stranded if it consists of two polymers of nucleotides joined together along their length by base-pairing of their side chains. In nature, DNA is usually double-stranded, and RNA single-stranded, though some viruses present exceptions to this rule.
Main article: DNA: Chemical structure of DNA
[edit] Duplication
Duplications are mutations in which part of a chromosome gets repeated.
Main article: Mutation
[edit] Enzyme
An enzyme is an organic molecule that causes (strictly speaking, "catalyses") a chemical reaction. In the cell, the enzymes are either proteins or functional RNA (mostly proteins, but with functional RNA playing a crucial role in such basic cellular activities as translation and splicing).
[edit] Eukaryote
A eukaryote is an organism the cells of which have nuclei to contain their DNA, with transcription taking place inside the nucleus, and translation taking place outside. Amongst other things, plants, animals and fungi are eukaryotes. Non-eukaryotes are called prokaryotes.
[edit] Eugenics
Eugenics is the application of artificial selection to humans.
Main article: Eugenics; see also Artificial Selection
[edit] Evolution
Evolution is the change in the gene pool of a population over time.
Main article: Theory of Evolution
[edit] Exon
An exon is a section of RNA which is not snipped out during splicing, and so is present in the mature RNA.
Main article: Splicing
[edit] Frame shift
A frame shift is a mutation involving the insertion or deletion of a number of nucleotides not divisible by three. Because the codons of the genetic code have three bases each, this radically changes the translation of the gene after the frame shift.
Main article: Mutation; see also Genetic Code
[edit] Functional RNA
Functional RNA is RNA that is chemically active in the cell, as opposed to messenger RNA.
Main article: DNA: transcription
[edit] G
G is the abbreviation for guanine.
Main article: DNA: Chemical structure of DNA
[edit] Gamete
A sperm or an egg: a haploid cell two of which will fuse together to form a diploid zygote.
Main article: Chromosomes: sexual reproduction
[edit] Gene
A gene is a sequence of DNA coding for a single polypeptide.
Main articles: Mendelian Genetics: genes and alleles; DNA and Mendelian genetics
[edit] Gene pool
A gene pool is all the genes of a population.
Main article: Population Genetics
[edit] Genetic code
The genetic code is the code specifying the relationship between the sequence of codons in a strand of DNA (or the corresponding messenger RNA) and the sequence of amino acids in the polypeptide produced when the messenger RNA is translated.
Main article: Genetic Code
[edit] Genetic drift
Genetic drift is variation in the composition of a gene pool due to chance factors.
Main article: Genetic Drift
[edit] Genome
A genome is the complete set of genes of an organism, or, by extension, a species.
[edit] Genotype
The genes controlling a set of traits in an organism, as distinct from the set of traits themselves, which are referred to as the phenotype
Main article: Mendelian Genetics: genes and alleles
[edit] Germ-line mutation
A germ-line mutation is a mutation that affects a gamete, which is therefore heritable and which will be present in every cell of an organism that inherits it.
Main article: Mutation: note on somatic and germ-line mutations
[edit] Guanine
Guanine (G) is one of five bases that can occur as side chains of nucleotides. It base-pairs with cytosine.
Main article: DNA: Chemical structure of DNA
[edit] Haploid
A cell is said to be haploid if it contains only one copy of each of its chromosomes, rather than having two homologous copies of each chromosome as in diploid cells.
Main article: Chromosomes
[edit] Heterozygote
A diploid organism which has two different alleles of the same gene is said to be heterozygous with respect to that gene.
Main article: Mendelian Genetics: genes and alleles
[edit] Heterozygote advantage
Heterozygote advantage is the description given to the situation where natural selection favors organisms which are heterozygous for some gene over organisms that are homozygous for that gene.
Main article: Heterozygote Advantage
[edit] Homology
Two pieces of DNA are said to be homologous if they contain the same genes, though not necessarily the same alleles of those genes.
Main article: Chromosomes
[edit] Homozygote
A diploid organism which has two identical alleles of the same gene is said to be homozygous with respect to that gene.
Main article: Mendelian Genetics: genes and alleles
[edit] Horizontal gene transfer
Horizontal gene transfer is another term for lateral gene transfer.
Main article: Lateral Gene Transfer
[edit] Independent assortment
Mendel's law of independent assortment states that the inheritance pattern for one trait will not affect the inheritance pattern for another trait. For example, if a plant has two different alleles of a gene for flower color, and two different alleles for leaf shape, then the fact that one particular allele for flower color is passed on in the gamete will be independent of which allele for leaf shape is passed on. Note that this "law" breaks down when the two genes lie on the same chromosome, because of linkage.
Main article: Mendelian Genetics: reproduction
[edit] Insertion
An insertion is a mutation in which nucleotides are added into a section of DNA.
Main article: Mutation
[edit] Intron
Introns are sections of RNA which are snipped out during splicing.
Main article: Splicing
[edit] Inversion
An inversion is a mutation in which a bit of DNA gets turned round in the chromosome.
Main article: Mutation
[edit] Lateral gene transfer
Lateral gene transfer is the transfer of genes from one organism to another, as opposed to the more usual vertical transfer in which an organism gets its genes from its parents or parents. Modes of lateral gene transfer include conjugation, transformation, and transduction.
Main article: Lateral Gene Transfer
[edit] Linkage
Linkage is the proximity of two genes on the same chromosome, and so also refers to the tendency of alleles of two genes on the same chromosome to be inherited together, thus forming an exception to the principle of independent assortment.
Main article: Recombination: linkage
[edit] Meiosis
Meiosis is a process of cell division which starts with one diploid cell and ends up with four haploid cells. In animals, such cells will always be gametes.
Main article: Chromosomes: meiosis
[edit] Mendel's laws
Mendel's laws, or principles, are the principle of segregation and the principle of independent assortment. They provide us with a statistical basis for predicting the inheritance of genes.
Main article: Mendelian Genetics
[edit] Messenger RNA
Messenger RNA is RNA which will be translated by a ribosome, directing the formation of a polypeptide.
Main article: DNA: transcription; DNA: translation
[edit] Mitosis
Mitosis is the process in the division of a eukaryotic cell by which a eukaryotic cell separates the chromosomes in its nucleus into two identical sets in two daughter nuclei.
Main article: Chromosomes: the cell cycle
[edit] Molecular clocks
Molecular clocks are methods for deducing the time since two populations diverged by comparing their DNA or the proteins that it codes for.
Main article: Molecular Clocks
[edit] Molecular phylogeny
Molecular phylogeny is the deduction of the form of the "tree of life" by comparison of the DNA or proteins of different species.
Main article: Molecular Phylogeny
[edit] Monomer
Monomers are the units that link together in a chain to form a polymer: e.g. amino acids are the monomers of which polypeptides are the polymers; nucleotides are the monomers of which nucleic acids are the polymers.
[edit] mRNA
Abbreviation for messenger RNA
Main articles: DNA: transcription; DNA: translation
[edit] Mutation
A mutation is a change in genetic material due to copying errors or other forms of damage or botched repair to the DNA. Mutations come in various types such as single nucleotide substitution, insertion, deletion, inversion and translocation. Mutations can also be classified as germ-line mutations and somatic mutations, depending on whether or not they affect gametes.
Main article: Mutation; see also Mutation and Evolution; Beneficial Mutations
[edit] Natural selection
Natural selection is the tendency for those genes producing phenotypes better adapted to the environment than other genes in the same population to spread through the gene pool of that population.
Main article: Natural Selection
[edit] Neutral mutation
A neutral mutation is one that has neither a beneficial nor a harmful effect on the organism; its fate in the gene pool therefore depends only on genetic drift and not on natural selection.
Main articles: Mutation; Genetic Drift
[edit] Nonsense codon
Nonsense codon is a slightly inaccurate term for a stop codon. They are not nonsensical, they mean "stop".
Main article: Genetic Code
[edit] Nucleic acid
A nucleic acid is a polymer of nucleotides: i.e. DNA or RNA
Main article: DNA
[edit] Nucleotide
A nucleotide consists of a sugar with a base and a phosphate group attached to it. Nucleotides can be joined together by linking the phosphate group of one to the sugar of another, producing a polymer. DNA and RNA are polymers of nucleotides.
Main article: DNA: Chemical structure of DNA
[edit] Nucleus
In a eukaryotic cell, the chromosomes and some of the cell machinery (e.g. the mechanisms for DNA replication, transcription, and splicing) come enclosed inside a double membrane: the membrane and what it encloses is the nucleus of the cell.
[edit] Operon
An operon is a set of genes which lie on the same stretch of DNA and are transcribed together onto a single piece of messenger RNA. Because the genes are divided by stop codons, they are translated into separate polypeptides.
[edit] Phenotype
A set of traits of an organism, as opposed to the set of genes that underlie those traits (the geneotype)
Main article: Mendelian Genetics: genes and alleles
[edit] Plasmid
A plasmid is a small loop of DNA in a bacterium, separate from its main chromosome.
[edit] Point mutation
In this text, a point mutation is a mutation that affects only a small number of bases, rather than making major changes to chromosome structure. Some authors, however, use the phrase "point mutation" as a synonym for "single nucleotide substitution".
Main article: Mutation
[edit] Polymer
A polymer is a sequence of similar molecules joined together in a chain to make one big molecule. DNA, RNA, and polypeptides are examples of polymers.
[edit] Polypeptide
A polypeptide is a polymer of amino acids. Proteins are made up of one or more polypeptides.
Main articles: DNA: translation; Genetic Code
[edit] Primary transcript
The primary transcript is the RNA intitally produced by transcription, before it undergoes processes such as splicing to produce mature RNA.
[edit] Prokaryote
Prokaryotes are organisms which have no nucleus to their cells, i.e. bacteria and archaea. Organisms that are not prokaryotes are called eukaryotes.
[edit] Promoter
A promoter is a region of DNA which comes just before a section of DNA that is meant to be transcribed, instructing the RNA polymerase to transcribe it.
Main article: DNA: transcription
[edit] Protein
One or more polypeptides attached together. Along with functional RNA , proteins are the enzymes that cause chemical activity in the cell.
Main article: DNA: translation
[edit] Recessive
An allele is said to be recessive if the trait associated with it is only expressed if the organism is homozygous for that allele.
Main article: Mendelian Genetics: dominant and recessive alleles
[edit] Recombination
Recombination is the exchange of genes between chromosomes: usually of homologous genes between homologous chromosomes.
Main article: Recombination
[edit] Reversion
A reversion is a mutation that cancels out the effect of another, previous mutation.
[edit] Ribonucleotide
Riboneucleotides are nucleotides in which the sugar is ribose; the nucleotides that polymerize to form RNA.
Main article: DNA: transcription
[edit] Ribosome
Ribosomes are bodies in the cell that translates messenger RNA into polypeptides.
Main article: DNA: translation
[edit] RNA
Ribonucleic acid is a polymer of ribonucleotides. RNA can either be functional, catalyzing reactions in the cell, or messenger RNA, which plays an intermediate role in the synthesis of proteins, being transcribed from DNA before being translated into a polypeptide.
Main article: DNA: transcription
[edit] RNA polymerase
RNA polymerase is the enzyme that directs the synthesis of RNA during transcription.
Main article: DNA: transcription
[edit] Segregation
Mendel's law of segregation states that for diploid organisms having two different alleles of some gene, the chances of either of those alleles being the one that gets passed on in some particular gamete are 50:50.
Main article: Mendelian Genetics: reproduction
[edit] Sexual selection
Sexual selection is the effect on the gene pool caused by competition within a population for breeding partners.
Main article: Sexual Selection
[edit] Sex chromosomes
Sex chromosomes are chromosomes the presence or absence of which determine the sex of an organism.
Main article: Sex Chromosomes
[edit] Side chain
Side chains are the bits of monomers that stick out at the side when they’re chained together to make polymers: for example, the bases of nucleotides are their side chains.
[edit] Single nucleotide substitution
A single nucleotide substitution, or SNS, is a mutation in which just one base (and the corresponding base on the complementary strand) of the DNA is altered.
Main article: Mutation
[edit] Sister chromosomes
Sister chromosomes are two homologous chromosomes in a diploid organism.
Main article: Chromosomes: diploidy
[edit] Somatic mutation
A somatic mutation is one that affects a normal cell of the body, rather than a gamete, and which therefore will not be passed on to future generations.
Main article: Mutation: note on somatic and germ-line mutations
[edit] Species
In our articles, two populations are said to belong to different species if they are reproductively isolated from one another. Other definitions are sometimes used by biologists.
Main articles: Species; Reproductive Isolation
[edit] Splicing
Splicing is the process occurring after transcription in which a body called the spliceosome snips out sections of the RNA (known as introns), joining together, as it goes, the bits that don't get snipped out (known as exons). It is this "mature RNA" that goes on to serve as functional RNA or messenger RNA.
Main article: Splicing
[edit] Spliceosomes
Spliceosomes are bodies in the cell that snip introns out of RNA and join the remaining exons together to form the mature RNA that goes on to be functional RNA or messenger RNA.
Main article: Splicing: how?
[edit] Start codon
In the standard genetic code, the codon AUG, when it occurs in messenger RNA performs a double role: it codes for the amino acid methionine, but it also serves as a start codon: that is to say, when the ribosome translates a strand of mRNA, it doesn't start translating until it reaches the first occurence of AUG.
In the coding strand of the corresponding DNA, the start codon is be represented by ATG, since DNA uses thymine (T) rather than uracil (U).
Main article: Genetic Code
[edit] Stop codon
A stop codon is any codon that instructs the ribosome to stop translating a piece of messenger RNA and release the completed polypeptide into the cell. In the standard genetic code , the stop codons are UAA, UAG, and UGA, as they appear in mRNA; in the coding strand of the corresponding DNA these appear as TAA, TAG and TGA, since DNA uses thymine (T) rather than uracil (U).
Main article: Genetic Code
[edit] T
T is the abbreviation for thymine.
Main article: DNA: Chemical structure of DNA
[edit] Terminator
A terminator is a region of DNA which instructs the RNA polymerase to stop [[Genetics Glossary#Transcription|transcribing] the DNA.
Main article: DNA: transcription
[edit] Theory of evolution
The theory of evolution is the explanation of how evolution occurs in terms of (but not limited to) such known genetic factors as natural selection, mutation, recombination, lateral gene transfer, and genetic drift.
Main article: Theory of Evolution
[edit] Thymine
Thymine (T) is one of five bases that can occur as side chains of nucleotides. It base-pairs with adenine. It is found only in DNA: in RNA the similar base uracil is used as a substitute.
Main article: DNA: Chemical structure of DNA
[edit] Trait
A trait is one of the possible variations in a character.
Main article: Mendelian Genetics: characters and traits
[edit] Transcription
Transcription is the process of copying a sequence of bases from one strand (the coding strand) of a piece of double-stranded DNA to a strand of RNA. This is carried out by an enzyme called RNA polymerase, which complements the strand not to be copied (the anticoding strand) with ribonucleotides. Because of the base-pairing rules, this results in a copy of the coding strand, except that the RNA copy will have the base uracil wherever the coding strand had thymine.
Main article: DNA: Transcription
[edit] Transduction
Transduction is a form of lateral gene transfer in which genes are moved from one organism to another as a result of a malfunction in the life-cycle of a virus.
Main article: Lateral gene transfer: examples
[edit] Transfer RNA
Transfer RNA is functional RNA that assists the ribosomes to translate messenger RNA by supplying it with the amino acid appropriate to the codon that it's translating.
Main article: DNA: translation
[edit] Transformation
Transformation is a form of lateral gene transfer in which an organism absorbs naked DNA and incorporates it into its own genome.
The term can also refer to the sequence of somatic mutations that turn a normal body cell into a cancer cell: this is a quite different process, not involving lateral gene transfer, and the two should not be confused.
Main article: Lateral gene transfer: examples
[edit] Translation
Translation is the process by which a strand of messenger RNA directs a ribosome in the synthesis of a polypeptide.
Main articles: DNA: translation; The Genetic Code: transcription and translation
[edit] Translocation
Translocations are mutations where two non-homologous chromosomes exchange parts, or where part of one chromosome gets inserted into another.
Main article: Mutation
[edit] tRNA
Abbreviation for transfer RNA
Main articles: DNA: translation
[edit] True-breeding line
A true-breeding line, with respect to a certain trait, is a population such that however long you go on breeding it with itself, every member exhibits that trait: hence the population must be homozygous for the alleles determining that trait.
Main article: Mendelian Genetics: Mendel’s experiments
[edit] U
U is the abbreviation for uracil.
Main article: DNA: Chemical structure of DNA
[edit] Uracil
Uracil (U) is one of five bases that can occur as side chains of nucleotides. It base-pairs with adenine. It is found only in RNA, as a substitute for the chemically similar base thymine.
Main article: DNA: RNA
[edit] Virus
A virus consists of a bit of DNA or RNA in protein packaging. The virus injects its genes into a cell, where the genes rely on the machinery of the host cell for replication, transcription, and translation, hijacking the cell machinery to manufacture more viruses.
[edit] X chromosome
One of two alternative sex chromosomes in species with an "XY" system of sex determination, in which the female has two X chromosomes and the male has one X chromosome and one Y chromosome.
Main article: Sex Chromosomes
[edit] Y chromosome
One of two alternative sex chromosomes in species with an "XY" system of sex determination, in which the female has two X chromosomes and the male has one X chromosome and one Y chromosome.
Main article: Sex Chromosomes
[edit] Zygote
A newly fertilized egg, produced by the fusion of two gametes: the single cell from which an organism develops.
Main article: Chromsomes: sexual reproduction
[edit] 3' end
DNA and RNA are made up of nucleotides linked together by attaching the sugar of one nucleotide to the phosphate group of the next. This means that any linear strand of DNA or RNA will have one free phosphate group at one end and one free sugar at the other. The end with the free sugar is the 3' end (pronounced "three prime end"); the other end is the 5' end.
Main article: DNA: Chemical structure of DNA
[edit] 5' end
DNA and RNA are made up of nucleotides linked together by attaching the sugar of one nucleotide to the phosphate group of the next. This means that any linear strand of DNA or RNA will have one free phosphate group at one end and one free sugar at the other. The end with the free phosphate group is the 5' end (pronounced "five prime end"); the other end is the 3' end.
Main article: DNA: Chemical structure of DNA
