P21
P21 Protein
Cyclin-dependent kinase inhibitor 1A (p21, Cip1), also known as CDKN1A, is a protein which in humans is encoded by the CDKN1A gene located on chromosome 6 (6p21.2).
This gene WAF1/CIP1 on human chromosome 6p, discovered by Wafik El-Deiry in 1993, encodes a potent/universal cyclin-dependent kinase inhibitor (CKI). The encoded protein p21(WAF1) binds to and inhibits the activity of cyclin-CDK2 or -CDK4 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 1990's 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. The p21(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, if anything p21 inhibits apoptosis and does not under any circumstances induce cell death. Two alternatively spliced variants, which encode an identical protein, have been reported.
p21(WAF1) is a CKI that directly inhibits the activity of cyclin E/CDK2 and cyclin D/CDK4 complexes. p21 functions as a regulator of cell cycle progression at S phase. 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.
Protein
Proteins (also known as polypeptides) are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded in the genetic code. In general, the genetic code specifies 20 standard amino acids; however, in certain organisms the genetic code can include selenocysteine - and in certain archaea - pyrrolysine. Shortly after or even during synthesis, the residues in a protein are often chemically modified by post-translational modification, which alters the physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins. Proteins can also work together to achieve a particular function, and they often associate to form stable complexes.
Chromosome 6
Chromosome 6 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 6 spans more than 170 million base pairs (the building material of DNA) and represents between 5.5 and 6% of the total DNA in cells. It contains the Major Histocompatibility Complex, which contains over 100 genes related to the immune response, and plays a vital role in organ transplantation.
Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. Chromosome 6 likely contains between 1,100 and 1,600 genes.