The DIFFERENCE is 32....sorry….just being stupid : )
Below is portions of BOTH from WIKI
p21
p21 is a potent cyclin-dependent kinase inhibitor (CKI). The p21 (CIP1/WAF1) protein binds to and inhibits the activity of cyclin-CDK2 or -CDK1 complexes, and thus functions as a regulator of cell cycle progression at G1. The expression of this gene is tightly controlled by the tumor suppressor protein p53, through which this protein mediates the p53-dependent cell cycle G1 phase arrest in response to a variety of stress stimuli. This was a major discovery in the early 1990s that revealed how cells stop dividing after being exposed to damaging agents such as radiation. In addition to growth arrest, p21 can mediate cellular senescence and one of the ways it was discovered was as a senescent cell-derived inhibitor. Interestingly, studies of human embryonic stem cells (hESCs) commonly describe the nonfunctional p53-p21 axis of the G1/S checkpoint pathway with subsequent relevance for cell cycle regulation and the DNA damage response (DDR). Importantly, p21 mRNA is clearly present and upregulated after the DDR in hESCs, but p21 protein is not detectable. In this cell type, p53 activates numerous microRNAs (like miR-302a, miR-302b, miR-302c, and miR-302d) that directly inhibit the p21 expression in hESCs.[4]
The p21(CIP1/WAF1) protein can also interact with proliferating cell nuclear antigen (PCNA), a DNA polymerase accessory factor, and plays a regulatory role in S phase DNA replication and DNA damage repair. This protein was reported to be specifically cleaved by CASP3-like caspases, which thus leads to a dramatic activation of CDK2, and may be instrumental in the execution of apoptosis following caspase activation. However p21 may inhibit apoptosis and does not induce cell death on its own.[5] Two alternatively spliced variants, which encode an identical protein, have been reported.
p21(CIP1/WAF1) is a CKI that directly inhibits the activity of cyclin E/CDK2 and cyclin D/CDK4/6 complexes. p21 functions as a regulator of cell cycle progression at S phase.[6] The expression of p21 is controlled by the tumor suppressor protein p53. Sometimes,it is expressed without being induced by p53. This kind of induction plays a big role in p53 independent differentiation which is promoted by p21. Expression of p21 is mainly dependent on two factors 1) stimulus provided 2) type of the cell. Growth arrest by p21 can promote cellular differentiation. p21 therefore prevents cell proliferation.
The p21 protein also is important in the stress response.[7] p21 is a transcriptional target of the tumor suppressor gene, p53; despite this, loss-of-function mutations in p21 (unlike p53) do not accumulate in cancer nor do they predispose to cancer incidence. Mice genetically engineered to lack p21 develop normally and are not susceptible to cancer at a higher rate than wild-type mice (unlike p53 knockout mice).
Mice that lack the p21 gene gain the ability to regenerate lost appendages.
p53
p53 (also known as protein 53 or tumor protein 53), is a tumor suppressor protein that in humans is encoded by the TP53 gene.[2][3][4][5] p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer. As such, p53 has been described as "the guardian of the genome" because of its role in conserving stability by preventing genome mutation.[6]
The name p53 is in reference to its apparent molecular mass: It runs as a 53-kilodalton (kDa) protein on SDS-PAGE. But, based on calculations from its amino acid residues, p53's mass is actually only 43.7 kDa. This difference is due to the high number of proline residues in the protein, which slows its migration on SDS-PAGE, thus making it appear heavier than it actually is.[7] This effect is observed with p53 from a variety of species, including humans, rodents, frogs, and fish.
In humans, p53 is encoded by the TP53 gene located on the short arm of chromosome 17 (17p13.1).[2][3][4][5] The gene spans 20 kb, with a non-coding exon 1 and a very long first intron of 10 kb.The coding sequence contains five regions showing a high degree of conservation in vertebrates, predominantly in exons 2, 5, 6, 7 and 8, but the sequences found in invertebrates show only distant resemblance to mammalian TP53.[8] TP53 orthologs[9] have been identified in most mammals for which complete genome data are available.
In humans, a common polymorphism involves the substitution of an arginine for a proline at codon position 72. Many studies have investigated a genetic link between this variation and cancer susceptibility, however, the results have been controversial. For instance, a meta-analysis from 2009 failed to show a link for cervical cancer.[10] A 2011 study found that the TP53 proline mutation did have a profound effect on pancreatic cancer risk among males.[11] A study of Arab women found that proline homozygosity at TP53 codon 72 is associated with a decreased risk for breast cancer.[12] One study suggested that TP53 codon 72 polymorphisms, MDM2 SNP309, and A2164G may collectively be associated with non-oropharyngeal cancer susceptibility and that MDM2 SNP309 in combination with TP53 codon 72 may accelerate the development of non-oropharyngeal cancer in women.[13] A 2011 study found that TP53 codon 72 polymorphism was associated with an increased risk of lung cancer.[14]
Meta-analyses from 2011 found no significant associations between TP53 codon 72 polymorphisms and both colorectal cancer risk[15] and endometrial cancer risk.[16] A 2011 study of a Brazilian birth cohort found an association between the non mutant arginine TP53 and individuals without a family history of cancer.[17] Another 2011 study found that the p53 homozygous (Pro/Pro) genotype was associated with a significantly increased risk for renal cell carcinoma.[18]
Function[edit]
p53 has many mechanisms of anticancer function, and plays a role in apoptosis, genomic stability, and inhibition of angiogenesis. In its anti-cancer role, p53 works through several mechanisms:
• It can activate DNA repair proteins when DNA has sustained damage.
• It can arrest growth by holding the cell cycle at the G1/S regulation point on DNA damage recognition (if it holds the cell here for long enough, the DNA repair proteins will have time to fix the damage and the cell will be allowed to continue the cell cycle).
• It can initiate apoptosis, the programmed cell death, if DNA damage proves to be irreparable.
p53 pathway: In a normal cell p53 is inactivated by its negative regulator, mdm2. Upon DNA damage or other stresses, various pathways will lead to the dissociation of the p53 and mdm2 complex. Once activated, p53 will induce a cell cycle arrest to allow either repair and survival of the cell or apoptosis to discard the damaged cell. How p53 makes this choice is currently unknown.
Activated p53 binds DNA and activates expression of several genes including microRNA miR-34a,[27] WAF1/CIP1 encoding for p21 and hundreds of other down-stream genes. p21 (WAF1) binds to the G1-S/CDK (CDK2) and S/CDK complexes (molecules important for the G1/S transition in the cell cycle) inhibiting their activity.
When p21(WAF1) is complexed with CDK2 the cell cannot continue to the next stage of cell division. A mutant p53 will no longer bind DNA in an effective way, and, as a consequence, the p21 protein will not be available to act as the "stop signal" for cell division.[28] Studies of human embryonic stem cells (hESCs) commonly describe the nonfunctional p53-p21 axis of the G1/S checkpoint pathway with subsequent relevance for cell cycle regulation and the DNA damage response (DDR). Importantly, p21 mRNA is clearly present and upregulated after the DDR in hESCs, but p21 protein is not detectable. In this cell type, p53 activates numerous microRNAs (like miR-302a, miR-302b, miR-302c, and miR-302d) that directly inhibit the p21 expression in hESCs.[29]
Recent research has also linked the p53 and RB1 pathways, via p14ARF, raising the possibility that the pathways may regulate each other.[30]
p53 by regulating LIF has been shown to facilitate implantation in the mouse model and possibly in humans.[31]
p53 expression can be stimulated by UV light, which also causes DNA damage. In this case, p53 can initiate events leading to tanning.[32][33]
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