Glossary
I’ll be the first to admit; we biologists are addicted to fancy words. I hope this resource is useful to you as you read the guides on the website.
- Alex
DNA101 Glossary
GLOSSARY OF NEW TERMS
3’ Polyadenylation - The addition of a polyA tail to a messenger RNA molecule after the process of transcription in Eukaryotes. It is a stretch of RNA Arginine bases that covers the end of the mRNA molecule, allowing for stable and reliable transport out of the nucleus, to the Rough Endoplasmic Reticulum.
5’ > 3’ – Directionality of DNA. Systems that run in this direction along a DNA strand have a forward motion, systems that run 3’ > 5’ have a reverse motion. When DNA is written as a raw sequence, always assume it is the 5’>3’ of the positive (coding) strand.
5’ Cap - The addition of a specially modified nucleotide (7-methylguanylate) to the 5’ end of an messenger RNA strand during the process of transcription. This results in a more stable mRNA molecule and more reliable translation of proteins.
Agarose Gel – Protocol for measuring the length of DNA by size-based electrophoresis. Works due to DNA carrying a linear negative charge.
Amino acid - The basic building block of polypeptides/proteins. This molecule contains a central carbon, flanked by a hydrogen, a Carboxyl (-COOH), an amino (-NH2) group and a variable side chain that gives it its unique characteristics. There are only 20 naturally occurring amino acids.
Aminoacyl-tRNA synthetase - Enzyme responsible for bonding Amino Acids to anticodon-bearing tRNAs for the ribosome to use during Translation. A unique aminoacyl-tRNA synthetase is produced for each tRNA, which allows for extremely high specificity. Different species will preferentially produce more or less of these enzymes, resulting in a codon preference. This can be overcome in genetic design via codon optimisation.
Anneal - Hydrogen bonds forming between the matching bases of two DNA strands (A-T, C-G) to form a double stranded sequence.
Anti-Codon - Three base-pair sequence that is the opposite (A becomes T (or U for RNA), C becomes G) of a codon. Anti-codons can be found on the non-coding strand of the original sequence and on the tRNA’s used for translation. The ‘frame’ of the anti-codons is set by the start codon on the coding strand.
Apoptosis - Carefully ordered self-destruction of a cell due to its failure to pass an important quality assurance checkpoint. Cells with DNA damage or replication errors are ordered to commit suicide, packaging active enzymes into vesicles to prevent them from causing collateral damage to neighbouring cells.
Aptamer - DNA or Protein molecules that selectively bind a target, but do not catalyse a reaction - a similar function to an antibody. Unlike antibodies; aptamers are smaller, are synthetically produced and bind their target with higher affinity.
ATP molecule - Adenosine 5’-triphosphate is a ubiquitous molecule in metabolic processes, responsible for moving readily available energy around the cell. Adenosine 5’ diphosphate (ADP) is combined with a phosphate during cellular respiration to produce an energy-rich bind. This bond can be easily broken by enzymes that require energy to perform their function. In Animals, Plants and Fungi, ATP is produced within the mitochondria, the powerhouse of the cell. Bacteria lack mitochondria and instead produce ATP via the Krebs cycle in aerobic conditions, or fermentation when oxygen is unavailable.
Backbone - Nickname for a functional plasmid that can host an inserted gene.
Bacteriophage - Virus that infects only bacteria. Bacteria and Viruses have been locked in an arms race that goes back aeons. A bad bacteriophage infection in a biotech space can be devastating for everyone in the lab. Keep DNA backups!
Base-pairs - Pairs of nucleic acids that are a feature of DNA and can occur during the self-association of RNA. Hydrogen bonds form between the nucleic acids, holding them together strongly (but not as strongly as the bonds holding the nucleic acid to the backbone). A always pairs with T with two Hydrogen Bonds, C always pairs with G with three Hydrogen Bonds. The extra bond makes the C-G base pair slightly stronger than the A-T base pair.
Buffered Solution - Combination of weak acid/strong base or strong acid/weak base that keeps a persistent pH even as additional ions are added. Extremely useful while handling biological substances that do not tolerate pH changes well.
Binary Fission - Bacterial asexual reproduction by replication of the genome and separating into two distinct ‘daughter cells’.
Cell Cycle - Process of cell division dictated by internal and external chemical signalling pathways. A correctly functioning cell cycle helps ensure that only cells with accurate DNA replication are able to divide.
Central Dogma of Molecular Biology - DNA > mRNA > Proteins
Centromere - DNA sequence that binds homologous chromosomes together prior to replication that is bound by the spindle during the Metaphase of Mitosis/Meiosis.
Chaperone Proteins - Proteins that assist in the correct folding of new proteins that may not fold correctly without assistance.
Chelating agents - Metal ions that bond to an organic molecule (DNA or protein) to produce a conformational change.
Chromosome - Packaged DNA that is part of the genome of an organism. Humans have 23 chromosomes. Human Chromosome 1 is our longest at 249 million nucleic acid base pairs. Bacteria have simpler chromosome, made up of a single circle containing 4.6 million base pairs (Mbp).
Coding Strand - Strand of DNA that codes for the protein. The sequence of the coding strand is copied by RNA polymerase reading the non-coding strand and adding matching nucleotides (A wherever there is a T, C wherever there is a G, and vice versa). This works because the non-coding strand is the exact opposite of the coding strand). The sequence of the coding strand will be made up of three base pair codons, with the frame set by the start codon AUG.
Codon - Sets of three nucleotides on the coding strand that each code for a specific amino acid according to a code universal to all forms of life. Translation of codons to amino acids can be easily performed by finding a codon table online. Despite this code being universal across all forms of life, different species preferentially use different codons more. This means that codon optimisation is necessary to prevent bottlenecks during the biosynthesis of proteins.
Codon Optimisation - Adjusting the codons in a DNA sequence while maintaining the resultant protein sequence. Codon optimisation uses species specific modelling to determine the best codons to use for your sequence in order to prevent bottlenecks by the Aminoacyl-tRNA synthetases.
Codon Usage Bias - The preferential use of codons in the genome of a particular species which indicates the levels of Aminoacyl-tRNA synthetases available to handle the workload of producing tRNAs for translating a specific codon.
Constitutive - Always on expression. A constitutive gene is never switched off, either because the mechanism is never flipped, is broken or does not exist.
Covalent Bond - A strong molecular bond resulting from the sharing of electron pairs between atoms. Covalent bonds vary in strength, but are always significantly stronger than hydrogen bonds. This is why the two DNA strands will denature (detach) from one another when heated, but the backbone remains intact.
Cytoplasm - Internal cellular liquid composed of dissolved salts and proteins in water. Extremely complex when scrutinized at the atomic level, but often simplified for models of intracellular interactions.
Cytokinesis - The separation of the cytoplasm of two daughter cells following mitosis/meiosis in Eukaryotes or Binary Fission in Prokaryotes. A new cellular membrane completes itself around the new daughter cells.
Degrees of Abstraction - Simplified versions of reality to help our monkey brains comprehend a complex reality.
Dehydration Polymerisation - Reaction that occurs during the synthesis of the DNA backbone. A triphosphate nucleotide is brought near the growing DNA strand by DNA polymerase and two phosphates are removed to provide the energy for the reaction. The polymerase facilitates the formation of covalent bonds between the phosphate and ribose sugar to add the nucleotide to the growing chain. A water molecule is released during the reaction, with an OH from ribose sugar and an H from the phosphate group.
Diploid - The number (2n) of chromosomes in a somatic (body) cell of an organism that reproduces via sexual reproduction. There is a copy of every gene in a diploid genome, although most of the time only one of the pair (the dominant gene) is actually expressed.
Disulphide Bonds - A bond between two cysteine amino acids within a polypeptide chain. Two sulphur molecules are bonded between the cysteines, resulting in a change in the folding of the protein.
DNA - Deoxyribonucleic acid, a chain of nucleic acids bonded within a sugar-phosphate backbone that codes for biological life.
DNA Ligase - An enzyme responsible for repairing breaks in the DNA backbone, repairing bonds between ribose sugars and phosphate molecules via a reaction known as dehydration polymerisation. T4 DNA ligase (hijacked from the T4 phage) is an examples of a ligase enzyme which can be used by a researcher to ligate DNA in the lab.
DNAzymes - Small stretches of DNA capable of catalysing or performing a biological reaction.
Downstream - In the 3’ direction of.
Dye Intercalation - The insertion of fluorescent dye molecules in between the bases of DNA within the double helix. The helix unwinds slightly, exposing an internal hydrophobic environment that the dye molecule can slip into.
Epigenetics - Changes to genetic expression caused by factors other than changes to the sequence of nucleic acids. Caused by a multitude of factors, such as the methylation of cytosines or histone tails. An emerging and poorly understood field.
Eukaryotes - Organisms with membrane-bound organelles and a distinct nucleus enclosing the nucleic acids. Range from single-celled yeast, to complex organisms such as humans.
Expression - Genes that are transcribed and translated into proteins are being ‘expressed’. Expression may refer to one protein, or a pattern of proteins that result in a particular allele or phenotype. Genetic sequencing and protein assays allow us to be highly specific about mutations and the resulting changes - however you may also encounter these less specific terms;
Allele - Alternative forms of expression, resulting from different gene sequences coding for a similar protein. A more specific way to describe an allele is with the mutations that created it.
Genotype - The genetic makeup of an organism, including the combinations of genes that make up the genome. Generally described in terms of dominant/recessive genes, or ‘types’. e.g. ABO blood types.
Phenotype - The observable features of an organism, resulting from the interaction of the genome with the environment. Mostly qualitative term that has led to plenty of misconceptions.
Expression Machinery - Proteins/Enzymes required to produce a protein, from transcription to translation to folding. The capacity of a cell to produce new proteins is limited by the capacity of these systems. Useful as a catch-all term when you’re unsure which specific enzyme is causing a problem.
Exon - Regions of the mRNA molecule in Eukaryotes that will be translated into protein. After transcription, Eukaryotic mRNA undergoes splicing, whereby non-coding introns are removed, leaving coding exons. This allows for adaptable recombination of protein sequences to perform specific functions as required. Certain regions of the protein are conserved, while the active region is swapped out for something more appropriate. Recombination of exons by splicing is an example of post-transcriptional regulation.
Folding - The contortions and overlapping within three-dimensional space of DNA or protein molecules. At this scale, folding becomes critical to the functional properties of organic molecules - similar how to metal might be folded into a useful shape.
Forward Mechanism - Runs in the 5’ > 3’ direction along DNA.
Gamete - A germ-line sex cell with half of the genetic information necessary to form a complete organism. Two gametes merge during sexual reproduction to form a zygote, the first omnipotent cell that will go on to form a complete organism.
Gblock™ - Nickname for an artificially synthesised strand of double stranded DNA (125-3000 bp) by IDT. Gblocks are also referred to as oligonucleotides, a term that also describes smaller fragments of DNA <125.
Gene Cascade - A series of genes switched on and off in a specific order, most often used during initial development of an organism. Gene cascades involve the intricately complex interactions of promoters and repressors in order to ensure an organism develops in the correct order.
Genetic Parts - Individual sections of DNA code that are responsible for the various functions required by the researcher for an experiment. During the expression of a protein, this might be: Promotor, Operator, Ribosome Binding Site, Gene of Interest with start and stop codon, Terminator, all flanked by a Multicloning Site Prefix and Suffix. Working from a database of standardised genetic parts (such as the iGEM parts database) is an extremely useful way to skip a lot of painful trial and error in genetic design.
Genome - The entire set of chromosomes, containing all of the DNA necessary to successfully recreate the organism for the next generation. The human genome is 6.4 billion base pairs in size, spread across 46 chromosomes. This is made up of 3.2 billion base pairs inherited from the original sperm and egg. For comparison, the entire E. coli genome fits within a single 4.6 million base pair chromosome that can replicate its entire length in less than 30 minutes.
Genomic DNA - Series of nucleic acids that codes for the entirety of an organism. Varies in structure from simple 1,000,000 base pair circular genomes in prokaryotes, to 6,200,000,000 base pairs across 42 chromosomes in diploid human cells.
Genomic Integration - The insertion of a gene into the genome of an organism. This is process that occurs naturally, such as with integrative retroviruses. Genomic integration can also be artificially reproduced in the lab, resulting in a permanently changed organism. Changes will be carried over to the offspring if changes are made to a gamete, zygote, germ-line cells or single-celled organism.
Golgi Apparatus - Eukaryotic organelle responsible for some protein folding (after synthesis in the Rough ER) as well as the transport and folding of protein and lipids. Small vesicles are budded off the membrane of the Golgi body, secreting the protein/lipid while maintaining the same mildly acidic and oxidative environment to ensure it holds its structure until it reaches its destination.
Haploid - The number (n) of chromosomes in a gamete (sex) cell of an organism that reproduces via sexual reproduction. There is only one copy of each gene, a second copy is required in order to initiate cell replication and the creation of a complete organism.
Histones - Basic (alkaline) proteins responsible for the wrapping and packaging of DNA in a chromosome. Histones form octamers that each contain two copies of the four histone proteins (H2A, H2B, H3 and H4).
Homeostasis - The maintenance of a consistent internal environment, e.g. pH, RedOx, Temperature, Osmotic pressure, etc.
Homologous - Share a matching structure, but not necessarily a matching sequence. You receive one homologous chromosome from each parent. Each will code for the same proteins albeit with slight variations, or errors that result in no protein at all.
Homologous Recombination - Occurs during the ‘crossing-over’ event in meiosis, matching chromosomes line up and exchange matching regions to create entirely new genetic combinations on the resultant chromosomes. When a double stranded break occurs in a mature organism, the homologous chromosome is used as a repair template (HDR) - potentially resulting in unintended homologous recombination.
Homology Directed Repair (HDR) - Repair of a double-stranded break in an organism using the homologous chromosome as a template for the process. Significant upstream and downstream homology is required for the template to be used, without a nearby homologous region the break will be repaired by Non-Homologous End-Joining (NHEJ), a very error prone process.
Host - The organism that contains the DNA, RNA or protein in question. In microbiology, molecular biology and synthetic biology this will most likely be a single celled organism (e.g. E. coli or Yeast) or an extremely simple multicellular organism. When referring to diseases, the host is the organism that has been infected by a pathogen (bacteria, virus, phage, prion, etc).
Hydrogen Bond - A weak intermolecular bond formed by the electrostatic attraction between an electropositive proton (hydrogen) in one molecule and a relatively electronegative atom in another molecule.
Independent Assortment - Source of variation during meiosis caused by the alignment of the chromosomes along the equator of the cell. Since they line up in pairs and can pick either side, each pair has only two possible alignments. However there are 23 chromosomes in humans, resulting in a large number of combinations (2^23). For other sexually reproducing organisms with n chromosomes, independent assortment introduces 2^n variation.
Intracellular Machinery - The internal organelles and proteins responsible for the day to day operations of a cell. This is a catch-all term used to refer to the productive capacities of a cell in general, often when specifics are yet to be inferred.
Intron - Transcribed region of DNA that will be excised from the messenger RNA molecule, leaving only the exons to be translated. Introns provide a vital regulatory role and should not be discounted as ‘junk’.
In Vivo - Occurring on or within a living organism.
In Vitro - Occuring in the lab, exterior to the living organism.
Nucleic Acids - Adenine, Guanine, Cytosine, Thymine (and Uracil for RNA). Four organic molecules that provide the ‘coding region’ of DNA/RNA. Combinations of three nucleic acids are known as codons. The ribosome uses the codon during translation to pick the correct corresponding amino acid for building the polypeptide/protein.
Major Groove - When a strand of DNA is twisted into a double helix, the gaps between the chains of DNA backbone alternate between small and large. The larger gap is the major groove and it is considered to be richer in information, since the base pairs are sufficiently exposed to read without being broken apart. Both grooves surround the same sequence, the only difference is accessibility.
Melting Temperature (Tm) - The temperature at which exactly 50% of DNA is single stranded and 50% is double stranded. Higher GC content in a DNA sequence results in a higher melting temperature, higher AT content results in a lower melting temperature (3 hydrogen bonds vs. 2 hydrogen bonds). The Tm is a relevant factor when calculating the annealing temperature of primers for the Polymerase Chain Reaction.
Minor Groove - The smaller groove in the double-helix model of DNA.
Mitosis - Cellular division in eukaryotic organisms that results in 2 x 2n somatic daughter cells.
Meiosis - Cellular division in eukaryotic organisms that results in 4 x n gamete daughter cells.
Mutation - A change in the base pair sequence of DNA. Mutations in somatic cells will only affect the original organism, whereas mutations in the gametes or zygote will affect all future generations. Mutations can be caused by any number of possible carcinogenic influences, or introduced by pathogens.
Native State - DNA or proteins that are correctly folded and functional, as they would be in their natural environment.
Nitrogenous Base - Alkaline, nitrogen containing molecule that is a component of DNA. Two types, (purines and pyrimidines) appear in DNA as the molecules that provide the actual code (A, C, T, G + U in RNA).
Non-Homologous End-Joining (NHEJ) - If a double stranded break occurs in DNA and there is no nearby homologous region to act as a scaffold for repair, the cell will instead perform Non-Homologous End-Joining. The gap will be filled in with random bases and the backbone repaired. This can cause deleterious effects in any nearby gene sequences, and will likely inactivate any gene that it occurs within.
Nucleus - A membrane bound organelle in Eukaryotes that holds the genomic DNA. The nucleus is the command centre of the cell, but kept carefully isolated from the external environment to prevent external influences (e.g. Viruses) from interfering with the operation of the DNA. Once messenger RNA is transcribed and given a Cap and Tail, it leaves the nucleus for the Endoplasmic Reticulum, an organelle dedicated to translation. Here, Ribosomes translate the mRNA into protein. Prokaryotes do not have a nucleus, adopting a more laissez-faire approach with everything floating around in the cytoplasm.
Nuclear Membrane - A double layer of phospholipid membranes that encapsulates the nucleus of a cell. The outer layer of the outer membrane is partly combined with the Rough Endoplasmic Reticulum, simplifying the passage of mRNA molecules to the Ribosomes.
Nucleosides - A purine or a pyrimidine bound to a ribose sugar, but not to any phosphate.
Nucleotides - A purine or a pyrimidine bound to a ribose sugar and phosphate group. A ‘single unit’ of DNA. Can be an A, C, T or G.
Open Reading Frame (ORF) - A region of DNA/RNA that is translated. In order for this to occur it must be flanked by a Start and Stop Codon. There should also be a Ribosome Binding Site 6-8 base-pairs upstream to make the ORF functional.
Operon - A unit of interdependent gene sequences responsible for regulating a separate gene. Sometimes used to describe the regulatory apparatus of a gene as a whole, including any genes within the cluster. e.g. Lac Operon, Cumate Operon.
Oxidative - An oxidative environment is likely to induce oxidation, or the loss of electrons from any molecules contained within. Reductive species may be oxidative, as they induce oxidation in others. OILRIG = Oxidation is Loss, Reduction is Gain.
Parentage - The origin of daughter cells or a whole organism.
Periplasmic Space - Region between the inner and outer membrane in gram-negative bacteria. This is an oxidative environment that may be favourable to the folding of some proteins.
Phosphate - A phosphorous atom covalently bound to 4 oxygen atoms.
Plasmid - A circular structure of DNA capable of replicating independently from the rest of the genome.
Plasmid Supercoiling - A unique structural capability of circular DNA strands whereby the circle of DNA twists and writhes into a significantly smaller shape than would be possible with a linear DNA strand.
Polypeptide - A sequence of amino acids assembled in a continuous polypeptide chain.
Potency
Omnipotent/Totipotent - Can become any cell in the body, found in the fertilised egg (zygote).
Pluripotent - Can become any cell within a particular system, slightly more specialised. Found during foetal development.
Multipotent - Can become one of a small set of cell types within a particular system. Found during foetal development and in areas of a mature organism that require regular replenishment, e.g. bone marrow stem cells.
Unipotent - Specialised cells that can only divide to produce cells of the same type.
Primer - Short sequence of DNA, generally 15-40 base-pairs but can be shorter or longer. Used to ‘prime’ the Polymerase Chain Reaction by annealing during the annealing step and then providing a double stranded region for DNA Polymerase to bind.
Primer hairpin - Internal structure caused by self-association within the primer, rather than with the target sequence. Not good!
Promotion - The switching on or speeding up of genetic expression, either at the transcription or translation level. mRNA polymerase and associated transcription factors are recruited to a specific DNA sequence to encourage the production of a protein. A ‘promoter region’ upstream of a gene is responsible for recruiting the polymerase and transcription factors. Once transcribed, the promotion of protein translation may occur via chaperone proteins,
Protein - One or more polypeptide chains that have been folded to form a functional molecular machine. Read the Proteins101 guide for more information.
Protein-DNA/Protein-RNA complexes - A combination of proteins bound to DNA or RNA that creates a functional unit. These complexes are most often found within enzymes with a DNA/RNA related function, such as within Ribosomes or CRISPR/CAS9.
Proteome - The full sum of proteins that can be produced by a cell. Deeper analysis of the proteome can reveal quantitative insights into the levels of these proteins over time.
Prokaryotes - Organisms that lack membrane-bound organelles, with no nucleus to contain the genome. Most molecular machinery freely floats within the cytoplasm.
Quiescent - Cell that has entered the G0 resting state. Quiescent cells will not divide or work towards division until an appropriate signal is given.
Reductive - A reductive environment is likely to induce reduction, or the gain of electrons from any molecules contained within. Oxidative species may be reductive, as they induce reduction in others. OILRIG = Oxidation is Loss, Reduction is Gain.
Repression - The switching off or toning down of genetic expression, either at the transcription or translation level.
Reverse Mechanism - Runs in the 3’ > 5’ direction along DNA.
Ribose Sugar - C5H10O5 pentose sugar. Component in the DNA backbone.
Ribosome - A molecule consisting of RNA and proteins that is used by prokaryotes and eukaryotes alike to produce new proteins according to the code provided by a messenger RNA molecule. While there is some variation in the structure between species, ribosome are nearly ubiquitous across living organisms.
RNA - Ribonucleic Acid - a single chained molecule similar to DNA, although all the Thymines (T) are replaced with Uracils (U). RNA’s single chained nature allows it to form more complex 3-D shapes, resulting in a wide variety of uses. Messenger RNA is a molecule used to copy the code from DNA and move it to the Ribosome where it can be translated into protein. Read the RNA101 guide for more information!
Rough Endoplasmic Reticulum - An organelle in Eukaryotic organisms that contains the ribosomes necessary for protein production within an oxidative environment that helps facilitate the correct folding of the new protein.
Sexual Reproduction - The production of a new organism from the combination of gametes from two separate organisms into a new zygote.
Silencing RNAs - Regulation of genes by short stretches of RNA that inhibit specific processes by competitive binding.
Splicing - The recombination of exons after the removal of introns. Occasionally used as a verb for describing the cutting and pasting of DNA.
Start Codon - AUG, codes for Methionine. The first three base pair sequence (codon) in any translated DNA sequence.
Somatic - Relating to the body. Most often used to describe ‘somatic cells’ - cells that make up an individual but are not contributed to the next generation like germ-line cells.
Template Strand - Opposite the coding strand, the template strand is used for attaching the growing strand of mRNA produced by RNA polymerase. The template contains anti-codons, sequences that perfectly opposite sequences to the codons required for making a protein. A replaces T, C replaces G & vice versa.
Transcription/Transcribed - The conversion of DNA code into a messenger RNA molecule by the protein RNA Polymerase.
Transcription Factors - Proteins that assist the RNA polymerase as it binds and transcribes the mRNA strand from the template strand. Transcription factors generally speed up the process of transcription with their presence or slow it down by their absence.
Transcription Level - Regulation that occurs at or before the stage of transcription.
Transcriptome - The levels of mRNA produced at any one time within an organism. Due to the additional layer of translation regulation, the transcription levels may not necessarily be related to the levels of produced protein. Analysis of the changes in transcription levels in an organism over time and after changes in environmental pressures.
Translation/Translated - The conversion of the code on the messenger RNA molecule into a protein by a Ribosome. In Eukaryotes this occurs in the rough endoplasmic reticulum, in Prokaryotes this occurs in the cytoplasm.
Translation Level - Regulation that occurs at or before the stage of translation.
Upstream - In the 5’ direction from.
Virus - ‘Non-living’ agent that consists of DNA or RNA wrapped in a protein coat. Viruses inject their genome into a host cell. The intracellular machinery of the cell is hijacked for the purpose of making copies of the virus. Once no more room for viral particles is available, the outer membrane of the host is lysed and the viruses spill out to seek new targets.
Vesicles - Small compartment isolated from the rest of the cell or external environment by a phospholipid bilayer. Vesicles can ‘bud off’ larger membranes and are often used as vehicles for the transport of proteins, lipids or DNA.
Zygote - The first omnipotent/totipotent cell in a multicellular organism, often formed by the fertilisation of an egg by a sperm. Once fertilised, the zygote immediately begins dividing into multiple, more specialised cells.