Life Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! == Composition == === Chemical elements === All life forms require certain core [[chemical element]]s for their [[biochemistry|biochemical]] functioning. These include [[carbon]], [[hydrogen]], [[nitrogen]], [[oxygen]], [[phosphorus]], and [[sulfur]]—the elemental [[nutrient|macronutrients]] for all organisms.<ref name=wsj20101203>{{cite news |first1=Robert Lee |last1=Hotz |title=New link in chain of life |work=[[The Wall Street Journal]] |date=3 December 2010 |url=https://www.wsj.com/articles/SB10001424052748703377504575650840897300342?mod=ITP_pageone_1#printMode |quote=Until now, however, they were all thought to share the same biochemistry, based on the Big Six, to build proteins, fats and DNA. |url-status=live |archive-url=https://web.archive.org/web/20170817163835/https://www.wsj.com/articles/SB10001424052748703377504575650840897300342?mod=ITP_pageone_1#printMode |archive-date=17 August 2017 }}</ref> Together these make up [[nucleic acid]]s, proteins and [[lipid]]s, the bulk of living matter. Five of these six elements comprise the chemical components of DNA, the exception being sulfur. The latter is a component of the amino acids [[cysteine]] and [[methionine]]. The most abundant of these elements in organisms is carbon, which has the desirable attribute of forming multiple, stable [[covalent bond]]s. This allows carbon-based (organic) molecules to form the immense variety of chemical arrangements described in [[organic chemistry]].<ref name="Lipkus Yuan Lucas Funk 2008 pp. 4443–4451">{{cite journal | last1=Lipkus | first1=Alan H. | last2=Yuan | first2=Qiong | last3=Lucas | first3=Karen A. | last4=Funk | first4=Susan A. | last5=Bartelt | first5=William F. | last6=Schenck | first6=Roger J. | last7=Trippe | first7=Anthony J.| title=Structural Diversity of Organic Chemistry. A Scaffold Analysis of the CAS Registry | journal=The Journal of Organic Chemistry | publisher=American Chemical Society (ACS) | volume=73 | issue=12 |year=2008 | doi=10.1021/jo8001276 | pages=4443–4451| pmid=18505297 | doi-access=free }}</ref> Alternative [[hypothetical types of biochemistry]] have been proposed that eliminate one or more of these elements, swap out an element for one not on the list, or change required [[Chirality (chemistry)|chiralities]] or other chemical properties.<ref>{{cite book |author1=Committee on the Limits of Organic Life in Planetary Systems |author2=Committee on the Origins and Evolution of Life |author3=National Research Council |date=2007 |publisher=National Academy of Sciences |title=The Limits of Organic Life in Planetary Systems |isbn=978-0-309-66906-1 |url=http://www.nap.edu/catalog.php?record_id=11919 |access-date=3 June 2012 |url-status=live |archive-url=https://web.archive.org/web/20120510213123/http://www.nap.edu/catalog.php?record_id=11919 |archive-date=10 May 2012 }}</ref><ref>{{cite journal |first1=Steven A. |last1=Benner |first2=Alonso |last2=Ricardo |first3=Matthew A. |last3=Carrigan |journal=Current Opinion in Chemical Biology |title=Is there a common chemical model for life in the universe? |volume=8 |issue=6 |date=December 2004 |pages=672–689 |doi=10.1016/j.cbpa.2004.10.003 |url=http://www.fossildna.com/articles/benner_commonmodelforlife.pdf |archive-url=https://web.archive.org/web/20121016220349/http://www.fossildna.com/articles/benner_commonmodelforlife.pdf |archive-date=16 October 2012 |url-status=dead |access-date=3 June 2012 |pmid=15556414}}</ref> === DNA === {{main|DNA}} Deoxyribonucleic acid or [[DNA]] is a [[molecule]] that carries most of the [[genetics|genetic]] instructions used in the growth, development, functioning and [[reproduction]] of all known living [[organism]]s and many viruses. DNA and [[RNA]] are [[nucleic acid]]s; alongside [[protein]]s and [[Polysaccharide|complex carbohydrates]], they are one of the three major types of [[macromolecules|macromolecule]] that are essential for all known forms of life. Most DNA molecules consist of two [[biopolymer]] strands coiled around each other to form a [[Nucleic acid double helix|double helix]]. The two DNA strands are known as [[polynucleotide]]s since they are composed of [[monomer|simpler units]] called [[nucleotide]]s.<ref>{{cite web |url=http://basicbiology.net/micro/genetics/dna |title=DNA |date=5 February 2016 |website=Basic Biology |access-date=15 November 2016 |last1=Purcell |first1=Adam |url-status=dead |archive-url=https://web.archive.org/web/20170105045651/http://basicbiology.net/micro/genetics/dna/ |archive-date=5 January 2017 }}</ref> Each nucleotide is composed of a [[nitrogenous base|nitrogen-containing]] [[nucleobase]]—either [[cytosine]] (C), [[guanine]] (G), [[adenine]] (A), or [[thymine]] (T)—as well as a [[monosaccharide|sugar]] called [[deoxyribose]] and a [[phosphate group]]. The nucleotides are joined to one another in a chain by [[covalent bond]]s between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating [[backbone chain|sugar-phosphate backbone]]. According to [[base pair]]ing rules (A with T, and C with G), [[hydrogen bond]]s bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA. This has the key property that each strand contains all the information needed to recreate the other strand, enabling the information to be preserved during reproduction and cell division.<ref name="NYT-20150718-rn">{{cite news |last=Nuwer |first=Rachel |author-link=Rachel Nuwer|date=18 July 2015 |title=Counting All the DNA on Earth |url=https://www.nytimes.com/2015/07/21/science/counting-all-the-dna-on-earth.html |work=The New York Times |location=New York |access-date=18 July 2015 |url-status=live |archive-url=https://web.archive.org/web/20150718153742/http://www.nytimes.com/2015/07/21/science/counting-all-the-dna-on-earth.html |archive-date=18 July 2015 }}</ref> Within cells, DNA is organised into long structures called [[chromosome]]s. During [[cell division]] these chromosomes are duplicated in the process of [[DNA replication]], providing each cell its own complete set of chromosomes. Eukaryotes store most of their DNA inside the [[cell nucleus]].<ref>{{cite book |last=Russell |first=Peter |title=iGenetics |url=https://archive.org/details/igenetics0000russ_v6o1 |url-access=registration |publisher=Benjamin Cummings |location=New York |year=2001 |isbn=978-0-8053-4553-7}}</ref> <!--DNA was first isolated by [[Friedrich Miescher]] in 1869.<ref>{{cite journal |last=Dahm |first=R. |title=Discovering DNA: Friedrich Miescher and the early years of nucleic acid research |journal=Hum. Genet. |volume=122 |issue=6 |pages=565–581 |year=2008 |pmid=17901982 |doi=10.1007/s00439-007-0433-0|s2cid=915930 }}</ref> Its molecular structure was identified by [[James Watson]] and [[Francis Crick]] in 1953, whose model-building efforts were guided by [[X-ray diffraction]] data acquired by [[Rosalind Franklin]].<ref name="pmid24840850">{{cite journal |last=Portin |first=P. |title=The birth and development of the DNA theory of inheritance: sixty years since the discovery of the structure of DNA |journal=Journal of Genetics |volume=93 |issue=1 |pages=293–302 |year=2014 |pmid=24840850 |doi=10.1007/s12041-014-0337-4 |s2cid=8845393 }}</ref>--> === Cells === {{main|Cell (biology)}} Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by [[Cell division|division]].<ref>{{cite web |date=2 June 2019 |title=2.2: The Basic Structural and Functional Unit of Life: The Cell |url=https://med.libretexts.org/Courses/American_Public_University/APUS%3A_An_Introduction_to_Nutrition_(Byerley)/Text/03%3A_Nutrition_and_the_Human_Body/2.2%3A_The_Basic_Structural_and_Functional_Unit_of_Life%3A_The_Cell |url-status=live |archive-url=https://web.archive.org/web/20200329060227/https://med.libretexts.org/Courses/American_Public_University/APUS:_An_Introduction_to_Nutrition_(Byerley)/Text/03:_Nutrition_and_the_Human_Body/2.2:_The_Basic_Structural_and_Functional_Unit_of_Life:_The_Cell |archive-date=29 March 2020 |access-date=29 March 2020 |publisher=LibreTexts}}</ref><ref>{{cite web |last=Bose |first=Debopriya |date=14 May 2019 |title=Six Main Cell Functions |url=https://sciencing.com/six-main-cell-functions-6891800.html |url-status=live |archive-url=https://web.archive.org/web/20200329060221/https://sciencing.com/six-main-cell-functions-6891800.html |archive-date=29 March 2020 |access-date=29 March 2020 |publisher=Leaf Group Ltd./Leaf Group Media}}</ref> [[Cell theory]] was formulated by [[Henri Dutrochet]], [[Theodor Schwann]], [[Rudolf Virchow]] and others during the early nineteenth century, and subsequently became widely accepted.<ref name=sapp2003>{{cite book |first1=Jan |last1=Sapp |title=Genesis: The Evolution of Biology |publisher=Oxford University Press |date=2003 |isbn=978-0-19-515619-5 |pages=[https://archive.org/details/genesisevolution00sapp/page/75 75]–78 |url=https://archive.org/details/genesisevolution00sapp |url-access=registration }}</ref> The activity of an organism depends on the total activity of its cells, with [[Cellular respiration|energy flow]] occurring within and between them. Cells contain hereditary information that is carried forward as a [[genetics|genetic]] code during cell division.<ref>{{cite journal |last1=Lintilhac |first1=P.M. |title=Thinking of biology: toward a theory of cellularity—speculations on the nature of the living cell |journal=BioScience |date=Jan 1999 |volume=49 |issue=1 |pages=59–68 |pmid=11543344 |url=https://www.rz.uni-karlsruhe.de/~db45/Studiendekanat/Lehre/Master/Module/Botanik_1/M1401/Evolution_Zellbiologie/Lintilhac%202003.pdf |access-date=2 June 2012 |doi=10.2307/1313494 |jstor=1313494 |url-status=dead |archive-url=https://web.archive.org/web/20130406043511/https://www.rz.uni-karlsruhe.de/~db45/Studiendekanat/Lehre/Master/Module/Botanik_1/M1401/Evolution_Zellbiologie/Lintilhac%202003.pdf |archive-date=6 April 2013}}</ref> There are two primary types of cells, reflecting their evolutionary origins. [[Prokaryote]] cells lack a [[Cell nucleus|nucleus]] and other membrane-bound [[organelle]]s, although they have circular DNA and [[ribosome]]s. Bacteria and [[Archaea]] are two [[domain (biology)|domains]] of prokaryotes. The other primary type is the [[eukaryote]] cell, which has a distinct nucleus bound by a nuclear membrane and membrane-bound organelles, including [[mitochondria]], [[chloroplasts]], [[lysosomes]], rough and smooth [[endoplasmic reticulum]], and [[vacuoles]]. In addition, their DNA is organised into [[chromosome]]s. All species of large complex organisms are eukaryotes, including animals, plants and fungi, though with a wide diversity of [[protist]] [[microorganism]]s.<ref>{{Cite journal |last1=Whitman |first1=W. |last2=Coleman |first2=D. |last3=Wiebe |first3=W. |title=Prokaryotes: The unseen majority |doi=10.1073/pnas.95.12.6578 |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=95 |issue=12 |pages=6578–6583 |year=1998 |pmid=9618454 |pmc=33863 |bibcode=1998PNAS...95.6578W|doi-access=free }}</ref> The conventional model is that eukaryotes evolved from prokaryotes, with the main organelles of the eukaryotes forming through [[endosymbiosis]] between bacteria and the progenitor eukaryotic cell.<ref>{{cite journal |first1=Norman R. |last1=Pace |title=Concept Time for a change |journal=Nature |volume=441 |page=289 |date=18 May 2006 |doi=10.1038/441289a |url=http://coursesite.uhcl.edu/NAS/Kang/BIOL3231/Week3-Pace_2006.pdf |archive-url=https://web.archive.org/web/20121016220349/http://coursesite.uhcl.edu/NAS/Kang/BIOL3231/Week3-Pace_2006.pdf |archive-date=16 October 2012 |url-status=dead |access-date=2 June 2012 |pmid=16710401 |bibcode=2006Natur.441..289P |issue=7091|s2cid=4431143 }}</ref> The molecular mechanisms of [[cell biology]] are based on [[protein]]s. Most of these are synthesised by the ribosomes through an [[Enzyme catalysis|enzyme-catalyzed]] process called [[protein biosynthesis]]. A sequence of amino acids is assembled and joined based upon [[gene expression]] of the cell's nucleic acid.<ref>{{cite web |title=Scientific background |website=The Nobel Prize in Chemistry 2009 |publisher=Royal Swedish Academy of Sciences |url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/2009/advanced.html |access-date=10 June 2012 |url-status=live |archive-url=https://web.archive.org/web/20120402150754/http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2009/advanced.html |archive-date=2 April 2012 }}</ref> In eukaryotic cells, these proteins may then be transported and processed through the [[Golgi apparatus]] in preparation for dispatch to their destination.<ref name="pmid20605430">{{cite journal|last1=Nakano |first1=A. |last2=Luini |first2=A. |year=2010 |title=Passage through the Golgi |journal=Current Opinion in Cell Biology |volume=22 |issue=4 |pages=471–478 |doi=10.1016/j.ceb.2010.05.003 |pmid=20605430 }}</ref> Cells reproduce through a process of [[cell division]] in which the parent cell divides into two or more daughter cells. For prokaryotes, cell division occurs through a process of [[Fission (biology)|fission]] in which the DNA is replicated, then the two copies are attached to parts of the cell membrane. In [[eukaryote]]s, a more complex process of [[mitosis]] is followed. However, the result is the same; the resulting cell copies are identical to each other and to the original cell (except for [[mutations]]), and both are capable of further division following an [[interphase]] period.<ref>{{cite book |first1=Joseph |last1=Panno |title=The Cell |series=Facts on File science library |publisher=Infobase Publishing |date=2004 |isbn=978-0-8160-6736-7 |pages=60–70 |url=https://books.google.com/books?id=sYgKY6zz20YC&pg=PA60 |access-date=10 August 2023 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194758/https://books.google.com/books?id=sYgKY6zz20YC&pg=PA60 |url-status=live }}</ref> === Multicellular structure === [[Multicellular organism]]s may have first evolved through the formation of [[Colony (biology)|colonies]] of identical cells. These cells can form group organisms through [[cell adhesion]]. The individual members of a colony are capable of surviving on their own, whereas the members of a true multi-cellular organism have developed specialisations, making them dependent on the remainder of the organism for survival. Such organisms are formed [[Clone (cell biology)|clonally]] or from a single [[germ cell]] that is capable of forming the various specialised cells that form the adult organism. This specialisation allows multicellular organisms to exploit resources more efficiently than single cells.<ref>{{cite book |first1=Bruce |last1=Alberts |first2=Dennis |last2=Bray |first3=Julian |last3=Lewis |first4=Martin |last4=Raff |first5=Keith |last5=Roberts |first6=James D. |last6=Watson |chapter=From Single Cells to Multicellular Organisms |title=Molecular Biology of the Cell |edition=3rd |location=New York |publisher=Garland Science |date=1994 |isbn=978-0-8153-1620-6 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK28332/ |access-date=12 June 2012 |url-access=registration |url=https://archive.org/details/molecularbiology00albe }}</ref> About 800 million years ago, a minor genetic change in a single molecule, the [[enzyme]] [[GK-PID]], may have allowed organisms to go from a single cell organism to one of many cells.<ref name="NYT-20160107">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=Genetic Flip Helped Organisms Go From One Cell to Many |url=https://www.nytimes.com/2016/01/12/science/genetic-flip-helped-organisms-go-from-one-cell-to-many.html |date=7 January 2016 |work=[[The New York Times]] |access-date=7 January 2016 |url-status=live |archive-url=https://web.archive.org/web/20160107204432/http://www.nytimes.com/2016/01/12/science/genetic-flip-helped-organisms-go-from-one-cell-to-many.html |archive-date=7 January 2016 }}</ref> Cells have evolved methods to perceive and respond to their microenvironment, thereby enhancing their adaptability. [[Cell signalling]] coordinates cellular activities, and hence governs the basic functions of multicellular organisms. Signaling between cells can occur through direct cell contact using [[juxtacrine signalling]], or indirectly through the exchange of agents as in the [[endocrine system]]. In more complex organisms, coordination of activities can occur through a dedicated [[nervous system]].<ref name=alberts2002>{{cite book |first1=Bruce |last1=Alberts |first2=Alexander |last2=Johnson |first3=Julian |last3=Lewis |first4=Martin |last4=Raff |first5=Keith |last5=Roberts |first6=Peter |last6=Walter |chapter=General Principles of Cell Communication |title=Molecular Biology of the Cell |location=New York |publisher=Garland Science |date=2002 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK26813/ |access-date=12 June 2012 |isbn=978-0-8153-3218-3 |url-status=live |archive-url=https://web.archive.org/web/20150904000612/http://www.ncbi.nlm.nih.gov/books/NBK26813/ |archive-date=4 September 2015 }}</ref> Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. If you do not want your writing to be edited mercilessly, then do not submit it here. 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