Editing Cell (biology) (section)

== Cellular processes ==

[[File:Three cell growth types.svg|thumb|upright=1.25|[[Prokaryotes]] divide by [[binary fission]], while [[eukaryotes]] divide by [[mitosis]] or [[meiosis]].]]

=== Replication ===

{{main|Cell division}}

Cell division involves a single cell (called a ''mother cell'') dividing into two daughter cells. This leads to growth in [[multicellular organism]]s (the growth of [[biological tissue|tissue]]) and to procreation ([[vegetative reproduction]]) in [[unicellular organism]]s. [[Prokaryote|Prokaryotic]] cells divide by [[binary fission]], while [[Eukaryote|eukaryotic]] cells usually undergo a process of nuclear division, called [[mitosis]], followed by division of the cell, called [[cytokinesis]]. A [[diploid]] cell may also undergo [[meiosis]] to produce haploid cells, usually four. [[Haploid]] cells serve as [[gamete]]s in multicellular organisms, fusing to form new diploid cells.

[[DNA replication]], or the process of duplicating a cell's genome,<ref name="NCBI"/> always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the [[cell cycle]].

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before [[meiosis I]]. DNA replication does not occur when the cells divide the second time, in [[meiosis II]].<ref>{{cite book|title=Campbell Biology{{snd}}Concepts and Connections|year=2009|publisher=Pearson Education|page=138}}</ref> Replication, like all cellular activities, requires specialized proteins for carrying out the job.<ref name="NCBI"/>

=== DNA repair ===

{{main|DNA repair}}

Cells of all organisms contain enzyme systems that scan their DNA for [[DNA damage (naturally occurring)|damage]] and carry out [[DNA repair|repair processes]] when it is detected. Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damage that could lead to [[mutation]]. [[Escherichia coli|''E. coli'']] bacteria are a well-studied example of a cellular organism with diverse well-defined [[DNA repair]] processes. These include: [[nucleotide excision repair]], [[DNA mismatch repair]], [[non-homologous end joining]] of double-strand breaks, [[homologous recombination|recombinational repair]] and light-dependent repair ([[photolyase|photoreactivation]]).<ref>{{cite book |last1=Snustad |first1=D. Peter |last2=Simmons |first2=Michael J. |title=Principles of Genetics |edition=5th |at=DNA repair mechanisms, pp. 364–368}}</ref>

=== Growth and metabolism ===

{{main|Cell growth|Metabolism|Photosynthesis}}

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: [[catabolism]], in which the cell breaks down complex molecules to produce energy and [[Reducing agent|reducing power]], and [[anabolism]], in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.

Complex sugars can be broken down into simpler sugar molecules called [[monosaccharides]] such as [[glucose]]. Once inside the cell, glucose is broken down to make adenosine triphosphate ([[adenosine triphosphate|ATP]]),<ref name="NCBI"/> a molecule that possesses readily available energy, through two different pathways. In plant cells, [[chloroplast]]s create sugars by [[photosynthesis]], using the energy of light to join molecules of water and [[carbon dioxide]].

=== Protein synthesis ===

{{main|Protein biosynthesis}}

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from [[amino acid]] building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: [[transcription (genetics)|transcription]] and [[translation (genetics)|translation]].

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give [[messenger RNA]] (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called [[ribosome]]s located in the [[cytosol]], where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to [[transfer RNA]] (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.

=== Motility ===

{{main|Motility}}

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include [[flagella]] and [[cilia]].

In multicellular organisms, cells can move during processes such as wound healing, the immune response and [[cancer metastasis]]. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.<ref name="Ananthakrishnan Ehrlicher 2007">{{cite journal |last1=Ananthakrishnan |first1=R. |last2=Ehrlicher |first2=A. |title=The forces behind cell movement |journal=International Journal of Biological Sciences |volume=3 |issue=5 |pages=303–317 |date=June 2007 |pmid=17589565 |pmc=1893118 |doi=10.7150/ijbs.3.303 |publisher=Biolsci.org }}</ref> The process is divided into three steps: protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.<ref name="Alberts2">{{cite book |last1=Alberts |first1=Bruce |title=Molecular biology of the cell |date=2002 |publisher=Garland Science |isbn=0815340729 |pages=973–975 |edition=4th}}</ref><ref name="Ananthakrishnan Ehrlicher 2007"/>

==== Navigation, control and communication ====

{{See also|Cybernetics#In biology}}

In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able to [[Chemotaxis|navigate]] efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused [[chemoattractant]]s which enable them to sense upcoming maze junctions before reaching them, including around corners.<ref>{{cite news |last1=Willingham |first1=Emily |title=Cells Solve an English Hedge Maze with the Same Skills They Use to Traverse the Body |url=https://www.scientificamerican.com/article/cells-solve-an-english-hedge-maze-with-the-same-skills-they-use-to-traverse-the-body/ |access-date=7 September 2020 |work=Scientific American |language=en |archive-date=4 September 2020 |archive-url=https://web.archive.org/web/20200904102655/https://www.scientificamerican.com/article/cells-solve-an-english-hedge-maze-with-the-same-skills-they-use-to-traverse-the-body/ |url-status=live }}</ref><ref>{{cite news |title=How cells can find their way through the human body |url=https://phys.org/news/2020-08-cells-human-body.html |access-date=7 September 2020 |work=phys.org |language=en |archive-date=3 September 2020 |archive-url=https://web.archive.org/web/20200903220400/https://phys.org/news/2020-08-cells-human-body.html |url-status=live }}</ref><ref>{{cite journal |last1=Tweedy |first1=Luke |last2=Thomason |first2=Peter A. |last3=Paschke |first3=Peggy I. |last4=Martin |first4=Kirsty |last5=Machesky |first5=Laura M. |last6=Zagnoni |first6=Michele |last7=Insall |first7=Robert H.|title=Seeing around corners: Cells solve mazes and respond at a distance using attractant breakdown |journal=Science |volume=369 |issue=6507 |date=August 2020 |page=eaay9792 |pmid=32855311 |doi=10.1126/science.aay9792 |s2cid=221342551 |url=https://www.science.org/doi/10.1126/science.aay9792 |access-date=2020-09-13 |archive-date=2020-09-12 |archive-url=https://web.archive.org/web/20200912234645/https://science.sciencemag.org/content/369/6507/eaay9792 |url-status=live }}</ref>